Abstract:
The present invention provides a group III-nitride light emitting device improved in operating voltage and electrostatic discharge characteristics. The group III-nitride light emitting device comprises a lower n-type clad layer, a current spreading layer, an upper n-type clad layer, an active layer and an p-type clad layer formed in their order on a substrate. The current spreading layer includes a SiC layer.

Description:
RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. 2005-0026736 filed on Mar. 30, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a group III-nitride light emitting device. More particularly, the present invention relates to a group III-nitride light emitting device such as a group III-nitride light emitting diode (LED) or a laser diode improved in operating voltage and electrostatic discharge characteristics. 
     2. Description of the Related Art 
     A device using a group III-nitride semiconductor is utilized in many areas such as light emitting devices including light emitting diodes (LEDs) and laser diodes (LDs) or light-receiving devices such as solar cell and optical censor. The device is also employed in electronic devices such as a transistor and a power device. The group III-nitride semiconductor is a compound semiconductor having a composition expressed by Al x Ga y In 1-x-y N, where 0≦x≦1, 0≦y≦1 and 0≦x+y≦1. The LED and LD devices using such material are essential for the development of various light sources such as a natural-color sign board, a traffic signal, a light source of image scanner, and a high-density optical recording medium. 
       FIG. 1  is a sectional view illustrating a group III-nitride light emitting device (especially LED device). Referring to  FIG. 1 , a conventional light emitting device  10  includes an undoped GaN layer  13 , an n-type clad layer  14  made of n-doped Al x Ga y In 1-x-y N, an active layer  16  and a p-type clad layer  18  made of p-doped Al x Ga y In 1-x-y N. An n-electrode  24  is formed on a surface of the n-type clad layer  14  exposed by mesa etching, and a transparent electrode layer  20  and a p-type electrode  22  made of ITO are formed on the p-type GaN-based clad layer  18 . Japanese Laid-Open Patent Application No. 10-135514 discloses an active layer of a multiple quantum well structure that has an undoped GaN barrier layer and an undoped InGaN well layer in order to enhance light emitting efficiency and brightness. 
     However, in order to use the LED device  10  described above as lighting source and outdoor display, output power should be enhanced more and operating voltage (Vf) should be lowered further to reduce heat generation and improve reliability and useful life of the device. Also, to utilize the LD device as stable light source for optical pickup or DVD, it is necessary to upgrade the LD device significantly and lengthen its useful life through reduction in operating voltage. 
     In addition to operating voltage problem as identified above, electrostatic discharge (ESD) is caused by a person or a machine handling or using the LED or LD. Extensive studies have been conducted to increase ESD resistance in order to prevent damage to the LED or LE by ESD, and various solutions have been proposed. For example, optical device such as the LED or LD can be connected in parallel to a separate zener diode, or an ESD protection diode is mounted on a single substrate in manufacturing of the LED (refer to U.S. Pat. No. 6,593,597). But these methods require purchasing or assembling of a separate protective device besides the LED (or LD) or providing a protective LED via a separate process, therefore complicating a manufacturing process and increasing costs. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the foregoing problems of the prior art and it is therefore an object of the present invention to provide a group III nitride light emitting device capable of enhancing light output and lowering operating voltage. 
     It is another object of the invention to provide a group III-nitride light emitting device capable of achieving high ESD resistance without a need for other devices to increase ESD resistance. 
     According to an aspect of the invention for realizing the object, there is provided a group III-nitride light emitting device comprising: a lower n-type clad layer, a current spreading layer and an upper n-type clad layer formed in their order on a substrate; and an active layer and a p-type clad layer formed in their order on the upper n-type clad layer, wherein the current spreading layer comprises a SiC layer. 
     According to the embodiment of the invention, the SiC layer may comprise an undoped SiC layer. The undoped SiC layer is basically n-type conductive. According to other embodiment, the SiC layer may comprise an n-doped SiC layer. In this case, a dopant added to the SiC layer comprises one selected from a group consisting of N, Ti, Cr and P. 
     According to the embodiment of the invention, the current spreading layer further comprises high-concentration n-doped Al x Ga y In 1-x-y N layers having electron concentration higher than that of the lower n-type clad layer, where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1, and formed on and underneath the SiC layer, respectively. Preferably, the high-concentration n-doped Al x Ga y In 1-x-y N layers have electron concentration of 1×10 21  cm −3  or less. Also, the lower n-type clad layer preferably has electron concentration of 1×10 18  to 5×10 18  cm −3 . The high-concentration n-doped Al x Ga y In 1-x-y N layer formed on the SiC layer has a composition identical to or different from the high-concentration n-doped Al x Ga y In 1-x-y N layer formed underneath the SiC layer. 
     According to one embodiment of the invention, the current spreading layer may further comprise at least one SiC layer on the high-concentration n-doped Al x Ga y In 1-x-y N layer formed on the SiC layer. For example, the current spreading layer may comprise a multilayer structure including the high-concentration n-doped Al x Ga y In 1-x-y N layer and the SiC layer stacked repeatedly. 
     According to the preferred embodiment, the upper n-type clad layer comprises a low-concentration n-doped Al x Ga y In 1-x-y N layer having electron concentration lower than that of the lower n-type clad layer, and formed just underneath the active layer. 
     Preferably, the low-concentration n-doped Al x Ga y In 1-x-y N layer has electron concentration of at least 1×10 16  cm −3 . 
     More preferably, the low-concentration n-doped Al x Ga y In 1-x-y N layer has electron concentration of 1×10 16  cm −3  to 1×10 18  cm −3 . 
     The group III-nitride light emitting device according to the invention may be a light emitting diode (LED) or a laser diode (LD). 
     The essential characteristics of the invention is that the current spreading layer including the SiC layer is inserted between the n-clad layers of the group III-nitride light emitting device. The current spreading layer may comprise only the SiC layer in the simplest way. However to attain sufficient ESD protection effects, preferably, the current spreading layer comprises high-concentration n-doped Al x Ga y In 1-x-y N layers formed on and underneath the SiC layer. According to embodiments, the current spreading layer may comprise a plurality of SiC layers. 
     In the specification, ‘group III-nitride’ designates a binary, ternary or quaternary compound semiconductor expressed by Al x Ga y In 1-x-y N, where 0≦x≦1, 0≦y≦1 and 0≦x+y≦1. Also ‘group III nitride light emitting device’ means that an n-type clad layer, an active layer and a p-type clad layer constituting a light emitting structure are made of group III-nitride. As used herein, the term “high-concentration” means electron concentration higher than that of the lower n-type clad layer. The term “low-concentration” means electron concentration lower than that of the lower n-type clad layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a sectional view illustrating an example of a conventional group III-nitride light emitting device; 
         FIG. 2  is a sectional view illustrating a group III-nitride light emitting device according to an embodiment of the invention; 
         FIG. 3  is a sectional view illustrating a group III-nitride light emitting device according to another embodiment of the invention; 
         FIG. 4  is a partial sectional view illustrating a current spreading layer of a group III-nitride light emitting device according to an embodiment of the invention; 
         FIG. 5  is a partial sectional view illustrating a current spreading layer of a group III-nitride light emitting device according to another embodiment of the invention; and 
         FIG. 6  is a partial sectional view illustrating a current spreading layer of a group III-nitride light emitting device according to further another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the shapes and dimensions may be exaggerated for clarity, and the same reference signals are used to designate the same or similar components throughout. 
       FIG. 2  is a sectional view of a group III-nitride light emitting device according to an embodiment of the invention. Especially,  FIG. 2  shows the group III-nitride light emitting device. Referring to  FIG. 2 , the group III-nitride light emitting device  100  includes an undoped GaN layer  103 , a lower n-type clad layer  104  made of n-doped Al x Ga y In 1-x-y N, a current spreading layer (CSL)  130 , an upper n-type clad layer  104  made of n-doped Al x Ga y In 1-x-y N sequentially formed on a sapphire substrate  101 . The upper and lower n-type clad layers  105 ,  104  and the current spreading layer  130  form an n-side area of the light emitting device  100 . 
     An active layer  107  with multiple quantum well structure (MQW), a p-type clad layer  109  made of p-doped Al x Ga y In 1-x-y N and a transparent electrode layer are formed on the upper n-type clad layer  104 . Also, a p-electrode  122  is formed on a surface area of the transparent electrode layer, and an n-electrode  124  is formed on the lower n-type clad layer  104  exposed by mesa etching. According to embodiments, the active layer with single quantum well (SQW) structure may be used. 
     An essential characteristic of the invention is that the current spreading layer  130  including a SiC layer is interleaved in an n-type clad layer  135  to boost current spreading and ESD resistance. On an interface of the SiC layer included in the current spreading layer  130 , 2-dimensional electron gas (2-DEG) layer is formed due to band gap difference. Such 2-DEG layer allows better spreading of current due to its high electron mobility. Further, this current spreading effect reduces operating voltage (V f ) and expands light emitting area. Consequently, the current spreading layer  130  leads to improvement in light emitting efficiency and light output. 
     In addition, the SiC layer in the current spreading layer  130  exhibits relatively higher dielectric constant compared to adjacent n-doped Al x Ga y In (1-x-y) N material, thus serving as a capacitor. As a result, the current spreading layer  130  protects the light emitting device  100  from a sudden voltage surge or static electricity to increase ESD resistance of the light emitting device. Detailed structure of the current spreading layer  130  is shown in  FIGS. 4 to 6 . 
       FIG. 3  is a sectional view illustrating a group III-nitride light emitting device according to another embodiment of  FIG. 3 . Especially,  FIG. 3  shows the group III-nitride laser diode (LD). Referring to  FIG. 3 , the group III-nitride light emitting device  200  includes a lower n-type clad layer  203 , a current spreading layer  230 , and an upper n-type clad layer  205  sequentially formed on a GaN substrate or a SiC substrate  201 . The upper and lower clad layers  205 ,  203  are made of n-doped Al x Ga y In 1-x-y N, and together with the current spreading layer  230 , constitute an n-side area  235 . 
     An active layer  207  and a p-type clad layer  209  are formed on the upper n-type clad layer  205 . The p-type clad layer  209  is ridge-structured and made of p-doped Al x Ga y In 1-x-y N. On one side of the ridge, a current blocking layer  215  made of insulating material is formed, and on the p-type clad layer  209 , a p-type contact layer  213  is formed. Although not shown in  FIG. 3 , a p-electrode is formed on the p-type contact layer  213  and an n-electrode is formed on a bottom of a substrate  210 . 
     In the embodiment of the invention, the current spreading layer  230  is interleaved between the n-type clad layers and includes the SiC layer. Therefore, as in the aforesaid embodiment, according to this embodiment, the 2-DEG layer resulting from energy band gap allows current spreading effect, and high dielectric constant of the SiC layer increases ESD resistance of the light emitting device  200 . Therefore, the invention is effectively applied to the LED and LD as well. 
       FIG. 4  is a partial sectional view illustrating a group III-nitride light emitting device according to an embodiment of the invention. Referring to  FIG. 4 , an upper n-type clad layer  304  made of n-doped Al x Ga y In 1-x-y N is formed on an undoped GaN layer  303 , and a current spreading layer  330   a  is formed on the upper n-type clad layer  304 . Also, an upper n-type clad layer  305  and an active layer  307  with MQW structure are formed on the current spreading layer  330   a.    
     According to the embodiment, the current spreading layer  330   a  is made of only the SiC layer. The SiC layer has energy band gap smaller than that of n-type clad layers  305 ,  304  positioned in the upper and lower part thereof. Consequently, on an interface of the SiC layer, a 2-DEG layer, which reportedly has very high electron mobility, is formed. Therefore, in activating the light emitting device, current spreads well due to high electron mobility of the 2-DEG layer. This current spreading effect reduces operating voltage and elevates light emitting efficiency. 
     Also, the SiC layer constituting the current spreading layer  330   a  has dielectric constant higher than that of the n-type clad layers  305 ,  304 . As a result, a layered structure of n-doped Al x Ga y In 1-x-y N/SiC/n-doped Al x Ga y In 1-x-y N serves as a capacitor. Therefore, such a capacitor accumulates electric charge momentarily when ESD voltage is impressed, thus protecting the light emitting device. 
     The SiC layer constituting the current spreading layer  330   a  may be an undoped SiC layer or an n-doped SiC layer. The undoped SiC layer is basically n-type conductive. At least one of N, Ti, Cr and P may be used as dopants added to the n-doped SiC layer. 
     As shown in  FIG. 4 , an upper n-type clad layer  305  just underneath an active layer  307  is made of low-concentration n-doped Al x Ga y In 1-x-y N layer having electron concentration lower than that of a lower n-type clad layer  304 . The low-concentration n-doped Al x Ga y In 1-x-y N layer (upper n-type clad layer  305 ) is placed just underneath the active layer  307  so that crystalline degradation of the active layer  307  can be prevented. If high-concentration n-doped Al x Ga y In (1-x-y) N layer is arranged just underneath the active layer  307 , crystalinity of the active layer deteriorates due to dopant impurities. 
     Preferably, the lower n-type clad layer  304  has electron concentration of about 1×10 18  to 5×10 18  cm −3 . The upper n-type clad layer  305  has electron concentration lower than that of the lower n-type clad layer  304 , but preferably at least 1×10 16  cm −3 . If the upper n-type clad layer  305  has electron concentration smaller than 1×10 16  cm −3 , electric resistance becomes too big. Preferably, the upper n-type clad layer  305  has electron concentration of 1×10 16 cm   −3  to 1×10 18  cm −3 . 
       FIG. 5  is a partial sectional view illustrating a current spreading layer of a group III-nitride light emitting device according to another embodiment of the invention. In this embodiment, a current spreading layer  330   b  includes high-concentration n-doped Al x Ga y In 1-x-y N layers  331 ,  333  and a SiC layer  332  interposed therebetween. By arranging high-concentration doped Al x Ga y In 1-x-y N layers  331 ,  333 , current is spread widely and ESD protection capacity is also improved. The SiC layer  332  with high dielectric constant is interleaved between high-concentration n-doped Al x Ga y In 1-x-y N layers  331 ,  333  to increase ESD resistance. The embodiment of  FIG. 5  indicates greater ESD than that of  FIG. 4 . This is because high-concentration n-doped Al x Ga y In 1-x-y N layers  331 ,  333  sandwiching the SiC layer  332  therebetween can be used as high-quality capacitor electrodes. 
       FIG. 6  is a partial sectional view illustrating a current spreading layer of a group III-nitride light emitting device according to further another embodiment of the invention. In this embodiment, the current spreading layer  330   c  includes a plurality of SiC layers  332  and high-concentration n-doped Al x Ga y In 1-x-y N layers  331  sandwiching the SiC layers  332  therebetween. Repeated stacking of a high-concentration n-doped Al x Ga y In 1-x-y N layer  331  and a SiC layer  332  allows formation of a greater number of 2-DEG layers. Consequently, current spreading effect is significantly heightened so that properties of operating voltage and useful life of the device are improved. In the embodiment of  FIG. 6 , the high-concentration n-doped Al x Ga y In 1-x-y N layer  331 , the SiC layer  332 , the high-concentration n-doped Al x Ga y In 1-x-y N layer  331  are stacked one on another to ensure ESD protection effect. 
     As stated above, according to the invention, the current spreading layer including the SiC layer is interleaved between n-type clad layers to spread current widely and expand light emitting area. This increases useful life, light emitting efficiency and optical output of the light emitting device, while reducing operating voltage and heat generation thereof. 
     Also, the light emitting device is protected from ESD voltage by inserting a SiC layer with high dielectric constant between the n-doped Al x Ga y In 1-x-y N layers. 
     While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.