Abstract:
In silicon-carbide based light emitting diodes (LEDs) and other similar applications, diamond crystals are used to provide an intermediate refractive index that permits more of the ultraviolet light generated in the diode to reach the phosphors, producing a brighter light with greater efficiency.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/623,283, filed Oct. 28, 2004, entitled “Diamond and/or Silicon Carbide Forming of Small and Microscale or Nanoscale Capsules and Hohlraums,” which disclosure is incorporated herein by reference for all purposes. 
   The present disclosure is related to the following commonly-assigned co-pending U.S. Patent Applications:
         application Ser. No. 11/067,517, filed Feb. 25, 2005, entitled “Diamond Capsules and Methods of Manufacture”; and   application Ser. No. 11/067,609, filed Feb. 25, 2005, entitled “Apparatus for Modifying and Measuring Diamond and Other Workpiece Surfaces with Nanoscale Precision.”       

   The respective disclosures of these applications are incorporated herein by reference for all purposes. 

   BACKGROUND OF THE INVENTION 
   The present invention relates in general to light coupling, and in particular to diamond and/or silicon carbide coatings applied to light-emitting diodes. 
   Light-emitting diodes (LEDs) are known in the art and can be made of various materials that radiate light at different wavelengths. For example, ultraviolet light (UV) can be produced by diodes fabricated from silicon carbide using conventional techniques. Where visible light is desired, phosphors that radiate visible light of a desired wavelength in response to the incident UV are placed in contact with the silicon carbide diode. Such diodes and LEDs are used in numerous applications. 
   Existing silicon carbide diodes convert a large percentage of the UV they generate back into heat because of the high index of refraction of silicon carbide (2.7) as compared to air(1), plastic(1.1) or in-contact phosphors(1.2-0.4). The large index difference results in UV being reflected back into the silicon carbide, heating it This effect limits the efficiency of silicon carbide diodes. 
   It would therefore be desirable to provide silicon carbide LEDs with higher efficiency. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to coupling light from a very high index of refraction material such as silicon carbide (index about 2.7) to a lower index material such as plastic (index about 1.1), glass or quartz (index about 1.4), or air (index about 1). In some embodiments, the invention relates to coupling ultraviolet light produced in a silicon carbide (SiC) light emitting diode (LED) to phosphors located on or adjacent to the diode, which in turn give off any mixture of colors including white (sufficient amounts of red, green and blue so as to appear white). In such diodes and other similar applications, diamond can be used to provide a region of intermediate refractive index (diamond index is 2.4) to permit much more of the UV light to reach the phosphors and thus produce a brighter light with greater efficiency. Further since UV light production in SiC is associated with heat generation the diamond coating provides a means for cooling the SiC substrate. 
   In one embodiment, CVD diamond crystals in size from a nanometer to a few microns are grown on the SiC substrate before the phosphor coating is applied thereto. These diamond crystals serve both to couple the UV light out of the SiC substrate; in addition, because the crystals form a rough surface they increase the surface area on which the phosphor is coating again increasing the total light output of the result. 
   The following detailed description together with the accompanying drawing will provide a better understanding of the nature and advantages of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a side view of a SiC LED apparatus  100  according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In accordance with an embodiment of the present invention, a single crystal silicon carbide (SiC) LED can be used with a diamond coating to index-match it such that UV light otherwise trapped by total internal reflection can escape into the diamond coating, where it can readily interact with visible-light-producing phosphors to substantially increase the efficiency of the SiC-based LED. 
   “Diamond,” as used herein, refers generally to any material having a diamond lattice structure on at least a local scale (e.g., a few nanometer), and the material may be based on carbon atoms, silicon atoms, or any other atoms capable of forming a diamond lattice. The diamond coating may be made of any type of diamond material, including crystalline, polycrystalline (ordered or disordered), nanocrystalline and amorphous diamond. In some embodiments, small (nanometer to micrometer) diamond crystals are used for the coating. 
   In some embodiments, silicon nitride may be used to further reduce by steps the index of refraction of the system to couple more UV from the SiC. Any number of additional decreasing or increasing index materials may be coated onto the device to optimally match any material (including phosphors or some particular object targeted for exposure to the UV light) to the output of the SiC UV source. 
     FIG. 1  is a side view of a SiC LED apparatus  100  according to an embodiment of the present invention. A SiC crystal diode  108  is coated with diamond crystals  106  and a phosphor layer  104 . A lens  102 , which may be made of plastic or other material, covers phosphor layer  104  and collimates the light emitted from phosphor layer  104 . 
   Diamond crystals  106  may be formed using chemical vapor deposition (CVD) or other known techniques for growing or depositing diamond crystals on a surface, and the crystals may range in size from a nanometer to tens of microns or more. In some embodiments, diamond crystals  106  form a microscopically rough surface, and this roughness increases the contact area between diamond crystals  106  and phosphor layer  104 . 
   Phosphor layer  104  contains phosphors (including any conventional phosphorescent material) that radiate light of a desired wavelength when stimulated by UV light. 
   Lens  102  may also be of generally conventional design and may focus, defocus, or otherwise direct the light. 
   In operation, crystal diode  108  generates UV light rays  110 ,  112 . Ray  112  strikes the surface of diode  108  at a large angle and is internally reflected. Ray  110  strikes the surface at a smaller angle and passes through to diamond coating  106 , which refracts ray  110  toward normal so that it can cross into phosphor layer  104 . Ray  110  is absorbed in phosphor layer  104 , which radiates visible light  114  into lens  102 . Lens  102  refracts light ray  114  to collimate it with other light rays (not explicitly shown) generated in phosphor layer  104  to form a visible light beam  116 . 
   Those skilled in the art will appreciate that diamond crystals  106  reduce the difference in index of refraction at the boundary layer between SiC diode  108  and phosphor layer  104 . This reduces the amount of total internal reflection in SiC diode  108 , allowing more light rays to enter phosphor layer  104 . The result is higher efficiency: at the same power consumption, more light is generated, or the same light can be generated at lower power consumption. Since more of the input energy is converted to light, heat generation is also produced. 
   Further, to the extent that the top surface of diamond layer  106  is rougher than the top surface of SiC diode  108 , diamond layer  106  increases the contact area through which light can enter phosphor layer  104 . This can further increase the total light output of apparatus  100 . 
   While the invention has been described with respect to specific embodiments, one skilled in the art will recognize that numerous modifications are possible. For instance, the diamond coatings described herein may also be applied to other crystal LEDs. Other index-matching coatings may also be used in conjunction with the diamond coating. 
   One skilled in the art will also recognize that the invention provides a number of advantageous techniques, tools and products, usable individually or in various combinations. These techniques, tools, and products include but are not limited to:
         a light-emitting or light-coupling device in which diamond is used to create an intermediate step in the index of refraction from a high index region (higher than diamond) to a lower index region (lower than diamond); and/or   a light-emitting or light-coupling device that uses diamond to create an intermediate index region in which the thermal properties of diamond are used to improve the power output or reduce the operating temperature of the device; and/or   a light-emitting or light-coupling device that uses diamond to create an intermediate index region in which the insulating and/or conducting properties of diamond are used isolate and/or electrically connect device regions and outside sensors or supplies; and/or   a SiC LED in which a diamond coating is used to couple UV light out of the LED and increase the efficiency with which such light is coupled to phosphors; and/or   an SiC LED with a diamond coating as described above in which silicon nitride is used to create a further reduced index of refraction for coupling UV from the SiC LED; and/or   an SiC LED with a diamond coating as described above in which any number of coatings are used to reduce by steps the index of refraction so as to match that of the material to be coupled to the LED UV such as a phosphor.       

   Thus, although the invention has been described with respect to specific embodiments, it will be appreciated that the invention is intended to cover all modifications and equivalents within the scope of the following claims.