Patent Publication Number: US-2006012284-A1

Title: Strontium silicate-based phosphor and method thereof

Description:
TECHNICAL FIELD  
      The present invention relates to a strontium silicate-based phosphor, and more particularly, to a strontium silicate-based phosphor having a very high luminous efficiency as applied to a light emitting diode (LED) or an active luminous LCD by adding europium oxide (Eu 2 O 3 ) as an activator to a base material of strontium silicate, mixing the two components, drying and performing a heat treatment the mixed two components under a specific condition.  
     BACKGROUND ART  
      In general, to fabricate LEDs of blue, green, red and the like, it is required to first fabricate different substrates, such as InGaN substrate, GaN substrate, GaAs substrate, ZnO substrate. This requirement needs to use different semiconductor thin films, which causes the fabrication costs and unit price to be increased. Accordingly, if these LEDs can be fabricated using an identical semiconductor thin film, their process is simplified, so that fabrication costs and investment costs can be remarkably reduced. In the meanwhile, a white LED is gaining the popularity as the back light for the LCD of a lighting device, a notebook computer, a handheld terminal and the like.  
      As a method for fabricating the white LED, there is a trial where a phosphor using ultraviolet rays around 470 nm as the excitation source is further coated on an InGaN-based LED. For instance, the white LED is fabricated by coating a YAG:Ce (cerium) phosphor emitting a yellow light (wavelength: 560 nm) on a blue InGaN-based LED.  
      However, since the blue LED emits a blue light of which emission peak is 450-470 nm, it is improper in realizing the white LED employing YAG:Ce phosphor. In other words, the excitation source of the blue LED causes the luminous efficiency of the yellow light of the YAG:Ce phosphor to be lowered.  
      To solve the aforementioned drawbacks, it is strongly requested to introduce a new material capable of realizing yellow light instead of the YAG:Ce phosphor.  
     DISCLOSURE OF THE INVENTION  
      Accordingly, the present invention has been made to substantially obviate one or more of the problems due to limitations and disadvantages of the related art.  
      An object of the present invention is to provide a strontium silicate-based phosphor having a wide wavelength spectrum and a main peak widely varied and fabrication method thereof.  
      To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, there is provided a strontium silicate-based phosphor expressed by the following chemical formula: Sr 2-x SiO 4 : Eu 2+   x  where x is 0.001≦x≦1.  
      According to another aspect of the present invention, there is provided a method for fabricating a strontium silicate-based phosphor, the method comprising the steps of: forming a mixture where strontium carbonate (SrCO 3 ), silica (SiO 2 ), and europium oxide (Eu 2 O 3 ) are mixed; drying the mixture; and performing a heat treatment of the dried mixture in a reducing atmosphere.  
      According to a further aspect of the present invention, there is provided a white LED chip comprising: an LED; and a strontium silicate-based phosphor, which is excited by a light emitted from the LED and expressed by the following chemical: Sr 2-x SiO 4 : EU 2+   x  where x is 0.001≦x≦1.  
      According to the present invention, there can be obtained a yellow phosphor showing a wide wavelength spectrum and having a main peak that is easily movable depending on the concentration of europium. Accordingly, when the yellow phosphor of present invention is applied in the long wavelength LED and the active luminous LCD, the color purity can be improved and the luminous efficiency can be enhanced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other features and advantages of the invention will be apparent from the following detailed description and the accompanying drawings, in which:  
       FIG. 1  is photoluminescence emission spectra of the strontium silicate-based phosphor of present invention under the 405 nm excitation wavelength.  
       FIG. 2  is a schematic sectional view of the LED to which the strontium silicate-based phosphor of present invention is applied.  
       FIG. 3  is the relative emmission spectra of a white-emitting InGaN-based YAG:Ce LED and GaN-based Sr 2-x SiO 4 : EU 2+   x  LED. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Hereinafter, preferred embodiments of the present invention will be described in detail with reference to accompanying drawings. It will be apparent to those skilled in the art that various modifications and variations can be made therein without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention that come within the scope of the appended claims and their equivalents.  
      Hereinafter, a concrete embodiment on a fabrication method of a strontium silicate-based phosphor according to the spirit of the present invention will be described.  
      First, strontium carbonate (SrCO 3 ), silica (SiO 2 ), and europium oxide (Eu 2 O 3 ) are weighed and are mixed with a solvent.  
      In detail, the europium oxide (Eu 2 O 3 ) used for doping the base material is added by a molar ratio of 0.001-1 with respect to the amount of the strontium constituting the base material of the strontium silicate. More preferably, the molar ratio of the europium oxide (Eu 2 O 3 ) is 0.01-0.3 with respect to the content of the strontium. This is because the europium oxide (Eu 2 O 3 ) molar ratio less than 0.001 is insufficient amount in functioning as an activator and the europium oxide (Eu 2 O 3 ) molar ratio more than 1 causes the luminance to be lowered due to concentration quenching phenomenon.  
      After that, the mixture is dried in an oven. At this time, the drying temperature is 100-150° C. and the drying time is 1-24 hours.  
      After that, the dried mixture is loaded into a high purity aluminum tube and is heat-treated in a reducing atmosphere of a hydrogen-mixed gas in an electric furnace. If the heat treatment temperature is below 800° C., strontium silicate crystal is not completely created and thereby luminous efficiency is reduced, whereas if the temperature is beyond 1500° C., lowering in the luminance is caused due to high response. Accordingly, the heat treatment temperature is set in a range of 800˜1500° C. for 1˜48 hours.  
      In detail, the hydrogen-mixed gas uses a nitrogen gas containing 2-25% by weight of hydrogen so as to make a reducing environment.  
     EXPERIMENTAL EXAMPLE  
      In the present experiment, to experiment the embodiment concretely, acetone is used as the solvent used for weighing and mixing strontium carbonate (SrCO 3 ), silica (SiO 2 ), and europium oxide (Eu 2 O 3 ), and then ball milling or agate mortar is used as a mixer of the solvent and the components of strontium carbonate (SrCO 3 ), silica (SiO 2 ), and europium oxide (Eu 2 O 3 ).  
      Also, the europium oxide (Eu 2 O 3 ) used for doping the base material was used by a molar ratio of 0.005, 0.03, 0.05 and 0.1 with respect to the amount of the strontium constituting the base material of the strontium silicate. Also, the drying temperature in the oven was 120° C., the drying time was 24 hours, the heat treatment temperature was 1,350° C., and the heat treatment time was 48 hours.  
       FIG. 1  shows the variation of photoluminescence spectra obtained by exciting a strontium silicate-based phosphor of present invention using an ultraviolet of 405 nm. In  FIG. 1 , ( a ), ( b ), ( c ) and ( d ) respectively correspond to 0.005, 0.03, 0.05 and 0.1 molar ratio europium oxide (Eu 2 O 3 ) with respect to the strontium constituting the base material of the strontium silicate.  
      As can be seen from  FIG. 1 , a strontium silicate-based phosphor according to the experiment shows a wide wavelength spectrum with a wavelength ranged from 450 nm to 650 nm. As the concentration of the europium increases, the main peak corresponding to a maximum value of the luminous spectrum intensity increases from 520 nm toward 550 nm. Also, it is seen that the spectrum has a relatively wide yellow light range.  
      By the above experiment, it is known that the strontium silicate-based phosphor shows a relatively wide wavelength spectrum. And, the main peak is varied with the concentration of the europium. Accordingly, when the above strontium silicate-based phosphor is applied to a long wavelength ultraviolet LED and an active luminous LCD as the yellow phosphor, it shows a very high efficiency.  
      Meanwhile, the present invention is not limited only to the aforementioned drying condition and heat treatment condition. In other words, in case the drying temperature is changed to a range of 110-130° C., the drying time is changed to a range of 8-12 hours, the heat treatment temperature is changed to a range of 1200-1400° C., and the heat treatment time is changed to a range of 2-5 hours, similar results can be obtained.  
      Hereinafter, the effects of the present invention will be described by comparing the experimental examples in which the strontium silicate-based phosphor is applied to a LED chip with the comparative examples in which the conventional YAG phosphor is applied to a LED chip.  
       FIG. 2  shows a structure of a long wavelength ultraviolet white LED to which the spirit of the invention is applied.  
      Referring to  FIG. 2 , a LED chip according to the spirit of the present invention is configured to include a reflection cup  202 , a GaN-based LED  204  placed on the reflection cup  202 , a phosphor  208 , which is excited by a light emitted from the LED  204 , an electrode line  206  connected to the LED  204 , and an exterior material  210  for molding and sealing the surrounding of the LED using a decolored or a colored transparent resin.  
      In detail, the GaN-based LED  204  is connected with an external power through the electrode line  206 . The phosphor  208  excited by the light emitted from the LED  204  is formed to cover the LED  204 . The phosphor  208  and its surrounding are molded and sealed by the exterior material of the decolored or a colored transparent resin. By the above construction, the long wavelength ultraviolet white LED is formed. Herein, the transparent resin uses epoxy or silicon resin.  
      Also, the phosphor  208  is formed on an outer surface of the LED  204 . By doing so, the light emitted from the LED  204  serves as the excitation light of the phosphor  208 .  
      Here, the GaN-based LED  204  emits an ultraviolet of 405 nm, and the phosphor  208  excited by the LED  204  uses the strontium silicate-based phosphor of present invention.  
      Next, the long wavelength ultraviolet white LED chip according to the experiment example and the LED chip according to the related art are compared. The LED chip used as the comparative example is a long wavelength ultraviolet LED chip using YAG:Ce yellow phosphor in which YAG phosphor and InGaN chip having the wavelength of 460 nm are used.  
       FIG. 3  is a graph comparing a white LED chip fabricated by using the strontium silicate-based phosphor (Sr 2 SiO 4 :Eu) of present invention with a conventional LED chip using the conventional InGaN chip. In the graph of  FIG. 3 , the solid line indicates the spectrum of the white LED chip fabricated by using strontium silicate-based phosphor (Sr 2 SiO 4 :Eu) of present invention, and the dotted line indicates the spectrum of the LED chip fabricated by using the conventional InGaN chip.  
      Referring to  FIG. 3 , the white LED chip fabricated using the strontium silicate-based phosphor of present invention shows the spectrum of a wide wavelength band of 450-650 nm, while the comparative example shows the spectrum of a narrow wavelength band of 450-470 nm, and shows that the main peak is formed in a narrow range.  
      Accordingly, by using the strontium silicate-based phosphor according to the present invention, color purity can be improved. Also, when the strontium silicate-based phosphor of present invention is employed in the long wavelength ultraviolet LED and the active luminous LCD, it can be used as a high efficiency yellow application material.  
     INDUSTRIAL APPLICABILITY  
      As described previously, according to the inventive strontium silicate-based phosphor and fabrication method thereof, the phosphor having a wide wavelength spectrum, and of which main peak is varied in a wide range by varying the concentration of the europium can be obtained. Especially, since the main peak is widely varied, the color purity is improved so that the phosphor of present invention can be applied to a high efficiency yellow phosphor.  
      Also, when the phosphor of present invention is employed in the long wavelength ultraviolet LED and the active luminous LCD, it can have a very high luminous efficiency.