Patent Publication Number: US-2023134581-A1

Title: Light-emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This disclosure claims the right of priority of TW Application No. 110140862 filed on Nov. 3, 2021, and the content of which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a light-emitting device, and in particular to a light-emitting device comprising an active structure of quantum well layers and barrier layers which are composed of a nitride semiconductor material. 
     Description of the Related Art 
     Light-emitting diode (LED) is a solid-state semiconductor light-emitting device, which has the advantages of low power consumption, low heat generation, long lifetime, shockproof, small size, high response speed and good optical-electrical characteristics, such as stable emission wavelength. Therefore, light-emitting diodes have been widely applied in household appliances, equipment indicator lights, and optoelectronic products, and so forth. 
     SUMMARY 
     A light-emitting device comprises a first nitride semiconductor structure; a stress relief structure on the first nitride semiconductor structure comprising narrow band gap layers and wide band gap layers alternately stacked, wherein one of the wide band gap layers comprises wide band gap sub-layers and one of the wide band gap sub-layers comprises aluminum; an active structure on the stress relief structure comprising quantum well layers and barrier layers alternately stacked, wherein one of the barrier layers comprises barrier sub-layers and one of the barrier sub-layers comprises aluminum, an aluminum composition of the wide band gap sub-layer is greater than or equal to that of the barrier sub-layer, and an average aluminum composition of the wide band gap layer is greater than that of the barrier layer; and an electron blocking structure on the active structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows a cross-sectional view of a light-emitting device  1  in accordance with an embodiment of the present disclosure. 
         FIG.  2    shows a transmission electron microscope (TEM) image of a portion of the light-emitting device  1  in accordance with an embodiment of the present disclosure. 
         FIG.  3    shows a schematic structural view of the periodic structure  10 . 
         FIG.  4    shows a schematic structural view of the stress relief structure  12 . 
         FIG.  5    shows a schematic structural view of the active structure  14 . 
         FIG.  6    shows a diagram of the secondary ion mass spectrometer (SIMS) of the light-emitting device  1  in accordance with an embodiment of the present disclosure. 
         FIG.  7    shows a diagram of the energy dispersive X-ray spectrometer (EDX) of the light-emitting device  1  in accordance with an embodiment of the present disclosure. 
         FIG.  8    shows a schematic view of a light emitting apparatus  2  in accordance with an embodiment of the present disclosure. 
         FIG.  9    shows a schematic view of a light emitting apparatus  3  in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In order to make the description of the present disclosure more detailed and complete, please refer to the description of the following embodiments with relevant figures. The embodiments shown below are for exemplifying the light-emitting device of the present disclosure, and the present disclosure is not limited to the following embodiments. In addition, the scope of the present disclosure is not limited thereto in the case that the dimensions, materials, shapes, relative arrangements, and so forth, of the constituent parts described in the embodiments of the present disclosure are not limited, which are merely for illustration. Furthermore, the sizes or positional relationships, and so forth, of the components shown in each of the figures may be enlarged for the sake of clarity. In addition, other layers/structures or steps may be incorporated in the following embodiments. For example, a description of “forming a second layer/structure on a first layer/structure” may comprise an embodiment which the first layer/structure directly contacts the second layer/structure, or an embodiment which the first layer/structure indirectly contacts the second layer/structure, namely other layers/structures exist between the first layer/structure and the second layer/structure. In addition, the spatial relative relationship between the first layer/structure and the second layer/structure may be varied depending on the operation or use of the apparatus, the first layer/structure itself is not limited to a single layer or a single structure, the first layer may comprise sub-layers, and the first structure may comprise layers. Furthermore, in the following description, in order to appropriately omit the detailed description, identical names and designations are used for the same or similar components. 
     Before describing the embodiments of the present disclosure, the following contents need to be described in advance. Firstly, in the present disclosure, Al x In y Ga (1-x-y) N represents that the chemical composition ratio of III group elements (the sum of Al, Ga and In) to N is 1:1, and Al, In and Ga of III group elements may be an arbitrary compound with a non-fixed composition ratio. Al x Ga (1-x) N represents that the chemical composition ratio of III group elements (the sum of Al and Ga) to N is 1:1, and Al and Ga of III group elements may be an arbitrary compound with a non-fixed composition ratio. In addition, if MN (or GaN) is referred to, it means that Ga (or Al) is not included in MN (or GaN), respectively. It is noted that compositions of Al, In or Ga in Al x In y Ga (1-x-y) N may be determined by known quantitative analysis, such as energy dispersive X-ray spectrometer (EDX) or X-ray diffractometer (XRD). In the present disclosure, Al x In y Ga (1-x-y) N as an example, the sum of Al, Ga, and In is 1. When a composition of Al is x, a composition of In is y, and a composition of Ga is (1-x-y). 
     In addition, in the present disclosure, a layer which electrically presents p-type characteristic is referred as a p-type layer, and a layer which electrically presents n-type characteristic is referred as an n-type layer. On the other hand, in the case that specific impurities such as magnesium (Mg), silicon (Si), and so forth, are not intentionally added to a layer and the layer does not electrically present a p-type or an n-type characteristic, the layer is referred as “i-type” or “undoped”. The undoped layer may be mixed with unavoidable impurities during the manufacturing process. Specifically speaking, when the doping concentration is less than 1×10 17 /cm 3 , it is referred as “undoped” in the present disclosure. In addition, values of concentration of impurities such as magnesium (Mg) and silicon (Si), and so forth, are obtained by the analysis of secondary ion mass spectrometer (SIMS). 
       FIG.  1    is a cross-sectional view of a light-emitting device  1  in accordance with an embodiment of the present disclosure. The light-emitting device  1  comprises a substrate  4 , a buffer structure  5 , a base layer  7 , an n-type nitride semiconductor structure  8 , a periodic structure  10 , a stress relief structure  12 , an active structure  14 , an electron blocking structure  16 , a p-type nitride semiconductor structure  17  and a contact layer  18  sequentially stacked on an upper surface  40 S of the substrate  4 . 
     The light-emitting device  1  comprises a mesa  40 , a portion of the n-type nitride semiconductor structure  8  is exposed outside the mesa  40 , and an n-type electrode  21  is formed on the exposed portion. A p-type electrode  25  is formed on the p-type nitride semiconductor structure  17  and the contact layer  18 . The transparent conductive layer  23  is formed between the p-type electrode  25  and the contact layer  18 . 
     The substrate  4  has a thickness which is thick enough to support layers and structures thereon, such as not less than 30 μm, or not more than 300 μm. The substrate  4  comprises a sapphire (Al 2 O 3 ) wafer, a gallium nitride (GaN) wafer, a silicon carbide (SiC) wafer or an aluminum nitride (AlN) wafer for the epitaxial growth of gallium nitride (GaN), indium gallium nitride (InGaN) or aluminum gallium nitride (AlGaN). The upper surface  40 S in contact with the buffer structure  5  may be a roughened surface. The roughened surface may be a surface with an irregular morphology or a surface with a regular morphology. As shown in  FIG.  1   , the substrate  4  comprises one or more protrusions  41  protruding from the upper surface  40 S, or comprises one or more recesses (not shown) recessed on the upper surface  40 S. In a cross-sectional view, the protrusions  41  or the concave portions (not shown) may be in the shape of a hemisphere or a polygonal cone. In one embodiment, the protrusions  41  comprise a material different from that of the substrate  4 , such as an insulating material or a conductive material. The semiconductor material comprises a compound semiconductor material, such as III-V group semiconductor materials, II-VI group semiconductor materials or silicon carbide (SiC). The insulating material comprises an oxide, a nitride, or an oxynitride. The oxide comprises silicon oxide, zinc oxide, aluminum oxide or titanium oxide. The nitride comprises silicon nitride, aluminum nitride or titanium nitride. The oxynitride comprises aluminum oxynitride. The conductive material comprises indium tin oxide. The protrusions  41  may be selected from a material which the refractive index thereof is between that of the substrate  4  and those of the semiconductor layers and the structures thereon to improve the light extraction efficiency of the light-emitting device  1 . In other embodiments, the upper surface  40 S in contact with the buffer structure  5  is a flat surface. 
     In one embodiment of the present disclosure, the buffer structure  5 , the base layer  7 , the n-type nitride semiconductor structure  8 , the periodic structure  10 , the stress relief structure  12 , the active structure  14 , the electron blocking structure  16 , the p-type nitride semiconductor structure  17  and/or the contact layer  18  are formed on the substrate  4  with metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), hydride vapor deposition (HVPE), physical vapor deposition (PVD) or ion plating method, wherein the physical vapor deposition comprises sputtering or evaporation. 
     The buffer structure  5  is for reducing defects and improving the quality of the epitaxial layer grown thereon. The buffer structure  5  comprises a single layer or multiple layers (not shown). When the buffer structure  5  comprises multiple layers (not shown), the multiple layers comprise an identical material or different materials. In one embodiment, the buffer structure  5  comprises a first layer and a second layer, wherein the growth method of the first layer is sputtering, and the growth method of the second layer is metal organic chemical vapor deposition (MOCVD). In one embodiment, the buffer structure  5  further comprises a third layer, wherein the growth method of the third layer is metal organic chemical vapor deposition (MOCVD), and the growth temperature of the second layer is higher than or lower than the growth temperature of the third layer. In one embodiment, the first layer, the second layer and the third layer comprise identical materials such as aluminum nitride (AlN), or comprise different materials, such as an arbitrary combination of aluminum nitride (AlN), gallium nitride (GaN) and aluminum gallium nitride (AlGaN). In other embodiments, the buffer structure  5  comprises PVD-aluminum nitride (PVD-AlN). The target material for forming PVD-AlN is composed of MN, or the target material composed of Al is used and is reactively formed aluminum nitride (AlN) in the environment of a nitrogen source. 
     In one embodiment, the buffer structure  5  may be undoped, namely unintentionally doped. In another embodiment, the buffer structure  5  may comprise a dopant, such as carbon (C), hydrogen (H), oxygen (O) or an arbitrary combination thereof, and the concentration of the dopant in the buffer structure  5  is not less than 1×10 17 /cm 3 . 
     The buffer structure  5  comprises Al x 1Ga (1-x1) N (0≤x1≤1), such as an MN layer or a GaN layer. The thickness of the buffer structure  5  is not particularly limited and can be greater than or equal to 3 nm and less than or equal to 150 nm, and can be further greater than or equal to 5 nm and less than or equal to 80 nm. 
     The base layer  7  comprises Al s1 In t1 Ga (1-s1-t1) N (0≤s1≤1, 0≤t1≤1), Al s1 Ga (1-s1) N (0≤s1≤1), or a GaN layer. The base layer  7  can prevent crystal defects existed in the buffer structure  5  from propagating from the base layer  7  to the active structure  14 . The base layer  7  may comprise an n-type impurity or not comprise an n-type impurity. When the base layer  7  does not comprise an n-type impurity, the crystallinity of the base layer  7  can be improved. Therefore, it is suitable that the base layer  7  does not comprise an n-type impurity to reduce the defects in the base layer  7 . Furthermore, the defects in the base layer  7  can also be reduced by increasing the thickness of the base layer  7 . If the thickness of the base layer  7  is increased above a certain level (for example, greater than 8 μm), the effect of the increase in the thickness of the base layer  7  corresponding to the defect reduction is saturated. Thus, the thickness of the base layer  7  can be greater than or equal to 2 μm and less than or equal to 8 μm, and can be less than or equal to 6 μm, or less than or equal to 4 μm alternatively. In one embodiment, when the substrate  4  comprises protrusions  41  protruding from the upper surface  40 S of the substrate  4 , the total thickness of the base layer  7  can be 0.5 μm thicker than the height of the protrusions  41  to completely cover the protrusions  41  and form a flat surface. 
     The n-type nitride semiconductor structure  8  comprises Al s2 In t2 Ga (1-s2-t2) N (0≤s2≤1, 0≤t2≤1) of an n-type impurity, or Al s2 Ga (1-s2) N of an n-type impurity (0≤s2≤1, 0≤s2≤0.1, or 0.001≤s2≤0.01 alternatively). The n-type nitride semiconductor structure  8  may be a single layer or layers which are formed by growth steps. The layers may have an identical composition or different compositions, and the layers may have an identical thickness or different thicknesses. In one embodiment, the n-type nitride semiconductor structure  8  comprises an n-type contact layer (not shown) and a modulation layer (not shown) located between the n-type contact layer and the base layer  7 . A portion of the n-type contact layer is exposed outside the mesa  40 , and the n-type electrode  21  may contact the n-type contact layer. The doping concentration of the n-type impurity of the n-type contact layer is the highest among the layers of the n-type nitride semiconductor structure  8 , and is also higher than the n-type doping concentration of the base layer  7 . The n-type contact layer may comprise a first n-type contact sub-layer and a second n-type contact sub-layer which are alternately stacked 7-40 times. The thickness of the n-type contact layer can be 0.4 μm-4 μm, 0.8 μm-3 μm, or 1 μm-2 μm alternatively, wherein the thicknesses of the first n-type contact sub-layer and the second n-type contact sub-layer are respectively 10 nm to 100 nm, 20 nm to 80 nm, or 30 nm to 70 nm in other embodiments. The first n-type contact sub-layer comprises Al x Ga (1-x) N, wherein 0≤x&lt;1, 0≤x&lt;0.1, 0&lt;x&lt;0.05, 0&lt;x&lt;0.005, or x is substantially 0 alternatively. The second n-type contact sub-layer comprises Al y Ga 1-y N, wherein 0&lt;y&lt;1, 0&lt;y≤0.1, 0&lt;y≤0.05, 0&lt;y≤0.01, or 0&lt;y≤0.005 alternatively. In one embodiment, y&gt;x. For example, the material of the first n-type contact sub-layer is gallium nitride (GaN), and the second n-type contact sub-layer is aluminum gallium nitride (AlGaN). The first n-type contact sub-layer comprises a higher n-type doping concentration and the second n-type contact sub-layer has a lower n-type doping concentration, which improves the lateral current dispersion and further enhances the anti-electrostatic discharge capability and luminous efficiency of the light-emitting device  1 . The n-type doping concentration of the modulation layer is between the n-type contact layer and the base layer  7 , and/or the lattice constant of the material of the modulation layer is between that of the n-type contact layer and that of the base layer  7 , and/or the growth temperature of the modulation layer is between that of the n-type contact layer and that of the base layer  7 , which is used to modulate the differences of doping concentration, materials, and growth condition parameters between the n-type contact layer and the base layer  7 , so that the epitaxial defects of each layer above the modulation layer are reduced and the quality of the epitaxial layer can be improved. In one embodiment, the modulation layer comprises AlInGaN series materials, such as Al z1 In z2 Ga (1-z1-z2) N, wherein 0≤z2&lt;z1≤1. In another embodiment, the modulation layer comprises Al z Ga 1-z N, wherein 0≤z≤1. In another embodiment, when the n-type contact layer comprises Al y Ga 1-y N, the modulation layer comprises Al z Ga 1-z N, wherein y≤z≤0.1 or y≤z≤0.05 alternatively, wherein the thickness of the modulation layer is less than the thickness of the first n-type contact sub-layer and/or the thickness of the second n-type contact sub-layer, such as less than 10 nm. 
     The n-type impurity in the n-type nitride semiconductor structure  8  comprises silicon (Si), carbon (C) or germanium (Ge). The n-type doping concentration in the n-type nitride semiconductor structure  8  can be less than or equal to 5×10 19  cm −3 , or less than or equal to 2×10 19  cm −3 , and can be greater than or equal to 1×10 18  cm −3 , or greater than or equal to 4×10 18  cm −3  alternatively. The greater the thickness of the n-type nitride semiconductor structure  8 , the more its resistance is lowered. Accordingly, the thickness of the n-type nitride semiconductor structure  8  can be thickened. As the thickness of the n-type nitride semiconductor structure  8  is increased, the production cost is also increased. Therefore, from the standpoint of manufacturing, the thickness of the n-type nitride semiconductor structure  8  can be 1 μm-6 μm, 1.5 μm-4.5 μm, or 2 μm-3.5 μm alternatively. 
     As shown in  FIG.  1   , the periodic structure  10  is disposed between the n-type nitride semiconductor structure  8  and the active structure  14 .  FIG.  3    is a structural schematic view of the periodic structure  10 . 
     As shown in  FIG.  3   , the periodic structure  10  may comprise several periods formed by alternately stacking first semiconductor layers  10 A and second semiconductor layers  10 B. The thickness of one period is the sum of the thickness of one first semiconductor layer  10 A and the thickness of one second semiconductor layer  10 B. In one embodiment, the thickness of one period in the periodic structure  10  is greater than the thickness of one period of the active structure  14  and the thickness of one period of the stress relief structure  12  described below. The thickness of the first semiconductor layer  10 A is less than the thickness of the second semiconductor layer  10 B. Specifically speaking, the first semiconductor layer  10 A comprises a thickness between 0.5 nm and 8 nm, or between 1 nm and 3 nm alternatively, and the second semiconductor layer  10 B comprises a thickness between 10 nm and 60 nm, or between 20 nm and 50 nm in other embodiments. The first semiconductor layer  10 A comprises In t3 Ga (1-t3) N (0&lt;t3&lt;1), wherein 0.005&lt;t3&lt;0.1 or 0.01&lt;t3&lt;0.05 alternatively. The second semiconductor layer  10 B can be In t4 Ga (1-t4) N (0≤t4&lt;1, t4&lt;t3), or a GaN layer not comprising indium (In) in other embodiments. The number of periods in which the first semiconductor layers  10 A and the second semiconductor layers  10 B are alternately stacked may be, for example, 2 to 20, 3 to 15 or 4 to 10 alternatively. 
     The first semiconductor layer  10 A and/or the second semiconductor layer  10 B comprises an n-type impurity or is undoped. In one embodiment, if the first semiconductor layer  10 A and the second semiconductor layer  10 B are both undoped or the doped n-type impurity concentrations thereof are too low, the driving voltage of the light-emitting device can be increased. Therefore, at least one of the first semiconductor layer  10 A and the second semiconductor layer  10 B comprises an n-type impurity. In one embodiment, when the n-type doping concentration of the periodic structure  10  is too high, the film quality of the periodic structure  10  is deteriorated, which further affects the film quality of the active structure  14  formed on the periodic structure  10  so the luminous efficiency of the active structure  14  may also be reduced accordingly. In one embodiment, the n-type doping concentration in the periodic structure  10  is less than the n-type doping concentration in the n-type nitride semiconductor structure  8 . In one embodiment, the n-type doping concentration in the periodic structure  10  can be one-tenth of the n-type doping concentration in the n-type nitride semiconductor structure  8  or can be one-half of the n-type doping concentration of the n-type nitride semiconductor structure  8 . In one embodiment, the n-type doping concentration of the periodic structure  10  can be less than 5×10 18  cm −3  but greater than or equal to 1×10 17  cm −3 . In one embodiment, the first semiconductor layer  10 A comprising indium (In) does not comprise an n-type impurity, and the second semiconductor layer  10 B which does not comprise indium (In) comprises an n-type impurity. The first semiconductor layer  10 A of the periodic structure  10  comprises In t3 Ga (1-t3) N (0&lt;t3&lt;1) not doped with an n-type impurity and the second semiconductor layer  10 B comprises GaN doped with an n-type impurity. When the periodic structure  10  comprises indium (In), the composition of indium (In) in the periodic structure  10  can be higher than that in the n-type nitride semiconductor structure  8  and lower than those in the stress relief structure  12  and the active structure  14  described below, so that the epitaxial lattice relaxes smoothly from the n-type nitride semiconductor structure  8  to the active structure  14 . 
     Although  FIG.  3    illustrates the second semiconductor layer  10 B as the lowermost layer and the first semiconductor layer  10 A as the uppermost layer, the lowermost layer of the periodic structure  10  may also be the first semiconductor layer  10 A and the uppermost layer is the second semiconductor layer  10 B. 
     As shown in  FIG.  1   , the stress relief structure  12  is disposed between the active structure  14  and the n-type nitride semiconductor structure  8 .  FIG.  4    is a structural schematic view of the stress relief structure  12 .  FIG.  5    is a structural schematic view of the active structure  14 . 
     The lattice mismatch between InGaN quantum well layer and GaN barrier layer of the active structure  14  can affect the epitaxial quality of the active structure  14 , so the stress relief structure  12  is grown before the active structure  14  to reduce lattice defects. Since the mobility of electrons is much faster than that of electron holes, electrons are uniformly distributed in the active structure  14 . The distribution of electron holes gradually decreases from a side close to the p-type nitride semiconductor structure  17  to another side away from the p-type nitride semiconductor structure  17 , causing that a portion of electrons cannot be radiatively recombined with electron holes to emit light so the luminous efficiency of the LED is reduced accordingly. In the present disclosure, the radiative recombination efficiency of electron holes and electrons in the active structure  14  is increased by adjusting the band gaps of the stress relief structure  12 , the active structure  14  and the electron blocking structure  16 . 
     As shown in  FIG.  4   , the stress relief structure  12  comprises several periods formed by alternately stacking narrow band gap layers  12 A and wide band gap layers  12 B. The thickness of one period is the sum of the thickness of one narrow band gap layer  12 A and the thickness of one wide band gap layer  12 B, which is less than the thickness of one period of the active structure  14  described below and less than the thickness of one period of the periodic structure  10  described above. The thickness of the narrow band gap layer  12 A is less than that of the wide band gap layer  12 B. Specifically speaking, the narrow band gap layer  12 A comprises a thickness between 1 nm and 3 nm, and the wide band gap layer  12 B comprises a thickness between 4 nm and 12 nm, or between 6 nm and 10 nm alternatively. The band gap of the wide band gap layer  12 B is greater than that of the narrow band gap layer  12 A. Specifically speaking, the narrow band gap layer  12 A comprises Al s5 In t5 Ga (1-s5-t5) N (0≤s5&lt;1, 0&lt;t5&lt;1) or In t5 Ga (1-t5) N (0&lt;t5&lt;1, or 0&lt;t5≤0.1 alternatively). The wide band gap layer  12 B comprises Al s6 In t6 Ga (1-s6-t6) N (0≤s6&lt;1, 0≤t6&lt;1, t6&lt;t5, s5&lt;s6), such as an Al s6 Ga (1-s6) N layer and/or a GaN layer, wherein 0&lt;s6≤0.08 or 0&lt;s6≤0.05 in other embodiments. The number of periods in which the narrow band gap layers  12 A and the wide band gap layers  12 B are alternately stacked may be, for example, 2 to 10, 3 to 8, or 4 to 6 in other embodiments. 
     The narrow band gap layers  12 A comprises a first narrow band gap layer  12 A 1  and a second narrow band gap layer  12 A 2 . The wide band gap layers  12 B comprise a first wide band gap layer  12 B 1  and a second wide band gap layer  12 B 2 . In one growth direction of the stress relief structure  12 , the numbers of the period formed by the first wide band gap layer  12 B 1  and the first narrow band gap layer  12 A 1  and the period formed by the second wide band gap layer  12 B 2  and the second narrow band gap layer  12 A 2  are respectively exemplified as one, but not limited to the number exemplified in the figures. For example, the period formed by the first wide band gap layer  12 B 1  and the first narrow band gap layer  12 A 1  may be two or more, and the first wide band gap layers  12 B 1  and the first narrow band gap layers  12 A 1  may be alternately stacked to form a first group of stress relief structures  12 . The period formed by the second wide band gap layer  12 B 2  and the second narrow band gap layer  12 A 2  may be two or more, and the second wide band gap layers  12 B 2  and the first narrow band gap layers  12 A 2  may be alternately stacked to form a second group of the stress relief structures  12 . In one embodiment, the number of periods and/or the total thickness of the narrow band gap layer  12 A and the wide band gap layer  12 B is not greater than the number of periods and/or the total thickness of the active structure  14  described below, so the light emitted from the active structure  14  is not absorbed and the light extraction efficiency of the light-emitting device  1  is not lowered. As shown in  FIG.  2   , in one embodiment, the stress relief structure  12  comprises a total thickness between 30 nm and 100 nm, which is less than the total thickness of the active structure  14  and less than the total thickness of the periodic structure  10 . 
     One of the wide band gap layers  12 B comprises wide band gap sub-layers. In the present embodiment, each of the wide band gap layers  12 B comprises a first wide band gap sub-layer  121   b , a second wide band gap sub-layer  122   b  and a third wide band gap sub-layer  123   b , wherein the second wide band gap sub-layer  122   b  is located between the first wide band gap sub-layer  121   b  and the third wide band gap sub-layer  123   b . The band gap of the second wide band gap sub-layer  122   b  is greater than that of the first wide band gap sub-layer  121   b  and that of the third wide band gap sub-layer  123   b . The first wide band gap sub-layer  121   b  and the third wide band gap sub-layer  123   b  may comprise GaN. The second wide band gap sub-layer  122   b  comprises Al s6 Ga (1-s6) N, while 0&lt;s6&lt;1, 0&lt;s6≤0.08, or 0&lt;s6≤0.05. 
     In another embodiment, the wide band gap layers  12 B further comprises a cladding layer (not shown) composed of GaN contacting the narrow band gap layer  12 A and an intermediate sub-layer (not shown) located between the cladding layer and the first wide band gap sub-layer  121   b . The intermediate sub-layer comprises a lattice constant less than that of other sub-layers of the wide band gap layer  12 B, and may be formed of a ternary compound semiconductor or a binary compound semiconductor having Al and N, such as AlGaN or MN. In one embodiment, an intermediate sub-layer is formed after each of the narrow band gap layers  12 A is formed, and the compressive stress of the narrow band gap layer  12 A is compensated by adjusting the thickness of the intermediate sub-layer. The intermediate sub-layer comprises a thickness less than those of other sub-layers of the wide band gap layer  12 B, such as 1 Å to 30 Å. 
     The first wide band gap sub-layer  121   b  and the third wide band gap sub-layer  123   b  respectively comprise a thickness less than that of the second wide band gap sub-layer  122   b . The first wide band gap sub-layer  121   b , the second wide band gap sub-layer  122   b  and the third wide band gap sub-layer  123   b  respectively have a thickness greater than 1 nm but less than 5 nm. A thickness of the second wide band gap sub-layer  122   b  and a thickness of the wide band gap layer  12 B have a first thickness ratio between 45% and 55%. In one embodiment, the second wide band gap sub-layer  122   b  comprising Al s6  Ga (1-s6) N is closer to the n-type nitride semiconductor structure  8  than the third wide band gap sub-layer  123   b  comprising GaN to block electrons in advance. The sum of a thickness of the first wide band gap sub-layer  121   b  and a thickness of the second wide band gap sub-layer  122   b  is greater than or less than a thickness of the third wide band gap sub-layer  123   b . In one embodiment, the first wide band gap sub-layer  121   b  and the third wide band gap sub-layer  123   b  comprise approximately identical or different thicknesses. 
     The wide band gap layer  12 B may comprise layers, wherein the second wide band gap layer  12 B 2  closer to the active structure  14  has a thickness greater than that of the first wide band gap layer  12 B 1  away from the active structure  14 , but the difference between a thickness of the first wide band gap layer  12 B 1  and a thickness of the second wide band gap layer  12 B 2  is not greater than 3 nm, or not greater than 2 nm alternatively. The narrow band gap layers  12 A, such as the first narrow band gap layer  12 A 1  away from the active structure  14  and the second narrow band gap layer  12 A 2  close to the active structure  14 , may comprise approximately identical thicknesses. 
     In the present embodiment, the first wide band gap sub-layer  121   b  is closer to the n-type nitride semiconductor structure  8  than the second wide band gap sub-layer  122   b , and the second wide band gap sub-layer  122   b  is closer to the n-type nitride semiconductor structure  8  than the third wide band gap sub-layer  123   b . The first wide band gap sub-layer  121   b  and the third wide band gap sub-layer  123   b  respectively contact two opposite sides of the narrow band gap layer  12 A. Doping an n-type impurity into the wide band gap layer  12 B can improve the injection efficiency of electrons. At least one of the first wide band gap sub-layer  121   b  and the third wide band gap sub-layer  123   b  comprises an n-type impurity, and the n-type impurity can be silicon (Si). In one embodiment, the n-type impurity can be doped before the narrow band gap layer  12 A is formed. For example, an n-type impurity is doped into the third wide band gap sub-layer  123   b , which is formed before the narrow band gap layer  12 A is formed and is in direct contact with the narrow band gap layer  12 A. The n-type doping concentration of the first wide band gap sub-layer  121   b  and/or the third wide band gap sub-layer  123   b  can be less than 1×10 18  cm −3  but greater than or equal to 1×10 17  cm −3 . When the n-type doping concentration of the first wide band gap sub-layer  121   b  and/or the third wide band gap sub-layer  123   b  is too high, the film quality of the stress relief structure  12  is easily deteriorated, and the film quality in the active structure  14  formed on the stress relief structure  12  may also be deteriorated. Therefore, the n-type doping concentration in the stress relief structure  12  is less than the n-type doping concentration in the n-type nitride semiconductor structure  8 . The n-type doping concentration of the stress relief structure  12  can be one-tenth of the n-type doping concentration in the n-type nitride semiconductor structure  8 . 
     As shown in  FIGS.  1  and  2   , the active structure  14  is disposed on the stress relief structure  12 .  FIG.  5    is a structural schematic view of the active structure  14 . The active structure  14  comprises several periods formed by alternately stacking quantum well layers  14 W and barrier layers  14 B. The thickness of one period is the sum of the thickness of one quantum well layer  14 W and the thickness of one barrier layer  14 B. The thickness of the barrier layer  14 B is 2-10 times the thickness of the quantum well layer  14 W. Specifically speaking, the quantum well layer  14 W comprises a thickness between 2 nm and 4 nm, and the barrier layer  14 B comprises a thickness between 4 nm and 40 nm, or between 6 nm and 20 nm alternatively. The band gap of the barrier layer  14 B is greater than the band gap of the quantum well layer  14 W. The quantum well layer  14 W comprises indium (In), such as Al s7 In t7 Ga (1-s7-t7) N (0≤s7≤1, 0&lt;t7≤1), or In t7 Ga (1-t7) N wherein 0.1&lt;t7&lt;0.25. The barrier layer  14 B comprises a nitride layer, and the composition ratio of indium (In) of the nitride layer is lower than that of the quantum well layer  14 W, such as Al s8 In t8 Ga (1-s8-t8) N (0≤s8≤0.1, 0≤t8≤0.1), wherein 0≤s8≤0.08, or 0&lt;s8&lt;0.05 in other embodiments. In one embodiment, the barrier layer  14 B can be an Al s8 Ga (1-s8) N layer, a GaN layer, or a laminated structure comprising an Al s8 Ga (1-s8) N layer and a GaN layer, wherein 0&lt;s8≤0.05. The number of periods in which the quantum well layers  14 W and the barrier layers  14 B are alternately stacked may be 2 to 20, 3 to 15, or 4 to 12. If the number of the periods is too large, the thickness of the active structure  14  can be too thick to deteriorate the epitaxial quality and further reduce the luminous efficiency of the LED. If the number of the periods is too small, the thickness of the active structure  14  is too thin, the recombination of electrons and electron holes cannot be effectively achieved, which reduces the luminous efficiency of the LED. As shown in  FIG.  2   , the active structure  14  comprises a total thickness between 100 nm and 200 nm, which is greater than the total thickness of the stress relief structure  12  but less than the total thickness of the periodic structure  10 . 
     In the present embodiment, the narrow band gap layer  12 A of the stress relief structure  12  comprises In t5 Ga (1-t5) N (0&lt;t5≤0.1), and the quantum well layer  14 W of the active structure  14  comprises In t7 Ga (1-t7) N (0.1&lt;t7&lt;0.15). With the narrow band gap layer  12 A of the stress release structure  12  comprising less indium (In) composition than that of the quantum well layer  14 W of the active structure  14 , the epitaxial lattice is smoothly relaxed towards the active structure  14 . Accordingly, the narrow band gap layer  12 A of the stress release structure  12  can further improve the diffusion of electrons to increase the luminous efficiency. 
       FIG.  6    is a secondary ion mass spectrometer (SIMS) diagram of the light-emitting device  1  in accordance with one embodiment of the present disclosure. The horizontal axis of  FIG.  6    corresponds to the distance away from the upper surface of the epitaxial structure of the light-emitting device  1 , for example, the distance away from the upper surface of the contact layer  18 . The closer to the left side in the diagram, the closer to the upper surface of the epitaxial structure of the light-emitting device  1 . The closer to the right side in the diagram, the farther from the upper surface of the epitaxial structure of the light-emitting device  1 . “1E+M” on the vertical axis of  FIG.  6    represents “1×10 M ”. The left side of the vertical axis represents the concentration of impurities, such as elements of C, H, O, Si and Mg, and the right side of the vertical axis represents the ionic strength, namely, the relative strength or the relative composition of the elements aluminum (Al) and indium (In), not quantitative composition of the elements aluminum (Al) and indium (In). From the relative strengths of the elements aluminum (Al) and indium (In) in the SIMS diagram, the relative average compositions of the elements aluminum (Al) and indium (In) of each layer could be determined. The average composition of each element will be described below. The area which shows the periodical changes of the indium (In) content of the periodic structure  10  corresponds to the first semiconductor layers  10 A and the second semiconductor layers  10 B. The indium (In) content of the first semiconductor layers  10 A is higher than that of the second semiconductor layers  10 B. The first semiconductor layers  10 A and the second semiconductor layers  10 B are alternately stacked to form a periodic structure  10  with six periods. The area which shows the periodical changes of the indium (In) content of the stress relief structure  12  corresponds to the narrow band gap layers  12 A and the wide band gap layers  12 B. The indium (In) content of the narrow band gap layer  12 A is higher than that of the wide band gap layer  12 B. The narrow band gap layers  12 A and the wide band gap layers  12 B are alternately stacked to form the stress relief structure  12  with six periods. The area which shows the periodical changes of the indium (In) content of the active structure  14  corresponds to the quantum well layers  14 W and the barrier layers  14 B. The indium (In) content of the quantum well layer  14 W is higher than the indium (In) content of the barrier layer  14 B. The quantum well layers  14 W and the barrier layers  14 B are alternately stacked to form the active structure  14  with ten periods. The quantum well layers  14 W and the barrier layers  14 B of the active structure  14 , the narrow band gap layers  12 A and the wide band gap layers  12 B of the stress relief structure  12 , and the first semiconductor layers  10 A and the second semiconductor layers  10 B of the periodic structure  10  may be distinguished by Indium (In) composition change rate measured by secondary ion mass spectrometer. As shown in  FIG.  6   , the quantum well layers  14 W of the active structure  14 , the narrow band gap layers  12 A of the stress relief structure  12  and the first semiconductor layers  10 A of the periodic structure  10  have different indium (In) ion strengths. In other words, the indium (In) ion strength of the quantum well layers  14 W of the active structure  14  is relatively greater than the indium (In) ion strength of the narrow band gap layer  12 A of the stress relief structure  12 , and the indium (In) ion strength of the narrow band gap layer  12 A of the stress relief structure  12  is relatively greater than the indium (In) ion strength of the first semiconductor layers  10 A of the periodic structure  10 . Therefore, the indium (In) average composition of the quantum well layer  14 W of the active structure  14  is greater than the indium (In) average composition of the narrow band gap layer  12 A of the stress relief structure  12 , and the indium (In) average composition of the narrow band gap layer  12 A of the stress relief structure  12  is greater than the indium (In) average composition of the first semiconductor layers  10 A of the periodic structure  10 . 
       FIG.  7    is an energy dispersive X-ray spectrometer (EDX) diagram of the light-emitting device  1  in accordance with one embodiment of the present disclosure. The horizontal axis of  FIG.  7    corresponds to the distance away from the upper surface of the epitaxial structure of the light-emitting device  1 . The closer to the left side of the diagram, the closer to the upper surface of the epitaxial structure of the light-emitting device  1 . The closer to the right side of the diagram, the farther from the upper surface of the epitaxial structure of the light-emitting device  1 . The vertical axis of  FIG.  7    represents the composition percentages of elements, such as the composition percentages of aluminum (Al) and indium (In). The area which shows the periodical changes of the indium (In) content and the aluminum (Al) content in the stress relief structure  12  corresponds to the narrow band gap layers  12 A and the wide band gap layers  12 B. The indium (In) content of the narrow band gap layer  12 A is higher than that of the wide band gap layer  12 B. The aluminum (Al) content of the wide band gap layer  12 B is higher than the aluminum (Al) content of the narrow band gap layer  12 A. The narrow band gap layers  12 A and the wide band gap layers  12 B are alternately stacked to form the stress relief structure  12  with  6  periods. The positions where the indium (In) content and the aluminum (Al) content in the active structure  14  periodically change are the positions of the quantum well layers  14 W and the barrier layers  14 B. The indium (In) content of the quantum well layer  14 W is higher than that of the barrier layer  14 B. The aluminum (Al) content of the barrier layer  14 B is higher than that of the quantum well layer  14 W. The quantum well layers  14 W and the barrier layers  14 B are alternately stacked to form the active structure  14  with ten periods. The quantum well layers  14 W and the barrier layers  14 B of the active structure  14 , and the narrow band gap layers  12 A and the wide band gap layers  12 B of the stress relief structure  12  may be distinguished with the indium (In) content change rate or the aluminum (Al) content change rate measured by energy dispersive X-ray spectrometer (EDX). As shown in  FIG.  7   , the indium (In) composition of the quantum well layers  14 W of the active structure  14  is greater than that of the narrow band gap layers  12 A of the stress relief structure  12 . 
     The quantum well layers  14 W comprise a first quantum well layer  14 W 1  and a second quantum well layer  14 W 2 . The barrier layers  14 B comprise a first barrier layer  14 B 1  and a second barrier layer  14 B 2 . As shown in  FIG.  5   , in order to identify each of the quantum well layers  14 W and each of the barrier layers  14 B, these layers are numbered as a first quantum well layer  14 W 1 , a first barrier layer  14 B 1 , a second quantum well layer  14 W 2 , a second barrier layer  14 B 2 , and so forth, in the direction from the n-type nitride semiconductor structure  8  toward the p-type nitride semiconductor structure  17 . In one growth direction of the active structure  14 , the numbers of the period formed by the first quantum well layer  14 W 1  and the first barrier layer  14 B 1  and the period formed by the second quantum well layer  14 W 2  and the second barrier layer  14 B 2  are respectively exemplified as one, but not limited to the number shown in the diagram. For example, the period formed by the first quantum well layers  14 W 1  and the first barrier layers  14 B 1  may be two or more, and the first quantum well layers  14 W 1  and the first barrier layers  14 B 1  may be alternately stacked to form a first group of the active structures  14 . The period formed by the second quantum well layers  14 W 2  and the second barrier layers  14 B 2  may be two or more, and the second quantum well layers  14 W 2  and the second barrier layers  14 B 2  may be alternately stacked to form a second group of the active structures  14 . 
     In the present embodiment, the first quantum well layer  14 W 1  and the second quantum well layer  14 W 2  may comprise substantially identical thicknesses and/or substantially identical indium (In) compositions. When each of the quantum well layers  14 W comprises an identical thickness and/or an identical indium (In) composition, it is beneficial to reduce the full width at half maximum (FWHM) of the LED, which is preferred for light-emitting devices used in specific applications such as lighting. In another embodiment, when the quantum well layers  14 W comprise different thicknesses and/or different indium (In) compositions, it is beneficial to increase the full width at half maximum (FWHM) of the LED, which is suitable for light-emitting devices used in specific applications such as a display. 
     If the thickness of the quantum well layer  14 W is too thin, the effective recombination of electrons and electron holes in the quantum well layer  14 W can be affected, which reduces the luminous efficiency of the LED. If the thickness of the quantum well layer  14 W is too thick, such large thickness may cause the stress of the quantum well layer  14 W which reduces the epitaxial quality and therefore affects the recombination efficiency of electrons and electron holes, and further affecting the luminous efficiency of the LED. In one embodiment, the thicknesses of each of the quantum well layers  14 W are identical to facilitate actual growth control. As shown in  FIGS.  2  and  5   , the first barrier layer  14 B 1  and the second barrier layer  14 B 2  may comprise substantially identical thicknesses. In another embodiment, the thickness of the first barrier layer  14 B 1  may be greater than the thickness of the second barrier layer  14 B 2  to increase the recombination efficiency of electrons and electron holes. The difference between the thickness of the first barrier layer  14 B 1  and the thickness of the second barrier layer  14 B 2  is maintained within 10% of the thickness of the second barrier layer  14 B 2 . 
     The thicknesses of the barrier layers  14 B gradually decrease along the growth direction of the active structure  14 . Compared with the first barrier layer  14 B 1  far from the p-type nitride semiconductor structure  17 , the thickness of the second barrier layer  14 B 2  close to the p-type nitride semiconductor structure  17  is smaller so that electron holes are more easily injected into the quantum well layer  14 W, which increases the transmission efficiency of electron holes in the quantum well layers  14 W, improves the distribution uniformity of electron holes in the quantum well layer  14 W, increases the radiation recombination efficiency of electrons and electron holes and further increases the luminous efficiency of the LED. In the present embodiment, the thickness of the barrier layer  14 B is about 6 nm-15 nm, and the difference between the thicknesses of the barrier layers  14 B is not greater than 2 nm. If the thickness of the barrier layers  14 B is too thin, the epitaxial quality may be degraded due to the too thin thickness of the barrier layers  14 B. If the thickness of the barrier layers  14 B is too thick, it is easy to affect the migration of electrons and electron holes and to block the recombination of electrons and electron holes, which reduces the luminous efficiency of the LED. 
     At least one of the barrier layers  14 B comprises barrier sub-layers. In the present embodiment, each of the barrier layers  14 B comprises barrier sub-layers, such as a first barrier sub-layer  141   b , a second barrier sub-layer  142   b  and a third barrier sub-layer  143   b . At least one or each of the barrier sub-layers in the barrier layer  14 B comprises a thickness greater than that of the quantum well layer  14 W. Each of the barrier sub-layers in the barrier layer  14 B comprises a band gap greater than that of the quantum well layer  14 W, and at least one barrier sub-layer in the barrier sub-layers of the barrier layer  14 B comprises a band gap greater than that of the other barrier sub-layers. For example, the first barrier sub-layer  141   b  comprises Al s8  Ga (1-s8) N (0&lt;s8≤0.05), and the second barrier sub-layer  142   b  and the third barrier sub-layer  143   b  comprise GaN. 
     In one embodiment of the present disclosure, a portion of the barrier layers  14 B further comprises a capping layer  144  located between the quantum well layer  14 W and the first barrier sub-layer  141   b , wherein the capping layer  144  directly contacts the quantum well layer  14 W, and the first barrier sub-layer  141   b  is located between the capping layer  144  and the second barrier sub-layer  142   b . The capping layer  144  can prevent the indium (In) in the quantum well layer  14 W from escaping due to the subsequent epitaxial temperature or gas condition difference, so the surface morphology deterioration and short wavelength shift of the quantum well layer  14 W can be prevented. The band gap of the first barrier sub-layer  141   b  is greater than the band gap of the second barrier sub-layer  142   b , the band gap of the third barrier sub-layer  143   b  and the band gap of the capping layer  144 , respectively. The second barrier sub-layer  142   b , the third barrier sub-layer  143   b , and the capping layer  144  may comprise GaN. The first barrier sub-layer  141   b  comprises Al s8 Ga (1-s8) N (0&lt;s8&lt;1), wherein s8≤0.08, s8≤0.05, or s8≤0.03. 
     In another embodiment, the barrier layers  14 B further comprises an intermediate sub-layer (not shown) located between the capping layer  144  and the first barrier sub-layer  141   b . The intermediate sub-layer comprises a lattice constant less than that of the other sub-layers of the barrier layer  14 B, and may be formed of a ternary compound semiconductor or binary compound semiconductor having Al and N, such as AlGaN or MN. In one embodiment, an intermediate sub-layer is formed after each of the quantum well layers  14 W, and the compressive stress of the quantum well layers  14 W is compensated by adjusting the thickness of the intermediate sub-layer. The intermediate sub-layer comprises a thickness less than that of the other sub-layers of the barrier layer  14 B, such as 1 Å to 30 Å. 
       FIG.  2    is a transmission electron microscope (TEM) image of a portion of the light-emitting device  1  in accordance with an embodiment of the present disclosure. When the composition of the adjacent layers is different, the interface of the adjacent layers may be distinguished under a transmission electron microscope so the thickness of each layer can be measured. In one embodiment of the present disclosure, the film thickness of each layer and each sub-layer of the stress relief structure  12  and the film thickness of each layer and each sub-layer of the active structure  14  may be measured by a transmission electron microscope (TEM). The aluminum (Al) composition of each sub-layer is detected by energy dispersive X-ray spectrometer (EDX), and the relative relationship of average aluminum (Al) composition of each layer is detected by secondary ion mass spectrometer (SIMS). 
     The definition of the average aluminum (Al) composition is described below by taking the wide band gap layer  12 B of the stress relief structure  12  as an example. As shown in  FIG.  4   , the wide band gap layer  12 B comprises a first wide band gap sub-layer  121   b , a second wide band gap sub-layer  122   b , and a third wide band gap sub-layer  123   b . In one embodiment, the first wide band gap sub-layer  121   b  and the third wide band gap sub-layer  123   b  comprise GaN. The second wide band gap sub-layer  122   b  comprises Al s6 Ga (1-s6) N. In order to simplify the description, firstly, the thicknesses of the first wide band gap sub-layer  121   b , the second wide band gap sub-layer  122   b , and the third wide band gap sub-layer  123   b  of the stress relief structure  12  are measured by a transmission electron microscope (TEM) and are defined as T a , T b  and T c , respectively. Since the composition of the second wide band gap sub-layer  122   b  comprises aluminum and the compositions of the first wide band gap sub-layer  121   b  and the third wide band gap sub-layer  123   b  do not comprise aluminum, the aluminum (Al) composition of the wide band gap sub-layer  12 B detected by EDX is defined as s6, namely, the aluminum composition of the second wide band gap sub-layer  122   b  is s6. Based on the ratio between the thickness T b  of the second wide band gap sub-layer  122   b  and the total thickness of the wide band gap layer  12 B (namely, the sum of thicknesses T a , T b  and T a ), the average aluminum (Al) composition A of the wide band gap layer  12 B is the product of the aluminum (Al) composition of the wide band gap layer  12 B and the film thickness ratio of Al s6  Ga (1-s6) N and can be determined by the following equation: 
     
       
         
           
             A 
             = 
             
               
                 Tb 
                 
                   Ta 
                   + 
                   Tb 
                   + 
                   Tc 
                 
               
               × 
               s 
               ⁢ 
               6 
             
           
         
       
     
     The definition of the average aluminum (Al) composition is described below by taking the wide band gap layer  12 B of the stress relief structure  12  as an example (not shown). As shown in  FIG.  4   , the wide band gap layer  12 B comprises a first wide band gap sub-layer  121   b , a second wide band gap sub-layer  122   b  and a third wide band gap sub-layer  123   b . The first wide band gap sub-layer  121   b  comprises Al x6 Ga (1-x6) N, the second wide band gap sub-layer  122   b  comprises Al s6 Ga (1-s6) N, and the third wide band gap sub-layer  123   b  comprises Al y6 Ga (1-y6) N. Firstly, in order to simplify the description, the thicknesses of the first wide band gap sub-layer  121   b , the second wide band gap sub-layer  122   b , and the third wide band gap sub-layer  123   b  of the stress relief structure  12  are measured by a transmission electron microscope (TEM) and are defined as thicknesses T A , T B , and T C , respectively. The aluminum (Al) compositions of the first wide band gap sub-layer  121   b , the second wide band gap sub-layer  122   b , and the third wide band gap sub-layer  123   b  detected by energy dispersive X-ray spectrometer are x6, s6 and y6. The average aluminum (Al) composition P of the wide band gap layer  12 B is obtained by the following equation by combining the analysis results of the energy dispersive X-ray spectrometer diagram and the film thickness of each sub-layer measured by a transmission electron microscope. Based on the respective thicknesses T A  and T B  of each of the wide band gap sub-layers  121   b  to  123   b  and the ratio between the thickness T C  and the total thickness of the wide band gap layer  12 B (namely, the sum of thicknesses T a , T b  and T c ), the average aluminum (Al) composition P of the wide band gap layer  12 B satisfies the following equation: 
     
       
         
           
             P 
             = 
             
               
                 
                   TA 
                   
                     TA 
                     + 
                     TB 
                     + 
                     TC 
                   
                 
                 × 
                 x 
                 ⁢ 
                 6 
               
               + 
               
                 
                   TB 
                   
                     TA 
                     + 
                     TB 
                     + 
                     TC 
                   
                 
                 × 
                 s 
                 ⁢ 
                 6 
               
               + 
               
                 
                   TC 
                   
                     TA 
                     + 
                     TB 
                     + 
                     TC 
                   
                 
                 × 
                 y 
                 ⁢ 
                 6 
               
             
           
         
       
     
     As shown in  FIG.  6   , with the detection results of the secondary ion mass spectrometer (SIMS), it can be qualitatively determined that the average aluminum composition comprised in the wide band gap layer  12 B is greater than that comprised in the barrier layer  14 B. The average aluminum composition comprised in the wide band gap layer  12 B and the barrier layer  14 B can be quantitatively obtained by the above equation. As shown in  FIG.  7   , in the energy dispersive X-ray spectrometer diagram, it can be detected that the Al composition of the second wide band gap sub-layer  122   b  comprising Al s6 Ga (1-s6) N in the wide band gap layer  12 B is greater than or equal to that of the first barrier sub-layer  141   b  comprising Al s8 Ga (1-s8) N in the barrier layer  14 B. 
     In the present embodiment, the wide band gap layer  12 B comprises a first wide band gap sub-layer  121   b , a second wide band gap sub-layer  122   b  and a third wide band gap sub-layer  123   b , wherein the second wide band gap sub-layer  122   b  comprising Al s6 Ga (1-s6) N is located between the first wide band gap sub-layer  121   b  and the third wide band gap sub-layer  123   b  which comprise GaN. As shown in  FIG.  7   , the second wide band gap sub-layer  122   b  comprising Al s6 Ga (1-s6) N is located approximately in the center between two adjacent narrow band gap layers  12 A. 
     In the present embodiment, as shown in  FIG.  7   , the barrier layer  14 B comprises a first barrier sub-layer  141   b , a second barrier sub-layer  142   b  and a third barrier sub-layer  143   b , wherein the first barrier sub-layer  141   b  comprising AlGaN is closer to the quantum well layer  14 W comprising InGaN than the second barrier sub-layer  142   b  and the third barrier sub-layer  143   b  which comprise GaN. 
     In one embodiment of the present disclosure, as shown in  FIG.  4    and  FIG.  5   , the wide band gap layer  12 B of the stress relief structure  12  comprises Al s6 Ga (1-s6) N (0&lt;s6≤0.05), and the barrier layer  14 B of the active structure  14  comprises Al s8  Ga (1-s8) N (0&lt;s8≤0.05). The wide band gap layer  12 B comprises a first wide band gap sub-layer  121   b , a second wide band gap sub-layer  122   b , and a third wide band gap sub-layer  123   b . There is a first film thickness ratio of the film thickness of the second wide band gap sub-layer  122   b  comprising Al s6  Ga (1-s6) N to the film thickness of the wide band gap layer  12 B. The barrier layer  14 B comprises a first barrier sub-layer  141   b , a second barrier sub-layer  142   b  and a third barrier sub-layer  143   b . There is a second film thickness ratio of the film thickness of the first barrier sub-layer  141   b  comprising Al s8 Ga (1-s8) N to the film thickness of the barrier layer  14 B. As shown in  FIG.  7   , when the aluminum (Al) composition of the second wide band gap sub-layer  122   b  comprising Al s6 Ga (1-s6) N in the wide band gap layer  12 B is equal to the aluminum (Al) composition of the first barrier sub-layer  141   b  comprising Al s8 Ga (1-s8) N in the barrier layer  14 B, namely, the aluminum (Al) composition s6 of the second wide band gap sub-layer  122   b  is the same as the aluminum (Al) composition s8 of the first barrier sub-layer  141   b , the first film thickness ratio can be greater than the second film thickness ratio, so that the average aluminum (Al) composition of the wide band gap layer  12 B is greater than the average aluminum (Al) composition of the barrier layer  14 B, which can be determined by  FIG.  6   . In the energy dispersive X-ray spectrometer diagram, when the aluminum (Al) composition of the second wide band gap sub-layer  122   b  of the wide band gap layer  12 B is greater than the aluminum (Al) composition of the first barrier sub-layer  141   b  of the barrier layer  14 B, the first film thickness ratio may also be greater than or equal to the second film thickness ratio, so that the average aluminum (Al) composition of the wide band gap layer  12 B is greater than the average aluminum (Al) composition of the barrier layer  14 B, which may also be determined by the secondary ion mass spectrometer diagram. 
     In one embodiment of the present disclosure, as shown in  FIG.  1    and  FIG.  5   , the first barrier layer  14 B 1  is closer to the stress relief structure  12  than the second barrier layer  14 B 2 . The first barrier layer  14 B 1  comprises a first barrier sub-layer  141   b   1 , a second barrier sub-layer  142   b   1 , and a third barrier sub-layer  143   b   1 . The second barrier layer  14 B 2  comprises a first barrier sub-layer  141   b   2 , a second barrier sub-layer  142   b   2 , and a third barrier sub-layer  143   b   2 . When the thickness of the first barrier layer  14 B 1  is greater than the thickness of the second barrier layer  14 B 2 , the thickness of any one or more barrier sub-layers of the first barrier layer  14 B 1  may be greater than that of any one or more barrier sub-layers of the second barrier layer  14 B 2 . In one embodiment, the thickness of the first barrier sub-layer  141   b   1  of the first barrier layer  14 B 1  is greater than the thickness of the first barrier sub-layer  141   b   2  of the second barrier layer  14 B 2 , the thickness of the second barrier sub-layer  142   b   1  of the first barrier layer  14 B 1  is approximately identical to the thickness of the second barrier sub-layer  142   b   2  of the second barrier layer  14 B 2 , and the thickness of the third barrier sub-layer  143   b   1  of the first barrier layer  14 B 1  is approximately identical to the thickness of the third barrier sub-layer  143   b   2  of the second barrier layer  14 B 2 . In one embodiment, the composition and aluminum (Al) composition of the first barrier sub-layer  141   b   1  of the first barrier layer  14 B 1  may be the same as the composition and aluminum (Al) composition of the first barrier sub-layer  141   b   2  of the second barrier layer  14 B 2 . The composition of the second barrier sub-layer  142   b   1  of the first barrier layer  14 B 1  is the same as that of the second barrier sub-layer  142   b   2  of the second barrier layer  14 B 2 , and the composition of the third barrier sub-layer  143   b   1  of the first barrier layer  14 B 1  is the same as the composition of the third barrier sub-layer  143   b   2  of the second barrier layer  14 B 2 . As shown in  FIG.  6    and  FIG.  7   , the average aluminum (Al) composition of the first barrier layer  14 B 1  close to the stress relief structure  12  may be greater than the average aluminum (Al) composition of the second barrier layer  14 B 2  far from the stress relief structure  12 . 
     In an embodiment of the present disclosure, as shown in  FIG.  4   , the first wide band gap layer  12 B 1  comprises a first wide band gap sub-layer  121   b   1 , a second wide band gap sub-layer  122   b   1 , and a third wide band gap sub-layer  123   b   1 . The second wide band gap layer  12 B 2  comprises a first wide band gap sub-layer  121   b   2 , a second wide band gap sub-layer  122   b   2 , and a third wide band gap sub-layer  123   b   2 . When the thickness of the second wide band gap layer  12 B 2  is greater than that of the first wide band gap layer  12 B 1 , the thickness of any one or more sub-layers of the second wide band gap layer  12 B 2  may be greater than the thickness of any one or more sub-layers of the first wide band gap layer  12 B 1 . In one embodiment, the thickness of the second wide band gap sub-layer  122   b   2  of the second wide band gap layer  12 B 2  is greater than the thickness of the second wide band gap sub-layer  122   b   1  of the first wide band gap layer  12 B 1 , but the thickness of the first wide band gap sub-layer  121   b   1  of the first wide band gap layer  12 B 1  is substantially the same as the thickness of the first wide band gap sub-layer  121   b   2  of the second wide band gap layer  12 B 2 , and/or the thickness of the third wide band gap sub-layer  123   b   1  of the first wide band gap layer  12 B 1  is substantially the same as the thickness of the third wide band gap sub-layer  123   b   2  of the second wide band gap layer  12 B 2 . In one embodiment, as shown in  FIG.  7   , the aluminum (Al) composition of the second wide band gap sub-layer  122   b   2  comprising AlGaN in the second wide band gap layer  12 B 2  may be greater than or equal to the aluminum (Al) composition of the second wide band gap sub-layer  122   b   1  comprising AlGaN in the first wide band gap layer  12 B 1 . The composition of the first wide band gap sub-layer  121   b   1  of the first wide band gap layer  12 B 1  is the same as that of the first wide band gap sub-layer  121   b   2  of the second wide band gap layer  12 B 2 , and the composition of the third wide band gap sub-layer  123   b   1  of the first wide band gap layer  12 B 1  is the same as that of the third wide band gap sub-layer  123   b   2  of the second wide band gap layer  12 B 2 . Furthermore, as shown in  FIG.  6   , the average aluminum (Al) composition of the second wide band gap layer  12 B 2  is greater than the average aluminum (Al) composition of the first wide band gap layer  12 B 1 . 
     As shown in  FIG.  5   , the first barrier sub-layer  141   b , the second barrier sub-layer  142   b , and the third barrier sub-layer  143   b  respectively have a thickness greater than 1 nm but less than 5 nm. The capping layer  144  has a thickness not greater than 1 nm. Compared with the thicknesses of the first barrier sub-layer  141   b , a second barrier sub-layer  142   b  and a third barrier sub-layer  143   b , the thickness of the capping layer  144  is relatively thin. A second thickness ratio of a thickness of the first barrier sub-layer  141   b  and a thickness of the barrier layer  14 B is between 35% and 45%. In the present embodiment, the second thickness ratio of the first barrier sub-layer  141   b  to the barrier layer  14 B in the active structure  14  may be less than or substantially the same as the first thickness ratio of the second wide band gap sub-layer  122   b  to the wide band gap layer  12 B in the stress relief structure  12 . The sum of a thickness of the first barrier sub-layer  141   b  and a thickness of the capping layer  144  is less than the sum of a thickness of the second barrier sub-layer  142   b  and a thickness of the third barrier sub-layer  143   b.    
     In the present embodiment, the first barrier sub-layer  141   b  is closer to the n-type nitride semiconductor structure  8  than the second barrier sub-layer  142   b , the second barrier sub-layer  142   b  is closer to the n-type nitride semiconductor structure  8  than the third barrier sub-layer  143   b , and the third barrier sub-layer  143   b  is closer to the n-type nitride semiconductor structure  8  than the capping layer  144 . The third barrier sub-layer  143   b  and the capping layer  144  respectively contact two opposite sides of the quantum well layer  14 W. The capping layer  144  can relieve stress and defects caused by lattice mismatch between the first barrier sub-layer  141   b  and the quantum well layer  14 W. 
     Since the mobility of electrons is higher than that of electron holes, electrons and electron holes generally recombine in the quantum well layer  14 W close to the p-type nitride semiconductor structure  17 . By doping an n-type impurity into the barrier layer  14 B, the injection efficiency of electrons can be improved so the forward voltage of the light-emitting device  1  can be reduced. One of the first barrier sub-layer  141   b , the second barrier sub-layer  142   b  and the third barrier sub-layer  143   b  may comprise an n-type impurity of relatively high concentration. The remaining first barrier sub-layer  141   b , the second barrier sub-layer  142   b  and the third barrier sub-layer  143   b  may be doped with an n-type impurity of a relatively low concentration or not be doped with an n-type impurity. In one embodiment, the third barrier sub-layer  143   b  comprises an n-type impurity. In one embodiment of the present disclosure, at least one of the first barrier sub-layer  141   b , the second barrier sub-layer  142   b , the third barrier sub-layer  143   b  and the capping layer  144  comprises an n-type impurity, and the n-type impurity comprises silicon (Si), carbon (C), or germanium (Ge). The concentration of the n-type impurity can be less than 1×10 18  cm −3  but greater than or equal to 1×10 17  cm −3 . When the concentration of the n-type impurity is less than 1×10 17  cm −3 , due to the reduced number of carriers, a polarization phenomenon is caused, which increases the operating voltage and reduces the luminous efficiency. When the concentration of the n-type impurity is greater than 1×10 18  cm −3 , the epitaxial quality is also affected and the luminous efficiency is reduced due to the too high concentration of the impurity. 
     In one embodiment of the present disclosure, the second barrier layer  14 B 2  close to the p-type nitride semiconductor structure  17  has a thickness thinner than that of the first barrier layer  14 B 1  far from the p-type nitride semiconductor structure  17 . The mobility of electrons is increased by doping silicon (Si) in the barrier layer  14 B of the active structure  14 , and the doping concentration of silicon (Si) is increased from a side close to the n-type nitride semiconductor structure  8  to another side away from the n-type nitride semiconductor structure  8 . In other words, the silicon (Si) doping concentration of the second barrier layer  14 B 2  close to the p-type nitride semiconductor structure  17  is higher than the silicon (Si) doping concentration of the first barrier layer  14 B 1  far from the p-type nitride semiconductor structure  17  so that the distribution of electrons in the barrier layer  14 B is uniform. In that case, the mobility of electrons and electron holes in the active structure  14  is increased so more electron holes and electrons can be radiatively recombined to emit light in the active structure  14  to improve the luminous efficiency of the LED. The doping concentration of silicon (Si) can be 5×10 17  to 10 18  cm −3 . Doping an appropriate amount of silicon (Si) into the barrier layers  14 B can also reduce the defects of the active structure  14  and improve the epitaxial quality of the active structure  14 , further improving the luminous efficiency of the LED. 
     In the present embodiment, the n-type impurity concentration of the wide band gap layer  12 B of the stress relief structure  12  is about 45%-60% of the n-type impurity concentration of the barrier layer  14 B of the active structure  14 . By increasing the n-type impurity concentration of the active structure  14 , the electron concentration of the active structure  14  is increased, which improves the recombination efficiency of electrons and electron holes in the active structure  14  and the luminous efficiency of the LED. 
     The active layer  14  may adjoin the stress relief structure  12  directly or indirectly. In one embodiment of the active structure  14  in direct contact with the stress relief structure  12 , the active structure  14  may be in contact with the narrow band gap layer  12 A of the stress relief structure  12  through the first barrier sub-layer  141   b , the second barrier sub-layer  142   b  or the third barrier sub-layer  143   b  of the barrier layer  14 B. For instance, the first barrier sub-layer  141   b  comprising Al s8  Ga (1-s8) N (0&lt;s8&lt;1) may be in contact with the narrow band gap layer  12 A of the stress relief structure  12 . In another embodiment, the active structure  14  may be in contact with the wide band gap layer  12 B of the stress relief structure  12  through the quantum well layer  14 W. 
     In the present embodiment, a final barrier layer  14 LB is disposed on the active structure  14  and is located between the electron blocking structure  16  and the active structure  14 . The final barrier layer  14 LB comprises an n-type impurity, which can be silicon (Si), carbon (C), or germanium (Ge). The concentration of the n-type impurity can be less than 5×10 17  cm −3 , such as less than or equal to 1×10 17  cm −3 . The final barrier layer  14 LB may comprise indium (In) to block electrons. The final barrier layer  14 LB may comprise Al s9 In t9 Ga (1-s9-t9) N (0≤s9≤1, 0≤t9≤1), or In 19 Ga (1-t9) N (0&lt;t9&lt;1 or 0.002&lt;t9&lt;0.02). In one embodiment, the indium (In) content of the quantum well layer  14 W is greater than the indium (In) content of the narrow band gap layer  12 A, and the indium (In) content of the narrow band gap layer  12 A may be greater than that of the final barrier layer  14 LB. 
     As shown in  FIG.  1    and  FIG.  6   , due to the influence of thermal diffusion during the growth of the p-type nitride semiconductor structure  17 , the stress relief structure  12 , the active structure  14  and/or the final barrier layer  14 LB comprise a p-type impurity, such as magnesium (Mg), having a concentration of greater than 1×10 17  cm −3 , which can be observed by the secondary ion mass spectrometer. 
     As shown in  FIG.  1   , the electron blocking structure  16  is disposed between the active structure  14  and the p-type nitride semiconductor structure  17 . The electron blocking structure  16  blocks electrons from overflowing from the active structure  14  to the p-type nitride semiconductor structure  17  and allows electron holes being injected into the active structure  14  so the luminous efficiency of the light-emitting device  1  is improved. The material of the electron blocking structure  16  has a band gap greater than that of the p-type nitride semiconductor structure  17  and the band gap of the electron blocking structure  16  is decreased along a direction toward the p-type nitride semiconductor structure  17 . The material of the electron blocking structure  16  may or may not comprise a p-type impurity of aluminum indium gallium nitride (AlInGaN), aluminum gallium nitride (AlGaN), and/or aluminum nitride (AlN), such as Al s10 In t10 Ga (1-s10-t10) N (0≤s10≤1, 0&lt;t10≤0.05), or Al s10 Ga (1-s10) N (0&lt;s10&lt;1 or 0.05&lt;s10≤0.5). The thickness of the electron blocking structure  16  may be 10 nm to 100 nm, 20 nm to 80 nm, or 30 nm to 60 nm. The electron blocking structure  16  may be a single layer or comprise multiple layers. As shown in  FIG.  6   , the average aluminum (Al) composition of the electron blocking structure  16  is greater than the average aluminum (Al) composition of the active structure  14 , and is greater than the average aluminum (Al) composition of the stress relief structure  12 . As shown in  FIG.  7   , the aluminum (Al) composition of the electron blocking structure  16  is greater than the aluminum (Al) composition of any one of the layers of the active structure  14 , and is greater than the aluminum (Al) composition of any one of the layers of the stress relief structure  12 . 
     In one embodiment, the electron blocking structure  16  may be doped with a p-type impurity, such as magnesium (Mg), and the p-type doping concentration of the electron blocking structure  16  is decreased along a direction toward the active structure  14 . The doping concentration of the p-type impurity in the electron blocking structure  16  may be lower than the doping concentration of the p-type impurity in the p-type nitride semiconductor structure  17 . As shown in  FIG.  6   , the concentration of the p-type impurity in the electron blocking structure  16  can be greater than or equal to 1×10 19  cm −3 , or greater than or equal to 1×10 20  cm −3 . 
     The p-type nitride semiconductor structure  17  may comprise a multilayer structure composed of a p-type AlGaN layer and/or a p-type GaN layer, or a single-layer structure composed of a p-type AlGaN layer or a p-type GaN layer. The p-type nitride semiconductor structure  17  comprises Al s11 In t11 Ga (1-s11-t11) N (0≤s11≤1, 0&lt;t11≤1) or Al s11 Ga (1-s11) N (0&lt;s11&lt;0.2 or 0.01&lt;s11&lt;0.05). The aluminum (Al) composition in the p-type nitride semiconductor structure  17  is less than the aluminum (Al) composition in the electron blocking structure  16 . If the aluminum (Al) molar fraction in the p-type nitride semiconductor structure  17  is greater than 20%, the driving voltage of the light-emitting device  1  is increased. The p-type impurity can be magnesium (Mg), but is not particularly limited to magnesium (Mg). As shown in  FIG.  6   , the concentration of the p-type impurity in the p-type nitride semiconductor structure  17  can be greater than or equal to 1×10 19  cm −3 , or greater than or equal to 1×10 20  cm −3 . 
     The thickness of the p-type nitride semiconductor structure  17  can be greater than or equal to 50 nm and less than or equal to 300 nm. By reducing the thickness of the p-type nitride semiconductor structure  17 , the heating time during growth can be reduced and the diffusion of the p-type impurity into the active structure  14  can be suppressed. 
     The contact layer  18  is formed on the p-type nitride semiconductor structure  17  to form an ohmic contact with a transparent conductive layer  23  described below. The contact layer  18  comprises an n-type impurity or a p-type impurity, and the n-type impurity can be silicon (Si), carbon (C) or germanium (Ge). The p-type impurity can be magnesium (Mg). The concentration of the n-type impurity or the p-type impurity can be greater than 5×10 19  cm −3 , or greater than or equal to 1×10 20  cm −3  alternatively. The contact layer  18  comprises a thickness less than or equal to 10 nm and greater than 0.1 nm. The contact layer  18  is a single-layer structure comprising Al s12 In t12 Ga (1-s12-t12) N (0≤s12≤1, 0≤t12≤1) or Al s12 Ga (1-s12) N (0&lt;s12&lt;1 or 0.03≤s12≤0.3). 
     The transparent conductive layer  23  comprises a transparent oxide as an ohmic contact layer. In order to reduce the contact resistance and improve the efficiency of current spreading, the material of the transparent oxide comprises a material which is transparent to the light emitted by the active layer. The transparent conductive layer  23  comprises a light-transmitting conductive oxide, such as indium tin oxide (ITO), zinc oxide (ZnO), zinc indium tin oxide (ZITO), zinc indium oxide (ZIO), zinc tin oxide (ZTO), gallium indium tin oxide (GITO), gallium indium oxide (GIO), gallium zinc oxide (GZO), aluminum doped zinc oxide (AZO), or fluorine tin oxide (FTO), and comprises one of metal layers, such as aluminum (Al), nickel (Ni) or gold (Au), having a thickness less than 500 angstroms. The light-transmitting conductive oxide may further comprise various dopants. 
     The insulating layer  27  may be a single-layer structure, which is composed of silicon oxide, silicon nitride or silicon oxynitride. The insulating layer  27  may also comprise two or more materials with different refractive indices stacked alternately to form a distributed Bragg reflector (DBR) structure, which selectively reflects the light of a specific wavelength. For example, an insulating reflective structure with high reflectivity may be formed by stacking layers such as SiO 2 /TiO 2  or SiO 2 /Nb 2 O 5 . When SiO 2 /TiO 2  or SiO 2 /Nb 2 O 5  forms a distributed Bragg reflector (DBR) structure, each layer of the distributed Bragg reflector (DBR) structure is designed to be one or an integer multiple of the optical thickness of one quarter of the wavelength of the light emitted by the active structure  14 . The optical thickness of each layer of the distributed Bragg reflector (DBR) structure may have a deviation of ±30% on the basis of one or an integer multiple of λ/4. Since the optical thickness of each layer of the distributed Bragg reflector (DBR) structure affects the reflectivity, E-beam evaporation is a suitable process to stably control the thickness of each layer of the distributed Bragg reflector (DBR) structure. 
     The n-type electrode  21  and the p-type electrode  25  comprise a metal material such as chromium (Cr), titanium (Ti), tungsten (W), gold (Au), aluminum (Al), indium (In), tin (Sn), nickel (Ni), platinum (Pt), silver (Ag), or an alloy of the previously-mentioned materials. The n-type electrode  21  and the p-type electrode  25  may be composed of a single layer or multiple layers. For example, the n-type electrode  21  and the p-type electrode  25  may comprise Ti/Au layer, Ti/Pt/Au layer, Cr/Au layer, Cr/Pt/Au layer, Ni/Au layer, Ni/Pt/Au layer, Cr/Al/Cr/Ni/Au layer or Ag/NiTi/TiW/Pt layer. The n-type electrode  21  and the p-type electrode  25  may be used as a current path for external power to supply electricity to the n-type nitride semiconductor structure  8  and the p-type nitride semiconductor structure  17 . The n-type electrode  21  and the p-type electrode  25  comprise a thickness between 1 and 100 μm, between 1.2 and 60 μm, or between 1.5 and 6 μm. 
       FIG.  8    is a schematic view of a light emitting apparatus  2  in accordance with one embodiment of the present disclosure. The light-emitting device  1  in the above-mentioned embodiment is mounted on a first pad  511  and a second pad  512  of a package substrate  51  in the form of flip-chip. The first pad  511  and the second pad  512  are electrically insulated by an insulating portion  53  comprising an insulating material. In flip-chip mounting, a side of the growth substrate facing the electrode pad formation surface is set to be the main light extraction surface. In order to increase the light extraction efficiency of the light emitting apparatus  2 , a reflection structure  54  may be disposed around the light-emitting device  1 . 
       FIG.  9    is a schematic view of a light emitting apparatus  3  in accordance with one embodiment of the present disclosure. The light emitting apparatus  3  is a bulb comprising a lampshade  602 , a reflector  604 , a light emitting module  611 , a lamp stand  612 , a heat sink  614 , a connecting portion  616  and an electrical connecting element  618 . The light emitting module  611  comprises a bearing portion  606  and light emitting units  608  located on the bearing portion  606 , wherein the light emitting units  608  may be the light-emitting device  1  or the light emitting apparatus  2  in the above-mentioned embodiments. 
     It is noted that each of the embodiments listed in the present application is merely used to describe the present application, not limiting the scope of the present application. It will be apparent to any one that obvious modifications or variations can be made to the devices in accordance with the present disclosure without departing from the spirit and scope of the present application. Identical or similar components in different embodiments or the components having identical reference numerals in different embodiments have identical physical properties or chemical properties. In addition, under suitable circumstances, the above-mentioned embodiments in the present application may be combined or replaced with each other, not limiting to the specific embodiments described above. In one embodiment, the connecting relationship of the specific component and other component described in detail may also be applied into other embodiments, falling within the scope of the following claims and their equivalents of the present application.