Abstract:
The present invention disclosed a light emitting diode (LED) and method for manufacturing the same. The light emitting diode includes a transparent substrate connected to an epitaxial layer with absorption substrate via a transparent adhesive layer. Then, the absorption substrate is removed to form a light emitting diode with the transparent substrate. Because of the low light absorption of the transparent substrate, the present invention provides high luminescence efficiency. Furthermore, because the first metal bonding layer is electrical connected with the first ohmic contact layer by the electrode connecting channel, the voltage is decreased and the current distribution is increased in the fixed current to improve the luminous efficiency of a light emitting diode.

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     The invention relates to a light emitting diode, and more specifically to an AlGalnP light emitting diode 
     2. Description of Prior Art 
     The conventional AlGalnP light emitting diode (LED) with a double heterostructure, as shown in FIG. 6, includes a n-type GaAs substrate  3 , a lower n-type (Al x Ga 1−X ) 0.5 ln 0.5 P cladding layer  4  with x=0.7˜1.0, a (Al x Ga 1−x ) 0.5 ln 0.5 P active layer  5 , an upper n-type (Al x Ga 1−x ) 0.5 ln 0.5 P cladding layer  6  with x=0.7˜1.0, and a p-type current spreading layer  7  with high energy band gap. The material of the p-type current spreading layer  7  is selected from GaP, GaAsP, GalnP and AlGaAs. 
     The light emitting wavelength of LED varies with the Al composition of the active layer  5  from green light of 555 μm to red light of 650 μm. However, when the light is emitted from the active layer  5  to the GaAs substrate  3 , because of the smaller energy band gap of substrate  3 , the light is absorbed by the substrate  3  resulting in forming a LED of low efficiency. 
     To prevent the light absorption of substrate  3 , in conventional techniques, a distributed Bragg reflector (DBR) is formed on the GaAs substrate to reflect the light. However, the DBR layer only reflects the incident light nearly perpendicular to the substrate. Therefore, the application of DBR layer is in efficient. 
     Besides, wafer-bonded transparent substrate (TS) of (Al x Ga 1−X ) 0.5 ln 0.5 P/GaP LED has been proposed to improve the luminous efficiency. The TS AlGalnP LED is fabricated by the VPE (Vapor Phase Epitaxy) technique to form a p-type GaP window layer with thickness of about 50 μm. And then, the GaAs substrate is removed to expose the n-type AlGalnP lower cladding layer. Furthermore, the exposed n-type AlGalnP lower cladding layer is connected to the n-type GaP substrate. 
     Because the wafer-bonded technique is directly connecting two types of III-V compound semiconductor together, the process is completed in pressuring and heating at higher temperature. The luminous efficiency of TS AlGalnP LED is twice brighter than that of the absorbing substrate AlGalnP LED. However, because of the complexity of manufacturing layers of TS AlGalnP LED, and high resistance in conductivity between the interface of non-ohmic contact layer, it&#39;s difficult to get high producing yield and lower the cost. 
     An AlGalnP/metal/SiO 2 /Si mirror-substrate (MS) LED is proposed in another prior art. The Si substrate and the epitaxial layer are connected by AuBe/Au. However, the luminous intensity of MS AIGalnP LED (about 90 mcd) is 40% less than the luminous density of TS AlGalnP LED in operation current of 20 mA. 
     SUMMARY OF INVENTION 
     The present invention presents a light emitting diode (LED) structure and a method for manufacturing the LED. The LED includes an epitaxial layer formed on an AIGalnP multi-layer epitaxial structure. The AlGalnP multi-layer epitaxial structure is connected to a transparent substrate by a transparent adhesive layer. The material of the AlGalnP multi-layer epitaxial structure is selected from a group consisting of homostructure, single heterostructure (SH), double heterostructure (DH), and multiple quantum well (MQW). 
     Furthermore, the LED comprises a first ohmic contact layer and a second ohmic contact layer, an electrode connecting channel for electrically coupling a first metal bonding layer to the first ohmic contact layer. Therefore, the first and the second metal bonding layers are in the same side related to the transparent substrate. 
     The present invention provides a method of manufacturing a light emitting diode. The method includes forming a first ohmic contact layer on an epitaxial structure. Then, the first ohmic contact and the epitaxial structure connect to a transparent substrate via a transparent adhesive layer, such as BCB (B-staged bisbenzocyclobutene), epoxy, and the like. Then, the substrate is removed. 
     Subsequently, the structure of the LED is etched in two steps. First, a portion of the multi-layer epitaxial structure is removed in width of about 3˜6 mils in etching process to expose the epitaxial layer. Then, the lower portion of the exposed epitaxial layer is removed in width of about 1˜3 mils to form a channel exposing the first ohmic contact layer. A second ohmic contact layer is formed on the lower cladding layer. Then, the first and the second metal bonding layers are connected to the first and the second ohmic contact layers, respectively. Therefore, the first and the second metal bonding layers are in the same side relative to the transparent substrate. 
     One advantage of the invention is to provide a high-brightness LED readily connected to a transparent substrate at lower temperature to prevent vaporization of group V elements during the adherence process. 
     Another advantage of the present invention is to provide a high-brightness LED integrated with low cost transparent substrate, such as glass, to improve the production yield at low cost. 
     Another advantage of the present invention is to provide an electrode channel of better current distribution and smaller voltage when operating at the same current. The electrode channel also improves the emitting efficiency in the same voltage. 
     Another advantage of the present invention is to provide a high-brightness LED connected to a transparent substrate by soft transparent adhesive layer. Even if the surface of the epitaxial layer is rough, the implement of the transparent adhesive layer is secure. 
    
    
     BRIEF DESCRIPTION OF THE APPENDED DRAWINGS 
     FIG. 1 to FIG. 3 show a cross-sectional view of one embodiment of AlGalnP light emitting diode according to the present invention; 
     FIG.  4  and FIG. 5 show a cross-sectional view of another embodiment of AlGaAs light emitting diode according to the present invention; and 
     FIG.6 shows a cross-sectional view of a conventional light emitting diode. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention discloses a structure of a light emitting diode (LED) and a method for manufacturing the LED. With reference to FIG. 1, the LED includes an n-type GaAs substrate  26 , an etching stop layer  24 , an n-type (Al x Ga 1−x ) 0.5 ln 0.5 P lower cladding layer  22  with x=0.5˜1.0, a (Al x Ga 1−x ) 0.5 ln 0.5 P active layer  20  with x=0˜0.45, and a p-type (Al x Ga 1−x ) 0.5 ln 0.5 P upper cladding layer  18  with x=0.5˜1.0, and a p-type epitaxial layer  16 . A p-type ohmic contact layer  28  is formed on the epitaxial layer  16 , sequentially arranged in a first direction. 
     The p-type epitaxial layer  16  is selected from AlGaAs, AlGalnP and GaAsP. The epitaxial layer  16  for preventing light absorption of the active layer  20  has a larger energy band gap than the active layer  20 , and a high carrier concentration for being the ohmic contact layer. 
     The foregoing active layer  20  is the AlGalnP with x=0˜0.45, and the upper cladding layer  18  and lower cladding layer  22  is the AlGalnP with x=0.5˜1.0. An example of the active layer  20  is Ga 0.5 ln 0.5 P with x=0 resulting in the wavelength of the light emitting diode of 635 nm of the LED. 
     While this invention has been described with reference to an illustrative embodiment, this embodiment is not intended to be construed in a limiting sense. The active layer  20  is selected from a group consisting of homostructure, single heterostructure (SH), double heterostructure (DH), and multiple quantum well (MQW). The DH structure, such as shown in FIG. 1, includes a (Al x Ga 1−x ) 0.5 ln 0.5 P lower cladding layer  22  with thickness of about 0.5˜3.0 μm, a (Al x Ga 1−x ) 0.5 ln 0.5 P active layer  20  with thickness of about 0.5˜2.0 μm and a (Al x Ga 1−x ) 0.5 ln 0.5 P upper layer  18  with thickness of about 0.5˜3.0 μm. 
     The etching stop layer  24  is selected from a group of III-V compound semiconductor, such as GalnP, or AlGaAs. Any material having lattice matched with the GaAs substrate  26  is suitable for the etching stop layer  24 . Besides, the etching rate of the etching stop layer  24  is lower than the etching rate of the substrate  26 . 
     In the first embodiment, as shown in FIG. 1, the etching rate of the lower cladding layer  22  is also lower than the etching rate of the substrate  26 . Therefore, if the thickness of the lower cladding layer  18  is thick enough, the etching stop layer  24  is not necessary to provide. 
     Then, as shown in FIG. 2, a transparent adhesive layer  14  and a transparent substrate (TS)  10  are illustrated. The transparent adhesive layer  14  is selected from BCB (B-staged bisbenzocyclobutene), or other adhesive materials of transparent character, such as epoxy. 
     The purpose of the transparent substrate  10  serves as a support to prevent the multi-layer epitaxial structure  20  of the LED from breaking during the process. Therefore, the transparent substrate  10  is not limited to single crystalline substrate. The transparent substrate  10  is selected from polycrystal substrate and amorphous substrate, such as sapphire, glass, GaP, GaAsP, ZnSe, ZnS, ZnSSe, or SiC, to lower the cost. 
     Then, the transparent substrate  10  is connected to the p-type ohmic contact layer  28  and the epitaxial layer  16  by pressuring and heating the transparent adhesive layer  14  at 250° C. for a while. In order to improve the connection of the epitaxial layer  16  and the transparent substrate  10 , an adhesive promoter is coated on the surface of the transparent substrate  10 , prior to the adhesive layer  14  coated on the transparent substrate  10 . Furthermore, for a better adhering effect, when the epitaxial layer  16  is connected to the transparent substrate  10 , the transparent adhesive layer  14  is heated at the temperature of about 60° C.˜100° C. to remove the organic solvent. Then, the temperature is rose to about 200° C.˜600° C. Therefore, the transparent substrate  10  is connected tightly to the epitaxial layer  16  by the transparent adhesive layer  14 . 
     Then, the substrate  26  is etched by a corrosive etchant, such as 5H 3 PO 4 :3H 2 O 2 :3H 2 O or 1NH 4 OH:35H 2 O 2 . If the etching stop layer  24  is made of light-absorption materials, such as GalnP or AlGaAs, the etching stop layer  24  must be removed by the same solution. 
     Then, the structure is etched in two steps. First, a portion of the multi-layer epitaxial structure, including an active layer  20  sandwiched between the upper cladding layer  18  and the lower cladding layer  22 , is removed in width of about 3˜6 mils by dry etching or wet etching process to expose the epitaxial layer  16 . Subsequently, the lower portion of the exposed epitaxial layer  16  is removed in width of about 1˜3 mils to form a channel exposing the p-type ohmic contact  28 . Then, an n-type ohmic contact layer  30  is formed on the lower cladding layer  22  in the second direction, and the second direction is opposite to the first direction. Subsequently, a first metal bonding layer  32  is formed on the epitaxial layer  16  and the channel is filled by Au or Al to form an electrode channel  31 , which is connected the p-type ohmic contact  28  in the second direction. A second metal bonding layer  34  is formed on the n-type ohmic contact layer  30  in the second direction. Therefore, the first and the second metal bonding layers  32 ,  34  are in the same side related to the transparent substrate  10 , as shown in FIG.  3 . 
     According to the invention, in the operation current of 20 mA, the wavelength light of the LED is 635 nm. And the output power of the light of the present invention is about 4 mW, which is twice larger than the power of light of the traditional AlGalnP LED with the light-absorbed substrate. 
     This embodiment of the AlGalnP LED is not intended to be construed in a limiting sense. The present invention can use other material, such as AlGaAs for red light LED, too. 
     Please referring to FIG. 4, in the second embodiment, a light emitting diode structure according to the present invention is formed on a GaAs substrate  51  in a first direction. The multi-layer epitaxial structure includes an n-type AlGaAs lower cladding layer  52 , an AlGaAs active layer  53 , and a p-type AlGaAs upper cladding layer  54 . The Al composition of the lower cladding layer  52  is about 70%˜80%, and the thickness of the lower cladding layer  52  is about 0.5˜3.0 μ% m. The Al composition of the upper cladding layer  54  is about 70%˜80% and the thickness of the upper cladding layer  54  is about 0.5˜3.0μm. The Al composition of the active layer  53  is about 35% and the thickness of the active layer  53  is about 0.5˜2.0μm. Then, as shown in FIG. 5, a p-type ohmic contact layer  57  is formed on the upper cladding layer  52  in the first direction. Then, a transparent substrate  56  connects the upper cladding layer  54  to the p-type ohmic contact layer  57  by a transparent adhesive layer  55 . 
     Subsequently, the substrate  51  is removed by a corrosive etchant, such as NH 4 OH: H 2 O 2 =1.7:1. Moreover, a portion of the multi-layer epitaxial structure is removed by wet etching or dry etching to form a channel exposing the p-type ohmic contact layer  57 . Then, an n-type ohmic contact layer  58  is formed on the lower cladding layer  52  in a second direction. And then, a first metal bonding layer  59  is formed on the upper cladding layer  54  in the second direction and an electrode channel  60  is formed in the upper cladding layer  54 . A second metal bonding layer  61  is formed on the n-type ohmic contact layer  58  in the second direction. Therefore, the first and the second metal bonding layers  59 ,  61  are in the same side related to the transparent substrate  10 , as shown in FIG.  5 . 
     According to the invention, in the operation current of 20 mA, the light wavelength of the red light AlGaAs LED is 650 nm. And the output power of the light of the present invention is twice larger than the output power of the light of the traditional AlGaAs LED with the light-absorbed substrate. 
     The present invention presents a light emitting diode with transparent substrate  10  and an electrode channel  31  connecting the p-type ohmic contact  28  to the first metal bonding layer  32 . As a result, the first and the second metal bonding layers  32  and  34  are in the same side related to the transparent substrate  10 . Therefore, the chip-package of flip chip technique is implemented, which eliminates the traditional wire bonding resulting in improvement of reliability of chip. Furthermore, the luminous efficiency is improved due to elimination light absorption of the transparent substrate  10 . Moreover, because the material, such as sapphire, glass, or SiC, of the transparent substrate  10  is hard, the thickness of the substrate is reduced to about 100 μm without breaking during the process. Therefore, the present invention provides a thin and small LED. 
     The present invention presents the transparent substrate  10  connected to the epitaxial structure via a soft transparent adhesive layer  14 . Therefore, even if the surface of the epitaxial structure is rough, the transparent substrate  10  is connected tightly to the epitaxial structure via the transparent adhesive layer  14 . 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the discovered embodiments. The invention is intended to 
     cover various modifications and equivalent arrangement included within the spirit and scope of the appended claims.