1. Field of the Invention
The present invention relates to a phosphor that can be excited by light having a peak wavelength in the range from near-ultraviolet light to visible light and can efficiently emit red light, and a light emitting device using this phosphor.
2. Description of the Related Art
Light emitting diodes are semiconductor light emitting elements formed of semiconductor materials such as gallium nitride (GaN). Various types of light emitting devices have been developed which include semiconductor light emitting elements and phosphors for emitting white light, electric bulb color light, orange light, and the like. The light emitting devices for emitting white light, and the like can be designed based on the additive color mixture principle. Two types of white light emitting devices are well known. One type of white light emitting devices includes an ultraviolet light emitting element, and three types of phosphors that emit red (R) light, green (G) light and blue (B) light. Another type of white light emitting devices includes a blue light emitting element, and a phosphor that emits yellow light, or the like. White light emitting devices are required in various applications such as ordinary lighting, automobile lighting, display, liquid crystal display backlight, and the like. In particular, high color purity as well as light emission efficiency is required for phosphors used in the display application in order to reproduce colors in the wide regions on the chromaticity coordinates. Since phosphors used in the display application are demanded to have suitability for a color filter to be used in combination with the phosphors, their half-value width of the peak light emission is necessarily narrow.
As a red phosphor that is excited by light in the blue range and has a narrow half-value width, for example, a fluoride phosphor is known which has a composition of K2SiF6:Mn4+, K2TiF6:Mn4+, K2SnF6:Mn4+, Na2TiF6:Mn4+, Na2ZrF6:Mn4+, K2Si0.5Ge0.5F6:Mn4+, or the like (see Japanese Patent Laid-Open Publication No. JP 2009-528,429 A1, for example).
It can be considered that a phosphor represented by the composition of 3.5MgO.0.5MgF2.GeO2:Mn4+ (See “Phosphor Handbook” edited by Phosphor Research Society, and published by Ohmsha, Ltd. (1987), for example. Hereinafter, the phosphor is occasionally referred to as “MGF phosphor”) is another possible red phosphor which can be used together with the above red phosphor or used alone for the same purpose.
Although the MGF phosphor is known as a red phosphor for mercury-vapor lamps, it can be also excited by a light emitting diode that emits light having a peak wavelength in the range from near-ultraviolet light to visible light (350 to 500 nm) as an excitation light source.
The MGF phosphor can be produced by calcination at about 1200° C. and normal atmospheric pressure in the air. Accordingly, there is an advantage that the MGF phosphor can be relatively easily manufactured.
In recent years, red phosphors have been developed which are obtained by substituting MgF2 in the composition of the MGF phosphor with other fluoride AF2, where A is at least one element selected from the group consisting of Ca, Sr, Ba, and Zn (see Japanese Patent Laid-Open Publication No. JP 2008-202,044 A1, for example). According to JP 2008-202,044 A1, the phosphors can have 150% of the light emission efficiency of the conventional MGF phosphor at the maximum.
In the case where the MGF phosphor is used for the liquid crystal display backlight, or the like, the persistence characteristics of the phosphor are important. In particular, in this case, short persistence time is required. For example, in the case where local dimming is used in a liquid crystal display, high speed of response is required. If the persistence time of phosphor is long, the refresh rate of the display may be low. Here, the persistence time of MGF phosphor is relatively longer than other phosphors.
It is said that the conventional MGF phosphor is efficiently excited by light having a wavelength of about 254 nm. However, in recent years, mercury free, and the like are demanded. For this reason, light emitting diodes are becoming widely used as ultraviolet excitation light sources instead of mercury-vapor lamps, or the like. Correspondingly, the MGF phosphor is required to efficiently emit luminescent radiation even when using the light emitting diodes that emit light having a peak wavelength in the range from near-ultraviolet light to visible light, which is longer than the wavelength of the mercury-vapor lamps. In addition, the persistence time is necessarily short for the MGF phosphor as stated above.
The present invention is devised to solve the above problems. It is a main object of the present invention to provide a red phosphor that can efficiently emit luminescent radiation even when excited by light having a peak wavelength in the range from near-ultraviolet light to visible light while maintaining the fundamental characteristics of the conventional MGF phosphor, and has short-persistence-time characteristics, and a light emitting device including the red phosphor.