Patent Publication Number: US-2015060913-A1

Title: Light-emitting diodes and fabrication methods thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Taiwan Patent Application No. 102131762, filed on Sep. 4, 2013, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to light-emitting diodes and more particularly to structures and fabrication methods of light-emitting diodes. 
     2. Description of the Related Art 
     A light-emitting diode (LED) is a semiconductor electronic component which can provide illumination. The light-emitting diode includes a p-doped semiconductor layer and an n-doped semiconductor layer. When the light-emitting diode is switched on, electrons are able to recombine with holes at the interface between the p-doped semiconductor layer and the n-doped semiconductor layer, releasing energy in the form of photons. This effect is called the electroluminescence of the light-emitting diode. Light-emitting diodes have many advantages over conventional incandescent light bulbs, including lower energy consumption, longer lifespan, smaller size, high brightness, etc. Thus, light-emitting diodes are widely used in applications such as various electronic devices and general lighting. 
     Generally, the semiconductor materials for forming the light-emitting diode have refractive indexes greater than that of the outside of the light-emitting diode. For example, the refractive indexes of the semiconductor materials are greater than the refractive indexes of packaging materials such as epoxy resin or the refractive index of air. Thus, a total reflection of light occurring at the interface between the semiconductor layers of the light-emitting diode and the outside of the light-emitting diode has a small critical angle. However, conventional light-emitting diode chips are fabricated to have a standard square outward appearance. The four cross-sections of the square outward appearance are parallel with each other, such that the probability of photons leaving the semiconductor layers at the interface between the semiconductor layers and the outside of the light-emitting diode is reduced. Thus, a large portion of light producing by the light-emitting diode is totally reflected from the interface between the semiconductor layers and the outside of the light-emitting diode and back into the interior of the semiconductor layers. Therefore, the luminous efficiency of the conventional light-emitting diodes is poor. 
     BRIEF SUMMARY OF THE INVENTION 
     The disclosure provides structure designs and fabrication methods of light-emitting diodes. The light-emitting diodes have a P-type epitaxial layer with a ladder-shaped sidewall. A rounded or a right-angled ladder-shaped sidewall can be formed on the P-type epitaxial layer by forming a photoresist pattern on the P-type epitaxial layer and performing an anisotropic-etching process to form the P-type epitaxial layer. The ladder-shaped sidewall of the P-type epitaxial layer can reduce a total reflection occurring at the interface between the P-type epitaxial layer and the outside of the light-emitting diode. Thus, the light extraction efficiency of the light-emitting diode is thereby improved and the luminous efficiency of the light-emitting diode is further enhanced. 
     In embodiments of the disclosure, a light-emitting diode is provided. The light-emitting diode includes an N-type epitaxial layer. A light-emitting layer is disposed on a portion of the N-type epitaxial layer to expose a partial surface of the N-type epitaxial layer. A P-type epitaxial layer is disposed on the light-emitting layer and the P-type epitaxial layer has a ladder-shaped sidewall. A P-type electrode is disposed on the P-type epitaxial layer and an N-type electrode is disposed on the exposed surface of the N-type epitaxial layer. 
     In embodiments of the disclosure, furthermore, a method of fabricating a light-emitting diode is provided. The method includes providing an N-type epitaxial layer; forming a light-emitting layer on the N-type epitaxial layer; forming a P-type epitaxial layer on the light-emitting layer; forming a first photoresist pattern on the P-type epitaxial layer to expose a portion of the P-type epitaxial layer; and performing an anisotropic-etching process to form a rounded or a right-angled first ladder at an edge of the P-type epitaxial layer. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows a schematic cross section of a light-emitting diode according to an embodiment of the disclosure; 
         FIG. 2  is a schematic diagram of a light emission mode at a circle area A of  FIG. 1 ; 
         FIG. 3  shows a schematic cross section of a light-emitting diode according to an embodiment of the disclosure; 
         FIG. 4  is a schematic diagram of a light emission mode at a circle area C of  FIG. 3 ; and 
         FIGS. 5A-5F  show schematic cross sections of several intermediate stages of fabricating the light-emitting diode of  FIG. 1  according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     Referring to  FIG. 1 , a cross section of a light-emitting diode  100  according to an embodiment of the disclosure is shown. The light-emitting diode  100  includes a substrate  101  and an N-type epitaxial layer  103  formed on the substrate  101 . A light-emitting layer  105  is formed on a portion of the N-type epitaxial layer  103  to expose a partial surface of the N-type epitaxial layer  103 . A P-type epitaxial layer  107  is formed on the light-emitting layer  105  and the P-type epitaxial layer  107  has a ladder-shaped sidewall  107 A. In the embodiment, the ladder-shaped sidewall  107 A is made up of three rounded ladders. As shown in  FIG. 1 , the ladder-shaped sidewall  107 A has three ladders. However, the number of ladders of the ladder-shaped sidewall  107 A is not limited to three. In other embodiments, the ladder-shaped sidewall  107 A can have two or more than three ladders. 
     In addition, the light-emitting diode  100  includes a current blocking layer  109  formed on the P-type epitaxial layer  107 . Furthermore, a transparent conductive film  110  is formed on the current blocking layer  109  and covers the P-type epitaxial layer  107 . Moreover, the light-emitting diode  100  further includes a P-type electrode  113  formed on the transparent conductive film  110  above the P-type epitaxial layer  107 . The light-emitting diode  100  further includes an N-type electrode  115  formed on the exposed partial surface of the N-type epitaxial layer  103 . As shown in  FIG. 1 , the current blocking layer  109  is disposed between the transparent conductive film  110  and the P-type epitaxial layer  107 . Furthermore, the current blocking layer  109  is correspondingly disposed under the P-type electrode  113 . The transparent conductive film  110  is disposed between the P-type epitaxial layer  107  and the P-type electrode  113 . 
     In an embodiment, the light-emitting diode  100  is for example a blue light-emitting diode, in which the substrate  101  can be a sapphire substrate. The material of the N-type epitaxial layer  103  can be an N-type gallium nitride (N-GaN). The material of the P-type epitaxial layer  107  can be a P-type gallium nitride (P-GaN). In other embodiments, the substrate  101 , the N-type epitaxial layer  103  and the P-type epitaxial layer  107  of the light-emitting diode  100  can be formed from other suitable materials, thus various colors of light-emitting diodes are obtained. 
     In an embodiment, the material of the current blocking layer  109  can be an organic or an inorganic insulating material, such as silicon dioxide. The material of the transparent conductive film  110  is, for example, indium tin oxide or other suitable transparent conductive materials. The transparent conductive film  110  can be used as a current spreading layer, such that a current applying onto the P-type electrode  113  can uniformly spread to the P-type epitaxial layer  107 . It can prevent the current from crowding and avoid a high-voltage issue. In addition, the P-type electrode  113  and the N-type electrode  115  can be formed from metal materials. Moreover, the light-emitting diode  100  can further include other element layers, such as a buffer layer disposed between the substrate  101  and the N-type epitaxial layer  103 . The structure as shown in  FIG. 1  is for the sake of simplifying, a structure of the light-emitting diode  100  is not limited to the structure as shown in  FIG. 1 . 
     According to an embodiment of the disclosure, the P-type epitaxial layer  107  of the light-emitting diode  100  can have a rounded ladder-shaped sidewall  107 A. As shown in  FIG. 1 , the rounded ladder-shaped sidewall  107 A can guide a lateral-light emitting from the light-emitting diode  100  to an axial-light emission. Therefore, the luminous efficiency of the light-emitting diode  100  is enhanced.  FIG. 2  shows a schematic diagram of a light emitting from the light-emitting diode  100  and passing through the circle area A of  FIG. 1 . As shown in  FIG. 2 , the rounded structure of the rounded ladder-shaped sidewall  107 A can reduce the probability of a light total reflection to achieve a minimum thereof. A light-emitting scope of the light emitting from the light-emitting diode  100  is thereby broadened. In the circle area B as shown in  FIG. 2 , light is still emitting. Thus, the light extraction efficiency of the light-emitting diode  100  is effectively enhanced through the structure design of the rounded ladder-shaped sidewall  107 A for the P-type epitaxial layer  107 . 
     Referring to  FIG. 3 , a cross section of a light-emitting diode  100  according to an embodiment of the disclosure is shown. The difference between  FIG. 3  and  FIG. 1  is that a P-type epitaxial layer  107  of the light-emitting diode  100  of  FIG. 3  has a right-angled ladder-shaped sidewall  107 C. The right-angled ladder-shaped sidewall  107 C as shown in  FIG. 3  is made up of four ladders. However, the number of ladders of the right-angled ladder-shaped sidewall  107 C is not limited to four. In other embodiments, the right-angled ladder-shaped sidewall  107 C can have two, three or more than four ladders. 
       FIG. 4  shows a schematic diagram of a light go forward mode for a light emitting from the light-emitting diode  100  and passing through the circle area C of  FIG. 3 . When light passes through a right-angled area of the right-angled ladder-shaped sidewall  107 C, such as the area D as shown in  FIG. 4 , a total reflection of the light occurs therein. Therefore, a light-emitting scope of the right-angled ladder-shaped sidewall  107 C as shown in  FIG. 3  is smaller than a light-emitting scope of the rounded ladder-shaped sidewall  107 A as shown in  FIG. 1 . However, compared with a light-emitting diode having no ladder shape formed on a sidewall of a P-type epitaxial layer, but having a square structure for the P-type epitaxial layer, the right-angled ladder-shaped sidewall  107 C as shown in  FIG. 3  can still reduce the probability of making a total reflection of the light and increase a light-emitting scope of the light. The light extraction efficiency of the light-emitting diode  100  is thereby enhanced. 
     Referring to  FIGS. 5A-5F , cross sections of several intermediate stages of fabricating the light-emitting diode  100  of  FIG. 1  according to an embodiment of the disclosure are shown. Referring to  FIG. 5A , firstly, an N-type epitaxial layer  103 , a light-emitting layer  105  and a P-type epitaxial layer  107  are grown on a substrate  101  in sequence. Then, a first photoresist pattern  201  is formed on the P-type epitaxial layer  107  to expose a portion of the P-type epitaxial layer  107 . 
     In an embodiment, the first photoresist pattern  201  is a photoresist pattern with a rounded corner  201 R at an edge thereof. The rounded corner  201 R at the edge of the first photoresist pattern  201  can be formed by a photoresist reflow method to shape the corner thereof. Moreover, the first photoresist pattern  201  can be formed by using a photo-mask for fabricating a mesa structure of the P-type epitaxial layer. Therefore, it can reduce the fabrication cost of one photo-mask. 
     Next, a first anisotropic-etching process  210 , for example an inductively coupled plasma reactive ion etching (ICP-RIE) process, is performed on the P-type epitaxial layer  107 . As shown in  FIG. 5B , the rounded corner  201 R at the edge of the first anisotropic-etching process  210  can be transferred to and printed on the P-type epitaxial layer  107  by performing the anisotropic-etching process  210  to etch the P-type epitaxial layer  107 . Thus, a rounded first ladder  107 S 1  is formed at the edge of the etched P-type epitaxial layer  107 . At the same time, the light-emitting layer  105  is also etched to expose a partial surface of the N-type epitaxial layer  103 . 
     In an embodiment, after the anisotropic-etching process  210  is completed, the first photoresist pattern  201  having the rounded corner  201 R at the edge thereof can be completely removed by etching. Thus, the rounded corner  201 R at the edge of the first photoresist pattern  201  is transferred to and printed on the P-type epitaxial layer  107  by etching to form the rounded first ladder  107 S 1 . In another embodiment, after the anisotropic-etching process  210  is completed, the first photoresist pattern  201  having the rounded corner  201 R at the edge thereof can remain on the P-type epitaxial layer  107 . Thus, a right-angled first ladder is formed at the edge of the etched P-type epitaxial layer  107 . After the anisotropic-etching process  210  is completed, whether the first photoresist pattern  201  is completely removed by etching or remains on the P-type epitaxial layer  107  is determined by an original thickness of the first photoresist pattern  201 . 
     Moreover, in other embodiments, the first photoresist pattern  201  can be a photoresist pattern with a right-angled corner at an edge thereof (not shown). When a photoresist is processed by an exposure and a development, and then the photoresist is not processed with a photoresist reflow treatment, a photoresist pattern having a right-angled corner at an edge thereof is thereby formed. Next, the anisotropic-etching process  210  is performed on the P-type epitaxial layer  107  to form a right-angled first ladder at the edge of the etched P-type epitaxial layer  107 . 
     Referring to  FIG. 5C , a second photoresist pattern  202  is formed on the P-type epitaxial layer  107  having the rounded first ladder  107 S 1  at the edge thereof. Moreover, the side surface and the upper surface of the first ladder  107 S 1  are exposed. In an embodiment, the second photoresist pattern  202  is a photoresist pattern with a rounded corner  202 R at an edge thereof. 
     Next, a second anisotropic-etching process  220 , for example an inductively coupled plasma reactive ion etching (ICP-RIE) process, is performed on the P-type epitaxial layer  107 . As shown in  FIG. 5D , the rounded corner  202 R at the edge of the second photoresist pattern  202  can be transferred to and printed on the P-type epitaxial layer  107  by performing the anisotropic-etching process  220  to etch the P-type epitaxial layer  107 . Then, a rounded second ladder  107 S 2  is formed at the edge of the second etched P-type epitaxial layer  107  and adjacent to the first ladder  107 S 1 . Now, the sidewall of the P-type epitaxial layer  107  has two rounded ladders  107 S 1  and  107 S 2  formed thereon. 
     In an embodiment, the second photoresist pattern  202  can be formed by using a photo-mask for fabricating a current blocking layer. Therefore, it can further save the fabrication cost of one photo-mask. If the second photoresist pattern  202  is formed by using the photo-mask for fabricating the current blocking layer, the second photoresist pattern  202  will have an opening (not shown) formed at a location corresponding to the current blocking layer to expose the P-type epitaxial layer  107 . Therefore, after the second anisotropic-etching process  220  is performed, a depression (not shown) is formed on an upper surface of the P-type epitaxial layer  107  at the location corresponding to the current blocking layer. 
     The step of forming a photoresist pattern and the step of performing an anisotropic-etching process over the P-type epitaxial layer  107  are repeated several times. Then, a plurality of rounded or right-angled ladders is formed on the sidewall of the P-type epitaxial layer  107 . The thickness of the photoresist patterns in each step can be determined by the thickness of the P-type epitaxial layer  107  and a predetermined amount of ladders formed on the sidewall of the P-type epitaxial layer  107 . Generally, if the thicknesses of the photoresist patterns in each step are the same, the thickness of the photoresist pattern in each step is almost equal to the thickness of the P-type epitaxial layer  107  divided by the number of ladders formed on the sidewall of the P-type epitaxial layer  107 . 
     As shown in  FIG. 5E , a current blocking layer  109  is formed on the upper surface of the P-type epitaxial layer  107  which has the first ladder  107 S 1  and the second ladder  107 S 2  formed on the sidewall thereof. Then, a transparent conductive film  111  is formed to cover the current blocking layer  109  and the P-type epitaxial layer  107 . Next, a third photoresist pattern  203  with a rounded corner  203 R at an edge thereof is formed on the transparent conductive film  111 . Moreover, the side surface and the upper surface of the second ladder  107 S 2  are exposed. In an embodiment, the third photoresist pattern  203  can be formed by using a photo-mask for fabricating the transparent conductive film  111 . Therefore, it can further reduce the fabrication cost of one photo-mask. 
     Next, a third anisotropic-etching process  230 , for example an inductively coupled plasma reactive ion etching (ICP-RIE) process, is performed on the P-type epitaxial layer  107 . As shown in  FIG. 5F , the rounded corner  203 R at the edge of the third photoresist pattern  203  can be transferred to and printed on the P-type epitaxial layer  107  by performing the anisotropic-etching process  230  to etch the P-type epitaxial layer  107 . Then, a rounded third ladder  107 S 3  is formed at the edge of the third etched P-type epitaxial layer  107  and adjacent to the second ladder  107 S 2 . Thus, the sidewall of the P-type epitaxial layer  107  has three rounded ladders  107 S 1 ,  107 S 2  and  107 S 3  formed thereon. The fabrication of the ladder-shaped sidewall  107 A of the P-type epitaxial layer  107  as shown in  FIG. 1  is completed. 
     Next, a P-type electrode  113  is formed on the transparent conductive film  111 . An N-type electrode  115  is formed on the exposed surface of the N-type epitaxial layer  103 . Then, the fabrication of the light-emitting diode  100  of  FIG. 1  is completed. 
     In other embodiments, the current blocking layer  109  and the transparent conductive film  111  can be formed after the fabrication of the ladder-shaped sidewall of the P-type epitaxial layer  107  is completed. 
     According to the embodiments of the disclosure, by performing the step of forming the photoresist pattern with a rounded or a right-angled corner at the edge thereof and the step of performing an anisotropic-etching process over the P-type epitaxial layer several times, a plurality of rounded or right-angled ladders can be formed on the sidewall of the P-type epitaxial layer of the light-emitting diode. The ladder-shaped sidewall of the P-type epitaxial layer can reduce the probability of a total reflection of light produced at the interface between the P-type epitaxial layer and the outside of the light-emitting diode. Therefore, the light extraction efficiency of the light-emitting diode is improved and the luminous efficiency of the light-emitting diode is further enhanced. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.