Abstract:
A light-emitting diode has a metal mesh pattern formed on an active layer without a transparent oxide conductive layer formed in between is disclosed. The mesh pattern is formed by using ion bombardment a metal layer so that myriad pits formed into the exposed portion of the active layer served as light emitting centers.

Description:
FIELD OF INVENTION 
     This invention relates to a light-emitting semiconductor device having enhanced brightness, particularly to a high power light emitting diode using a metal mesh directly onto the upper cladding layer of the active layer instead of on a transparent oxide layer. 
     BACKGROUND 
     The principles lying behind luminance of light emitting diodes relate to injecting an electric current sequentially through P-N junctions of a semiconductor to generate light, wherein AlGaInP is implemented in high brightness red, orange, yellow and yellowish green LEDs, AlGaInN is in blue and green LEDs. The process of metal organic vapor phase epitaxy (MOVPE) is commonly adopted in the mass production of the LEDs, while the light-emitting components are of the structures, including: homo-junction (HOMO), single-heterostructure (SH), double-heterostructure (DH), single-quantum well (SQW) and multiple-quantum well (MQW) or other appropriate structures. 
     The structure of a conventional light emitting diode is illustrated in  FIG. 1A , including, from the top down, a front electrical electrode, a transparent oxide layer or a window layer  14  to disperse the current, an active layer  12 , a substrate  10  and a back contact  13 . Among them, the active layer  12  is formed by a light-emitting material, such as AlGaInP or AlGaInN by adopting MOVPE and the transparent conductive oxide layer is a transparent conductive oxide layer typically formed of an indium tin oxide (ITO) layer. After a current is injected through the front contact  11 , the current will pass through the transparent conductive oxide layer  14  to disperse and then through the active layer  12  and the substrate  10  to flow towards the back contact  13 . Light is emitted when the current flows through the active layer  12 . The active layer  12  is a sandwich layer, including a p-type upper cladding layer, an intrinsic layer and an n-type lower cladding layer. However, the low carrier mobility and high resistance of the active layer made of AlGaInP or AlGaInN results in poor electric conductivity of the AlGaInP or AlGaInN. Apart from that, the transparent conductive oxide layer  14  though can improve the current dispersion, its conductive ability is however weaker than a metal layer. Consequently, a metal grid layer  16  is generally embedded in the ITO layer  14 , e.g., a metal grid layer formed on the active layer, and then covered it by forming an ITO layer thereover. The current distribution is getting improvement, never less, the primary emitting regions are mainly concentrated at and next to the lower portion of the electrode, as shown in  FIG. 1A . 
     To enhance the current distribution, improvements have been made to the structures and materials, such as that disclosed in U.S. Pat. No. 5,008,718 by Fletcher et al., where a capping layer  15  (or window layer), made of GaP, GaAsP and AlGaAs having a low resistance value and being pervious to light, is added between the front contact and active layer, as shown in  FIG. 1B . The objective of using this capping layer is to enhance the current distribution flowing from the front contact. As described in the &#39;718 patent, to improve the current distribution, the capping layer is preferred to be in the range from 150 to 200 micrometers thick to enhance the luminous intensity by 5 to 10 times. However, the increasing thickness of the capping layer also increases the time and cost required for MOVPE epitaxy thereby significantly increasing the cost of the epitaxy. In addition, the distribution ability is extremely relevant to the thickness. Hence, to ensure even current distribution, the thickness must be at least 10 micrometers or the current crowding problem cannot be effectively resolved. 
     Another embodiment is to change the design of the electrode. The structure metal electrode is a mesh. Please refer to  FIG. 1C ; a substrate  100  thereover is formed with an active layer, which may be a double hetero junction or a quantum well to improve the emitting efficacy. A transparent layer such as GaP, AlgaAs or an ITO layer  140  is then formed on the active layer to improve the current distribution. The back electrode  130  is formed on the opposite face of the substrate  100 . On the other hand, an upper electrode  210 , which is a metal mesh formed on the transparent layer  140  and an extra metal pad  110  is formed on the metal mesh  210 . 
     The material of the substrate  100  is dependent on the material of the active layer  120 . When the active layer  120  is made of AlGaInP, GaAs is chosen as the substrate. When the active layer  120  is made of AlGaInN. Any of sapphire, may be selected as the substrate. The active layer is preferred to be in the range from 0.3 to 3 μm in thick. The thickness of the transparent layer  140  is preferred to be in the range from 10 to 50 μm. Both the active layer  120  and the transparent layer  140  are formed by MOVPE or molecular beam epitaxy (MBE). 
     The metallic mesh layer  210  suggested are dimensioned to 0.5 to 5 micrometers and evenly distributed above the substrate. If the meshes are dimensioned to 2 μm with a capping layer having a thickness of 15 μm, the light-emitting angle θ is calculated by, tan s2θ c =2/15→2θ c ≈7.6°→θ c ≈3.8°. 
     SUMMARY OF INVENTION 
     It is a primary objective of this invention to provide a light-emitting diode to save the cost. 
     It is another object of the present invention is to improve the injecting current distribution and less light be shadowed by the metal mesh line. 
     The present invention discloses a light-emitting diode having a metal mesh pattern formed directly on an active layer without a transparent oxide conductive layer formed in between to save the manufacture cost. The mesh pattern is formed by using ion bombardment a metal layer so that myriad pits formed into the exposed portion of the active layer served as light emitting centers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other modifications and advantages will become even more apparent from the following detained description of preferred embodiments of the invention and from the drawings in which: 
         FIG. 1A  is a cross-sectional view of a conventional light emitting diode structure having an ITO as transparent conductive oxide layer and metal grid layer embedded therein. 
         FIG. 1B  is a cross-sectional view of a conventional light emitting diode structure having a transparent window layer selected from GaP, GaAsP, or AlGaAs for current redistribution. 
         FIG. 1C  is a cross-sectional view of a conventional light emitting diode structure having a metal mesh pattern formed thereon a transparent oxide layer formed thereunder. 
         FIG. 2A  is a cross-sectional view of a light emitting diode structure having a metal mesh pattern formed. Directly on an active layer without an ITO layer or a transparent window layer. 
         FIG. 2B  is a top view of a metal mesh having a round-hole pattern. 
         FIG. 2C  is a top view of a metal mesh having a lattice-hole pattern. 
         FIG. 2D  shows myriad pits  125  formed into the active layer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred Embodiments 
     According to a preferred embodiment, the light emitting diode has a metal meshes directly formed on the active layer without a transparent layer there between and in the openings of the meshes has myriad pits formed therein to service as emitting centers. 
     Referring to  FIG. 2A , it is a cross-sectional view of a light emitting diode. First, an active layer  120  is formed on a substrate  100 . The active layer is the primary structure of the LED component that may be a single-heterostructure, double-heterostructure, single-quantum well or multiple-quantum well to improve the emitting efficacy. When the active layer  120  is made of AlGaInP, GaAs is selected to form the substrate. When the active layer  120  is made of AlGaInN, a sapphire is selected as the substrate  100 . The active layer is preferred to be in the range from 0.3 to 3 micrometers thick. The active layer  120  is formed by adopting MOVPE or molecular beam epitaxy (MBE). 
     In accordance with a preferred embodiment, a metal layer is directly formed on the active layer  120  without a transparent layer  140  in between. The metal layer has a thickness between about 10˜100 nm. A photoresist pattern having round holes or mesh pattern is formed on the metal layer as an etching mask. The coverage of the photoresist layer is about 20% to 30%. Thereafter, a dry etching such as an ion bombardment technique is carried out to etch the metal layer so as to generate a mesh pattern  210  having round holes  200 A or lattice holes, as shown in  FIG. 2B  and  FIG. 2C , respectively. The ion bombardment may use argon gas as plasma source. The photoresist pattern is then stripped off. The result surface of the active layer  120  becomes rough, particularly to those of exposed portions from the metal mesh pattern, as shown in  FIG. 2D . That is myriad pits  125  acts as current centers or light emitter centers with sizes between nanometers or micrometers are formed on the exposed active layer  120  so that the main light emits will smooth out without shadow. 
     In another preferred embodiment, an ion beam etching technique is performed to pattern the metal layer by using a reticle having rounded holes or a mesh pattern as the etching mask. The rounded holes in the mesh have a diameter between about 1 μm to hundreds μm. In case of lattice holes in the mesh layer  210 , the width of the mesh line is between 100 nm to 5000 nm so that the remnant metal with an area-coverage is between about 20-26%. 
     The material of the metal mesh layer  210  is selected from copper or silver. The conductivity of the Cu and Ag are, respectively, of about 6.3×10 7  s/m (or 1.728×10 −8  Ωm in resistivity) and 5.85×10 7  s/m in SI unit in compare with the 10 4  s/m for ITO. For a case of metal area-coverage 25%, the transparency would be 75%. The transparency of the ITO is of about 80% for blue light or red light. Thus the total light emitting out from the metal mash layer is anticipated better than out from the ITO layer since the conductivity of metal mesh pattern is significantly surpass than that of the ITO. 
     The benefits of the present invention are: 
     1. The metal mesh pattern is directly formed on the active layer. No widow layer such as an ITO, GaP, GaAsP or AlGaAs layer is demanded. It is thus save the cost and time. 
     2. The metal mash layer is patterned by ion bombardment, myriad pits and become current centers are resulted and formed on the active layer where the centers are at the openings of the mesh pattern. 
     3. The current injection is directly through the metal mesh lines. It is thus the current distribution is uniform. 
     This invention is related to a novel creation that makes a breakthrough in the art. Aforementioned explanations, however, are directed to the description of preferred embodiments according to this invention. Since this invention is not limited to the specific details described in connection with the preferred embodiments, changes and implementations to certain features of the preferred embodiments without altering the overall basic function of the invention are contemplated within the scope of the appended claims.