Abstract:
Disclosed is a GaN LED structure with a p-type contacting layer using Al—Mg-codoped In 1−y Ga y N grown at low temperature, and having low resistivity. The LED structure comprises, from the bottom to top, a substrate, a buffer layer, an n-type GaN layer, an active layer, a p-type shielding layer, and a p-type contacting layer. In this invention, Mg and Al are used to co-dope the In 1−y Ga y N to grow a low resistive p-type contacting layer at low temperature. Because of the Al—Mg-codoped, the light absorption problem of the p-type In 1−y Ga y N layer is improved. The product, not only has the advantage of convenience of the p-type contacting layer for being manufactured at low temperature, but also shows good electrical characteristics and lowers the operating voltage of the entire element so that the energy consumption during operation is reduced and the yield rate is increased.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a structure for InGaN/GaN multiquantum well (MQW) light emitting diodes (LED), and more specifically to an LED structure with a p-type contacting layer made with Al—Mg-codoped In 1−y Ga y N grown at low temperature and having low resistivity. 
     BACKGROUND OF THE INVENTION 
     Conventional multiquantum well (MQW) In 1−y Ga y N/GaN light emitting diodes (LED) use a p-type GaN grown at high temperature as a contacting layer. The contacting layer is usually grown on top of the active (light-emitting) layer. During the manufacturing, it is found that the contacting layer grown at high temperature will affect the epitaxial characteristics of the active layer grown at low temperature in the previous step, so that the epitaxy will not keep the regular structure formed during its growth. The result of this effect is that the LED is either dysfunctional, or has poor electrical characteristics, such as having higher operating voltage so that the energy consumption is increased. Therefore, it is imperative to provide a new structure so that a p-type contacting layer grown at low temperature and having low resistivity can be obtained. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to overcome the above-mentioned drawback of conventional GaN MQW LED structures that need a high temperature to grow a p-type contacting layer. The primary object of the present invention is to provide a GaN MQW LED structure with a low resistive p-type contacting layer that is made of In 1−y Ga y N codoped with Mg and Al, and can be grown at low temperature. The resistivity of the p-type contact layer of the present invention is lower than that of the conventional p-type contacting layer is because the Al-doped p-type contacting layer has more two-dimensional hole carriers and their mobility. Also, the In 1−y Ga y N has a lower energy gap than the GaN, so that the lower resistivity can be achieved. Otherwise, if an ITO (Indium Tin Oxide) transplant contacting layer is used instead of a metal conductive layer, which is made of Ni/Au and so on to be a p-type electrode layer, the In 1−y Ga y N on the top surface co-doped with Mg and Al will have better reliability, because Al can block In and Sn inter-diffusion. 
     The second object of the present invention is to improve the light absorption problem caused by the material of the p-type contacting layer. In general, the p-type contacting layer is on top of the active layer; therefore, the p-type contacting layer will absorb the light and lower the external quantum efficiency of the GaN MQW LED. Because the p-type contacting layer made of Al—Mg-codoped In 1−y Ga y N can be grown at a low temperature in the present invention, the lateral growth rate of the GaN material is reduced so that the coarsened surface can be achieved, which, in turn, reduces the possibility of light being reflected and increases the external quantum efficiency. 
     The third object of the present invention is to provide a convenient step of growing a p-type contacting layer at a low temperature after manufacturing the active layer, to improve the overall electrical characteristics so as to have a lower operating voltage and lower energy consumption, and to increase the yield rate. Compared to the conventional p-type contacting layer, the growth temperature of the p-type contacting layer of the present invention is lower. Therefore, the In 1−y Ga y N active layer and the In 1−y Ga y N grown p-type contacting layer can both be grown at the same low temperature in order to protect the In 1−y Ga y N active layer and increase the external quantum efficiency of the element. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be understood in more detail by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein: 
         FIG. 1  shows a first embodiment of the present invention of a GaN LED structure having a p-type contacting layer; 
         FIG. 2  shows a second embodiment of the present invention of a GaN LED structure having a p-type contacting layer; 
         FIG. 3  shows a third embodiment of the present invention of a GaN LED structure having a p-type contacting layer; and 
         FIG. 4  shows a fourth embodiment of the present invention of a GaN LED structure having a p-type contacting layer. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a first embodiment of the present invention of a GaN LED structure having a p-type contacting layer grown at low temperature and having low resistivity. The embodiment includes a substrate  10 , a buffer layer  11 , an n-type GaN layer  12 , an active layer  13 , a p-type shielding layer  14 , and a p-type contacting layer  15 . Substrate  10  can be made of sapphire (including C-Plane, R-Plane and A-Plane), SiC (6H-SiC or 4H—SiC), Si, ZnO, GaAs, and MgAl 2 O 4 . It can also be made of a single crystal oxide having a lattice constant close to that of an N-compound semiconductor. But, in general, it is made of sapphire and SiC. Buffer layer  11  is located on top of substrate  10 , and is made of Al 1−x−y Ga x In y N, where 0≦x&lt;1, 0≦y&lt;1and x+y≦1. 
     Located on top of buffer layer  11  is n-type GaN layer  12 . Active layer  13 , which is located on top of n-type GaN layer  12 , is made of Al 1−x−y Ga x In y N. Located on top of the active layer  13  is a p-type shielding layer  14 , which is made of Mg-doped p-type Al 1−x In x N, where 0&lt;X&lt;1, but preferably 0.1≦X≦0.4. The thickness of p-type shielding layer  14  is between 50Å and 3000Å, and it is grown at temperature between 600° C. and 1100° C. Located on top of p-type shielding layer  14  is p-type contacting layer  15 , which is made of Al—Mg-codoped p-type In y Ga 1−y N, where 0≦Y≦1. The thickness of p-type contacting layer  15  is between 200Å and 3000Å, and it is grown at temperature between 600° C. and 1100° C. 
     According to the first embodiment, the present invention can further include an n-type electrode layer  16  on top of n-type GaN layer  12 . In addition, according to the first embodiment, the present invention can further include a p-type electrode layer  17  on top of p-type contacting layer  15 . The p-type electrode layer  17  further includes a metal conductive layer, which is made of Ni/Au, Ni/Pt, Ni/Pd, Ni/Co, Pd/Au, Pt/Au, Ti/Au, Cr/Au, Sn/Au, Ta/Au, TiN, TiWNx, WSix, or a transparent conductive oxide layer (TCO), which is made of ITO, CTO, ZnO:Al, ZnGa 2 O 4 , SnO 2 :Sb, Ga 2 O 3 :Sn, AgInO 2 :Sn, In 2 O 3 :Zn, CuAlO 2 , LaCuOS, NiO, CuGaO 2 , SrCu 2 O 2 . 
       FIG. 2  shows a second embodiment of the present invention of a GaN LED structure having a p-type contacting layer grown at low temperature and having low resistivity. The embodiment includes a substrate  20 , a buffer layer  21 , an n-type GaN layer  22 , an active layer  23 , a p-type shielding layer  24 , and a p-type contacting layer  25 . Substrate  20  can be made of sapphire (including C-Plane, R-Plane and A-Plane), SiC (6H—SiC or 4H—SiC), Si, ZnO, GaAs, and MgAl 2 O 4 . It can also be made of a single crystal oxide having a lattice constant close to that of an N-compound semiconductor. But, in general, it is made of sapphire and SiC. Buffer layer  21  is located on top of substrate  20 , and is made of Al 1−x−y Ga x In y N, where 0≦X&lt;1, 0≦Y&lt;1 , and X+Y≦1. 
     Located on top of buffer layer  21  is n-type GaN layer  22 . Active layer  23 , which is located on top of n-type GaN layer  22 , is made of Al 1−x−y Ga x In y N. Located on top of active layer  23  is p-type shielding layer  24 , which is made of Mg—Ga-codoped p-type Al 1−x In x N, where 0≦X&lt;1, but preferably 0.1≦X≦0.4. The thickness of p-type shielding layer  24  is between 50Å and 3000Å, and it is grown at temperature between 600° C. and 1100° C. Located on top of p-type shielding layer  24  is p-type contacting layer  25 , which is made of Al—Mg-codoped p-type In y Ga 1−y N, where 0≦Y&lt;1. The thickness of p-type contacting layer  25  is between 200Å and 3000Å, and it is grown at temperature between 600° C. and 1100° C. 
     According to the second embodiment, the present invention can further include an n-type electrode layer  26  on top of n-type GaN layer  22 . In addition, according to the second embodiment, the present invention can further include a p-type electrode layer  27  on top of p-type contacting layer  25 . The p-type electrode layer  27  further includes a metal conductive layer, which is made of Ni/Au, Ni/Pt, Ni/Pd, Ni/Co, Pd/Au, Pt/Au, Ti/Au, Cr/Au, Sn/Au, Ta/Au, TiN, TiWNx, WSix, or a transparent conductive oxide layer (TCO), which is made of ITO, CTO, ZnO:Al, ZnGa 2 O 4 , SnO 2 :Sb, Ga 2 O 3 :Sn, AgInO 2 :Sn, In 2 O 3 :Zn, CuAlO 2 , LaCuOS, NiO, CuGaO 2 , SrCu 2 O 2 . 
       FIG. 3  shows a third embodiment of the present invention of a GaN LED structure having a p-type contacting layer grown at low temperature and having low resistivity. The embodiment includes a substrate  30 , a buffer layer  31 , an n-type GaN layer  32 , an active layer  33 , a double shielding layer  34 , and a p-type contacting layer  35 . Substrate  30  can be made of sapphire (including C-Plane, R-Plane and A-Plane), SiC (6H—SiC or 4H—SiC), Si, ZnO, GaAs, and MgAl 2 O 4 . It can also be made of a single crystal oxide having a lattice constant close to that of an N-compound semiconductor. But, in general, it is made of sapphire and SiC. Buffer layer  31  is located on top of substrate  30 , and is made of Al 1−x−y Ga x In y N, where 0≦X&lt;1, 0≦Y&lt;1, and X+Y=1. 
     Located on top of buffer layer  31  is n-type GaN layer  32 . Active layer  33 , which is located on top of n-type GaN layer  32 , is made of Al 1−x−y Ga x In y N. Located on top of active layer  33  is double shielding layer  34 , which further includes a first shielding layer  340 , and a second shielding layer  342 . First shielding layer  340 , which is located on top of active layer  33 , is made of Mg—Ga-codoped p-type Al 1−x In x N, where 0&lt;X&lt;1, but preferably 0.1≦X≦0.4. The thickness of first shielding layer  340  is between 50Å and 3000Å, preferably between 50Å and 1000Å, and it is grown at temperature between 600° C. and 1100° C. Second shielding layer  342 , which is located on top of first shielding layer  340 , is made of Mg-doped p-type Al 1−z In z N, where 0&lt;Z&lt;1, but preferably 0.1≦Z≦0.4. The thickness of second shielding layer  342  is between 50Å and 3000Å, preferably between 50Å and 1000Å, and it is grown at temperature between 600° C. and 1100° C. Located on top of double shielding layer  34  is p-type contacting layer  35 , which is made of Al—Mg-codoped p-type In y Ga 1−y N, where 0≦Y&lt;1. The thickness of p-type contacting layer  35  is between 200Å and 3000Å, and it is grown at temperature between 600° C. and 1100° C. 
     According to the third embodiment, the present invention can further include an n-type electrode layer  36  on top of n-type GaN layer  32 . In addition, according to the third embodiment, the present invention can further include a p-type electrode layer  37  on top of p-type contacting layer  35 . The p-type electrode layer  37  further includes a metal conductive layer, which is made of Ni/Au, Ni/Pt, Ni/Pd, Ni/Co, Pd/Au, Pt/Au, Ti/Au, Cr/Au, Sn/Au, Ta/Au, TiN, TiWNx, WSix, or a transparent conductive oxide layer (TCO), which is made of ITO, CTO, ZnO:Al, ZnGa 2 O 4 , SnO 2 :Sb, Ga 2 O 3 :Sn, AgInO 2 :Sn, In 2 O 3 :Zn, CuAlO 2 , LaCuOS, NiO, CuGaO 2 , SrCu 2 O 2 . 
       FIG. 4  shows a fourth embodiment of the present invention of a GaN LED structure having a p-type contacting layer grown at low temperature and having low resistivity. The embodiment includes a substrate  40 , a buffer layer  41 , an n-type GaN layer  42 , an active layer  33 , a double shielding layer  44 , and a p-type contacting layer  45 . Substrate  40  can be made of sapphire (including C-Plane, R-Plane and A-Plane), SiC (6H—SiC or 4H—SiC), Si, ZnO, GaAs, and MgAl 2 O 4 . It can also be made of a single crystal oxide having an lattice constant close to that of an N-compound semiconductor. But, in general, it is made of sapphire and SiC. Buffer layer  41  is located on top of substrate  40 , and is made of Al 1−x−y Ga x In y N, where 0≦X&lt;1, 0≦Y&lt;1, and X+Y≦1. 
     Located on top of buffer layer  41  is n-type GaN layer  42 . Active layer  43 , which is located on top of n-type GaN layer  42 , is made of Al 1−x−y Ga x In y N. Located on top of active layer  43  is double shielding layer  44 , which further includes a first shielding layer  440 , and a second shielding layer  442 . First shielding layer  440 , which is located on top of active layer  43 , is made of Mg-doped p-type Al 1−x In x N, where 0&lt;X&lt;1, but preferably 0.1≦X≦0.4. The thickness of first shielding layer  440  is between 50Å and 3000Å, preferably between 50Å and 1000Å, and it is grown at temperature between 600° C. and 1100° C. Second shielding layer  442 , which is located on top of first shielding layer  440 , is made of Mg—Ga-codoped p-type Al 1−z In z N, where 0&lt;Z&lt;1, preferably 0.1≦Z≦0.4. The thickness of second shielding layer  442  is between 50Å and 3000Å, but preferably between 50Å and 1000Å, and it is grown at temperature between 600° C. and 1100° C. Located on top of double shielding layer  44  is p-type contacting layer  45 , which is made of Al—Mg-codoped p-type In y Ga 1−y N, where 0≦Y&lt;1. The thickness of p-type contacting layer  45  is between 200Å and 3000Å, and it is grown at temperature between 600° C. and 1100° C. 
     According to the fourth embodiment, the present invention can further include an n-type electrode layer  46  on top of n-type GaN layer  42 . In addition, according to the fourth embodiment, the present invention can further include a p-type electrode layer  47  on top of p-type contacting layer  45 . The p-type electrode layer  47  further includes a metal conductive layer, which is made of Ni/Au, Ni/Pt, Ni/Pd, Ni/Co, Pd/Au, Pt/Au, Ti/Au, Cr/Au, Sn/Au, Ta/Au, TiN, TiWNx, WSix, or a transparent conductive oxide layer (TCO), which is made of ITO, CTO, ZnO:Al, ZnGa 2 O 4 , SnO 2 :Sb, Ga 2 O 3 :Sn, AgInO 2 :Sn, In 2 O 3 :Zn, CuAlO 2 , LaCuOS, NiO, CuGaO 2 , SrCu 2 O 2 . 
     In summary, the GaN LED structure of the present invention comprises a p-type In y Ga 1−y N contacting layer which is Al—Mg-codoped. As is well known in the semiconductor process, the goal of doping or co-doping is to introduce dopants (impurities) in the material. The Al—Mg-codoped In y Ga 1−y N contacting layer contains both Al and Mg dopants (or impurities) in the In y Ga 1−y N material. The Al and Mg dopants result in lower band-gap for the Al—Mg-codoped In y Ga 1−y N in comparison to Al-doped contact layer commonly used in the prior arts. The low band-gap is good for the reverse electrical property of the LED structure because it avoids higher forward voltage. 
     Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.