Patent Publication Number: US-2012043567-A1

Title: Led structure with bragg film and metal layer

Description:
FIELD OF THE INVENTION 
     The present invention relates to an LED structure, particularly to an LED structure with a Bragg film and a metal layer. 
     BACKGROUND OF THE INVENTION 
     Refer to  FIG. 1  for a conventional blue light LED (Light Emitting Diode) structure. In the conventional blue light LED structure, an N-type GaN (gallium nitride) layer  2 , an activation layer  3  and a P-type GaN layer  4  are sequentially formed on a sapphire substrate  1 . Then, an N-type electrode  5  and a P-type electrode  6  are respectively coated on the N-type GaN layer  2  and the P-type GaN layer  4 . Thus is formed an LED structure. 
     In the conventional blue light LED structure, the emitted light is projected nondirectionally. Thus, about half of the light is scattered from the sapphire substrate  1 . Such an LED structure has inferior light-extraction efficiency and is less likely to be the light source. 
     In the conventional LED package structure, a metal layer with high refractive index is coated on the back side of an LED chip to reflect the light, which is originally projected downward, from the front side or lateral side of the LED chip so that the light-extraction efficiency can be increased. Due to the metal layer itself can absorb the light, such the LED structure decreases the light reflection efficiency of the bottom and makes the light-extraction efficiency hard to be effectively increased. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a high-reflectivity LED structure to increase the brightness of an LED. 
     To achieve the abovementioned objective, the present invention proposes an LED structure with a Bragg film and a metal layer, which comprises a sapphire substrate, a Bragg film, a light emitting layer and a metal layer. The light emitting layer is formed on the sapphire substrate. The Bragg film is arranged on one side of the sapphire substrate and opposite to the light emitting layer. The Bragg film includes at least two layers which are alternately stacked and have different refractive indexes. The metal layer is arranged on the Bragg film. 
     The Bragg film and metal layer arranged below the sapphire substrate have very high reflective indexes to function as high-reflectivity areas to reflect the light generated by the light emitting layer. The light projected downward is emitted from the top or lateral of the LED structure. Therefore, the present invention can effectively enhance the light-extraction efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view schematically showing a conventional LED structure; 
         FIG. 2  is a sectional view schematically showing an LED structure according to the present invention; 
         FIG. 3  is a sectional view schematically showing a Bragg film according to the present invention; 
         FIG. 4  is a sectional view schematically showing the light emitting condition according to the present invention; 
         FIG. 5  is a diagram showing the curves of relationships between the wavelength and reflectivity; and 
         FIG. 6  is a diagram showing the curves of relationships between the incident angle and reflectivity. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments are described in detail to demonstrate the technical contents of the present invention. However, it should be understood that the embodiments are only to exemplify the present invention but not to limit the scope of the present invention. 
     Refer to  FIG. 2 . The present invention proposes an LED structure with a Bragg film and a metal layer, which comprises a sapphire substrate  10 , a Bragg film  20 , a light emitting layer  30  and a metal layer  40 . The light emitting layer  30  is formed on the sapphire substrate  10 . The Bragg film  20  is arranged on one side of the sapphire substrate  10  and opposite to the light emitting layer  30 . The metal layer  40  is arranged on the Bragg film  20 . 
     The light emitting layer  30  further comprises an N-type semiconductor layer  31 , an activation layer  32  and a P-type semiconductor layer  33 . An N-type electrode  34  and a P-type electrode  35  are respectively coated on the N-type semiconductor layer  31  and the P-type semiconductor layer  33 . The N-type semiconductor layer  31  and the P-type semiconductor layer  33  are respectively made of a material selected from a group consisting of gallium nitride (GaN), indium gallium nitride (InGaN), aluminum indium gallium nitride (AlInGaN), gallium phosphide (GaP), aluminum indium gallium phosphide (AlInGaP), aluminum indium phosphide (AlInP), and aluminum gallium arsenide (AlGaAs). The activation layer  32  is a periodical structure formed of quantum wells and barrier layers. The quantum well is made of a material selected from a group consisting of gallium nitride (GaN), indium gallium nitride (InGaN), aluminum indium gallium nitride (AlInGaN), gallium phosphide (GaP), aluminum indium gallium phosphide (AlInGaP), aluminum indium phosphide (AlInP), and aluminum gallium arsenide (AlGaAs). 
     Refer to  FIG. 3 . The Bragg film  20  includes at least two layers  21  and  22 , which are alternately stacked and made of materials having different refractive indexes. In other words, the Bragg film  20  is a multi-layer structure. In  FIG. 3 , a four-layer structure is used to exemplify the Bragg film  20 . The layer  21  with high refractive index is made of a material having a refractive index greater than 1.7, such as titanium dioxide (TiO2), silicon nitride (SiNx), tantalum pentoxide (Ta 2 O 5 ) or zirconium oxide (Zr 2 O 3 ). The layer  22  with low refractive index is made of a material having a refractive index smaller than 1.7, such as silicon dioxide (SiO 2 ) or magnesium fluoride (MgF 2 ). The metal layer  40  is made of aluminum or silver. 
     Refer to  FIG. 4  for the light emitting condition according to the present invention. The Bragg film  20  is made of two different materials which are alternately stacked and have different refractive indexes. Thus the Bragg film  20  and the metal layer  40  can function as high-reflectivity areas to reflect the light generated by the light emitting layer  30 . The light projected downward is emitted from the top or lateral of the LED structure so that the light-extraction efficiency can be increased. 
     Refer to  FIG. 5  and  FIG. 6 . The Bragg film  20  can be optimized to have optimized thickness and materials stacked by computer simulation software similar to a Bragg reflector. The conventional Bragg reflector is made of two dielectric layers having two different materials alternately stacked, wherein the optical thicknesses of the two materials are identical during stacking. The periodical and alternately-stacked structure is called a Bragg reflector set. The optimized Bragg film  20  includes at least one Bragg reflector set, wherein the optical thicknesses of different layers in the Bragg reflector set are different. After the optimized Bragg film  20  is coated, a metal layer  40  made of aluminum or silver is then coated on the Bragg film  20 . Thereby, the high-reflectivity characteristic is provided for various incident angles. 
     In one embodiment, the optimized Bragg film  20  has a structure sequentially containing layers made of different materials and having different thicknesses: a first layer: SiO 2 , λ/(4n); a second layer: TiO 2 , λ/(4n); a third layer: SiO 2 , λ/(4n); a fourth layer: TiO 2 , λ/(4n); a fifth layer: SiO 2 , 5λ/(4n); a sixth layer: TiO 2 , 3λ/(4n); a seventh layer: SiO 2 , 2.41λ/(4n); an eighth layer: TiO 2 , 1.2λ/(4n); a ninth layer: SiO 2 , 2.43λ/(4n); and a tenth layer: TiO 2 , 0.5λ/(4n), wherein λ is the wavelength of the incident light, and n is a positive integer. 
     Refer to  FIG. 5 . The light vertically projects to the LED structure downward. Curve  5 A is a traditional reflectivity curve using aluminum. Curve  5 B is a reflectivity curve using the Bragg film  20  and the metal (aluminum) layer  40  of the present invention. Curve  5 C is a reflectivity curve using the optimized Bragg film  20  and metal (aluminum) layer  40  of the present invention. From  FIG. 5 , it is known that the Curve  5 B and Curve  5 C are obviously higher than the traditional Curve  5 A. Therefore, it is proved that the present invention can increase reflectivity and increase light-extraction efficiency. 
     Refer to  FIG. 6 . Curve  6 A is a traditional reflectivity curve having a wavelength of 460 nm using aluminum for different incident angles. Curve  6 B is a reflectivity curve using the Bragg film  20  and the metal (aluminum) layer  40  of the present invention. Curve  6 C is a reflectivity curve using the optimized Bragg film  20  and metal (aluminum) layer  40  of the present invention. From  FIG. 6 , it is known that the LED using the optimized Bragg film  20  has better reflectivity for different incident angles. Therefore, the present invention can greatly increase the light-extraction efficiency.