Patent Publication Number: US-2009224226-A1

Title: Light emitting device of group iii nitride based semiconductor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light emitting device of Group III nitride based semiconductor, and relates more particularly to a light emitting device of Group III nitride based semiconductor, the active layer of which has increased lumen output and high optical efficiency. 
     2. Description of the Related Art 
     With wide application of light emitting diode (LED) devices in different products, semiconductor materials used for fabricating blue light LEDs are becoming the focus of much research in the optoelectronic industry. At present, semiconductor materials such as zinc selenide (ZnSe), silicon carbide (SiC), and indium gallium nitride (InGaN) are preferred for blue light LEDs, and these semiconductor materials have wide band gaps of above 2.6 eV. Because gallium nitride is a direct gap semiconductor, it can have high luminous flux, and compared to zinc selenide, which is also a direct gap semiconductor, the GaN LED can last longer. 
       FIG. 1A  shows a light-emitting apparatus, disclosed in U.S. Pat. No. 7,067,838.  FIG. 1B  is an illustrative diagram of the magnitudes of band gaps of the light-emitting apparatus of  FIG. 1A . The light-emitting apparatus  10  comprises a sapphire substrate  11 , a buffer layer  19 , an N-type contact layer  12 , an N-type cladding layer  13 , an active layer  15 , a P-type block layer  16 , a P-type cladding layer  17  and a P-type contact layer  18 , wherein the active layer  15  comprises an N-type first barrier layer  153 , a plurality of N-type InGaN well layers  151 , and a plurality of N-type second barrier layers  152 . More specifically, when the band gap energy of the P-type block layer  16  is Egb, the band gap energy of the N-type second barrier layer  152  is Eg 2 , the band gap energy of the N-type first barrier layer  153  is Eg  1 , and the band gap energy of the N-type cladding layer  13  and the P-type cladding layer  17  is Egc, the relationship Egb&gt;Eg 2 &gt;Eg 1 &gt;Egc must be satisfied, as shown in  FIG. 1B . Due to the confinement of carriers from a P-type semiconductor layer by the P-type block layer  16  and the confinement of carriers from an N-type semiconductor layer by the N-type first barrier layer  153 , the electrons and carriers are confined in the active layer  15  and the recombination of electrons and holes in the active layer  15  can be facilitated. However, the structure is complex, and increases the difficulty of mass production. 
       FIG. 2A  is a schematic diagram of an active region of a light emitting diode, disclosed in U.S. Pat. No. 6,955,933.  FIG. 2B  is a simulated band structure for the light emitting diode of  FIG. 2A . The active region  20  comprises quantum well layers ( 12 ,  23 , and  25 ) and barrier layers ( 22 ,  24 , and  26 ). The quantum well layers ( 12 ,  23 , and  25 ) and the barrier layers ( 22 ,  24 , and  26 ) are formed from a III-Nitride semiconductor alloy of Al x In y Ga 1−x−y N where 0≦x&lt;1, 0≦y&lt;1, x+y≦1. Specifically, the compositions of the quantum well layers ( 12 ,  23 , and  25 ) and the barrier layers ( 22 ,  24 , and  26 ) are graded (gradually increasing or gradually decreasing) in a direction substantially perpendicular to the surface of the N-type semiconductor layer of the light emitting diode. Due to the gradation of the composition of the layers, each layer has a graded band gap, as shown in  FIG. 2B . However, this type of the structure will lower the total energy of the band gap of the active region  20  and results in variations of wavelength emitted. 
       FIG. 3  is a band structure for an active layer, disclosed in U.S. Pat. No. 6,936,838. The active layer comprises an N-type semiconductor layer  31 , a barrier layer  32 , a quantum well layer  33 , and a P-type semiconductor layer  34 . The barrier layer  32  comprises an internal layer portion doped with N-type impurities  321  and an anti-diffusion film  332 . Specifically, the band gap of the barrier layer  32  is greater than that of the quantum well layer  33 . The anti-diffusion film  332  prevents N-type impurities from being diffused into the quantum well layer  33 , so that it may achieve an improvement in optical power of the quantum well layer  33 . The band structure for the active layer is similar to conventional multiple quantum well structures, but an anti-diffusion film  332  is added between the barrier layer  32  and the quantum well layer  33 . 
       FIG. 4  is a band structure for an active layer, disclosed in U.S. Pat. No. 7,106,090. The active layer comprises at least one quantum well layer  42  and two barrier layers  41  and  43  sandwiching the quantum well layer  42 . The quantum well layer  42  having a step-like energy band gap profile includes four single layers  421 - 424 . The indium content gradually increases step by step from one layer to the next layer  421 ,  422 ,  423 ,  424 , and finally the last single layer  424  has the highest indium content. Compared to conventional quantum well layer with uniform energy band gap profile, the quantum well layer with a step-like energy band gap profile or with graded energy band gap profile will reduce the total band gap energy so as to change the wavelength and other characteristics of emitting light. (See FIG. 4 of U.S. Pat. No. 7,106,090.) 
     Therefore, a light emitting diode with none of the above-mentioned issues that can guarantee the quality and increase the power of the emitting light from the active layer thereof is required by the market. 
     SUMMARY OF THE INVENTION 
     The primary aspect of the present invention is to provide a light emitting device of Group III nitride based semiconductor, which includes a stress relieving layer disposed between the quantum well layer and the barrier layer such that the lattice mismatch stress in the active layer can be relieved, and the optical efficiency can be increased. 
     In view of the above aspect, the present invention proposes a light emitting device of Group III nitride based semiconductor, which comprises a substrate, an N-type semiconductor layer formed on the substrate, an active layer formed on the N-type semiconductor layer, and a P-type semiconductor layer formed on the quantum well layer. The active layer comprises at least one quantum well layer, at least two barrier layers formed to sandwich the quantum well layer therebetween and at least one stress relieving layer, wherein the stress relieving layer is interposed between the quantum well layer and one of the at least two barrier layers, and the composition of the stress relieving layer, made of Group III nitride based material, is a graded distribution along the direction from the quantum well layer to the barrier layers adjacent thereto. 
     According to one embodiment, the Group III nitride based material of the stress relieving layer is represented by the formula Al x In y Ga 1−x−y N, wherein 0≦x&lt;1, 0≦y&lt;1 and x+y≦1, wherein the composition ratio among components, Al (aluminum), Ga (gallium), and In (indium), is graded along the direction from the quantum well layer to the barrier layers adjacent thereto. 
     According to one embodiment, the grading distribution is monotonic increase, which can be linearly graded or non-linearly curvature graded. 
     According to one embodiment, the grading distribution is equally stepwise graded or is unequally stepwise graded. 
     According to one embodiment, the stress relieving layer comprises a multiple layer structure, and each layer is made of a Group III nitride based material with different composition ratio. The stress relieving layer is a Group III nitride based semiconductor layer doped with N-type impurities or is an undoped Group III nitride based semiconductor layer. 
     According to one embodiment, the light emitting device of Group III nitride based semiconductor further comprises a buffer layer disposed between the substrate and the N-type semiconductor layer, and also further comprises a current block layer disposed between the active layer and the P-type semiconductor layer. 
     According to one embodiment, the active layer includes a single quantum well layer or multiple quantum well layers. 
     According to another embodiment, the present invention proposes a light emitting device of Group III nitride based semiconductor, which comprises a substrate, an N-type semiconductor layer formed on the substrate, an active layer, and a P-type semiconductor layer. The active layer comprises at least one quantum well layer, at least two barrier layers formed to sandwich the quantum well layer, and at least two stress relieving layers, wherein stress relieving layers are separately interposed between the quantum well layer and the at least two barrier layers, and each stress relieving layer has a greater band gap energy than that of the quantum well layer and has a smaller band gap energy than that of the barrier layer adjacent thereto. Each stress relieving layer has a graded band gap along the direction from the quantum well layer to the barrier layers adjacent thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described according to the appended drawings in which: 
         FIG. 1A  shows a light-emitting apparatus, disclosed in U.S. Pat. No. 7,067,838; 
         FIG. 1B  is an illustrative diagram of the magnitudes of band gaps of the light-emitting apparatus of  FIG. 1A ; 
         FIG. 2A  is a schematic diagram of an active region of a light emitting diode, disclosed in U.S. Pat. No. 6,955,933; 
         FIG. 2B  is a simulated band structure for the light emitting diode of  FIG. 2A ; 
         FIG. 3  is a band structure for an active layer, disclosed in U.S. Pat. No. 6,936,838; 
         FIG. 4  is a band structure for an active layer, disclosed in U.S. Pat. No. 7,106,090; 
         FIG. 5  is a schematic diagram of a light emitting diode device of Group III nitride based semiconductor according to the first embodiment of the present invention; 
         FIG. 6A  is an illustrative diagram of the magnitudes of band gaps of the active layer with a single quantum well layer according to one embodiment of the present invention; 
         FIG. 6B  is an illustrative diagram of a prior art active layer with a single quantum well layer; 
         FIG. 7A  is an illustrative diagram of the magnitudes of band gaps of the active layer with a single quantum well layer according to another embodiment of the present invention; 
         FIG. 7B  is an illustrative diagram of a prior art active layer with a single quantum well layer; 
         FIGS. 8 to 11  are illustrative diagrams of the magnitudes of band gaps of the active layers each having a single quantum well layer according to other embodiments of the present invention; 
         FIG. 12  is a schematic diagram of a light emitting device of Group III nitride based semiconductor according to the second embodiment of the present invention; 
         FIG. 13A  and  FIG. 13B  are illustrative diagrams of the magnitudes of band gaps of the active layer with multiple quantum well layers according to another embodiment of the present invention; 
         FIG. 14  shows a comparison graph of the output intensities of a light emitting device of Group III nitride based semiconductor according to one embodiment of the present invention and of a prior art device; and 
         FIG. 15  is a schematic diagram of a light emitting device of Group III nitride based semiconductor according to the third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 5  is a schematic diagram of a light emitting diode device of Group III nitride based semiconductor according to the first embodiment of the present invention. The light emitting diode device of Group III nitride based semiconductor  50  comprises a substrate  51 , a buffer layer  52 , an N-type semiconductor layer  53 , an active layer  54 , a current block layer  57  and a P-type semiconductor layer  58 . The active layer  54  comprises at least one quantum well layer  56 , a first barrier layer  541  and a second barrier layer  542 . The first barrier layer  541  and the second barrier layer  542  are formed to sandwich the quantum well layer  56  therebetween. In addition, the active layer  54  further comprises a first stress relieving layer  551  and a second stress relieving layer  552 . The first stress relieving layer  551  is disposed between the first barrier layer  541  and the quantum well layer  56 , and the second stress relieving layer  552  is disposed between the second barrier layer  542  and the quantum well layer  56 . The N-type semiconductor layer  53  further comprises an N-type electrode layer  592 , and the P-type semiconductor layer  58  further comprises a P-type electrode layer  591 . 
     The stress relieving layers  551  and  552  are made of Group III nitride based material, and the compositions of the stress relieving layers  551  and  552  are graded along the direction from the quantum well layer  56  to the barrier layers  541  or  542  adjacent to the quantum well layer  56 . The stress relieving layer  551  or  552  can be a Group III nitride based semiconductor layer doped with N-type impurities or can be an undoped Group III nitride based semiconductor layer. The Group III nitride based semiconductor material can be, for example, a material represented by the formula Al x In y Ga 1−x−y N, wherein 0≦x&lt;1, 0≦y&lt;1 and x+y≦1, and the composition ratio among components, Al (aluminum), Ga (gallium), and In (indium), is graded in a thickness-wise direction. Alternatively, the thickness of the stress relieving layer  551  or  552  is greater than the thickness of the quantum well layer  56 , but less than the thickness of the barrier layer  541  or  542 . Moreover, the stress relieving layer  551  or  552  may comprise a multiple layer structure, and each layer is made of a Group III nitride based material with different composition ratio. 
       FIG. 6A  is an illustrative diagram of the magnitudes of band gaps of the active layer with a single quantum well layer according to one embodiment of the present invention. Referring to  FIG. 6A , the upper is the conduction band variation profile, Ec, of the active layer  54 , and the lower is the valence band variation profile, Ev, of the active layer  54 , the energy difference between the Ec and the Ev is the band gap energy, Eg. The band gap energy of the stress relieving layer  551  is greater than that of the quantum well layer  56 , and the band gap of the stress relieving layer  551  is smaller than that of the adjacent first barrier layer  541 . The stress relieving layer  551  has a graded band gap along the direction from the quantum well layer  56  to the first barrier layer  541 . In the present invention, the first stress relieving layer  551  has a monotonically linearly increasing band gap toward the first barrier layer  541 . 
     The active layer  54  has band gap energy, Eg 1 , which is equal to the sum of the conduction band difference ΔEc 1  and valence band difference ΔEv 1 , and namely, Eg 1 =ΔEc 1 +ΔEv 1 . As shown in  FIG. 6B , compared to prior art active layers, it can be found that ΔEc 1 &gt;ΔEc 2  and ΔEv 1 &gt;ΔEv 2 . Therefore, the active layer  54  of the present invention has a greater conduction band difference than a prior art active layer, and namely, Eg 1 &lt;Eg 2 , and consequently, the active layer  54  can emit light of longer wavelength, which is something the above-mentioned prior art active layers cannot achieve. 
       FIG. 7A  is an illustrative diagram of the magnitudes of band gaps of the active layer with a single quantum well layer according to another embodiment of the present invention. The first stress relieving layer  551  has a monotonically linearly increasing band gap toward the first barrier layer  541 ; however, the band gap becomes discontinuous and smaller at the interface between the quantum well layer  56  and the adjacent stress relieving layer  551 . As shown in  FIG. 7B , compared to prior art active layers, it can be found that ΔEc 1 =ΔEc 2  and ΔEv 1 =ΔEv 2 . Thus, the band gap energy of the active layer  54  of the present invention is equal to the band gap energy of prior art active layers, namely, Eg 1 =Eg 2 , and consequently, the active layer  54  can emit light having the same wavelength, and prior art active layers can only emit light of shorter wavelength. 
     In consideration of the possibility of the non-linear growth of an epitaxial film, the stress relieving layer  551  shown in  FIG. 7A  has a monotonically increasing band gap toward the first barrier layer  541  such that the active layer  551  in  FIG. 7A  can have similar light emitting characteristics to the active layer  551  of  FIG. 6A . 
     Compared to  FIG. 7A , the band gap profiles of the first stress relieving layer  551  and the second stress relieving layer  552  in the embodiments of  FIG. 8  and  FIG. 9  are non-linear profiles different from the linear profile shown in  FIG. 7A ; however, the active layer  54  having a non-linear profile can have the similar light emitting characteristics to the active layer  54  having the profile shown in  FIG. 7A . 
     Compared to  FIG. 6A , the band gap profiles of the first stress relieving layer  551  and the second stress relieving layer  552  in  FIG. 10  are stepwise increasing, which are different from the monotonic increasing band gap shown in the above-mentioned embodiments. However, the active layer  54  having an increasing stepwise profile can have light emitting characteristics similar to those of the active layer  54  having the profile shown in  FIG. 6A . In the present embodiment, the first stress relieving layer  551  and the second stress relieving layer  552  can be a multiple layer structure, and each layer is made of a Group III nitride based material with different composition ratio. 
     Similarly, the band gap profiles of the first stress relieving layer  551  and the second stress relieving layer  552  in  FIG. 11  are stepwise increasing. The only difference between the profile of  FIG. 10  and the profile of  FIG. 11  is that the profile of  FIG. 10  is an equally stepwise graded profile, and the profile of  FIG. 11  is not. However, the active layer  54  having an unequally stepwise graded profile still can have light emitting characteristics similar to those of the active layer  54  having the profile shown in  FIG. 7A . 
       FIG. 12  is a schematic diagram of a light emitting device of Group III nitride based semiconductor according to the second embodiment of the present invention. Compared to  FIG. 5 , the light emitting device of Group III nitride based semiconductor  120  has a structure including a plurality of quantum well layers. The active layer  54 ′ comprises three quantum well layers  56 , and each quantum well layer  56  is sandwiched by a first stress relieving layer  551  and a second stress relieving layer  552 . The first barrier layer  541  and the second barrier layer  542  are separately disposed outside of the first stress relieving layer  551  and the second stress relieving layer  552  such that the first stress relieving layer  551  and the second stress relieving layer  552  are sandwiched therebetween. The multiple quantum well layer structure can include different stacked layers of embodiments, for example, from 2 stacked layers to 30 stacked layers (in the present embodiment, the number of staked layers is 3). However, the structures having 6 to 18 stacked layers are preferred. 
       FIG. 13A  and  FIG. 13B  are illustrative diagrams of the magnitudes of band gaps of the active layer with multiple quantum well layers according to another embodiment of the present invention. The structures of  FIG. 13A  and  FIG. 13B  are similar to the above-mentioned structure with a single quantum well layer, and the difference is that in the present embodiment, three quantum well layers are serially connected, and the detailed description of the present embodiment can refer to the description of the embodiments of  FIG. 6A  and  FIG. 7A . 
       FIG. 14  shows curves of the output power of a light emitting device of Group III nitride based semiconductor according to one embodiment of the present invention and of a prior art device. Compared to the prior art light-emitting device, the light-emitting device of Group III nitride based semiconductor of the present invention can attain higher luminous intensity when the same current density is applied thereto. As a result, the light-emitting device of Group III nitride based semiconductor of the present invention has better optical efficiency. 
       FIG. 15  is a schematic diagram of a light emitting device of Group III nitride based semiconductor according to the third embodiment of the present invention. The light emitting device of Group III nitride based semiconductor  150  comprises a substrate  51 , a buffer layer  52 , an N-type semiconductor layer  53 , an active layer  54 ″, a current block layer  57 , and a P-type semiconductor layer  58 . The active layer  54 ″ comprises at least one quantum well layer  56  and the first barrier layer  541  and the second barrier layer  542  formed to sandwich the quantum well layer  56  therebetween. In addition, the active layer  54 ″, moreover, comprises a stress relieving layer  551 ′, and the stress relieving layer  551 ′ is disposed between the first barrier layer  541  and the quantum well layer  56 , or is disposed between the second barrier layer  541  and the quantum well layer  56 . The N-type semiconductor layer  53  further comprises an N-type electrode layer  592 , and the P-type semiconductor layer  58  further comprises a P-type electrode layer  591 . 
     The difference between the present embodiment from the embodiment of  FIG. 5  is that one stress relieving layer is formed between the quantum well layer and the barrier layer adjacent to the quantum well layer rather than two stress relieving layers separately formed between the quantum well layer and the barrier layers. However, the embodiment of  FIG. 5 , in which two stress relieving layers are disposed separately on both sides of the quantum well layer and are respectively sandwiched by the quantum well layer and the corresponding barrier layer, is preferred. Moreover, persons skilled in the art will understand from the above-mentioned embodiments that there can be one, two or more than two stress relieving layers, and the stress relieving layer(s) can be disposed on both sides or one side of the quantum well layer. 
     The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by persons skilled in the art without departing from the scope of the following claims.