Abstract:
A light emitting diode (LED) device is provided. The LED device includes a device substrate, a first doped layer of a first conductivity type, a light emitting layer, a second doped layer of a second conductivity type, a transparent conductive oxide layer, a reflecting layer and two electrodes. The first doped layer is deposited on the device substrate, the light emitting layer is deposited on a portion of the first doped layer, and the second doped layer is deposited on the light emitting layer. The first and the second doped layers are comprised of III-V semiconductor material respectively. The transparent conductive oxide layer is deposited on the second doped layer, and the reflecting layer is deposited on the transparent conductive oxide layer. The two electrodes are deposited on the reflecting layer and the first doped layer respectively.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the priority benefit of Taiwan application serial no.92120195, filed on Jul. 24, 2003.  
       BACKGROUND OF INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates in general to a structure of a semiconductor light emitting device, and more particularly, to a structure of a light emitting diode (LED) device, to a package structure of a flip-chip LED device, and to a light reflective structure being applicable for a LED.  
         [0004]     2. Related Art of the Invention  
         [0005]     In general, a light emitting diode (LED) constructed by an III-V semiconductor material can be provided as a wide bandgap light emitting device. The wavelength of the light emitted from the wide bandgap light emitting device ranges from infrared (IR) to ultraviolet (UV); therefore the entire spectrum of visible light is also covered. In recent years, due to the rapid development of the high illumination of the gallium nitride (GaN) LEDs having a blue/green light, the full-color LED display, white light LED and the LED for traffic signals are put into practice. Therefore, the application of a variety of LED also becomes more popular.  
         [0006]     In principle, a fundamental structure of a LED device includes an epitaxy layer of a P-type and a N-type III-V group compound and a light emitting layer in-between. The light emitting efficiency of the LED device is dependent on the internal quantum efficiency of the light emitting layer and the light extraction efficiency of the device. A method of increasing the internal quantum efficiency includes, for the most part, improving the quality of the light emitting layer and the design of the structure. The method of increasing the light extraction efficiency includes, for the most part, decreasing the light loss caused by the absorption of the light emitted from the light emitting layer due to the reflection of the light inside the LED device.  
         [0007]     In a conventional gallium nitride (GaN) LED device grown on the first substrate, such as sapphire, having an insulating property, since the positive and the negative electrodes of a gallium nitride (GaN) LED device are deposited on, in general, the same side of a first surface, and the positive electrode will screen out the emitted light from light emitting layer. Therefore, the packaging for a gallium nitride (GaN) LED normally uses the flip chip method. Thus, the emitted light will pass through the second surface. Moreover, a reflecting layer is formed on the topmost surface of GaN LED that faces the second substrate, in order to emit most of the emitted light towards the second surface of a GaN LED. Another advantage of using the flip-chip package process is, if a proper surface mount (so called surmount) substrate, for example, a silicon substrate is provided, the heat dissipation of the LED device is enhanced, especially under a high current operation. Accordingly, not only the light extraction efficiency is increased, the internal quantum efficiency of the light emitting layer will also be maintained.  
         [0008]     Moreover, in order to improve the electrical property of the LED device, a semi-transparent nickel (Ni)/gold (Au) ohmic contact layer is first formed on the epitaxy layer surface, and a thermal process is performed to form a desirable ohmic contact, followed by forming a reflecting layer thereon. However, since the absorption of light of the Ni/Au layer is high (the transparency of that is about 60% to about 70%), and due to the thermal process, the interface between the epitaxy layer and the Ni/Au layer becomes too rough to reflect light. Accordingly, the light reflective efficiency of the bottom of the flip-chip LEDs device will be reduced.  
       SUMMARY OF INVENTION  
       [0009]     Accordingly, the present invention is to provide a light reflective structure, which is applicable for a LED device to enhance the extraction efficiency of light.  
         [0010]     Another object of the present invention is to provide a LED device having a light reflective structure of the present invention, wherein the extraction efficiency of light is enhanced.  
         [0011]     It is yet another object of the present invention to provide a flip-chip LED package structure having a light reflective structure of the present invention, wherein the extraction efficiency of light is enhanced.  
         [0012]     In order to achieve the above objects and other advantages of the present invention, a light reflective structure for a LED device is provided. The light reflective structure includes, for example but not limited to, a transparent conductive oxide layer deposited on a semiconductor layer, a transparent insulating layer deposited on the transparent conductive oxide layer, and a reflecting layer deposited on the transparent insulating layer. The transparent conductive oxide layer is provided as an ohmic contact layer for the semiconductor layer. The transparent insulating layer is provided as a passivation layer for the transparent conductive oxide layer. When the wavelength of the light emitted from the LED device is λ, and the refractive index of the transparent conductive oxide layer is n, the thickness of the transparent conductive oxide layer is preferably to be (2 m+1)λ/2n (m is 0 or an positive integer). When the refractive index of the transparent insulating layer is k, the thickness of the transparent insulating layer is preferably to be (2 m+1)λ/2k (m is 0 or an positive integer). Therefore, a constructive interference of the lights is achieved.  
         [0013]     In order to achieve the above objects and other advantages of the present invention, a light reflective structure applicable for a LED device is provided. The light reflective structure includes a transparent conductive oxide layer deposited on a semiconductor layer, and a reflecting layer deposited on the transparent conductive oxide layer. The transparent conductive oxide layer is provided as an ohmic contact layer for the semiconductor layer. When the wavelength of the light emitted from the LED device is λ, and the refractive index of the transparent conductive oxide layer is n, the thickness of the transparent conductive oxide layer is preferably to be (2 m+1)λ/2n (m is 0 or a positive integer). Therefore, a constructive interference of the lights is achieved.  
         [0014]     The LED device of the present invention includes a first substrate called device substrate, a first doped layer, a light emitting layer, a second doped layer, a transparent conductive oxide layer, a reflecting layer, and two electrodes. The first doped layer is deposited on the device substrate, the light emitting layer is deposited on the first doped layer, and the second doped layer is deposited on the light emitting layer. The second doped layer and the first doped layer are constructed from an III-V group compound of semiconductor material with different conductivity type. The transparent conductive oxide layer is deposited on the second doped layer, and is provided as an ohmic contact layer. The transparent insulating layer is deposited on the ohmic contact layer to serves as a passivation layer. The reflecting layer is deposited on the transparent insulating layer. The two electrodes are formed on the reflecting layer and the first doped layer, respectively.  
         [0015]     The LED device of the present invention includes a first substrate called device substrate, a first doped layer, a light emitting layer, a second doped layer, a transparent conductive oxide layer, a reflecting layer, and two electrodes. The first doped layer is deposited on the device substrate, the light emitting layer is deposited on the first doped layer, and the second doped layer is deposited on the light emitting layer. The second doped layer and the first doped layer are constructed from an III-V group compound of semiconductor material with different conductivity type. The transparent conductive oxide layer is deposited on the second doped layer, and is provided as an ohmic contact layer. The reflecting layer is deposited on the transparent conductive oxide layer. The two electrodes are formed on the reflecting layer and the first doped layer, respectively.  
         [0016]     The flip-chip LED package structure of the present invention includes a package substrate called second substrate or submount substrate and a LED structure on the first substrate, in which the LED is faced-down over the package substrate and is electrically connected to the package substrate. The LED includes a first substrate (device substrate), a first doped layer, a light emitting layer, a second doped layer, a transparent conductive oxide layer, a transparent insulating passivation layer, a reflecting layer, and two electrodes. The first doped layer is deposited on the first substrate, the light emitting layer is deposited on the first doped layer, and the second doped layer is deposited on the light emitting layer. The second doped layer and the first doped layer are constructed from an III-V group compound of semiconductor material with different conductivity type. The transparent conductive oxide layer is deposited on the second doped layer, and is provided as an ohmic contact layer. The transparent insulating layer is deposited on the ohmic contact layer to serves as a passivation layer. The reflecting layer is deposited on the transparent insulating layer. The two electrodes are deposited on the reflecting layer and the first doped layer, respectively.  
         [0017]     The flip-chip LED package structure of the present invention includes a package substrate called second substrate or submount substrate and a LED structure on the first substrate, in which the LED is faced-down over the package substrate and is electrically connected to the package substrate. The LED includes a first substrate (device substrate), a first doped layer, a light emitting layer, a second doped layer, a transparent conductive oxide layer, a reflecting layer, and two electrodes. The first doped layer is deposited on the first substrate, the light emitting layer is deposited on the first doped layer, and the second doped layer is deposited on the light emitting layer. The second doped layer and the first doped layer are constructed from an III-V group compound of semiconductor material with different conductivity type. The transparent conductive oxide layer is deposited on the second doped layer, and is provided as an ohmic contact layer. The reflecting layer is deposited on the transparent insulating layer. The two electrodes are deposited on the reflecting layer and the first doped layer, respectively.  
         [0018]     Accordingly, in the present invention, the material of the ohmic contact layer includes a transparent conductive metal oxide, and a thermal process for achieving a good ohmic contact is not required for the transparent conductive metal oxide. Therefore, the interface between the ohmic contact layer and the second doped layer is smooth, and thus the interface can be provided as a reflecting surface. Moreover, in the present invention, the absorption to visible light of the transparent conductive metal oxide can be reduced to less than 10% (for example, when the oxide is an indium tin oxide (ITO; therefore, the absorption of the ohmic contact layer to the LED device is reduced drastically.  
         [0019]     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0020]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0021]      FIG. 1  is a cross-sectional view illustrating a structure of a LED device and a enlarged view of a portion adjacent to a interface of the transparent conductive oxide layer of the LED device according to a preferred embodiment of the present invention.  
         [0022]      FIG. 2  is a cross-sectional view illustrating another structure of a LED device.  
         [0023]      FIG. 3  is a cross-sectional view illustrating a flip-chip LED package structure achieved after a flip-chip package process of the LED device of  FIG. 1  and  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0024]     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. Like numbers refer to like elements throughout.  
         [0025]      FIG. 1  is a cross-sectional view illustrating a structure of a LED device and a enlarged view of a portion adjacent to a interface of the transparent conductive oxide layer of the LED device according to a preferred embodiment of the present invention. Referring to  FIG. 1 , the LED device includes a device substrate  100 , a N-type doped layer  110 , a light emitting layer  120 , a P-type doped layer  130 , a strained-layer superlattice (SLS) contact layer  135 , a transparent conductive oxide layer  140 , a reflecting layer  150 , and an anode  160  and a cathode  170 . In  FIG. 1 , an active layer constructed by a N-type doped layer  110 , a light emitting layer  120  and a P-type doped layer  130  is formed, for example but not limited to, by performing a series of epitaxy processes sequentially on the device substrate  100 . Moreover, in the succeeding process, a portion of the N-type doped layer  110 , a portion of the light emitting layer  120  and a portion of the P-type doped layer  130  are removed, for example but not limited to, by etching or by another method. Therefore, each of the layers  110 ,  120 ,  130  and  135  are patterned to form a plurality of isolated island structure (MESA). It is noticed that, in the isolated island structure above, a portion of the P-type doped layer  130  and SLS contact layer  135  over the cathode  170 , the light emitting layer  120  and a portion of the N-type doped layer  110  are removed. The cathode  170  thus can be electrically connected with the N-type doped layer  110 .  
         [0026]     Referring to  FIG. 1 , in the present embodiment, the transparent conductive oxide layer  140  is deposited on the SLS contact layer  135 , while the reflecting layer  150  is deposited on the transparent conductive oxide layer  140  and the anode  160  is deposited on the reflecting layer  150 .  
         [0027]     The device substrate  100  includes, for example but not limited to, a sapphire substrate. The materials of the N-type doped layer  110 , light emitting layer  120 , the P-type doped layer  130 , and SLS contact layer  135  are comprised of a III-V group compound of semiconductor material, including but not limited to, a gallium nitride (GaN), a gallium phosphide (GaP) or a gallium phosphide arsenide (GaAsP). The light emitting layer  120  includes, for example but not limited to, a single or a multi quantum well structure, to enhance the light emitting efficiency. A material of the transparent conductive oxide layer  140  preferably includes an indium tin oxide (ITO), but also may include, for example but not limited to, such as ITO, CTO, IZO, 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 , and so on or other transparent conductive material having similar properties. A material of the reflecting layer  150  includes, for example but not limited to, an aluminum (Al), a silver (Ag), Ni/Ag, Ni/Al, Mo/Ag, Mo/Al, Ti/Ag, Ti/Al, Nd/Al, Nd/Ag, Pd/Al, Pd/Ag, Cr/Al, Cr/Ag and materials of the anode  160  and the cathode  170  include, for example but not limited to, a bi-layer or tri-layer metal system, such as Cr/Au, Ti/Au, Cr/Pt/Au and Ti/Pt/Au.  
         [0028]     As shown in the enlarged view of  FIG. 1 , since the transparent conductive oxide layer  140  does not require a thermal process for increasing the ohm contact efficiency, the interface between the transparent conductive oxide layer  140  and the SLS contact layer  135  is smooth. A desirable reflecting effect is thereby achieved. Moreover, according to the theory of light interference, when the light emitting wavelength of the LED device is λ, and the refractive index of the transparent conductive oxide layer  140  is n, the thickness of the transparent conductive oxide layer  140  is preferably to be (2 m+1)λ/2n (m is 0 or an positive integer such as 1, 2, 3, etc.). Thus, the reflecting light from the interface between the transparent conductive oxide layer  140  and the reflecting layer  150 , and the reflecting light from the interface of the SLS contact layer  135  and the transparent conductive oxide layer  140  can generate a constructive interference effect.  
         [0029]      FIG. 2  is a cross-sectional view illustrating another structure of a LED device. Referring to  FIG. 2 , the LED device includes a device substrate  200 , a N-type doped layer  210 , a light emitting layer  220 , a P-type doped layer  230 , a strained-layer superlattice (SLS) contact layer  235 , a transparent conductive oxide layer  240 , a transparent insulating passivation layer  245 , a reflecting layer  250 , and an anode  260  and a cathode  270 . In  FIG. 2 , an active layer constructed by a N-type doped layer  210 , a light emitting layer  220  and a P-type doped layer  230  is formed, for example but not limited to, by performing a series of epitaxy processes sequentially on the device substrate  200 . Moreover, in the succeeding process, a portion of the N-type doped layer  210 , a portion of the light emitting layer  220 , a portion of the P-type doped layer  230  and a SLS contact layer  235  are removed, for example but not limited to, by etching or by another method. Therefore, each of the layers  210 ,  220 ,  230  and  235  are patterned to form a plurality of isolated island structure (MESA). It is noticed that, in the isolated island structure above, a portion of the P-type doped layer  230  and SLS contact layer  235  over the cathode  270 , the light emitting layer  220  and a portion of the N-type doped layer  210  are removed. The cathode  270  thus can be electrically connected with the N-type doped layer  210 .  
         [0030]     Referring to  FIG. 2 , in the present embodiment, the transparent conductive oxide layer  240  is deposited on the SLS contact layer  235 , and the transparent insulating passivation layer  245  is deposited on the transparent conductive oxide layer  240  while the reflecting layer  250  is deposited on the transparent insulating passivation layer  245  and the anode  260  is deposited on the reflecting layer  250 .  
         [0031]     The device substrate  200  includes, for example but not limited to, a sapphire substrate. The materials of the N-type doped layer  210 , light emitting layer  220 , the P-type doped layer  230 , and SLS contact layer  235  are comprised of a III-V group compound of semiconductor material, including but not limited to, a gallium nitride (GaN), a gallium phosphide (GaP) or a gallium phosphide arsenide (GaAsP). The light emitting layer  220  includes, for example but not limited to, a single or a multi quantum well structure, to enhance the light emitting efficiency. A material of the transparent conductive oxide layer  140  preferably includes an indium tin oxide (ITO), but also may include, for example but not limited to, such as ITO, CTO, IZO, 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 , and so on. or other transparent conductive material having similar properties. A material of the transparent insulating passivation layer  245  includes, for example but not limited to, a SiO 2 , SiN x , Al 2 O 3 , AlN, BeO, ZnO, and so on. A material of the reflecting layer  250  includes, for example but not limited to, an aluminum (Al), a silver (Ag), Ni/Ag, Ni/Al, Mo/Ag, Mo/Al, Ti/Ag, Ti/Al, Nd/Al, Nd/Ag, Pd/Al, Pd/Ag, Cr/Al, Cr/Ag and materials of the anode  260  and the cathode  270  include, for example but not limited to, a bi-layer or tri-layer metal system, such as Cr/Au, Ti/Au, Cr/Pt/Au and Ti/Pt/Au.  
         [0032]     As shown in the enlarged view of  FIG. 2 , since the transparent conductive oxide layer  240  does not require a thermal process for increasing the ohm contact efficiency, the interface between the transparent conductive oxide layer  240  and the SLS contact layer  235  is smooth. A desirable reflecting effect is thereby achieved. Moreover, according to the theory of light interference, when the light emitting wavelength of the LED device is λ, and the refractive index of the transparent conductive oxide layer  140  is n, the thickness of the transparent conductive oxide layer  240  is preferably to be (2 m+1)λ/2n (m is 0 or an positive integer such as 1, 2, 3, etc.). Moreover, according to the theory of light interference, when the light emitting wavelength of the LED device is λ, and the refractive index of the transparent insulating passivation layer  245  is k, the thickness of the transparent insulating passivation layer  245  is preferably to be (2 m+1)λ/2k (m is 0 or an positive integer such as 1, 2, 3, etc.). Thus, the reflecting light from the interface between the transparent insulating passivation layer  245  and the reflecting layer  250 , and the reflecting light from the interface of the SLS contact layer  235  and the transparent conductive oxide layer  240  can generate a constructive interference effect.  
         [0033]      FIG. 3  is a cross-sectional view illustrating a flip-chip LED package structure obtained after the flip-chip packaging of the LED device of  FIG. 1  and  FIG. 2 . Referring to  FIG. 3 , the LED device of  FIG. 1  or  FIG. 2  is flipped over a package substrate  300 , the package substrate  300  includes, for example but not limited to, a silicon substrate. The LED device of  FIG. 1  and the package substrate  300  are electrically connected via a bump  380  and a bump  390 . The bump  380  is electrically connected with the anode  160  and the package substrate  300 , and the bump  390  is electrically connected with the cathode  170  and the package substrate  300 . Since the reflecting layer  150  is between the top layer of the  FIG. 1  and the package substrate  300 , and faces to the package substrate  200 . Thus, the light emitted from the light emitting layer  120  is reflected by the multi-layer structures including the layer  135 , layer  140 , and layer  150  and emits through the device substrate  100 . Similar concept is also suitable for a device consisting of a transparent insulating passivation layer, as shown in  FIG. 2 .  
         [0034]     Moreover, the device structure of the embodiments described above, for example, a LED device having a flip-chip package structure, is only an example for describing the present invention. The scope of the invention is not limited to the above embodiments. Moreover, the present invention can also be provided for all of the LED devices that are formed with an ohmic contact layer and a reflecting layer and are packaged by a process other than the flip-chip package process for increasing the light reflecting efficiency. In addition, although the present invention is described with a N-type doped layer being formed on the device substrate, and a P-type doped layer being formed on the light emitting layer and, the present invention is also applicable with the conductive type of the doped layers being exchanged. That is, a P-type doped layer is formed on the device substrate, and a N-type doped layer is formed on the light emitting layer. Therefore, the electrode formed on the reflecting layer is served as a cathode, and the electrode formed on the P-type doped layer is served as an anode.  
         [0035]     In accordance to the present invention, the material of the ohmic contact layer includes a transparent conductive metal oxide, wherein a thermal process for increasing the ohmic contact efficiency is not required for the transparent conductive metal oxide. Therefore, the interface between the ohmic contact layer and the SLS contact layer is smooth, and thus the interface can be provided as a reflecting surface. Moreover, in the present invention, the absorption to visible light of the transparent conductive metal oxide can be reduced to less than 10% (for example, when the oxide is a indium tin oxide (ITO); therefore, the absorption of the ohmic contact layer to the LED device is reduced drastically.  
         [0036]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.