Y[sub 3]Al[sub 5]O[sub 12]:Ce[sub 0.05] Phosphor Coatings on Gallium Nitride for White Light Emitting Diodes

Abstract

White light was obtained by mixing blue light from the emission of a gallium nitride (GaN) chip and yellow light from the fluorescence of a Y/sub 3/Al/sub 5/O/sub 12/:Ce/sub 0.05/ yellow phosphor. A uniform coating and an optimized thickness of yellow phosphor layer on a GaN chip were necessary for achieving an efficient white light emitting diode. The phosphor particles were coated on a GaN chip or indium tin oxide by several methods including the slurry method, the settling method, and electrophoretic deposition (EPD). The properties of the phosphor layers prepared by these methods were examined using scanning electron microscope and photoluminescence. The chromaticity of white light was dependent upon the thickness of the phosphor layer. The properties of the phosphor layer prepared by EPD such as packing density, thickness, and uniformity could be more easily controlled than those by the slurry and settling methods. Further high packing density of the EPD could compensate for the typical thick phosphor layer, allowing the thin layer to be fabricated. To overcome the weak adhesion strength of phosphor particles by the EPD, an aqueous solution including poly(vinyl alcohol)+ammonium dichromate was coated on the phosphor layer and cured by exposure to ultraviolet light