Patent Publication Number: US-2011057219-A1

Title: Nitride-based semiconductor light emitting device

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to solid state light emitting devices and, more particularly, to a nitride-based semiconductor light emitting device with high light extraction efficiency. 
     2. Discussion of Related Art 
     Nowadays, nitride-based semiconductor light emitting devices such as gallium nitride LEDs (i.e., light emitting diodes) have the advantages of low-power consumption and long life-span, etc, and thus are widely used for display, backlight, outdoor illumination, automobile illumination, etc. However, in order to achieve high luminous brightness, an improvement of light extraction efficiency of the conventional nitride-based LEDs is required. 
     Kao et al. has published a paper on IEEE photonics technology letters, vol. 19, No. 11, page 849-851 (June, 2007) entitled “light-output enhancement of nano-roughened GaN laser lift-off light-emitting diodes formed by ICP dry etching”, the disclosure of which is fully incorporated herein by reference. Kao et al. has proposed an approach for the improvement of the light extraction efficiency of the GaN LED, by way of forming a number of grooves on a light-emitting region of the GaN LED via an ICP-RIE (i.e., inductively coupled plasma-reactive ion etching) dry etching. However, side-surfaces of the grooves are usually perpendicular to an active layer and can not be used as light emitting surfaces; therefore, it is difficult to improve light extraction efficiency of the GaN LED due to the limitative area of the light emitting surface of the LED. 
     Therefore, what is needed is a nitride-based semiconductor light emitting device with high light extraction efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top plan view of a nitride-base semiconductor light emitting device, in accordance with an embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view of the nitride-base semiconductor light emitting device of  FIG. 1 , taken along line II-II thereof. 
         FIG. 3  is a graph of light extraction efficiency vs. angle for the nitride-base semiconductor light emitting device of  FIG. 1 . 
         FIG. 4  is a graph of light extraction efficiency vs. current for the nitride-base semiconductor light emitting device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference will now be made to the drawings to describe various embodiments of the present nitride-base semiconductor light emitting device in detail. 
     Referring to  FIGS. 1-2 , a nitride-base semiconductor light emitting device  10 , such as a gallium nitride light emitting diode (GaN LED), in accordance with the present embodiment, is provided. The light emitting device  10  includes a substrate  11 , a nitride-based multi-layered structure  12  epitaxially formed on the substrate  11 , an N-type electrode  14  and a P-type electrode  13  formed on the nitride-based multi-layered structure  12 . 
     The substrate  11  beneficially is a single crystal plate and can be made from a material of sapphire, silicon carbide (SiC), silicon (Si), gallium arsenide (GaAs), lithium aluminate (LiAlO 2 ), magnesium oxide (MgO), zinc oxide (ZnO), GaN, aluminum nitride (AlN) or indium nitride (InN), etc. The substrate  11  has a crystal face  121  facilitating the epitaxial growth of the nitride-based multi-layered structure  12  thereon. A crystal growth orientation of the crystal face  121  matches with a crystal growth orientation of the multi-layered structure  12 . 
     The multi-layered structure  12  includes an N-type layer  122 , an active layer  124  and a P-type layer  123  arranged along a direction away from substrate  11 , in the order written. That is, the active layer  124  is sandwiched between the N-type layer  122  and the P-type layer  123 . The N-type layer  122  is of an opposite conductive type with respect to the P-type layer  123 . The N-type layer  122 , the active layer  124  and the P-type layer  123  individually can be a single layer structure or a multi-layered structure, and suitably made from group III-nitride compound materials. The group III element can be aluminum (Al), gallium (Ga), indium (In) and so on. In this embodiment, the N-type layer  122 , the active layer  124  and the P-type layer  123  respectively are an N-type GaN layer, an InGaN layer and a P-type GaN layer. The multi-layered structure  12  has a developed mesa structure, whereby the N-type layer  122  is partially exposed to form an exposed portion  125  at a side facing away from the substrate  11 . The P-type layer  123  has a top surface  126  facing away from the substrate  11 . In one embodiment, the multi-layered structure  12  may include a P-type layer, an active layer and an N-type layer arranged along a direction away from substrate  11 . 
     The P-type layer  123  defines a number of grooves  15  at the top surface  126  thereof. The grooves  15  each have a side surface  151  and a bottom surface  152  adjoining the side surface  151 . The side surface  151  and the bottom surface  152  cooperatively form an included angle θ, and the angle θ ranges from 140 degree to 160 degree. The grooves  15  each can have a shape of a conversed truncated-cone, or a conversed truncated-pyramid. In this embodiment, the grooves  15  are arranged on the top surface  126  in an array and spaced from each other. The grooves  15  each have a shape of a conversed truncated-pyramid with six edges  154  on the top surface  126  of the P-type layer  123 . The edges  154  of each groove  15  cooperatively define a hexagon, that is, each groove  15  has a hexagonal shape as viewed from a top of the light emitting device  10 . Lengths of the edges  154  are equivalent. The length of each edge  154  ranges from 0.5 to 2 micron. The hexagon of each groove  15  has an imaginary center, and a length D between centers, such as O 1 , O 2 , of two adjacent hexagons ranges from 0.85 to 3.5 micron. In the present embodiment, a height H 1  of the groove  15  is a half of that of the P-type layer  123 . 
     The grooves  15  are defined in the top surface  126  of the P-type layer  123  by ICP-RIE dry etching. An exemplary method for fabricating the grooves  15  will be described in detail: providing a substrate  11 ; epitaxially growing a nitride-based multi-layered structure  12  on the substrate  11 ; providing with strong oxidation air, such as chlorine and argon, thereby etching a light-emitting region of the nitride-based multi-layered structure  12  via an ICP-RIE to form a number of the grooves  15  on the P-type layer  123  of the nitride-based multi-layered structure  12 . Furthermore, it can adjust the angle θ via changing the concentration of chlorine and argon. 
     The N-type electrode  14  is formed on the exposed portion  125  of the N-type layer  122  so as to electrically connect (e.g., ohmic contact) with the N-type layer  122 . The N-type electrode  14  usually includes at least one metallic layer which is in ohmic contact with the N-type layer  122 . 
     The P-type electrode  13  is formed on the top surface  126  of the P-type layer  123  so as to electrically connect (e.g., ohmic contact) with the P-type layer  123 . The P-type electrode  13  can be a single metallic layer or a multi-layered structure consisting of a metallic layer and a transparent conductive film. 
     The grooves  15  of the P-type layer  123  are configured for eliminating total-reflection to improve the light extraction efficiency of the light emitting device  10 . 
     Furthermore, the angle θ is in a range from 140 degree to 160 degree; therefore, the side surface  151  can be as a light emitting surface, and the light extraction efficiency of the light emitting device  10  is improved due to the increased area of the light emitting surface. 
     Referring to  FIG. 3 , a graph of light extraction efficiency of the light emitting device  10  is provided. X-axis represents the angle θ cooperatively formed by the side surface  151  and the bottom surface  152 , and Y-axis represents the light extraction efficiency of the light emitting device  10 . It can be seen from  FIG. 3 , when the angle θ is within the range from 140 degree to 160 degree, the light extraction efficiency of the light emitting device  10  has a larger value. When the angle θ is 150 degree, the light extraction efficiency of the light emitting device  10  achieves a peak value. 
     Referring to  FIG. 4 , a graph of the light extraction efficiency of the light emitting device  10  from another aspect is provided. X-axis represents the current of the light emitting device  10 , and Y-axis represents the light extraction efficiency of the light emitting device  10 . Curves A 1 , A 2 , A 3 , A 4  and A 5  respectively indicate the light extraction efficiencies of the light emitting device  10  in condition that the height of the P-type layer  123  is H 2 , and the height H 1  of the grooves  15  is zero, 
     
       
         
           
             
               
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     respectively. It can be seen from the  FIG. 4 , when driven by a current of 100 microampere, the light emitting device  10  has a light extraction efficiency of 62%, in condition that 
     
       
         
           
             
               
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     the light emitting device  10  has a light extraction efficiency of 57%, in condition that 
     
       
         
           
             
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     Therefore, the light emitting device  10  has a higher light extraction efficiency in condition that the height H 1  of the grooves  15  is in a range from 
     
       
         
           
             
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     It is to be further understood that even though numerous characteristics and advantages have been set forth in the foregoing description of embodiments, together with details of the structures and functions of the embodiments, the disclosure is illustrative only; and that changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.