An optical semiconductor device, which is provided with a semiconductor light-emitting element and a fluorescent material, is known as a small-sized and low power-consuming illumination source. The semiconductor light-emitting element radiates light to the fluorescent material so that the fluorescent material emits white-grouped color-mixed light. Such a kind of optical semiconductor device is disclosed in Japanese Patent Application Publication No. 2004-179644 and Japanese Patent Application Publication No. 2001-210874, for example.
According to the optical semiconductor device disclosed in the former patent publication, first and second fluorescent material layers are stacked on a semiconductor light-emitting element. The second fluorescent material layer contains a fluorescent material which emits fluorescent light having a wavelength shorter than that of the fluorescent material contained in the first fluorescent material layer.
In the optical semiconductor device disclosed in the former patent publication, the first fluorescent material layer can efficiently emit light. It is because the first fluorescent material layer is directly excited by the light radiated from a semiconductor light-emitting element. However, the second fluorescent material layer can not always efficiently emit light. It is because the light radiated from the semiconductor light-emitting element includes a light beam which directly excites the second fluorescent material layer and a light beam which is reflected by the first fluorescent material. For this reason, the brightness of the second fluorescent material layer may be reduced by the same amount as the reflection amount caused by the first fluorescent material.
The optical semiconductor device disclosed in the latter patent publication is provided with a semiconductor light-emitting element having a light-emitting layer and first and second fluorescent material layers. The semiconductor light-emitting element is disposed and fixed in a depression of a container. The first and second fluorescent material layers contain first and second fluorescent materials respectively. The first and second fluorescent materials are uniformly dispersed in the first and second fluorescent material layers respectively. The first fluorescent material layer is formed on a lower surface of a substrate of a semiconductor light-emitting element. The second fluorescent material layer is formed on the entire surface of the semiconductor light-emitting element including a side surface of the same. The second fluorescent material layer has a light-emission wavelength different from that of the first fluorescent material layer. The light-emitting layer of the semiconductor light-emitting element is fully enveloped by the first and second fluorescent material layers.
According to the optical semiconductor device disclosed in the latter patent publication, brightness of a yellow color provided by the first fluorescent material layer is reduced, when the color of the light emitted from the first and fluorescent material layers are yellow and red respectively. It is because portion of the yellow light emitted from the first fluorescent material layer is absorbed in the second fluorescent material layer.
Furthermore, the inner surface of the depression of the container may be discolored and deteriorated by the yellow light which is emitted from the side surface of the semiconductor light-emitting element, when the colors of the lights emitted from the first and second fluorescent material layer are red and yellow respectively. The discoloration and deterioration of the container may reduce reliability of the optical semiconductor device.