Abstract:
The present invention provides a light-emitting diode (LED) device and a fabrication method thereof. The LED device has a double-layered contact layer structure with a surface of one contact layer being patterned to increase ohmic contact area of the double-layered contact layer structure to lower an operation voltage of the LED device, and hence reducing power consumption.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a light-emitting device and a fabrication method thereof, and more particularly to a light-emitting diode and a fabrication method thereof. 
         [0003]    2. Description of the Related Art 
         [0004]    With reference to  FIG. 1 , which is a schematic cross-sectional view showing the structure of a conventional light-emitting diode device. The light-emitting diode device  1  includes a substrate  10 , a buffer layer  11  on the substrate  10 , a first confining layer  12  on the buffer layer  11 , an active layer  13  on the first confining layer  12 , a second confining layer  14  on the active layer  13 , a contact layer  15  on the second confining layer  14 , a transparent electrically conductive layer  16  on the contact layer  15  and a set of electrode units  17 . The set of electrode units  17  is comprised of a first electrode  17   a  located on the first confining layer  12  and a second electrode  17   b  located on the transparent electrically conductive layer  16 , which work in coordination to provide the light-emitting diode device  1  with electric energy. 
         [0005]    For the above-mentioned structure, the first confining layer  12  has a type of electrically conductivity different from that of the second confining layer  14 . For example, the first confining layer  12  and the second confining layer  14  are made of n-type doped and p-type doped gallium nitride (GaN) semiconductor materials, respectively. 
         [0006]    The first and second electrodes  17   a  and  17   b  of the set of electrode units  17  are made of metals such as gold, chromium and/or the alloy thereof. The first electrode  17   a  is arranged on the n-typed first confining layer  12 , and an ohmic contact therebetween is formed, while the second electrode  17   b  is arranged on the top face of the transparent electrically conductive layer  16 , and an ohmic contact thereof is formed with the contact layer  15 , so as to provide the n-typed first confining layer  12  and the p-typed second confining layer  14  with electric energy. 
         [0007]    While the electric energy is applied to the light-emitting diode device  1  through the first and second electrodes  17   a  and  17   b , the current will pass through the n-typed first confining layer  12  and the p-typed second confining layer  14 . The current flowing from the second electrode  17   b  into the light-emitting diode device  1  is uniformly distributed whereby the electron and the hole are combined within the active layer  13  so that the energy is released and converted into light energy for light-emitting. The active layer  13  has a structure of multi-quantum wells including a plurality of barrier layers  13   a  and plural layers of quantum well  13   b  which are stacked on each other. 
         [0008]    In order to enhance the light-extraction efficiency of the light-emitting diode device  1 , the p-typed second confining layer  14  and the contact layer  15  are patterned to form a plurality of openings therein, so as to enable a sufficient contact of the second electrode  17   b  with the p-typed second confining layer  14  and the contact layer  15 . In this case, the lighting brightness of the light-emitting diode device  1  can be increased. 
         [0009]    Nevertheless, the working voltage of the patterned p-typed second confining layer  14  is relatively increased as well. In view of this, an improved structure for the light-emitting diode device and the fabrication method thereof are provided in the present invention, by which not only the issue relating to the increased working voltage is addressed but the fabrication process therefor is simplified and the product cost thereof is reduced. 
       SUMMARY OF THE INVENTION 
       [0010]    One object of the present invention is to provide a light-emitting diode (LED). 
         [0011]    Moreover, a further object of the present invention is to provide a fabrication method for an LED. 
         [0012]    According to an aspect of the present invention, the provided LED device includes a substrate; a buffer layer on the substrate; a confining layer of a first type of electrical conductivity on the buffer layer; an active layer on the confining layer; a first confining layer of a second type of electrical conductivity on the active layer; a first electrically conductive contact layer on the first confining layer; a second confining layer of the second type of electrical conductivity patterned and located on the first electrically conductive contact layer; a second electrically conductive contact layer patterned and located on the second confining layer; a transparent electrically conductive layer on the second electrically conductive contact layer, the transparent electrically conductive layer having a plurality of contacts passing through the second electrically conductive layer and the second confining layer to electrically contact with the first electrically conductive contact layer; a first electrically conductive electrode on the confining layer; and a second electrically conductive electrode on the transparent electrically conductive layer. 
         [0013]    According to a further aspect of the present invention, the provided LED device includes a substrate; a buffer layer on the substrate; a confining layer of a first type of electrical conductivity on the buffer layer; an active layer on the confining layer; a first confining layer of a second type of electrical conductivity on the active layer; a first electrically conductive contact layer on the first confining layer; a second confining layer of the second electrical conductivity patterned and located on the first electrically conductive contact layer; a second electrically conductive contact layer patterned and located on the second confining layer; a transparent electrically conductive layer on the second electrically conductive contact layer, the transparent electrically conductive layer having a plurality of first contacts passing through the second electrically conductive contact layer and the second confining layer to electrically contact with the first electrically conductive contact layer; a first electrically conductive electrode on the first confining layer; and a second electrically conductive electrode on the transparent electrically conductive layer, the second electrically conductive electrode having a plurality of second contacts passing through the transparent electrically conductive electrode and the second confining layer to electrically contact with the first electrically conductive contact layer. 
         [0014]    Preferably, the first electrically conductive contact layer and the second electrically conductive contact layer have a respective thickness of less than 50 nm. 
         [0015]    Preferably, the first electrically conductive contact layer and the second electrically conductive contact layer have a respective thickness of less than 10 nm. 
         [0016]    Preferably, the first electrically conductive contact layer and the second electrically conductive contact layer have a respective thickness of about 3 nm. 
         [0017]    Preferably, the light-emitting diode device is a gallium nitride (GaN) based light-emitting diode device. 
         [0018]    Preferably the first electrically conductive contact layer and the second electrically conductive contact layer are selected from any one of the following materials: heavy-doped P ++ -type or N ++ -type nitrides of group IIIA, heavy-doped P ++ -type or N ++ -type materials of group IIA-VA having an element of group IIA selected from beryllium (Be), magnesium (Mg) and calcium (Ca) and an element of group VA selected from nitrogen (N), arsenic (As), phosphorous (P) and antimony (Sb), and heavy-doped P ++ -type or N ++ -type materials of group IIA-VIA having an element of group VIA selected from oxygen (O), sulfur (S), selenium (Se), tellurium (Te) and polonium (Po). 
         [0019]    Preferably, the active layer has a structure of multi-quantum wells. 
         [0020]    Preferably, the first electrically conductive contact layer and the second electrically conductive contact layer have the first type of electrical conductivity or the second type of electrical conductivity. 
         [0021]    According to still another aspect of the present invention, the provided fabrication method for an LED includes the steps of: forming a buffer layer on a substrate; forming a confining layer of a first type of electrical conductivity on the buffer layer; forming an active layer on the first confining layer; forming a first confining layer of a second type of electrical conductivity on the active layer; forming a first electrically conductive contact layer on the first confining layer; forming a second confining layer of the second type of electrical conductivity on the first electrically conductive contact layer; forming a second electrically conductive contact layer on the second confining layer; removing a portion of the second electrically conductive contact layer, second confining layer, the first electrically conductive contact layer, the first confining layer, the active layer and the confining layer, so as to form a mesa on the confining layer; forming a first electrically conductive electrode on the mesa of the confining layer; forming a plurality of first openings passing through the second electrically conductive contact layer and the second confining layer to make a portion of the first electrically conductive contact layer exposed; forming a transparent electrically conductive layer on the second electrically conductive contact layer; and forming a second electrically conductive contact layer on the transparent electrically conductive contact layer. 
         [0022]    Preferably, the provided fabrication method further includes the step of forming a plurality of second openings passing through the transparent electrically conductive layer, the second electrically conductive contact layer and the second confining layer to make a portion of the first electrically conductive contact layer exposed after forming the transparent electrically conductive layer. 
         [0023]    In the fabrication method according to the present invention, the second confining layer and the second electrically conductive contact layer are patterned in such a way that the plurality of openings are formed therein so as to expose the first electrically conductive contact layer. 
         [0024]    Preferably, the transparent electrically conductive layer is located above the second electrically conductive layer and has a plurality of contacts to the second electrically conductive layer and the second confining layer, so as to establish the electrical contacts to the first electrically conductive layer. 
         [0025]    Preferably, the electrodes are separately arranged from each other, and are respectively ohmically contacted with the two confining layers of different electrical conductivities, so as to provide the two confining layers with the electric energy to cause the electron-hole combination, release the energy, and convert the energy into the light energy within the active layer as well. 
         [0026]    Preferably, the first electrically conductive contact layer and the second electrically conductive contact layer have a respective thickness of less than 50 nm. More preferably, the thickness is less than 10 nm, and the most preferably, the first electrically conductive contact layer and the second electrically conductive contact layer have a respective thickness of less than 3 nm. 
         [0027]    Preferably, the light-emitting diode device is a gallium nitride (GaN) based light-emitting diode device, wherein the first and the second electrically conductive contact layers are made of a material selected from gallium indium nitride, divalent phosphorous doped gallium nitride and divalent nitrogen doped gallium nitride. 
         [0028]    Preferably, the active layer has a structure of multi-quantum wells, and the first electrically conductive contact layer and the second electrically conductive contact layer have the first type of electrical conductivity or the second type of electrical conductivity. 
         [0029]    In the present invention, the area of ohmic contact of the contact layer is increased owing to a double-layered structure thereof which is formed by the arrangement of a further contact layer in the LED device, so that the working voltage of the LED can be lowered according to the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a schematic cross-sectional view showing the structure of a conventional light-emitting diode device; 
           [0031]      FIGS. 2A to 2D  are schematic cross-sectional views corresponding to various stages of a method for forming a light-emitting diode device according to a preferred embodiment of the present invention; and 
           [0032]      FIGS. 3A to 3E  are schematic cross-sectional views corresponding to various stages of a method for forming a light-emitting diode device according to a further preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0033]    With reference to the following disclosures combined with the accompanying drawings, the light-emitting diode device and the fabrication method thereof according to the present invention are illustrated and understood. 
         [0034]    It should be noted that in the following descriptions and the accompanying drawings, similar elements are indicated by the same reference numeral. 
         [0035]    With reference to  FIGS. 2A to 2D , the structure of the light-emitting diode device corresponding to various stages of the fabrication method thereof according to a preferred embodiment of the present invention is illustrated. 
         [0036]    As shown in  FIG. 2A , a buffer layer  21  made of gallium nitride based material is provided on the substrate  20  by means of any known method in this art, and thereon the gallium nitride based semiconductor material is formed. That is, on the buffer layer  21 , the confining layer of a first type of electrical conductivity  22 , the active layer  23 , the first confining layer of a second type of electrical conductivity  24   a , a first electrically conductive contact layer  25   a , a second confining layer of the second type of electrical conductivity  24   b  and a second electrically conductive layer  25   b  are formed in sequence. According to a preferred embodiment of the present invention, the active layer  23  has a structure of multi-quantum wells constructed by plural barrier layers  23   a  and plural quantum well layers  23   b , which are mutually stacked on each other. The material and composition of respective quantum well layers  23   b  can be identical or not. The confining layer of the first type of electrical conductivity  22  is doped and exhibits an n-typed electrical conductivity, which is thus termed as an n-typed confining layer. The first and second confining layers of the second type of electrical conductivity  24   a  and  24   b  are doped and exhibit a p-typed electrical conductivity, which are respectively termed as a first p-typed confining layer and a second p-typed confining layer. 
         [0037]    After forming the mentioned structure, the step of  FIG. 2B  is carried out to form the first electrode of the light-emitting diode device. First, a portion of the second electrically conductive contact layer  25   b , second confining layer  24   b , first electrically conductive contact layer  25   a , first confining layer  24   a , active layer  23  and the confining layer  22  is removed so as to form the so-called mesa. 
         [0038]    Then, the second electrically conductive contact layer  25   b  and the second p-typed confining layer  24   b  are patterned by means of wet-etching, inductively coupled plasma (ICP) etching or photoelectrochemical (PEC) etching so as to form a plurality of openings therein and to expose the electrically conductive contact layer  25   a . Subsequently, the transparent electrically conductive layer  27  is formed on the patterned second electrically conductive contact layer  25   b  and fills the recess and thus contacts to the second electrically conductive contact layer  25   b  and the second p-typed confining layer  24   b , as shown in  FIG. 2C . The transparent electrically conductive contact layer  27  can be a layer of indium tin oxide (ITO). 
         [0039]    After the transparent electrically conductive layer  27  is formed, an electrode of a first type of electrically conductivity  26   a  is further formed thereon. The electrode  26   a  is an n-typed electrode which is ohmically contacted to the confining layer  22 . The light-emitting diode device  2  of the present invention is thus fabricated. In this preferred embodiment, the first electrically conductive contact layer  25   a  and the second electrically conductive contact layer  25   b  can be an N ++ -typed contact layer or a P ++ -typed contact layer having a dopant concentration ranged in 1E17/cm 3  to 5E22/cm 3 . The thickness of the respective first electrically conductive contact layer  25   a  and second electrically conductive contact layer  25   b  is less than 50 nm, and is preferably less than 10 nm. In a most preferred embodiment, the thickness of the respective first electrically conductive contact layer  25   a  and second electrically conductive contact layer  25   b  is less than 3 nm. Moreover, in this preferred embodiment, the first type of electrical conductivity is n-typed and the second type is p-typed for illustration. Nevertheless, the types of electrical conductivities of the light-emitting diode device  2  can be interchanged. 
         [0040]    With reference to  FIGS. 3A to 3E , the structure of the light-emitting diode device corresponding to various stages of the fabrication method thereof according to a preferred embodiment of the present invention is illustrated. In this embodiment, the front portion of the stages of fabrication method for the light-emitting diode device  3 , as shown in  FIGS. 3A to 3C , is the same as that of the prior embodiment. In other words, in this embodiment, a buffer layer  31  made of gallium nitride based material is also provided on the substrate  30  by means of any known method in this art, and thereon a layer sequence of gallium nitride based semiconductor material including a confining layer of a first type of electrical conductivity  32 , an active layer  33 , a first confining layer of a second type of electrical conductivity  34   a , a first electrically conductive contact layer  35   a , a second confining layer of the second type of electrical conductivity  34   b  and a second electrically conductive layer  35   b  is formed, wherein the active layer  33  has a structure of multi-quantum wells constructed by plural barrier layers  33   a  and plural quantum well layers  33   b , which are mutually stacked on each other. The material and composition of respective quantum well layers  33   b  can be identical or not. The confining layer of the first type of electrical conductivity  32  is doped and exhibits an n-typed electrical conductivity, which is thus termed as an n-typed confining layer. The first and second confining layers of the second type of electrical conductivity  34   a  and  34   b  are doped and exhibit a p-typed electrical conductivity, which are respectively termed as a first p-typed confining layer and a second p-typed confining layer, as shown in  FIG. 3A . 
         [0041]    After forming the mentioned structure, the step of  FIG. 3B  is carried out to form the first electrode of the light-emitting diode device. First, a portion of the second electrically conductive contact layer  35   b , second confining layer  34   b , first electrically conductive contact layer  35   a , first confining layer  34   a , active layer  33  and the confining layer  32  is removed so as to form the so-called mesa. 
         [0042]    Then, the second electrically conductive contact layer  35   b  and the second p-typed confining layer  34   b  are patterned by means of wet-etching, inductively coupled plasma (ICP) etching or photoelectrochemical (PEC) etching so as to form a plurality of openings therein and to expose the electrically conductive contact layer  35   a . Subsequently, the transparent electrically conductive layer  37  is formed on the patterned second electrically conductive contact layer  35   b  and fills the recess and thus contacts to the second electrically conductive contact layer  35   b  and the second p-typed confining layer  34   b , as shown in  FIG. 3C . 
         [0043]    This embodiment is different from the prior one in that, after forming the transparent electrically conductive layer  37 , a portion of the transparent electrically conductive layer  37 , second electrically conductive contact layer  35   b  and second p-typed confining layer  34   b  is patterned by etching again, so as to form a plurality of openings therein for exposing the first electrically conductive contact layer  35   a , as shown in  FIG. 3D . 
         [0044]    When the second etching is completed, a second electrode  36   b  is formed within the transparent electrically conductive layer  37  and fills into the mentioned openings and thus ohmically contacts to the transparent electrically conductive layer  37 , the second electrically conductive contact layer  35   b  and the second p-typed confining layer  34   b , so that the light-emitting diode device  3  is fabricated, as shown in  FIG. 3E . 
         [0045]    In the present invention, the substrates  20 ,  30  of the respective light-emitting diode devices  2 ,  3  are made of sapphire. In order to improve the lattice matching between the semiconductor material of gallium nitride and the substrate, a respective GaN buffer layer  21 ,  31  is grown on the substrate  20 , before the confining layers  22 ,  24 ,  32  and  34  are grown thereon. In this case, the film defect resulting from the mismatch of lattice can be avoided. 
         [0046]    The confining layers  22 ,  24 ,  32  and  34  are made of gallium indium nitride, and are respectively doped with the dopant combination of divalent phosphorous and divalent nitrogen so as to exhibit two different types of electrical conductivities. In this embodiment, the first confining layers  22 ,  32  are n-typed while the second confining layers  24 ,  34  are p-typed, and vice versa. 
         [0047]    The first electrodes  26   a ,  36   a  and the second electrodes  26   b ,  36   b  are made of such as gold, chromium and/or the alloy thereof. The first electrodes  26   a ,  36   a  are configured to form an ohmic contact with the n-typed confining layers  21 ,  31 , and are thus termed as n-typed electrodes, while the second electrodes  26   b ,  36   b  are configured to form an ohmic contact with the p-typed confining layers  24 ,  34 , and are thus termed as p-typed electrodes. By applying a voltage through the n-typed electrodes  26   a ,  36   a  and the p-typed electrodes  26   b ,  36   b , the current can pass through the n-typed confining layers  21 ,  31  and the p-typed confining layers  24 ,  34 , and thereby the electrons and holes are combined within the active layer and thus results in the light energy. 
         [0048]    In the present invention, the second electrically conductive contact layers  25   b ,  35   b  and the second p-typed confining layers  24   b ,  34   b  are patterned to create a plurality of openings therein. Preferably, the transparent electrically conductive layers  27 ,  37 , the second electrically conductive contact layers  25   b ,  35   b  and the second p-typed confining layers  24   b ,  34   b  are patterned and thus define a plurality of openings therein, so that the first electrically conductive contact layers  25   a ,  35   a  can be exposed, and the contact area between the contact layers  25 ,  35  and the second electrodes  26   b ,  36   b  is significantly increased. In this case, the efficient current is enlarged, which is sufficient to maintain the low working voltage of the light-emitting diode device  2 ,  3  of the present invention. In comparison with the conventional light-emitting diode device  1  in this art, which has only one contact layer therein, the light-emitting diode device and the fabrication method thereof according to the present invention are advantageous in an increased efficiency in light-emitting and a reduced working voltage of the light-emitting diode device. 
         [0049]    While the invention has been described by way of examples and in terms of preferred embodiments, it is to be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.