Conventionally widely known are light emitting devices, each of which utilizes a light emitting diode (LED) or laser diode (LD) as a light source, converts a wavelength of emission light from the light source and thereby generate light of desired color or wavelength, as discussed in Patent Documents 1 to 4.
In recent years, various light emitting devices using LEDs or LDs as light sources have been developed. As one such type of light emitting device, there has been developed a light emitting device that emits white light by the use of e.g. LED as a light source and realizes power-saving performance and high-color-rendering properties. In general, currently commercially available white light emitting devices utilize blue GaN LEDs as light sources in combination of yellow phosphors so as to convert parts of blue emission light from the LEDs to yellow light by the action of the yellow phosphors, mix the blue emission light with the converted yellow light and generate pseudo white light. Cerium-doped YAG oxide phosphors are widely used as the above yellow phosphors.
In the case of using a phosphor in combination with a LED or LD light source, it is common practice to seal the light source with a mixture of the phosphor and an encapsulating resin such as epoxy resin, silicon resin or fluorine resin. In this case, there is a problem that the resin may be deteriorated due to heat generation from the LED or LD or by ultraviolet or blue light radiation from the LED or LD to cause discoloration, deterioration of light transmission characteristics etc. Depending on the kind of the phosphor, there is also a problem that the phosphor may be damaged and deactivated by the entry of ambient moisture through the encapsulating resin.
Under these circumstances, attention is being focused on the use of glass materials as encapsulants in view of the fact that the glass materials have higher resistance to heat and light than resin materials and show high water-barrier properties. For example, it is conceivable to mix a glass powder with a phosphor powder, sinter the glass-phosphor powder mixture and seal a LED with the sintered glass-phosphor body (also referred to as “phosphor-dispersed glass”). As an application example of such seal structure, Patent Document 5 discloses a white light emitting device that utilizes a sintered body formed by mixing a SnO—P2O5—ZnO glass powder with an oxide phosphor powder and sintering the glass-phosphor powder mixture. Patent Document 6 discloses a weather-resistant emission color conversion material that utilizes a SiO2—TiO2—Nb2O5—R2O (R=Li, Na, K) glass composition substantially free of PbO and having a softening point of 650° C. or lower.
Patent Document 5 discloses a method of sealing a LED with the use of a glass powder and a phosphor powder by sintering the glass-phosphor powder mixture into a phosphor-dispersed glass, placing the phosphor-dispersed glass on the LED, softening the phosphor-dispersed glass and thereby allowing the phosphor-dispersed glass to flow over and seal the LED, or by densely covering the LED with the glass-phosphor powder mixture, softening the glass-phosphor powder mixture and thereby allowing the glass-phosphor powder mixture to flow over and seal the LED simultaneously with forming the glass-phosphor mixture into a phosphor-dispersed glass. Patent Document 6 discloses a method of sealing a LED with the use of a glass powder and a phosphor powder by kneading the glass powder and the phosphor powder together with a binder, a solvent and the like, applying the resulting paste to the LED and sintering the applied paste into a phosphor-dispersed glass so as to allow sealing of the LED simultaneously with formation of the phosphor-dispersed glass, or by forming a green sheet of the same material as the above glass-phosphor paste, stacking the green sheet on the LED, thermocompression bonding the green sheet to the LED and then sintering the green sheet.
Further, Patent Document 7 discusses that a sulfide phosphor, an aluminate phosphor and a silicate phosphor are inferior in moisture resistance and thus proposes a method of producing a phosphor-dispersed glass by mixing a powder of such a phosphor with a glass powder and sintering the glass-phosphor powder mixture, rather than by a sol-gel technique using water, so as to prevent damage of the phosphor by water during production of the phosphor-dispersed glass.
By sealing the light source such as LED or LD with the phosphor-dispersed glass in which the phosphor is dispersed in the glass material as mentioned above, it is possible to solve the problem of the conventional resin seal structure and realize the light emitting device with improved resistance to heat, light and ambient moisture. However, the glass-phosphor powder mixture needs to be sintered by heating at a temperature higher than or equal to the glass transition temperature for production of the phosphor-dispersed glass and sealing of the light source. There is a possibility that the phosphor may be deactivated by such heating.
It has been reported that a nitride phosphor is deactivated when heated in the presence of oxygen. For example, Non-Patent Document 1 reports that, when a Sr2-xSi5N8:Eu2+ phosphor is heated in the presence of oxygen, Eu2+ of the phosphor is oxidized to Eu3+. In other words, there is a problem that the nitrogen phosphor may significantly decrease in emission efficiency during sintering of a mixture of the nitrogen phosphor with an oxygen-containing glass material.
In addition to the above problem of the nitrogen phosphor, there is also a problem that a sulfide phosphor, a halide phosphor and an aluminate phosphor significantly decrease in emission efficiency due to heat during sintering of a mixture of a powder of the phosphor with a glass powder. Patent Document 8 thus discloses a phosphor-dispersed glass in which a low heat-resistant phosphor of the above type is dispersed in a Zn—B2O3—SiO2 glass material having a softening point of about 600° C. so as to suppress deactivation of the phosphor during sintering of a mixture of a powder of the phosphor with a powder of the glass material.