Abstract:
A GaN-based light-emitting device and the fabricating method for the same are described. The light-emitting device is a light-emitting body with a light extraction layer thereon. The light-emitting body has some GaN-based layers and is capable of emitting a light when energy is applied. The light extraction layer is a double layered structure having a current spreading layer and a micro-structure layer, or a single layered structure without the current spreading layer. The micro-structure layer is a TiN layer with a nano-net structure obtained by nitridation of a Ti layer or a Pt layer with metal clusters thereon obtained by annealing of a Pt layer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The present invention is related to an improvement of luminous efficiency of a gallium nitride (GaN) based light-emitting diode (LED). in particular, the present invention is related to a GaN LED with a metal micro-structure as a light extraction layer and a manufacturing method for the same. 
   2. Related Art 
   Semiconductor light-emitting diodes (LEDs) have been developed for several decades and the luminous efficiency thereof plays a key role in whether LEDs may be further applied in lighting facilities generally used in ordinary living. Therefore, LED research, for the past decades, has been focused on improvement of luminous efficiency. Generally, luminous efficiency varies with the following factors: semiconductor material adopted, device structure devised, transparency of material used, total reflection existed, etc. 
   Of the semiconductor LEDs, a gallium nitride (GaN) based material may be the most commonly used. To let the GaN-based material irradiate light, a voltage or a current has to be applied to the corresponding LED. To apply a voltage or a current to the LED, a pair of positive and negative electrodes are disposed on the LED structure. 
   The positive electrode is also called a p-electrode while the negative electrode is also called an n-electrode, since charges provided by the p-electrode first flow into a p-type semiconductor material layer and charges supplied by the n-electrode first flow into an n-type semiconductor material layer. The p-type electrode is where positive charges flow into the LED structure, and holes are carriers for conductivity. On the other hand, the n-type electrode is where negative charges flow into the LED structure, and electrons are carriers for conductivity. Owing to the poorer mobility of the hole carriers as compared to the electron carriers, a current spreading layer is generally disposed under the p-type electrode so that the hole carriers may be uniformly distributed in the p-type semiconductor layer. In this case, electric force lines between the p-type electrode and the n-type electrode may also be uniformly distributed so as to enhance excitation of light in the LED. The afore-mentioned current spreading layer may be any suitable material, and a Ni/Au double layered structure is the most commonly used. Referring to  FIG. 1 , which illustrates an LED structure  10 , the LED structure  10  comprises a substrate  11 , a buffer layer  12 , an n-type GaN-based layer  13 , a semiconductor active layer  14 , a p-type GaN-based layer  15 , a p-type semiconductor layer  16 , a current spreading layer  17  and a p-type electrode  18 . The process thereof may be found in ROC patents 558848, 419837, etc. In the figure, the current spreading layer  17  is disposed between the p-type semiconductor layer  16  and the p-electrode  18  and used to distribute uniformly positive charges thereon and then enter into the p-type semiconductor layer  16 . 
   However, a serious total reflection issue is closely related to the current spreading layer since the current spreading layer has flat surface and thus reflects light back to the LED structure and has a poor light extraction. Some technologies for roughening the current spreading layer are provided. For example, roughening structures are disposed where the emitted light is output so that most emission angles of the emitted lights are smaller than a critical angle, which is defined by Snell&#39;s Law. These roughening structures are generally shaped as hemispheres or truncated pyramids. However, these roughening shapes are hard to form and may be expensive. 
   Other roughening technologies are available. For example, an etch process is applied onto the upper flat surface of the LED to form small, roughened facets on the flat surface so that most emission angles of emitted light may output without reflection to the LED structure. Such a roughening method comprises a process of randomly etching a surface. For example, particles are deposited on the surface and then used as masks in the random etching. However, there are at least two major disadvantages. First of all, some small islands may exist in the p-type electrode. Since the lower parts of the island structures do not contact the p-type electrode contact, no light will be emitted by these portions and the total light output is reduced. Second, since the upper surface of the LED structure is very close to the light-generating area below, the light generating area may be very likely broken. 
   In view of the disadvantages of the prior GaN-based LED, there is a need to provide a GaN-based LED with high light extraction efficiency. 
   SUMMARY OF THE INVENTION 
   The present invention is aimed to provide a GaN-based LED with high light extraction efficiency and a method for manufacturing the GaN-based LED. 
   In view of the purpose above, a micro-structure surface is formed on a current spreading layer of the GaN-based LED structure according to the present invention. With the micro-structure, a total reflection of light associated with the current spreading layer is reduced and the corresponding luminous efficiency is improved. 
   The LED structure with the micro-structure according to the present invention has two primary embodiments. In the first embodiment, a TiN layer is formed on the current spreading layer and the two layers jointly form a light extraction double layered structure. The double structure is disposed on a light-emitting body. The TiN layer has a micro-structure and the micro-structure is a nano-net structure. With the nano-net structure, lights generated by an active layer in the light-emitting body are more immune to total reflection. 
   In the first manufacturing embodiment according to the present invention, a GaN-based light-emitting body is first formed, a current spreading layer is next formed on the light-emitting body, and a Ti layer is formed on the current spreading layer. Next, the Ti layer is subjected to nitridation so that a TiN micro-structure surface with a nano-net structure is formed. 
   In the second manufacturing embodiment according to the present invention, a GaN-based light-emitting body is first formed, a current spreading layer is next formed on the light-emitting body, and a Pt layer is formed on the current spreading layer. Next, the Pt layer is annealed so that a Pt micro-structure surface with metal clusters is formed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow for illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a front view of a prior gallium nitride (GaN) based light emitting diode (LED); 
       FIG. 2  is an illustration of a first device structure embodiment according to the present invention; 
       FIG. 3  is an illustration of a first device structure embodiment according to the present invention; 
       FIG. 4  is a first manufacturing method embodiment according to the present invention used to manufacture the first device structure according to the present invention; 
       FIG. 5  is a second manufacturing method embodiment according to the present invention used to manufacture the second device structure according to the present invention; 
       FIG. 6  is an illustration of an alternative LED with a micro-structure layer used according to the present invention; 
       FIG. 7  is an illustration of a tunneling junction LED with the micro-structure layer used according to the present invention; and 
       FIG. 8  an illustration of a vertical LED with the micro-structure layer used according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides a light extraction layer with a micro-structure surface to reduce total reflection of light and enhance a light extraction efficiency of the light extraction layer, which is illustrated below. 
   The gallium nitride (GaN) light-emitting device (LED) primarily comprises two major embodiments. Referring to  FIG. 2 , which illustrates a first device structure  20  embodiment according to the present invention, a substrate  21  is first prepared, which may be sapphire, GaN or SiC or other suitable materials. An n-type GaN-based layer  23 , a semiconductor active layer  24  and a p-type GaN-based layer  25  are sequentially formed over the substrate  21  to generate light when a voltage or a current is applied. The three layers  23 ,  24 ,  25  comprise a light-emitting body. 
   The semiconductor active layer may be an AlGaNInN layer or an InGaN/GaN layer. The substrate  21  and the n-type GaN-based layer  23  may be selectively disposed on a buffer layer  22  to let the two layers  21 ,  23  adjacent to the buffer layer  22  have better lattice matching. A p-type contact layer  26  and a light extract layer  27  are sequentially disposed over the contact layer  26 . The contact layer  26  may be p-InGaN or p-AlInGaN layer and the light extraction layer  27  is a double layer structure composed of a current spreading layer  28  and a micro-structure layer  29 . 
   The current spreading layer  28  is made of transparent and conductive material and is at least a Ni/Au double layered structure, Ni, Pt, Pd, Rh, Ru, Os, Ir, Zn, In, Sn, Mg or oxides thereof, which may be blended with additives to increase conductivity thereof, such as aluminum. The transparent wavelength of the current spreading layer  28  depends on the light-emitting body used, so the lights generated by the light-emitting body may largely penetrate the current spreading layer  28 . 
   In  FIG. 2 , the micro-structure layer  29  is a TiN layer with a nano-net structure and is not shown for clarity. Since the TiN nano-net structure includes considerably fine roughening structures, more photons generated by the semiconductor active layer  24  are output with emission angles smaller than a critical angle. As compared to the prior roughening structure, the micro-structure is much smaller, and hence the total reflection may be greatly reduced. 
   Further, a p-type electrode  40  is disposed on the micro-structure  29  and an n-type electrode  32  on the n-type GaN-based layer for supplying a current into the light-emitting body. 
   Referring to  FIG. 3 , which shows a second device embodiment according to the present invention, the device  40  comprises a substrate  41 , a buffer layer  42 , an n-type GaN-based layer  43 , a semiconductor active layer  44 , a p-type GaN-based layer  45 , a p-type contact layer  46 , a current spreading layer  48 , a micro-structure layer  49  and electrodes  50 ,  51 . The second device embodiment is the same as the first device embodiment except for the micro-structure layer  49  of the light extraction layer  47 . In the embodiment, the micro-structure  49  is an annealed Pt layer with metal clusters, which is not shown for clarity. Similarly, since the dimension of the metal clusters is much smaller than that of the prior roughening structure, the number of photons emitted with an emission angle smaller than the critical angle increases greatly. Therefore, the light extraction efficiency may be considerably enhanced. 
   Referring to  FIG. 4 , which illustrates a first manufacturing embodiment for the first device according to the present invention, a substrate is first prepared ( 61 ). Next, a buffer layer is selectively formed over the substrate ( 62 ) by a molecular beam epitaxial (MBE) method, metal organic chemical vapor deposition (MOCVD) or other suitable technologies. Next, an n-type GaN-based layer is formed over the buffer layer ( 63 ), and a semiconductor active layer is formed over the n-type GaN-based layer  66 . Next, a current spreading layer is formed over the contact layer ( 67 ) and a Ti layer is formed over the current spreading layer ( 68 ). Next, the Ti layer is subjected to nitridation to form a TiN layer with a micro-structure ( 69 ). In addition, the micro-structure layer may be formed with a p-type electrode thereon and an n-type electrode may be formed over a portion of the n-type GaN-based layer. 
   Reference is made to  FIG. 5 , which shows a second manufacturing method embodiment for the second device according to the present invention. In the second manufacturing method, Step  71  to Step  77  is the same as Step  61  to Step  67  in  FIG. 4 . After Step  77 , in which a current spreading layer is formed, a Pt layer is then formed over the current spreading layer ( 78 ). Then, the Pt layer is annealed to have a metal cluster formed thereon ( 79 ). In addition, a p-type electrode may be formed over the micro-structure layer and an n-type electrode may be formed over a portion of the n-type GaN-based layer. 
     FIG. 6  illustrates a light-emitting device structure  80  where a p-type metal electrode is disposed beside a current spreading layer  86  but not over the current spreading layer  86 . The device structure  80  comprises a substrate  81 , an n-type GaN-based layer  82 , a semiconductor active layer  83 , a p-type GaN-based layer  84 , a p-type contact layer  85 , a GaN-based current spreading layer  86  formed over the p-type contact layer  85 , and a p-type metal electrode  88  disposed beside the GaN-based current spreading layer  86 . In addition, an inventive micro-structure layer  87  is finally disposed on the GaN-based current spreading layer  86  and beside the metal electrode  88 . In the device structure embodiment, since the micro-structure  87  is not involved with current spreading, the micro-structure  87  may be separately formed with the p-type metal electrode  88 . 
   The inventive micro-structure with the above nano-net structure or metal clusters may be used in a tunneling junction structure  90 , which is described in  FIG. 7 . In the figure, the structure  90  comprises a substrate  91 , an n-type GaN-based layer  92 , a semiconductor  93 , p-type GaN-based layer  94  and a p+ and an n+-type GaN-based layer  95 ,  96 . The p+-type GaN-based layer is disposed over the n+-type GaN-based layer  96  and the two layers  95 ,  96  will be referred to as a p+/n+-type GaN-based layer. Then, the inventive micro-structure  97  is directly disposed over the GaN-based layer  96  without need of the current spreading layer in the foregoing embodiments since the GaN-based layer  96  has electrons as its major carriers. However, a current spreading layer may also be selectively included in the structure  90 . Finally, a p-type electrode is disposed on the micro-net structure  97  and an n-type electrode on the n-type GaN-based layer  92 . 
   Referring to  FIG. 8 , the micro-structure layer according to the present invention is used in a vertical LED structure  100 . In the figure, a metal reflective layer  102  is disposed over a substrate  101  to restrict the light generated by the p-type GaN-based layer  103 , the semiconductor active layer  104  and the n-type GaN-based layer  105  above the metal reflective layer  102 . 
   The substrate  101  is a conductive metallic material used to supply charges to the p-type GaN-based layer  103  since the n-type electrode  107  is disposed on the most upper part of the structure  100 . In such a structure, since no low mobility issue of holes exists, no current spreading layer is required. A current spreading layer may be selectively incorporated. Therefore, the inventive micro-structure layer  106  may be formed directly over the n-type GaN-based layer  105 . Then an n-type electrode  107  is disposed on the micro-structure layer  106 . 
   In addition to the advantage of reduction of total reflection and improvement of light extraction efficiency, the manufacturing of the device structure of the present invention is not complicated and thus the light extraction layer of the present invention is suitable for use in any type of light emitting device. 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.