Abstract:
The luminous element includes a luminescence lamination, a second transparent oxidative conducting layer and a composite conducting layer. The composite conducting layer includes first transparent oxidative conducting layer and a metal layer. The second transparent oxidative conducting layer is positioned between the metal layer and luminescence lamination the second transparent oxidative conducting layer forms good ohmic contact with the luminous element and with metal layer. Thus, the metal layer will not be influenced by interfusion so as to maintain good light transmissivity and raise luminous efficiency of luminous element.

Description:
TECHNICAL FIELD 
     The invention relates to a light-emitting device, and more particularly to a semiconductor light-emitting device having stacked transparent electrodes. 
     REFERENCE TO RELATED APPLICATION 
     This application claims the right of priority based on TW application Ser. No. 096111705, filed Mar. 30, 2007, and the content of which is hereby incorporated by reference. 
     DESCRIPTION OF BACKGROUND ART 
     An important issue arose in designing a structure of a light-emitting diode (LED) is to evenly spread current from bonding pad to p-n junction in order to reach a better light-emitting efficiency. The known technologies such as semiconductor window layer, transparent conductive oxide film, and patterned electrode, are already used to boost the current-spreading performance. 
     GaP window layer is usually adopted in AlGaInP series LED. GaP has an energy band gap (Eg) of 2.26 eV, and is transparent to red, orange, yellow light, and part of green light spectrum, and is an indirect band gap semiconductor, which absorbs less light in comparison with a direct band gap semiconductor. A GaP layer of a sufficient thickness such as 2 μm˜30 μm exhibits an acceptable current-spreading behavior, and the thicker the GaP window layer is, the better the current-spreading performance can be achieved. However, it takes much time to grow a thicker window layer and the throughput is reduced. 
     The transparent conductive oxides such as ITO, CTO, and InO are also used to enhance the current-spreading performance. For instance, ITO has a 90% transmittance in the range of 500 nm˜800 nm in wavelength with a resistivity of 3×10 −4  Ω-cm and a sheet resistance of 10Ω/□. Generally speaking, a small scale chip with an ITO layer of 0.1 μm˜1 μm can acquire an acceptable current-spreading outcome. The required thickness can be formed in a short time by using the known manufacturing method such as sputtering and electron beam evaporation. However, the ITO also falls short of the current-spreading requirement in response to the increasing area of light-emitting diode chip (for example, chip size≧15×15 mil) and the development of rectangular chip. 
     Patterned electrode is another way that is often used to elevate current-spreading performance. In the method, to evenly spread current from the patterned electrode to p-n junction, the electrode is arranged by outwardly extending from a connection, interdigitating p- and n-electrodes, or forming as dots, mesh or other patterns. To form a patterned electrode, extra quantity of electrode material is often needed in order to cover larger area of the upper surface of the light-emitting diode. Moreover, the material adopted in the patterned electrode is usually an opaque metal, and consequently the light-emitting efficiency is greatly degraded. 
     SUMMARY OF THE DISCLOSURE 
     The application provides a semiconductor light-emitting device able to spread current to increase the light-emitting efficiency. 
     In one embodiment in accordance with the invention, the disclosed semiconductor light-emitting device includes a substrate; a semiconductor epitaxial layer over the substrate and having an outer surface distant from the substrate; a first transparent conductive layer over the outer surface; and a second transparent conductive layer over a first surface of the first transparent conductive layer; wherein the area of the interface between the second and first transparent conductive layers is smaller than the total area of the first surface of the first transparent conductive layer. 
     Preferably, an area ratio of the surface of the second transparent conductive layer to that of the first transparent conductive layer is not greater than ½. The composition of the material of the first transparent conductive layer and the second transparent conductive layer are identical or similar. The area of the semiconductor light-emitting device is not smaller than 15×15 mil. 
     In several embodiments, the first transparent conductive layer covers whole or part of the outer surface. The first transparent conductive layer has a greater transmittance to light emitted from the semiconductor epitaxial layer than that of the second transparent conductive layer. The conductivity of the first transparent conductive layer is smaller than that of the second transparent conductive layer. The thickness of the first transparent conductive layer is greater than that of the second transparent conductive layer. The second transparent conductive layer comprises a plurality of electrically-connected segments. 
     In another embodiment, the semiconductor light-emitting device further includes a groove sunken from a surface of the semiconductor epitaxial layer to a bottom surface to expose part of at least one layer of the semiconductor epitaxial layer, and extending from a first side of the semiconductor epitaxial layer toward an opposite second side; and a second connection layer in the groove and having an extension segment over the bottom surface. In a modified embodiment, at least one of the first transparent conductive layer and the second transparent conductive layer surrounds the groove. In addition, the semiconductor light-emitting device further includes a first connection electrically connected to the second transparent conductive layer. 
     In a further embodiment, the semiconductor epitaxial layer includes a first type semiconductor layer; a second type semiconductor layer; and a light-emitting layer between the first type semiconductor and the second type semiconductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates a cross section of a light-emitting diode in accordance with an embodiment of present invention. 
         FIG. 1B  illustrates a top view of the light-emitting diode of  FIG. 1A . 
         FIG. 2  illustrates a cross section of a light-emitting diode in accordance with another embodiment of present invention. 
         FIGS. 3A and 3B  illustrate layouts of the transparent conductive layer in accordance with an embodiment of present invention. 
         FIGS. 4A and 4B  illustrate layouts of the transparent conductive layer in accordance with another embodiment of present invention. 
         FIG. 5  illustrates a layout of the transparent conductive layer in accordance with a further embodiment of present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The embodiments of the invention are described in accompany with drawings. The term “layer” used in this specification is meant to be a single layer, or two or more layers having identical or different composition materials. The layers can be directly or indirectly connected. 
     The First Embodiment 
     As shown in  FIG. 1A , the semiconductor light-emitting device  10  includes an electrode  15 , a substrate  11 , a semiconductor epitaxial layer  12 , a first transparent conductive layer  1301 , a second transparent conductive layer  1302 , and a connection  14 . The semiconductor epitaxial  12  includes at least a first type semiconductor layer  1201 , a second type semiconductor layer  1203 , and a light-emitting layer  1202  between the first type semiconductor layer  1201  and the second type semiconductor  1203 . The first type semiconductor layer  1201  and the second type semiconductor layer  1203  have different conductivities, for example, the conductivities are selected from at least two of p-type, n-type, and i-type. In a double heterostructure (DH), the energy band gaps of the first type semiconductor layer  1201  and the second type semiconductor layer  1203  are greater than that of light-emitting layer  1202 . When a bias voltage is applied to the semiconductor epitaxial layer  12 , the electrons and holes recombine in the region and/or neighborhood of the light-emitting layer, and then light is radiated. 
     In present invention, the first transparent conductive layer  1301  and the second transparent conductive layer  1302  are sequentially formed on the semiconductor epitaxial layer  12 . The first transparent conductive layer  1301  covers whole or part of the upper surface of the semiconductor epitaxial layer  12 . The second transparent conductive layer  1302  covers part of the upper surface of the first transparent conductive layer  1301 , i.e. the second transparent conductive layer  1302  is smaller in area than the first transparent conductive layer  1301 . In present embodiment, only two layers are displayed, however, a transparent conductive layer having more than two area-shrinking layers may be adopted in present invention. 
     In one preferable embodiment, the first transparent conductive layer  1301  and the second transparent conductive layer  1302  are made of the same material but with different electrical and/or optical properties, and the compositions or proportions of elements in the material can be different from each other, therefore, the resistivity and the transmittance of the first transparent conductive layer  1301  are greater than those of the second transparent conductive layer  1302 . 
     The second transparent conductive layer  1302  has a resistivity effective enough to spread current to the underneath larger-area first transparent conductive layer  1301 . The sheet resistance or the resistivity of the first transparent conductive layer  1301  is greater than that of the second transparent conductive layer  1302 . One function of the second transparent conductive layer  1302  is to make current flow towards the distant connection  14  under a suitable transmittance range. Under the prerequisite, the material composition, thickness, and the layout of the second transparent conductive layer  1302  can be adapted to the requirement. 
     Current flows into the second transparent conductive layer  1302  through the connection  14 , and flows into the first transparent conductive layer  1301  through the second transparent conductive layer  1302 , and then flows into the semiconductor epitaxial layer  12  through the first transparent conductive layer  1301 . With the combination of the two transparent conductive layers, current can flow outwardly, and the current crowding effect is further alleviated. Due to a more complete photoelectric transformation in the light-emitting layer  1202 , one can obtain a more efficient light-emitting region. 
       FIG. 1B  illustrates a top view of the semiconductor light-emitting device  10  of  FIG. 1A . In the drawing, the areas of the connection  14 , the second transparent conductive layer  1302 , and the first transparent conductive layer  1301  are gradually increased. With an appropriate arrangement of the thickness, sheet resistance, and/or the resistivity, current from the connection  14  gradually flows outwards and downwards, and spreads into the light-emitting layer  1202 . 
     In one embodiment, both of the first transparent conductive layer  1301  and the second transparent conductive layer  1302  are made by ITO, while the proportions of at least one of In, O, and Sn in the two layers are at different levels, or the two ITO layers are made by different process conditions, for example, the first transparent conductive layer  1301  is made by sputtering, and the second transparent conductive layer  1302  is made by electron beam evaporation, or vice versa. Preferably, the first transparent conductive layer  1301  is made by ITO having a higher transmittance such as above 90%, 80%, 70%, or 60%, and the second transparent conductive layer  1302  is made by ITO having a lower transmittance such as below 50% and a lower resistivity. With the condition, one can obtain a light-emitting device having appropriate optical and electrical performance (such as the levels of transmittance and resistivity). 
     In one embodiment, the first transparent conductive layer  1301  and the second transparent conductive layer  1302  can be also made by ITO and metal such as Ni/Au or Au respectively. The metal is better to have a thickness between 0.005 μm˜0.2 μm in order to sustain both of the transmittance and the resistivity in appropriate levels. In another embodiment, the thickness of the second transparent conductive layer  1302  is smaller than the surface roughness Ra of the first transparent conductive layer  1301 . However, within the manufacturing tolerance, the thickness of the second transparent conductive layer  1302  can be greater than the surface roughness Ra of the first transparent conductive layer  1301 . The thin metal has a smaller resistivity than that of ITO, and therefore, can provide help in spreading current within ITO layer and obstructs less quantity of light. 
     In one embodiment, part or whole of at least one of the first transparent conductive layer  1301  and the second transparent conductive layer  1302  is transparent or has a more than 50% transmittance to light from light-emitting layer  1202 , and the first transparent conductive layer  1301  has a higher transmittance than that of the second transparent conductive layer  1302 . An individual transparent conductive layer may also be composed of two or more portions having different transmittances. The second transparent conductive layer  1302  has a lower resistivity to reach a better current-spreading performance, and should absorb light from the light-emitting layer  1202  as less as possible. In another embodiment, the thickness of the first transparent conductive layer  1301 , which has a higher transmittance, is greater than that of the second transparent conductive layer  1302 , which has a lower resistivity. But the invention is not limited to aforementioned cases; the thickness arrangement of the two transparent conductive layers is dependent upon the characteristic of the adopted material. 
     In the foregoing embodiments, the material of the substrate  11  includes, but not limited to, SiC, GaAs, AlGaAs, GaAsP, ZnSe, III-nitride (e.g. GaN), sapphire, Si, and glass. The materials of the first type semiconductor layer  1201  and the second type semiconductor  1203  include, but not limited to, AlGaInP series and III-nitride series. The structure of the light-emitting layer  1202  includes, but not limited to, single heterostructure (SH), double heterostructure (DH), double-side double heterostructure (DDH), single quantum well (SQW), and multi-quantum well (MQW). 
     The material of the first transparent conductive layer  1301  includes, but not limited to, ITO, IZO, ZnO, CTO, In 2 O 3 , SnO 2 , MgO, CdO, and other transparent oxide. The material of the second transparent conductive layer  1302  includes, but not limited to, ITO, IZO, ZnO, CTO, In 2 O 3 , SnO 2 , MgO, CdO, and other transparent oxide. The material of the second transparent conductive layer  1302  includes, but not limited to, Au, Ni, Ti, In, Pt, Al, Cr, Rh, Ir, Co, Zr, Hf, V, Nb, an alloy or a stack of aforementioned materials, and other metal with acceptable optical and electrical properties. 
     The Second Embodiment 
     Refer to  FIG. 2 , the semiconductor light-emitting device  20 , in accordance with another embodiment of present invention, includes a substrate  21 , a semiconductor epitaxial layer  22 , a first transparent conductive layer  2301 , a second transparent conductive layer  2302 , a first connection  24 , and a second connection  25 , wherein the first and second connections are positioned on the same side of the substrate  21 . The semiconductor epitaxial layer  22  includes at least a first type semiconductor layer  2201 , a second type semiconductor layer  2203 , and a light-emitting layer  2202  between the first type semiconductor layer  2201  and the second type semiconductor layer  2203 . 
     In the embodiment, current injected from the first connection  24  firstly flows into the second transparent conductive layer  2302 , and then the first transparent conductive layer  2301  through the second transparent conductive layer  2302 , and further flows into the semiconductor epitaxial layer  22  through the first transparent conductive layer  2301 . With the combination of the two transparent conductive layers, current can flow outwardly, and light-emitting efficiency is increased. 
     In the embodiment, the materials used in the semiconductor light-emitting device  20 , and the relation between the first transparent conductive layer  2301  and the second transparent conductive layer  2302  can refer to the description of the first embodiment. Furthermore, structures or design principles similar to those of the first transparent conductive layer  2301  and the second transparent conductive layer  2302  can also be introduced among the second connection  25  and the first type semiconductor layer  2201  to improve the current-spreading performance. 
     The first and second transparent conductive layers may have following modifications, however, the following embodiments are not used to limit the invention, and any suitable arrangement may be incorporated with the concept of the invention. 
     The Third Embodiment 
       FIGS. 3A and 3B  illustrate top views of the first transparent conductive layer  1301  and the second transparent conductive layer  1302  of the semiconductor light-emitting device  10 . In the two views, the second transparent conductive layer  1302  is formed on the first transparent conductive layer  1301 , and has an area smaller than that of the first transparent conductive layer  1301 . 
     In present embodiment, the second transparent conductive layer  1302  has a surrounding segment  1304  and a cross segment  1303 . As shown in  FIG. 3A , the surrounding segment  1304  includes two circular segments surrounding the connection  14 . The cross segment  1303  extends outwardly from the connection  14  and passes through the first circular segment near the connection  14 . Those segments are electrically connected with each other, but not limited to a physical connection. The current from the connection  14  flows through the cross segment  1303  and then spreads outwards into the first and second circular segments. The current further spreads to farther regions after passing through the two circular segments. 
     In present embodiment, the surrounding segment  1304  and cross segment  1303  are not limited to the quantity described in the specification or shown in the drawings. The surrounding segment may also be substituted by segments in polygon shapes, such as triangle, quadrangle, pentagon, and hexagon. As shown in  FIG. 3B , The surrounding segment  1304  includes two quadrangles. The cross segment  1303  can penetrate or not the surrounding segment  1304 . The surrounding segment  1304  can be arranged in a radial symmetry or bilateral symmetry with respect to the connection  14 . The connection  14  may also be in any position of the surrounding segment  1304 . 
     The cross segment and surrounding segment need to be have a predetermined width in order to spread current and not to absorb too much light. The widths of the surrounding segment  1304  and the cross segment  1303  are respectively between 0.1 μm and 50 μm, preferably, less than 20 μm. The thicknesses of the two segments depend on the adopted materials. For metallic material, the thickness is between 0.001 μm and 1 μm, preferably, between 0.005 μm and 0.05 μm, in order to sustain an appropriate transmittance. As to the transparent oxide, e.g. ITO, the segment may be thicker. 
     The present embodiment is only illustrated by, but not limited to, the semiconductor light-emitting device  10  of  FIG. 1A . The aforementioned arrangements of the transparent conductive layer can be applied to one or both of the first transparent conductive layer  2301  and the second transparent conductive layer  2302  of the semiconductor light-emitting device shown in  FIG. 2   
     The Fourth Embodiment 
     As shown in  FIG. 4A , two current-spreading segments  2302   a  and  2302   b  are positioned on the first transparent conductive layer  2301 , and respectively extend outwards from the first connection  24  to the direction of the second connection  25 . In the drawing, the current-spreading segments  2302   a  and  2302   b  are formed in a curve, like a U-shape. However, the current-spreading segments  2302   a  and  2302   b  may also be formed in a straight line, a curve, or the combination thereof. The current-spreading segments  2302   a  and  2302   b  preferably approach the boundary of the first transparent conductive layer  2301  in order to spread current to farther region. 
     The second connection  25  is located on a bottom surface of a groove  26 . The bottom surface of the groove  26  and the first connection  24  are respectively on the opposite sides of the light-emitting layer  2202  to constitute an electrical path. The groove  26  is formed by etching from any position of the semiconductor epitaxial layer to exceed the depth of the light-emitting layer. The etching process is performed by chemical or physical etching. Therefore, the groove  26  is sunken from a surface of the first type second type semiconductor layer  2203  to the first type semiconductor layer  2201  to expose the first type semiconductor layer  2201 , and extending from a first side of the semiconductor epitaxial layer  12  toward an opposite second side. 
     A modification of the present embodiment is shown in  FIG. 4B . The groove  26  is extended to the direction of the first connection  24 . An extension segment  2501  is formed in the groove  26 , and extends from the second connection  25  to the direction of the first connection  24 . The disposition of the current-spreading segments  2302   a  and  2302   b  are shown in  FIG. 4A . The current can evenly spread in the region between the first and second connections by the assistance of the extension segment  2501 , and accordingly more active light-emitting zones are introduced and the light-emitting efficiency is improved. 
     In present embodiment, the covering area of the segment depends on the size of the semiconductor light-emitting device. The distance between the closest neighboring individual segments is between 1 μm and 500 μm. The thickness of the segment is between 0.1 μm and 50 μm. The thickness of metallic segment is between 0.001 μm and 1 μm, preferably, between 0.005 μm and 0.05 μm; the thickness of ITO segment is larger, e.g. 0.6 μm or above. 
     The Fifth Embodiment 
     As shown in  FIG. 5 , the current-spreading segment  2302  is located on the first transparent conductive layer  2301 , and extends outwards from the first connection  24  to the direction of the second connection  25 . In the drawing, the current-spreading segment is illustrated as a straight line, but a curve, a zigzag, or the combination thereof can also be adopted herein. Specifically, from the top view, the semiconductor light-emitting device is in a rectangular shape with a ratio of length to width in 1.1˜3, preferably more than 1.5. The detailed description of groove  26  can be referred to the aforementioned embodiments. 
     The foregoing description has been directed to the specific embodiments of this invention. It will be apparent; however, that other variations and modifications may be made to the embodiments without escaping the spirit and scope of the invention.