JP2012046590A - White fluorescent thin film, method for production thereof, and light-emitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a perovskite type fluorescent oxide thin film which has excellent chemical stability and white fluorescence property and is useful as thin-film electroluminescent devices for illuminators/light sources, display and the like.SOLUTION: The characteristic of emitting white fluorescence in which the fluorescent intensity is high throughout the whole wavelength range of 450-700 nm is obtained by a perovskite type inorganic oxide thin film prepared by adding to CaTiO, a Bi element in an amount of 0.1-0.4 atom%. The thin film is preferably obtained by using a target comprising CaTiOcontaining the Bi element to form a thin film by a pulsed laser deposition method at a temperature of 600-1,000°C in an atmosphere having an oxygen pressure of 200-1,000 mTorr and by heat-treating the thin film in the atmosphere at a temperature of 900-1,100°C.

Description

  The present invention relates to a white phosphor thin film of a perovskite oxide having excellent chemical stability and excellent color rendering properties, and a method for producing the same, and a light emitting device using the white phosphor thin film.

  Conventionally, luminaires mainly used are incandescent bulbs and fluorescent lamps. A fluorescent lamp can efficiently emit ultraviolet rays of 254 nm and 185 nm by discharging mercury at a low vapor pressure. The ultraviolet light is converted into white light by the phosphor applied in the glass tube. Fluorescent lamps generate much less heat than incandescent bulbs and require several percent of power to obtain the same brightness. However, in recent years, regulations have been issued in Europe to prohibit the use of harmful substances such as lead and mercury in electrical and electronic equipment-related devices and devices in accordance with the RoHS directive. This is expected to limit the use of fluorescent lamps that have been used so far, and the development of new lighting devices is urgently needed. Candidates for future lighting include white LEDs, organic / inorganic EL, etc., but what has been determined at this time has not been determined, and the development of white phosphors with excellent chemical stability is required. It has been.

In recent years, development of three primary colors of red, green, and blue, which are the basis for display production with a thin film, has been promoted. The following research and development has been made on phosphors made of oxide thin films having a perovskite structure (see Patent Documents 1 and 2; see Non-Patent Documents 1 to 3). Non-Patent Document 1 shows that red, blue, green, and white fluorescence characteristics can be obtained by ultraviolet excitation in a tin-based perovskite structure-related oxide thin film. Non-Patent Document 3 shows that red, blue, and green fluorescence appear by ultraviolet excitation when an appropriate rare earth element is added in a zirconia-based perovskite oxide thin film. Further, the present inventors have shown that red fluorescence appears by ultraviolet excitation when Pr is added in a (CaSr) TiO ₃ perovskite oxide thin film (see Non-Patent Document 2).

In Patent Document 1 by the present inventors, an excellent red phosphor oxide epitaxial thin film formed by a pulse laser deposition method using a polycrystalline target material in which an aluminum element is added to a Pr-substituted SrTiO ₃ perovskite oxide. It has been shown that the fluorescence characteristics of Specifically, it is shown that an oxide phosphor epitaxial thin film is formed on a substrate by epitaxial growth at a temperature of 600 ° C. or higher and 800 ° C. or lower by a pulse laser deposition method.

  In addition, Patent Document 2 by the present inventors shows that red, blue, green, and white fluorescence characteristics can be obtained by ultraviolet excitation in a tin-based perovskite structure-related oxide thin film. Specifically, it is shown that an oxide phosphor epitaxial thin film is formed on a substrate by epitaxial growth at a temperature of 600 ° C. or higher and 800 ° C. or lower by a pulse laser deposition method.

Further, when the prior art is investigated for the white phosphor, Patent Document 3 shows the white phosphor. Patent Document 3 includes aM ¹ O · bM ² ₂ O ₃ · cM ³ O ₂ (where M ¹ is one or more elements selected from the group consisting of Ba, Sr, Ca, Mg, and Zn; ² is one or more elements selected from the group consisting of Al, Sc, Ga, Y, In, La, Gd and Lu, and M ³ is selected from the group consisting of Si, Ti, Ge, Zr, Sn and Hf. An activator, wherein a is 8 or more and 10 or less, b is 0.8 or more and 1.2 or less, and c is 5 or more and 7 or less. Describes a phosphor containing one or more elements selected from the group consisting of rare earth elements, Mn and Bi. However, the oxide of Patent Document 3 does not have a perovskite structure.

JP 2009-155376 A JP 2008-19317 A JP 2007-77307 A

Appl. Phys. Express. 1, (2008) 150031-150033 Applied Physics Letters. 89 (2006) 2619151-3 Mater. Sci. Eng. B, 161 (2009) 100-103

  Conventionally, a large number of phosphors such as organic EL and inorganic EL have been obtained, but there has been a problem that the crystallinity is lowered by oxidation and the aging of the fluorescence characteristics is remarkably deteriorated. In addition, in the oxide polycrystal, good phosphors have been obtained in recent years, and the development of three primary colors of red, green, and blue, which are the basis for display production with thin films, has been carried out. There was a problem that the color rendering was not sufficient. In addition, in the application of lighting and backlights for liquid crystal monitors, EL development using thin films is indispensable, and development of oxide white phosphor epitaxial thin films is desired.

  As described above, a white phosphor thin film excellent in chemical stability is required. However, a white phosphor characteristic cannot be realized in a perovskite inorganic oxide thin film having a simple crystal structure. In Non-Patent Document 1 and Patent Document 2, tin-based perovskite structure-related oxide thin films show white fluorescence characteristics, but the problem of poor color rendering because white was realized by the correlation of a plurality of sharp emission spectra. was there.

  The present invention is intended to solve these problems, and an object of the present invention is to provide a phosphor epitaxial thin film having a white fluorescent property with excellent color rendering by ultraviolet excitation using a perovskite oxide. . It is another object of the present invention to provide a manufacturing method that improves white fluorescent characteristics. It is another object of the present invention to provide a light emitting device using a perovskite oxide white phosphor epitaxial thin film.

The present invention introduces a small amount of bismuth Bi having an atomic number of 83 into CaTiO ₃ , which is known as a perovskite-type oxide, and has a white fluorescent property with excellent broad color rendering in a wavelength region having a center near 540 nm. To achieve a phosphor thin film. In order to achieve the above object, the present invention has the following features.

The present invention is a method for producing a white phosphor thin film of a perovskite oxide, wherein a perovskite oxide obtained by adding Bi element to CaTiO ₃ in an amount of 0.1 atomic% to 0.4 atomic% is used as a target material. A thin film is formed by epitaxial growth on a substrate. It can be said that the added amount of Bi is mol% with respect to CaTiO ₃ . In CaTiO ₃ , the ratio of Ca to Ti is 1: 1, and the amount of Bi added is atomic% when Ca or Ti is 100%. In the method for producing a white phosphor thin film of the present invention, it is preferable to perform heat treatment at 900 ° C. or higher and 1100 ° C. or lower in the air after forming the thin film. In addition, the thin film is preferably formed at a temperature of 600 ° C. or higher and 1000 ° C. or lower. Further, it is preferable to form the thin film in an oxygen pressure atmosphere of 200 mTorr or more and 1000 mTorr or less. Further, it is preferable to use a pulse laser deposition method.

The present invention is a white phosphor thin film of a perovskite oxide, characterized in that it is an epitaxial thin film in which Bi element is added in an amount of 0.1 atomic% to 0.4 atomic% to CaTiO ₃ . The epitaxial thin film can be produced by the manufacturing method. The perovskite oxide phosphor thin film of the present invention has an emission spectrum with white fluorescence characteristics over the entire wavelength range of 450 to 700 nm. In the white phosphor thin film of the perovskite oxide of the present invention, the substrate used for the epitaxial thin film is preferably any one of SrTiO ₃ , LaAlO ₃ , LaGaO ₃ , MgO, and LaSrGaO ₄ .

The present invention is a light-emitting device, and is characterized in that the epitaxial thin film of the present invention in which Bi element is added in an amount of 0.1 atomic% to 0.4 atomic% to CaTiO ₃ is used as a phosphor thin film.

According to the present invention, a white phosphor thin film with excellent color rendering can be obtained by adding Bi element to a CaTiO ₃ thin film of a perovskite oxide. By using a perovskite oxide thin film, a white phosphor thin film excellent in chemical stability can be obtained. In particular, when the addition amount of Bi element is 0.1 atomic% or more and 0.4 atomic% or less, a broad emission spectrum can be obtained in the entire wavelength range of 450 to 700 nm, and the emission intensity is large. By using any one of SrTiO ₃ , LaAlO ₃ , LaGaO ₃ , MgO, and LaSrGaO ₄ as the substrate, an epitaxial thin film having excellent fluorescence characteristics can be formed.

In the present invention, a perovskite oxide in which Bi element is added to CaTiO ₃ by 0.1 atomic% or more and 0.4 atomic% or less can be epitaxially grown on the substrate, so that it is almost white. A Bi-added CaTiO ₃ thin film exhibiting fluorescence characteristics can be obtained. After the thin film is formed, excellent fluorescence characteristics can be obtained by performing a heat treatment at 900 ° C. to 1100 ° C. in the air. The target material can be deposited with its stoichiometric composition by pulsed laser deposition. Further, by forming the thin film at a temperature of 600 ° C. or higher and 1000 ° C. or lower, cluster growth becomes dominant, and the target material can be formed with its stoichiometric composition. Further, by forming a thin film in an oxygen pressure atmosphere of 200 mTorr or more and 1000 mTorr or less, a broad emission spectrum can be obtained over the entire wavelength range of 450 to 700 nm.

(A) Diffuse reflection spectrum and (b) Excitation / emission spectrum of CaTiO ₃ : Bi (0.2 at.%) Of the present invention In the present invention, Bi is set to 0, 0.1, 0.2, 0.3, 0.4, 0.6 at. % Diffused reflection spectrum (upper) and excitation emission spectrum (lower) of CaTiO ₃ : Bi Luminescence intensity with respect to Bi concentration in the present invention X-ray diffraction pattern when grown at an oxygen pressure of 700 mTorr 600 ° C. in Example 2 Fluorescence characteristics when film formation was performed under three conditions of oxygen pressure of 10 mTorr, 100 mTorr, and 700 mTorr in Example 2.

Embodiments of the present invention will be described below. The present inventors first conducted the following investigation on the possibility of white fluorescence characteristics in bulk perovskite oxides. We searched for new perovskite phosphors under optimized synthesis conditions. CaTiO ₃ , SrTiO ₃ , and BaTiO ₃ were used as the base material, and an element expected as a luminescence center was added thereto. Table 1 shows the combinations of elements that can be expected as the base material and the emission center.

Luminescence was observed from a sample in which Bi was added to the combination of CaTiO ₃ in Table 1. Luminescence could not be observed from other combinations.

By introducing a small amount of bismuth Bi having an atomic number of 83 into CaTiO ₃ , which is a perovskite oxide, the present inventors have a white fluorescent property with excellent broad color rendering in a wavelength region having a center near 540 nm. An application has already been filed for what is obtained (Japanese Patent Application No. 2010-178264). The application mainly examines the white fluorescence characteristics in the structure of powder and bulk bodies. Hereinafter, the white fluorescence characteristic of the perovskite oxide obtained by adding a small amount of Bi to CaTiO ₃ will be described.

Production of a sample in which Bi is added to CaTiO ₃ will be described. CaTiO ₃ : Bi is the following using CaCO ₃ (99.99%), TiO ₂ (99.99%), B ₂ O ₃ (99.9%), Bi ₂ O ₃ (99.9%). The procedure was as follows. CaCO ₃ powder and TiO ₂ powder were always dried in an oven at 120 ° C. to prevent moisture absorption. First, CaCO ₃ and B ₂ O ₃ were dry-mixed and fired at 850 ° C. for 12 h to synthesize CaB ₂ O ₄ serving as a flux (confirmed that there were no impurities by powder X-ray diffraction). Next, Bi ₂ O ₃ was dissolved in nitric acid, excess nitric acid was evaporated to bismuth nitrate, and this was dissolved in ethanol to prepare an ethanol solution of Bi (NO ₃ ) ₃ . CaCO ₃ , TiO ₂ , and CaB ₂ O ₄ were weighed at a molar ratio of 0.975: 1: 0.025 so that 1 g of CaTiO ₃ : Bi was synthesized, and a Bi (NO ₃ ) ₃ ethanol solution was added thereto. Bi is added to CaTiO ₃ so that it becomes 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 mol%, and a few ml of ethanol is further added. Wet mixing was performed until ethanol evaporated in the mortar. The mixed powder was put into a platinum boat, heated to 1100 ° C. at 200 ° C./h, fired at that temperature for 6 h, and then cooled to room temperature to synthesize a CaTiO ₃ : Bi powder sample. The obtained powder was slightly agglomerated, but this was lightly pulverized and used as a measurement sample.

The detailed light emission characteristics of CaTiO ₃ : Bi were examined below. First, it was found that white light was emitted when CaTiO ₃ : Bi was irradiated with ultraviolet rays (330 nm). FIG. 1 (a) and FIG. 1 (b) show diffuse reflection and excitation / emission spectra of CaTiO ₃ : Bi (0.2 at.%), Respectively. A broad emission was observed from 450 to 700 nm, which is a factor that appears white in the emission spectrum. From the excitation spectrum, a peak A ′ at 335 nm and a shoulder B ′ at 365 nm were observed. From the reflection spectrum, absorption due to interband transition of CaTiO ₃ was observed below 335 nm and absorption due to Bi 3+ sp transition was observed near 365 nm.

Next, the Bi concentration dependency of the emission intensity will be described. The amount of Bi added is 0, 0.1, 0.2, 0.3, 0.4, 0.6 at. % Samples were synthesized and the optimum composition ratio of Bi was examined. In addition, whether the lattice constant of CaTiO ₃ was changed by adding Bi was examined by comparing the diffraction peak positions obtained by powder X-ray diffraction experiments. In Table 2, Bi is set to 0 at. % Of the sample synthesized with the addition of 0.6 at. The lattice constant of the sample synthesized by adding% is shown. In addition, the numerical value in parenthesis shows the error of the numerical value of the last digit.

  In Table 2, 0 at. % Of the sample synthesized with the addition of 0.6 at. %, The lattice constants of the samples synthesized by the addition were equal within the error, so that 0.6 at. It can be seen that no change in the lattice constant is observed when% is added.

In FIG. 2, the amount of Bi added is 0, 0.1, 0.2, 0.3, 0.4, 0.6 at. The diffuse reflection spectrum (upper part) and excitation emission spectrum (lower part) of the sample (CaTiO ₃ : Bi) in% are shown. Bi is set to 0.2 at. %, The optimum amount of Bi added to CaTiO ₃ is 0.2 at. %.

  FIG. 3 shows the emission intensity with respect to the Bi concentration. The emission intensity is 0 at. % To 0.2 at. It can be seen that it has increased to% and has since decreased.

According to the above powder / bulk examples, it can be seen that by adding Bi to CaTiO ₃ , it has characteristics as a white phosphor. In general, the light emission of a phosphor using Bi is assumed to be an sp transition, and thus the light emission of the present invention is considered to be due to the Bi sp transition. Since the wavelength of the peak A ′ in the excitation / emission spectrum shown in FIG. 1 corresponds to the band gap of CaTiO ₃ , the energy of the electron-hole pair generated by the interband transition of the base material moves to Bi and emits light. It is thought that there is. As the additive concentration of Bi increases, the absorption near 365 nm in the reflection spectrum increases, and this absorption is considered to be due to the sp transition of Bi3 +. Therefore, it is considered that the peak B ′ observed near 365 nm in the excitation / emission spectrum is due to the Bi 3+ sp transition.

From the above, the amount of Bi added to CaTiO ₃ is 0.1 to 0.4 at. % Is preferred. Furthermore, 0.1 to 0.3 at. % Indicates that the emission intensity is high (FIGS. 2 and 3), which is preferable.

Example 1
With respect to the perovskite oxide obtained by adding Bi to CaTiO ₃ as described above, a thin film shape was obtained in this example. This will be described below with reference to the drawings. In this example, a thin film was formed using a pulsed laser deposition method.

The pulse laser deposition method can form a thin film of about 500 nm in a short time (typical film formation time is 30 minutes), and is a film formation method expected to be applied to engineering. In addition, since the film can be formed in an oxygen stream, deterioration of electrical characteristics and fluorescence characteristics due to oxygen deficiency or the like can be extremely reduced during oxide thin film growth. In the pulsed laser deposition method, an ArF (wavelength of 193 nm) excimer laser is irradiated to a target material made of oxide in low-pressure oxygen of 1 Torr or less, and the target material is turned into plasma to form a plume, which is opposed to the target material. A superheated substrate is placed on the surface and a thin film is deposited. Cluster growth is dominant at a temperature of 1000 ° C. or lower, and the target material can be deposited with its stoichiometric composition. In this example, a perovskite oxide in which Bi was added to CaTiO ₃ was used as a polycrystalline target material, and the target material was irradiated with a pulse laser, and a thin film having the same composition as the target material was epitaxially grown on the substrate. The distance between the substrate and the target was 32 mm. The laser irradiation frequency was 8 Hz, the film formation time was 30 minutes, and the film thickness was 300 nm. The laser energy is about 120 mJ.

As the substrate, a SrTiO ₃ single crystal substrate polished so that the substrate surface was (001) was used. The crystal structure of SrTiO ₃ is tetragonal and the lattice constant is 3.905 nm. An oxide epitaxial thin film having excellent crystallinity can be grown because it has a lattice constant in the vicinity of the perovskite oxide obtained by adding Bi to CaTiO _{3 of} this example and has good lattice matching.

As the substrate, in addition to the SrTiO ₃ substrate, LaAlO ₃ , LaGaO ₃ , MgO, LaSrGaO ₄ having good lattice matching can be used.

(Example 2)
In this example, manufacturing conditions for manufacturing a white phosphor thin film made of CaTiO ₃ to which Bi is added will be discussed below. The case where the best fluorescence characteristic was obtained and the addition amount of Bi was 0.2 atomic% was examined.

In the pulse laser deposition method, the film was formed under three conditions: the substrate temperature was 600 ° C., and the oxygen pressure during film formation was 10 mTorr, 100 mTorr, and 700 mTorr. As the substrate, a SrTiO ₃ (001) single crystal polishing substrate was used. X-ray diffraction was measured to investigate the crystal structure. As a result, it was confirmed that the (001) thin film was grown at all oxygen pressures. FIG. 4 shows an X-ray diffraction pattern during growth at 600 ° C. and an oxygen pressure of 700 mTorr as a typical example. In the figure, STO _sub means SrTiO ₃ substrate, and BiCTO means Bi-added CaTiO ₃ . Appearance other than the peak indexed by (00l) is not confirmed, and no impurity peak is confirmed. As shown enlarged in the frame in FIG. 4, it can be seen that the peak of (002) is clearly separated into the peak of the substrate and the peak of the thin film, and the lattice constant of the thin film BiCTO is higher than that of the substrate STO. Since it is small, a peak appears on the high angle side. As a result, the lattice mismatch of SrTiO ₃ (002) / CaTiO ₃ : Bi (002) is about 2.5%, but it can be seen that the epitaxial growth is achieved.

  When the fluorescence characteristics of the sample immediately after thin film growth were examined, no clear fluorescence characteristics were obtained. However, when they were heat-treated at 1000 ° C. in the atmosphere, a broad peak was observed around 540 nm when excited at an ultraviolet wavelength of 330 nm in a sample grown at 700 mTorr. FIG. 5 shows fluorescence characteristics after film formation was performed under three conditions of a substrate temperature of 600 ° C. and an oxygen pressure of 10 mTorr, 100 mTorr, and 700 mTorr, and a heat treatment in the atmosphere at 1000 ° C. This is considered to be caused by the Bi3 + sp transition. From FIG. 5, it can be seen that a broad fluorescence characteristic cannot be obtained at an oxygen pressure of 10 mTorr or 100 mTorr, but a remarkable broad characteristic can be obtained at 700 mTorr.

From FIG. 5, the oxygen pressure condition is preferably 200 mTorr or more and 1000 Torr or less, and more preferably 400 mTorr or more and 1000 mTorr or less. Furthermore, the oxygen pressure conditions will be examined. The pulse laser deposition method is a method by which a material having the same composition as the target material composition can be formed on a substrate by matching the distance between the substrate and the target and the mean free path of the target element particles. The melting point of Bi element is 271 ° C., and a material having a low melting point temperature has a feature that it is easily scattered in plasma. Since the mean free path of particles becomes long in low-pressure oxygen of 100 mTorr or less, only the Bi element in the Bi-added CaTiO _{3 as the} target material is scattered over the distance between the substrate and the target, and the Bi element is scattered in the thin film on the substrate. Lack. As a result, it is considered that fluorescence characteristics cannot be obtained. When the oxygen pressure is 700 mTorr, the mean free path and the distance between the substrate and the target match, so that the film adheres on the substrate with the composition of the target material. As a result, a more preferable degree of vacuum (oxygen pressure) is 400 mTorr to 1000 mTorr.

  From the above results, it was found that heat treatment in the atmosphere at 1000 ° C. was necessary after producing in an atmosphere having an oxygen pressure of 200 mTorr or more in order to obtain the same fluorescence characteristics as the bulk material. In addition, when the heat treatment temperature in the atmosphere is 900 ° C. and 1100 ° C., similar fluorescence was obtained, and thus the heat treatment temperature is preferably 900 ° C. or higher and 1100 ° C.

  Although the case where the substrate temperature during film formation is 600 ° C. has been described, a thin film can be formed at a substrate temperature of 600 ° C. or higher and 1000 ° C. or lower. This is because cluster growth is dominant at a temperature of 1000 ° C. or lower, and the target material can be deposited with its stoichiometric composition. In addition, at 500 degrees C or less, it becomes amorphous and a fluorescence characteristic is not acquired.

  Although the case where excitation is performed at an ultraviolet wavelength of 330 nm has been described, the same result can be obtained when excitation is performed in a region where the excitation wavelength is 250 nm or more and 350 nm or less.

  In this example, the case where the addition amount of Bi was set to 0.2 atomic% was examined, but the same tendency can be obtained even when the addition amount of Bi is 0.1 to 0.4 atomic%.

  In the embodiment, an example of forming a film by a pulse laser deposition method has been shown. However, as a film formation method for growing an epitaxial film on a substrate, a vapor phase growth method such as a sputtering method is used in addition to the pulse laser deposition method. Can do. At that time, the thin film forming temperature, the oxygen pressure, and the heat treatment temperature are manufactured at the same temperature as the temperature shown in the embodiment, so that the same fluorescence characteristics as in the embodiment can be obtained.

  The examples shown in the embodiment and the like are described for easy understanding of the invention, and are not limited to this embodiment.

  The perovskite-type inorganic oxide phosphor thin film of the present invention realizes white fluorescence characteristics with excellent color rendering properties and excellent chemical stability, so that it is a thin film electroluminescence device for illumination, light source, display, etc. Available to:

Claims (9)

A method for producing a white phosphor thin film of perovskite oxide,
A white phosphor thin film characterized in that a thin film is formed on a substrate by epitaxial growth using a perovskite oxide in which Bi element is added at 0.1 atomic% or more and 0.4 atomic% or less to CaTiO ₃ as a target material. Production method.   2. The method for producing a white phosphor thin film according to claim 1, wherein after the thin film is formed, heat treatment is performed at 900 ° C. to 1100 ° C. in the air.   The method for producing a white phosphor thin film according to claim 1 or 2, wherein the thin film is formed at a temperature of 600 ° C or higher and 1000 ° C or lower.   4. The method for producing a white phosphor thin film according to claim 1, wherein the thin film is formed in an oxygen pressure atmosphere of 200 mTorr or more and 1000 mTorr or less.   5. The method for producing a white phosphor thin film according to claim 1, wherein the thin film is formed by a pulse laser deposition method. A white phosphor thin film of perovskite oxide,
A white phosphor thin film characterized by being an epitaxial thin film in which Bi element is added in an amount of 0.1 atomic% to 0.4 atomic% to CaTiO ₃ .   7. The white phosphor thin film according to claim 6, wherein the white phosphor thin film has an emission spectrum of white fluorescence characteristics over a wavelength range of 450 to 700 nm. The white phosphor thin film according to claim 6 or 7, wherein the substrate used for the epitaxial thin film is any one of SrTiO ₃ , LaAlO ₃ , LaGaO ₃ , MgO, and LaSrGaO ₄ .   A light emitting device comprising the white phosphor thin film according to claim 6.

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