Abstract:
A GaN-based LED includes: a substrate with front and back sides; an epitaxial layer formed over the front side of the substrate and including, from top down, a P-type layer, a light-emitting area, and an N-type layer; a current spreading layer formed over the P-type layer; a P electrode formed over the current spreading layer; a first reflecting layer between the current spreading layer and the epitaxial layer, disposed at a peripheral area of the epitaxial layer in a band-shaped distribution; and a second reflecting layer over the back side the substrate. The band-shaped or annular distribution can increase a probability light extraction of the LED sideways. By controlling the ratio of lights extracted upwards and sideways, the light-emitting distribution evenness can be adjusted and the uneven heat dissipation can be improved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of, and claims priority to, PCT/CN2013/077609 filed on Jun. 21, 2013, which claims priority to Chinese Patent Application No. CN 201210206024.3 filed on Jun. 21, 2012. The disclosures of these applications are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Light Emitting Diode (LED) is a semiconductor light-emitting device fabricated by adopting P-N junction electroluminescence principle. LEDs have advantages such as being—polluting, high brightness, low power consumption, long service life, low working voltage, easy miniaturization, etc. Since its successful development in 1990s, the GaN-based LED gets luminance improved and has wider applications with continuous research and development. Much studies are made on improving LED light-emitting efficiency, mainly concerning such technologies as pattern substrate technology, distributed current blocking layer (i.e., current blocking layer), distributed Bragg reflector (DBR) structure, transparent substrate, surface roughening, photonic crystal technology, etc. 
     Referring to  FIG. 1 , a normal LED structure, comprising a substrate  100 , a bottom-up-laminated N-type layer  101 , a light-emitting area  102 , a P-type layer  103 , a current spreading layer  104 , a P electrode  106 , an N electrode  107  on the exposed surface of the N-type layer  101  and a back-plating reflecting layer  108  under the substrate  100 . The light emitted from the light-emitting layer (as shown in  1   a ) can be extracted from the front side of the chip. Light  1   b , however, due to total reflection, cannot be extracted from the front of the chip but from the side; Light  1   c  is directly extracted from the side; Lights  1   d  and  1   e  are extracted from the front side of the chip after reflection of the back-plating reflecting layer  108 . 
     Referring to  FIG. 2 , an improved conventional LED structure, comprising a substrate  100 , a bottom-up-laminated N-type layer  101 , a light-emitting area  102 , a P-type layer  103 , a current spreading layer  104 , a metal reflecting layer  105 , a P electrode  106 , an N electrode  107  on the exposed surface of the N-type layer  101  and a back-plating reflecting layer  108  under the substrate  100 . The light emitted from the light-emitting layer (as shown in  1   a ) can be extracted from the front side of the chip. Light  1   b , however, due to total reflection, cannot be extracted from the front of the chip but from the side; Light  1   c  is directly extracted from the side; Lights  1   d  and  1   e  are extracted from the front side of the chip after reflection of the back-plating reflecting layer  108 ; light  1   f , due to the dual reflection of the metal reflecting layer  105  (normally, Al or Ag) and the back-plating reflecting layer  108  (normally, Al, Ag or DBR), is finally extracted from the front side of the chip. In the above two normal LED structures, most light from the light-emitting layer is extracted from the front of the chip and less from the side of the chip, leading to uneven light distribution of LED, over strong light emitting at axial direction at front side, uneven heat dissipation and small light emitting angle. 
     SUMMARY 
     The technical problem to be solved by the present disclosure is to provide a GaN-based HBLED with reflecting layers and fabrication method so as to overcome the defects of the prior art. A first reflecting layer in a band-shaped distribution is added between the LED epitaxial layer and the P electrode (i.e., the peripheral area of the epitaxial layer), and a second reflecting layer is formed on the back of the substrate. The structure may also comprise a third reflecting layer formed between the current spreading layer and the P electrode, right at the bottom of the P electrode, thus effectively extracting the light emitted from the light-emitting layer, eliminating light absorption of the P electrode and improving light extraction efficiency. The first reflecting layer in a band-shaped distribution at the peripheral area of the epitaxial layer surface can extract part of light sideways, which is originally to be extracted upwards, thereby increasing opportunity of side light extraction of the LED. By controlling the ratio of lights (emitted from the light-emitting layer) extracted upwards and sideways, the light-emitting distribution evenness is adjusted and the uneven heat dissipation is improved. 
     To achieve the above object, the technical scheme disclosed is to firstly grow an epitaxial layer on front of the substrate and to form a first reflecting layer in a band-shaped distribution on the epitaxial layer before fabrication of the current spreading layer and the P and N electrodes. The last step is to fabricate a second reflecting layer on the back of the substrate. Before fabrication of the P and N electrodes, a third reflecting layer can be formed between the current spreading layer and the P electrode right at the bottom of the P electrode. 
     The fabrication in the present disclosure mainly comprises: 1) growing an epitaxial layer on the front of the substrate; 2) forming a first reflecting layer in a band-shaped distribution on the peripheral area of the epitaxial layer surface; 3) forming a current spreading layer on the first reflecting layer in a band-shaped distribution and the exposed epitaxial layer; 4) plating a third reflecting layer on the surface layer of the current spreading layer; 5) fabricating a P electrode and an N electrode on the third reflecting layer and the exposed N-type layer; and 6) forming a second reflecting layer on the back of the substrate, 
     wherein, in step 2), bonding the edges of the first reflecting layer in a band-shaped distribution and the epitaxial layer; in step 4) locating the third reflecting layer at right bottom of the P electrode. 
     A GaN-based HBLED with dual reflecting layers, comprising a substrate with front and back sides; an epitaxial layer formed on the front surface of the substrate, comprising a P-type layer, a light-emitting area and an N-type layer from top-down; a current spreading layer formed on the P-type layer; a P electrode formed on the current spreading layer; a first reflecting layer located between the current spreading layer and the epitaxial layer at the peripheral area of the epitaxial layer in a band-shaped distribution distribution; and a second reflecting layer on the back of the substrate. 
     The GaN-based HBLED with reflecting layers can also comprise a third reflecting layer formed between the current spreading layer and the P electrode, right at the bottom of the P electrode. 
     The first reflecting layer locates at the peripheral area of the epitaxial layer and forms a closed annular. 
     The P electrode locates at the peripheral area of the current spreading layer. The first reflecting layer locates at the peripheral area of the epitaxial layer away from the P electrode. 
     The stripe width of the first reflecting layer is 5-30 μm. 
     The first reflecting layer accounts for 5%-30% of the light emitting area of the epitaxial layer. 
     The diameter of the third reflecting layer is 50-200 μm. 
     The first reflecting layer can be a DBR, a metal reflecting layer or an ODR. 
     The second reflecting layer can be a DBR, a metal reflecting layer or an ODR. 
     The third reflecting layer can be a DBR, a metal reflecting layer or an ODR. 
     The first and second reflecting layers are composed of alternating high refractive index and low reflective index material layers, wherein, the high refractive index layer is selected from TiO, TiO 2 , Ti 3 O 5 , Ti 2 O 3 , Ta 2 O 5 , ZrO 2  or any of their combinations and the low reflective index layer is selected from SiO 2 , SiN x , Al 2 O 3  or any of their combinations. 
     The first, second and third reflecting layers can be Al, Ag or Ni or any of their combinations. 
     The substrate can be sapphire (Al 2 O 3 ) or silicon carbide (SiC). 
     The current spreading layer material can be Ni/Au alloy, Ni/ITO alloy, ITO, ZnO or In-mixed ZnO, Al-mixed ZnO, Ga-mixed ZnO or any of their combinations. 
     Compared with the conventional LEDs, the LEDs disclosed herein can have one or more of the following advantages: a first reflecting layer in a band-shaped distribution is arranged at the peripheral area of the LED epitaxial layer surface and extracts the light (originally to be extracted from upwards of the chip) sideways, thus increasing opportunity of sideways light extraction of LED, improving light-emitting distribution evenness and providing high brightness and even light-emitting source. The LEDs can be used in a light-emitting system such as displays and signage, which can include a plurality of such LEDs forming an array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a conventional LED. 
         FIG. 2  is a diagram of an improved LED structure. 
         FIG. 3  is a cross-sectional view of the GaN-based HBLED disclosed in Embodiment 1. 
         FIG. 4  is a top view of the GaN-based HBLED disclosed in Embodiment 1. 
         FIG. 5  is a cross-sectional view of the GaN-based HBLED disclosed in Embodiment 2. 
         FIG. 6  is a top view of the GaN-based HBLED disclosed in Embodiment 2. 
         FIG. 7  is a cross-sectional view of the GaN-based HBLED disclosed in Embodiment 3. 
         FIG. 8  is a top view of the GaN-based HBLED disclosed in Embodiment 3. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments of the present disclosure will be described in detail with reference to the accompanying drawings and examples, to help understand and practice the disclosed embodiments, regarding how to solve technical problems using technical approaches for achieving the technical effects. In specific device design and manufacture, the LED structures according to the present disclosure will be adjusted and changed in terms of structure, dimension and the material based on specific application fields and process. 
     The embodiments disclose a GaN-based HBLED with reflecting layers, comprising a substrate, an epitaxial layer, a current spreading layer, a reflecting structure, a P electrode and an N electrode. 
     More specifically, the substrate can be sapphire (Al 2 O 3 ), silicon carbide (SiC) or silicon wafer (Si). Insulation material is applied for horizontal LED devices and conductive material for vertical LED devices. 
     The epitaxial layer can be formed on the substrate surface through epitaxial growth, comprising at least an N-type layer, a light-emitting layer and a P-type layer from bottom up, also comprising a buffer layer and an electron blocking layer. The material is GaN-based semiconductor. 
     The current spreading layer, formed on the P-type layer, can be Ni/Au alloy, Ni/ITO alloy, ITO, ZnO or In-mixed ZnO, Al-mixed ZnO, Ga-mixed ZnO or any of their combinations. 
     The P electrode is formed on the electrode extension layer and is used for provision of current injection for the light-emitting layer. The horizontal LED device may etch part of the P-type layer and the light-emitting layer and expose the N-type layer. The N electrode is formed on the exposed N-type layer surface. In the vertical LED device, the N electrode is on the back of the conductive substrate. 
     The reflecting structure comprises a first reflecting layer and a second reflecting layer, wherein, the first reflecting layer locates between the current spreading layer and the epitaxial layer and at the peripheral area of the epitaxial layer in a band (or stripe)-shaped distribution, either forming in a closed annular or a non-closed shape at the peripheral area away from the P electrode. More specifically, a first reflecting layer is formed on the p-type layer and locates between the p-type layer and the current spreading layer. It may be included in the current spreading layer or be implanted in the epitaxial layer. It can be a DBR, a metal reflecting layer or an ODR. The structure size and position parameters for the first reflecting layer can be adjusted and designed based on chip size and specific optical path. According to some embodiments, the stripe width of the first reflecting layer is 5-30 μm and the area accounts for 5%-30% of the light emitting area of the epitaxial layer. The second reflecting layer locates on the back of the substrate and can be a DBR, a metal reflecting layer or an ODR. A third reflecting layer can be arranged at the right bottom of the P electrode between the current spreading layer and the P electrode. The diameter is 50-200 μm. The third reflecting layer can be a DBR, a metal reflecting layer or an ODR. The reflecting layers in the reflecting structure can be made from Alternating high refractive index and low reflective index material layers. The high refractive index layer is selected from TiO, TiO 2 , Ti 3 O 5 , Ti 2 O 3 , Ta 2 O 5 , ZrO 2  or any of their combinations. The low reflective index layer is selected from SiO 2 , SiN x , Al 2 O 3  or any of their combinations. The reflecting layers in the reflecting structure can also be pure-metal reflecting layer like Al, Ag or Ni. 
     In the following, detailed descriptions will be given in combination with Embodiments 1-3 and  FIGS. 3-8 . 
     Embodiment 1 
     As shown in  FIGS. 3-4 , a GaN-based HBLED, comprising a sapphire substrate  200 , an N-type layer  201 , a light-emitting area  202 , a P-type layer  203 , a non-closed annular first reflecting layer  204 , a current spreading layer  205 , a P electrode  207 , an N electrode  208  and a second reflecting layer  209 . 
     More specifically, the LED structure has a sapphire substrate  200  at the bottom; an N-type layer  201 , formed on the sapphire substrate  200 ; a light-emitting area  202 , formed on the N-type layer  201 ; a P-type layer  203 , formed on the light-emitting area  202 ; a first reflecting layer  204 , selected as a DBR, formed on the P-type layer  203  and at the peripheral area of the P-type layer  203  away from the P electrode, wherein, the stripe width is 15 μm and the area accounts for about 20% of the light emitting area of the epitaxial layer; an ITO current spreading layer  205 , formed on the first reflecting layer  204  and the exposed P-type layer  203  surface; a P electrode  207 , formed on the current spreading layer  205 ; an N electrode  208 , formed on the exposed N-type layer  201 ; a second reflecting layer  209 , selected as an ODR, formed on the back of the sapphire substrate  200 ; wherein the DBR  204  comprises Alternating high refractive index TiO 2  material and low refractive index SiO 2  material. 
     The beneficial effects of the present embodiment are that: a non-closed annular DBR  204  on the peripheral area of the P-type layer  203  surface of the LED epitaxial layer, apart from extracting Lights  2   a  and  2   e  from the front of the chip and Lights  2   b  and  2   c  from the side of the chip, also extracts the light (originally to be extracted from upwards of the chip) sideways (as shown in Light  2   d ), thus increasing opportunity of sideways light extraction of LED, improving light-emitting distribution evenness and providing high brightness and even light-emitting source. 
     Embodiment 2 
     As shown in  FIGS. 5-6 , a GaN-based HBLED, comprising a sapphire substrate  200 , an N-type layer  201 , a light-emitting area  202 , a P-type layer  203 , a closed annular first reflecting layer  204 , a current spreading layer  205 , a third reflecting layer  206 , a P electrode  207 , an N electrode  208  and a second reflecting layer  209 . 
     More specifically, the bottom layer in the LED structure is a sapphire substrate  200 ; an N-type layer  201  is formed on the sapphire substrate  200 ; a light-emitting area  202  is formed on the N-type layer  201 ; and a P-type layer  203  is formed on the light-emitting area  202 . A first reflecting layer  204 , selected as a DBR, is formed on the P-type layer  203  and located at the peripheral area of the P-type layer  203  surface. The stripe width of the first reflecting layer is 20 μm and the area accounts for 25% of the light emitting area of the epitaxial layer. An ITO current spreading layer  205  is formed on the closed DBR  204  and the exposed P-type layer  203  surface. A third reflecting layer  206 , selected as an Al reflecting layer, is formed on the ITO current spreading layer  205  surface, wherein, diameter of the third reflecting layer (90 μm) is larger than the P electrode diameter; a P electrode  207  is formed on the third reflecting layer  206 ; an N electrode  208  is formed on the exposed N-type layer  201 ; a second reflecting layer  209 , selected as an Al metal reflecting layer, is formed on the back of the sapphire substrate  200 . The DBR  204  comprises alternating high refractive index TiO 2  material and low refractive index SiO 2  material. 
     The beneficial effects of the present embodiment are that: a closed annular DBR  204  on the peripheral area of the surface of the P-type layer  203  of the LED epitaxial layer, apart from extracting Lights  2   a  and  2   e  from the front of the chip and Lights  2   b  and  2   c  from the side of the chip, also extracts the light (originally to be extracted from upwards of the chip) sideways, thus increasing opportunity of sideways light extraction of LED (as shown in Light  2   d  and  2   f ), improving light-emitting distribution evenness and providing high brightness and even light-emitting source. 
     It should be understood that in the above structure, the third reflecting layer  206  may be inside the current spreading layer or on the current spreading layer. 
     Embodiment 3 
     In comparison with Embodiment 2, the present embodiment discloses a vertical GaN-based HBLED with reflecting layers. In the present embodiment, Si serves as the substrate  200 . The N electrode  208  forms on the back of the substrate and constitutes a vertical LED device. The third reflecting layer  206  is an ODR. The diameter is 70 μm, less than the P electrode diameter, for the convenient contact conduction of the P electrode and the current spreading layer  205 . 
     Advantages of the reflecting layer structure disclosed herein may include one or more of: (1) the first reflecting layer can be in closed annular shape or in non-closed annular shape; and (2) the peripheral area of the a band-shaped (annular) reflecting layer overlaps with the peripheral area of the epitaxial layer surface. Through reasonable design and distribution of the a band-shaped (annular) reflecting layer, part of light (originally to be extracted from upwards of the chip) can be extracted sideways, thus improving light-emitting distribution evenness of the LED chip.