Abstract:
A light emitting diode and a light emitting diode (LED) manufacturing method are disclosed. The LED comprises a substrate; a first n-type GaN layer; a second n-type GaN layer; an active layer; and a p-type GaN layer formed on the substrate in sequence; the second n-type GaN layers has a bottom surface interfacing with the first n-type GaN layer, a rim of the bottom surface has a roughened exposed portion, and Ga—N bonds on the bottom surface has an N-face polarity.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to semiconductor devices and, particularly, to a light emitting diode and a method for manufacturing the light emitting diode. 
     2. Description of Related Art 
     Light emitting diodes (LEDs) have many beneficial characteristics, including low electrical power consumption, low heat generation, long lifetime, small volume, good impact resistance, fast response and excellent stability. These characteristics enable the LEDs to be used as light sources in electrical appliances and electronic devices. 
     In general, the light output of an LED depends on the quantum efficiency of the active layer and the light extraction efficiency. As the light extraction efficiency increases, the light output of the LED is enhanced. In order to improve the light extraction efficiency, efforts have been made to overcome a significant photon loss resulting from total reflection inside the LED after emission from the active layer. 
     A typical method for increasing the light extraction efficiency of the LED is to roughen the surface of the LED by etching. However, it may be difficult to roughen the surface of the conventional LED, and the etching process may be time-consuming. 
     What is needed is an LED and a method for manufacturing the LED which may overcome the disadvantages discussed above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  shows a step of providing a substrate in accordance with a first embodiment of a method for making an LED. 
         FIG. 2  shows a step of forming a buffer layer, an updoped GaN layer, a first n-type GaN layer and an AlN layer on the substrate in  FIG. 1 . 
         FIG. 3  shows a step of patterning of the AlN layer in  FIG. 2 . 
         FIG. 4  shows a top view of the AlN layer after patterning in  FIG. 3 . 
         FIG. 5  shows a step of forming a second n-type GaN layer on an upper surface of the first n-type GaN layer uncovered by the AlN layer in  FIG. 3 . 
         FIG. 6  shows a step of forming an active layer and a p-type GaN layer on an upper surface of the second n-type GaN layer in  FIG. 5 . 
         FIG. 7  shows a step of etching the AlN layer and the second n-type GaN layer in  FIG. 6  by alkaline solution. 
         FIG. 8  shows a step of forming an electrode on an upper surface of the p-type GaN layer. 
         FIG. 9  shows a top view of a patterned AlN layer in accordance with a second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will now be described in detail below, with reference to the accompanying drawings. 
     Referring to  FIG. 1  wherein a substrate  110  is first provided. The substrate  110  may be made of a material selected from a group consisting of Si, SiC and sapphire. 
     Referring also to  FIG. 2 , a buffer layer  120 , an undoped GaN layer  130 , a first n-type GaN layer  140  and an AlN layer  150  are formed on the substrate  110  in sequence. A thickness of the AlN layer  150  ranges from 5 nm to 500 nm. If the thickness of the AlN layer  150  is less than 5 nm, the AlN layer  150  may not be easily removed by etching solution. If the thickness of the AlN layer  150  is larger than 500 nm, a GaN layer formed above the AlN layer  150  in following steps may be cracked due to different lattice constants between the GaN layer and the AlN layer  150 . In this embodiment, the thickness of the AlN layer  150  is about 50 nm. In this embodiment, the buffer layer  120  and the updoped GaN layer  130  are configured to improve the quality of the first n-type GaN layer  140 . In alternative embodiments, the first n-type GaN layer  140  may be directly formed on the buffer layer  120 , or directly formed on the substrate  110 . The first n-type GaN layer  140  has a first surface  1400  away from the substrate  110 . The first surface  1400  of the first n-type GaN layer  140  has a Ga-face polarity, in which Ga atoms are formed on an upper surface of a GaN lattice structure. In contrast, an N-face polarity means that N atoms are formed on an upper surface of the GaN lattice structure. A GaN layer having N-face polarity can be easily etched by alkaline solution under a temperature less than 100 degrees centigrade, and a GaN layer having Ga-face polarity is hard to react with alkaline solution under a temperature less than 100 degrees centigrade. 
     Referring to  FIG. 3 , a middle portion of the AlN layer  150  is removed by inductively coupled plasma (ICP) technology, thereby exposing a middle region of the first surface  1400  of the first n-type GaN layer  140 , and a remaining portion of the AlN layer  150  covering a rim of the first surface  1400  of the first n-type GaN layer  140 . Referring also to  FIG. 4 , the AlN layer  150  is rectangular shaped in this embodiment. 
     Referring to  FIG. 5 , a second n-type GaN layer  160  is formed on the first n-type GaN layer  140 . The second n-type GaN layer  160  not only covers the middle portion of the first surface  1400  uncovered by the AlN layer  150 , but also totally covers the AlN layer  150 . A bottom surface of the second n-type GaN layer  160  facing the first n-type GaN layer  140  has an N-face polarity; therefore it can be easily etched by alkaline solution. A thickness of the second n-type GaN layer  160  ranges from 300 nm to 500 nm. 
     Referring to  FIG. 6 , an active layer  170  and a p-type GaN layer  180  are formed on an upper surface of the second n-type GaN layer  160 , in sequence. In this embodiment, the active layer  170  is an InGaN/GaN multiple quantum well structure. 
     Referring to  FIG. 7 , alkaline solution is used to remove the AlN layer  150 , thereby exposing a portion of the bottom surface of the second n-type GaN layer  160 . Then the alkaline solution is used to etch and roughen the portion of the bottom surface of the second n-type GaN layer  160 . An annular roughened portion  161  thus is formed on the bottom surface of the second n-type GaN layer  160 . In order to accelerate the etching, the alkaline solution may be strong alkaline solution, such as KOH solution and NaOH solution, etc. In this embodiment, the AlN layer  150  and the second n-type GaN layer  160  are etched by KOH solution under a temperature of 85 degrees centigrade for 30 to 60 minutes. 
     Referring to  FIG. 8 , an electrode  190  is formed on the p-type GaN layer  180 . The electrode  190  is made of a material selected from a group consisting of Au, Ag, Cu, Al, Sn, Ni, Co, and alloys thereof. In this embodiment, the electrode  190  is formed on the p-type GaN layer  180  by sputtering or vacuum evaporating. As shown in  FIG. 8 , an LED comprises the substrate  110 , the buffer layer  120 , the undoped GaN layer  130 , the first n-type GaN layer  140 , the second n-type GaN layer  160 , the active layer  170 , the p-type GaN layer  180  and the electrode  190  sequentially formed on the substrate  110 . The second n-type GaN layer  160  comprises the annular roughened portion  161  formed on the rim of the bottom surface thereof, wherein the annular roughened portion  161  has an N-face polarity. Facets of the annular roughened portion  161  may reflect back light from the active layer  170 , thereto increase the light extraction efficiency of the LED. 
     Because the AlN layer  150  is easily removed by alkaline solution, the alkaline solution may penetrate into an interface between the first n-type GaN layer  140  and the second n-type GaN layer  160 , and may preferentially etch, or roughen, the bottom surface of the second n-type GaN layer  160  which has an N-face polarity. Therefore, the roughening of the bottom surface of the second n-type GaN layer  160  may be accelerated. Also, the second n-type GaN layer  160  is directly grown on the first n-type GaN layer  140 , and the AlN layer  150  surrounds the bottom surface of the second n-type GaN layer  160 . After the AlN layer  150  is removed, the second n-type GaN layer  160  is still connected with the first n-type GaN layer  140 . As a result, removal of the AlN layer  150  will not affecting electrical connections between the first n-type GaN layer  140  and the second n-type GaN layer  160 . 
     In alternative embodiments, the AlN layer may not be limited to rectangular shaped as shown in  FIG. 4 . Referring to  FIG. 9 , an AlN layer  250  in accordance with a second embodiment includes an annular portion  251  and a plurality of finger portions  252  extending inwardly from the annular portion  251 . In this embodiment, the annular portion  251  has a rectangle shape and the plurality of finger portions  252  extending inwardly from four corners of the annular portion  251 . The AlN layer  250  of the second embodiment may replace the AlN layer  150  of the first embodiment. When the AlN layer  250  is etched by alkaline solution, the alkaline solution may penetrate deep inside the LED from the annular portion  251  to the plurality of finger portions  252 . As a result, roughened surfaces with light reflecting facets between the first and second n-type GaN layers may be further increased, according to the second embodiment of the present disclosure. An increased area of roughened surfaces of the second n-type GaN layer may further improve light extracting efficiency. 
     While certain embodiments have been described and exemplified above, various other embodiments will be apparent to those skilled in the art from the foregoing disclosure. The disclosure is not limited to the particular embodiments described and exemplified, and the embodiments are capable of considerable variation and modification without departure from the scope and spirit of the appended claims.