In recent years, studies are being aggressively made to develop a White light-emitting device using a blue light-emitting element as a light-emitting source. In particular, a white light-emitting diode using a blue light-emitting diode is lightweight and mercury-free and has long life, and therefore demands therefor are expected to rapidly expand in the future. A most commonly employed method for converting blue light of a blue light-emitting element into white light is a method of obtaining a pseudo-white color by mixing a blue color and a yellow color which are in a complementary relationship. For example, as described in Japanese Unexamined Patent Publication (Kokai) No 2000-208815, a coating, layer containing a fluorescent material capable of absorbing a part of blue light and emitting yellow light is provided on the front surface of a light-emitting diode that emits blue light, and a mold layer or the like for mixing blue light of the light source and yellow light, from the florescent material is provided ahead of the coating layer, whereby a white light-emitting diode can be fabricated. As for the fluorescent material, a cerium-activated YAG (Y3Al5O12) powder or the like is used.
However, in the structure of a white light-emitting diode used at present as typified by the structure described in Japanese Unexamined Patent Publication (Kokai) No. 2000-208815, the fluorescent material powder is mixed with a resin such as epoxy and then coated, and therefore, for example, control for ensuring uniformity of the mixed state of the fluorescent material powder with the resin or stabilizing the thickness of the coated film is difficult and the white light-emitting diode readily undergoes color unevenness or fluctuation. The resin required when using a fluorescent material powder is poor in leaf resistance compared with a metal or ceramic and may cause reduction in the transmittance due to alteration by heat from the light-emitting element, hindering realization of a high-output white light-emitting diode that is currently demanded.
The present inventors have proposed a white light-emitting device fabricated using a blue light-emitting element and a light-converting ceramic composite comprising a solidified body in which a plurality of oxide phases including a fluorescence-emitting crystal phase are formed to be continuously and three-dimensionally entangled with each other (International Publication No. WO2004/065324, pamphlet). This light-converting ceramic composite enables stably obtaining homogeneous yellow fluorescence by virtue of uniform distribution of fluorescent phases and is excellent in heat resistance because of the ceramic. The ceramic composite itself is a bulk body and requires no resin for the fabrication of a white light-emitting device, so that the fabricated white light-emitting device can be reduced in the color unevenness or fluctuation and is very suitable for realizing high-output light emission.