1. Field of the Invention
This invention relates to a luminescent material to be employed in a light-emitting device and to a light-emitting device.
2. Description of the Related Art
LED lamps utilizing light-emitting diodes are now employed in various kinds of display devices such as portable devices, PC peripherals, OA equipment, various kinds of switches, light sources for backlights, and display panels. A high-load LED is known to become hot when energized, thus increasing the temperature of the luminescent material to about 100 to 200° C. and, due to this rise in temperature, the emission intensity of the luminescent material deteriorates. It is desirable that the luminescent material barely deteriorates even when caused to rise in temperature.
As the luminescent material which is capable of green emission as it is excited by blue light and which is designed to be employed in the aforementioned LED lamp, there is known an Eu-activated β-sialon luminescent material. This luminescent material is known to exhibit excellent emission efficiency as it is excited with light having a wavelength of 450 nm. Namely, when this luminescent material is excited with light having a wavelength of 450 nm, it exhibits an absorption coefficient of 65%, an internal quantum efficiency of 53% and an emission efficiency of 35%.
It should be noted, with regard to the development of a flat panel display device, that although much vigorous effort has been made in the development of a plasma display panel (PDP) and liquid crystal display (LCD), a field-emission-type display is expected to be more advantageous than the PDP and the LCD in that it is capable of providing a clearer image.
This field-emission-type display has a screen in which red, green and blue luminescent materials are arrayed and with a cathode which is disposed to face the screen at smaller intervals than those of the CRT. The cathode is provided with a plurality of electron sources as an emitter element, enabling the electron sources to emit an electron in conformity with a potential difference between the electron sources and the gate electrodes disposed near the electron sources. The electron thus emitted is accelerated by an anode voltage (accelerating voltage) of the luminescent materials side and enabled to impinge against the luminescent materials, thereby enabling the luminescent materials to emit light.
As the luminescent materials to be employed in the field-emission-type display constructed as described above, it is required that the luminescent materials are capable of exhibiting a sufficiently high luminous efficiency, so that even when the luminescent materials are saturated with electric charge as excited by a high current density, they are still capable of exhibiting a sufficiently high luminous efficiency. The conventional sulfide luminescent materials (ZnS:Cu, ZnS:Ag) which have been employed as a luminescent material for CRT may be a prospective candidate as meeting the aforementioned requirements. It is however reported that the sulfide luminescent materials such as ZnS are subject to decomposition under the conditions where a low energy cathode-ray display screen is excited. The decomposed matters thus generated may badly degrade a heat filament which is designed to emit electron beam. In particular, ZnS-based blue luminescent materials that have been conventionally employed are more liable to suffer from luminance deterioration as compared with red luminescent materials and green luminescent materials, thereby raising the problem that the display color of a color picture plane changes with time.