LIGHT-EMISSIVING NITRIDE SEMICONDUCTOR ELEMENT

Abstract

Light-emitting nitride semiconductor element comprising: an n-sided layer (13), a p-sided layer (15), and an active layer (14) arranged between the n-sided layer (13) and the p-sided layer (15), comprising a trough layer (142) containing Al, Ga and N, and a barrier layer (141, 143) containing Al, Ga and N, the Al content being higher than that of the trough layer (142), wherein the light-emitting nitride semiconductor element has an electron-blocking structure layer (20) between the active layer (14) and the p-side layer (15), and the electron-blocking structure layer (20) contains the following a first electron blocking layer (21) located between the p-side layer (15) and the active layer (14) is arranged and has a band gap that is larger than that of the barrier layer (141, 143), a second electron blocking layer (23) which is arranged between the p-side layer (15) and the first electron blocking layer (21) and has a band gap, which is larger than that of the barrier layer (141, 143), but smaller than that of the first electron blocking layer (21), and an intermediate layer (22) located between the first electron blocking layer (21) and the second electron blocking layer (23) is arranged and has a band gap that is smaller than that of the second electron blocking layer (23), and where the band gap of the intermediate layer (22) is smaller than the band gap of the barrier layer (141, 143).

Description

TECHNICAL AREA

[0001] The present invention relates to light-emitting nitride semiconductor elements, in particular those light-emitting nitride semiconductor elements that emit deep ultraviolet light. Discussion of the relevant state of the art

[0002] Nitride semiconductors, including GaN, are direct transition-type semiconductors. Three-element and four-element solid solutions containing Al and In can emit light in the infrared to deep ultraviolet range by appropriately adjusting their composition to vary the band gap. Since light-emitting nitride semiconductor devices using a nitride semiconductor containing Al with a large band gap as its active layer can be specifically configured to emit deep ultraviolet light with wavelengths in the 220–350 nm range by appropriately adjusting the Al composition, their practical application as deep ultraviolet light sources, such as LEDs or LDs, is promising.

[0003] For example, patent literature 1 discloses a light-emitting nitride semiconductor element with improved emission efficiency by forming an electron-blocking layer on the active layer, which consists of a nitride semiconductor containing Al with a large band gap. DOCUMENT OF THE CONVENTIONAL STATE OF TECHNOLOGY Patent Literature

[0004] Patent literature 1: WO2011 / 104 969 A1 DE 10 2013 017 275 A1 describes an optoelectronic semiconductor device comprising a layer stack based on a nitride compound semiconductor, which has an n-type semiconductor region, a p-type semiconductor region and an active layer arranged between the n-type semiconductor region and the p-type semiconductor region. US 2010 / 0019256A1 describes a light-emitting device with an electron-blocking combination layer comprising an active layer, an n-type GaN layer, a p-type GaN layer, and an electron-blocking combination layer having two Group III-V semiconductor layers with different band gaps, which can be periodically and repeatedly deposited onto the active layer to block an overflow of electrons from the active layers. EFFECT OF THE INVENTION PROBLEM TO BE SOLVED BY THE INVENTION

[0005] However, conventional light-emitting nitride semiconductor elements that use a large bandgap Al-containing nitride semiconductor as the active layer tend to have a shorter lifespan than blue LEDs and similar devices.

[0006] Accordingly, it is an object of the embodiments of the present invention to provide a long lifetime light-emitting nitride semiconductor element configured with an Al-containing nitride semiconductor with a large band gap as the active layer. MEANS TO SOLVE THE PROBLEM

[0007] The invention is defined in independent claim 1. Dependent claims define embodiments thereof. The light-emitting nitride semiconductor element comprises: an n-sided layer, a p-sided layer, and an active layer arranged between the n-sided layer and the p-sided layer, comprising a well layer or pot layer containing Al, Ga, and N, and a barrier layer orA boundary layer containing Al, Ga and N, wherein the Al content is higher than that of the trough layer, wherein the light-emitting nitride semiconductor element has an electron blocking structure layer between the active layer and the p-side layer, and the electron blocking structure includes a first electron blocking layer with a band gap larger than that of the depletion layer, and a second electron blocking layer located between the p-side layer and the first electron blocking layer, having a band gap larger than that of the depletion layer but smaller than that of the first electron blocking layer, and an intermediate layer located between the first electron blocking layer and the second electron blocking layer, having a band gap smaller than that of the second electron blocking layer. EFFECT OF INVENTION

[0008] By having an intermediate layer between the first electron blocking layer and the second electron blocking layer, the light-emitting nitride semiconductor element, constructed as above, can provide a long lifetime light-emitting nitride semiconductor element built with an Al-containing nitride semiconductor with a large band gap as the active layer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a schematic sectional view showing the structure of the light-emitting nitride semiconductor element according to an embodiment of the present invention. Fig. Figure 2 is a graphical representation that schematically shows the band structure of the electron blocking structure layer 20 of a form 1 of the light-emitting nitride semiconductor element according to the embodiment. Fig.Figure 3 is a graphical representation that schematically shows the band structure of the electron blocking structure layer 20 according to a form 2 of the light-emitting nitride semiconductor element according to the embodiment. Fig. Figure 4 is a graphical representation that schematically shows the band structure of the electron blocking structure in the light-emitting nitride semiconductor element of a reference example. Fig. Figure 5 is a graphical representation that schematically shows the band structure of the electron blocking structure in the light-emitting nitride semiconductor element of a conventional example. Fig. Figure 6 is a curve showing the results of the lifetime test performed in Example 1 of the invention and a comparison example. Fig.Figure 7 is a curve showing the results of the evaluation of the emission output maintenance rates after a certain time period performed on the light-emitting nitride semiconductor elements of an example 2 of the invention. DETAILED DESCRIPTION OF EXECUTION FORMS

[0009] The light-emitting nitride semiconductor element according to certain embodiments of the invention will be explained below with reference to the drawings. However, the present invention is not limited to the embodiments described below. Designs

[0010] The light-emitting nitride semiconductor element of the in Fig.In the embodiment shown in Figure 1, a substrate 10 comprises an n-sided layer 13, a p-sided layer 15, and an active layer 14 arranged between the n-sided layer 13 and the p-sided layer 15, and is structured to emit, for example, deep ultraviolet light in a range between 220 nm and 350 nm. In the light-emitting nitride semiconductor element of this embodiment, the active layer 14 has a quantum well structure, which includes, for example, a trough layer 142 containing Al, Ga, and N, and barrier layers 141 and 143, also containing Al, Ga, and N.In the active layer 14, for example, the trough layer 142 has an aluminum composition x that is set to result in the band gap corresponding to a given deep ultraviolet wavelength of light, and the barrier layers 141 and 143, for example, have an aluminum composition x that is set to result in a larger band gap than that of the trough layer 142.

[0011] For example, in the light-emitting nitride semiconductor element that emits deep ultraviolet light with a peak wavelength of 280 nm, the trough layer 142 is, for example, constructed with a nitride semiconductor having an aluminum composition x of 0.45, i.e., Al 0,45 Ga 0,55 N, while the barrier layers 141 and 143 are constructed with a nitride semiconductor having an aluminum composition y of 0.56, i.e., Al 0,56 Ga 0,44 N.

[0012] In this description, a nitride semiconductor made of a mixed crystal containing Al, Ga and N is occasionally referred to simply as AlGaN.

[0013] The light-emitting nitride semiconductor element of this embodiment also has an electron-blocking structure layer 20, which is arranged between the p-side layer 15 and the active layer 14, to prevent the electrons injected from the n-side layer 13 into the active layer 14 from flowing out into the p-side layer 14 without recombinating in the well layer 142.

[0014] Here, the electron-blocking structure layer 20 in the light-emitting nitride semiconductor element of this embodiment contains in particular: (i) a first electron blocking layer 21 with a larger band gap than that of the barrier layer 141 or 143, which is located on the side of the active layer 14, (ii) a second electron blocking layer 23 with a band gap larger than that of the barrier layer 141 or 143 but smaller than that of the first electron blocking layer 21, which is located on the side of the p-side layer 15, and (iii) an intermediate layer (charge-reducing layer) 22 between the first electron blocking layer 21 and the second electron blocking layer 23 having a smaller band gap than that of the first electron blocking layer 21 and the second electron blocking layer 23.

[0015] If the first electron-blocking layer 21 and the second electron-blocking layer 23 are present, as described above, the electron-blocking structure layer 20 can improve the effectiveness of blocking electrons; that is, it can increase the efficiency of electron injection into the active layer 14 by reducing electron overflow, thereby increasing emission efficiency. On the other hand, the strong electric field generated by the electron-blocking effect predictably degrades the second electron-blocking layer 23. However, in this embodiment, the intermediate layer 22 contained in the electron-blocking structure layer 20 can reduce the degradation of the second electron-blocking layer 23.More specifically, it is believed that the holes that have migrated through the second electron-blocking layer 23 are blocked by the first electron-blocking layer 21 and remain in the intermediate layer 22 closer to the electron-rich active layer 14, which consequently moderates or mitigates the electric field applied to the second electron-blocking layer 23. Accordingly, the light-emitting nitride semiconductor element of this embodiment, which has an electron-blocking structure layer 20, can increase the emission efficiency and extend its lifetime.

[0016] The electron-blocking structure layer 20 can have an additional layer between the first electron-blocking layer 21 and the active layer 14, and another layer between the second electron-blocking layer 23 and the p-side layer 15, as long as it achieves electron-blocking effects. In the light-emitting nitride semiconductor element according to this embodiment, however, the first electron-blocking layer 21 is preferably in contact with the active layer 14. This allows the first electron-blocking layer 21 to achieve high levels of electron-blocking effects. In the case where the first electron-blocking layer 21 is in contact with the active layer 14, it can be in contact with either the boat layer or the barrier layer. Furthermore, the second electron-blocking layer 23 is preferably in contact with the p-side layer 15.This can mitigate the decrease in electron hole injection efficiency into the active layer 14, thereby reducing the decrease in optical output. Furthermore, the electron blocking structure layer can include an additional layer between the intermediate layer 22 and the first electron blocking layer 21, and between the intermediate layer 22 and the second electron blocking layer 23, as long as it can mitigate the degradation of the second electron blocking layer 23. In this embodiment, however, the intermediate layer 22 is preferably in contact with the first electron blocking layer 21 or the second electron blocking layer 23, and more preferably in contact with both the first electron blocking layer 21 and the second electron blocking layer 23.Bringing the intermediate layer 22 into contact with one, or preferably both, of the first electron blocking layer 21 and the second electron blocking layer 23 can more effectively moderate the application of an electric field to the second electron blocking layer 23, thereby improving the effectiveness of reducing the deterioration of the second electron blocking layer 23.

[0017] Since the electron-blocking structure layer 20, according to this embodiment, is composed of the first electron-blocking layer 21, the intermediate layer 22, and the second electron-blocking layer 23, good electron-blocking effects can be achieved even if the first electron-blocking layer 21 is thinner than the second electron-blocking layer 23. Making the first electron-blocking layer 21 thinner than the second electron-blocking layer 23 allows electrons to tunnel through the first electron-blocking layer 21 and reach the active layer 14. This allows the intermediate layer to retain electron holes while preventing emissions in the intermediate layer, which can reduce the concentration of an electric field in the second electron-blocking layer 23 and thus mitigate the degradation of the second electron-blocking layer 23.Furthermore, forming the first electron blocking layer 21, which has a large band gap, with a small thickness can improve the electron blocking effect while attenuating a forward voltage increase.

[0018] An excessively thick first electron blocking layer 21 can increase the distortion occurring in the band gap of the trough layer, thus enabling a concentration of the electric field. In other words, because the band gap becomes small, most of the radiation recombination occurs in this small band gap portion. This consequently concentrates the electric field and shortens the lifetime of the light-emitting element. From this perspective as well, the first electron blocking layer 21 preferentially has a small thickness. However, an excessively thin first electron blocking layer 21 can reduce the electron blocking effects, thereby mitigating the effect of the degradation of the second electron blocking layer 23.Furthermore, by making the intermediate layer 22 smaller than the thickness of the second electron-blocking layer 23, emissions through the intermediate layer 22 can be prevented more effectively, while simultaneously preventing an electric field from concentrating in the second electron-blocking layer 23. The combined thickness of the first electron-blocking layer 21 and the intermediate layer 22 is preferably smaller than the thickness of the second electron-blocking layer 23.

[0019] The first electron-blocking layer 21 can be made with a nitride semiconductor Al z1 Ga 1-z1N may be constructed with, for example, an aluminum composition z1 that is higher than the aluminum composition y of the barrier layers 141 and 143. For the first electron-blocking layer 21, it is preferred that the aluminum composition z1 be higher in order to improve the electron-blocking effect; preferably, an aluminum composition z1 of 1, AlN, is used to assemble the layer.

[0020] The second electron-blocking layer 23 can be made with a nitride semiconductor Al z2 Ga 1-z2 N may be constructed with, for example, an aluminum composition z2 that is higher than the aluminum composition y of the barrier layers 141 and 143, but lower than the aluminum composition z1 of the first electron-blocking layer 21. If the trough layer 142 is constructed with a nitride semiconductor made of Al 0,45 Ga 0,55N is composed of barrier layers 141 and 143, which are constructed with a nitride semiconductor made of Al 0,56 Ga 0,44 N is composed, and the first electron blocking layer 21 is made of AlN, the second electron blocking layer 23 is, for example, made of a nitride semiconductor which has an aluminum composition z2 of 0.78, e.g. Al 0,78 Ga 0,22 N.

[0021] The intermediate layer 22 can be covered with a nitride semiconductor Al r Ga 1-r N is constructed, for example, with an aluminum composition that is lower than the aluminum composition z2 of the second electron blocking layer 23, preferably lower than the aluminum composition y of the barrier layers 141 and 143, and is preferably composed of a nitride semiconductor Al r Ga 1-r N constructed with an aluminium composition r that is equal to or higher than the aluminium composition x of the trough layer 142.

[0022] In the light-emitting nitride semiconductor element of this embodiment, the electron-blocking structure layer 20 can assume various forms, which are described below, as long as it satisfies the conditions (i) to (iii) specified above. Examples of the electron-blocking structure layer 20 are given below with reference to the Fig. 2 and the Fig. 3 will be explained.

[0023] Fig. 2 and Fig. Figure 3 shows the band structures of the electron blocking structure layer 20 of forms 1 and 2. Fig. 2 and Fig. 3 also include an n-sided second composition gradient layer 132, an active layer 14, a p-sided mantle layer 151, a p-sided composition gradient mantle layer 152 and a p-sided low concentration doped layer 153, which are described later. Electron blocking structure layer 20 of form 1

[0024] Fig.Figure 2 shows the band structure of the electron blocking structure layer 20 of form 1.

[0025] In the electron-blocking structure layer 20 of form 1, the band gap of the intermediate layer 22 is set identically to the band gap of the trough layer 142. For example, when the trough layer 142 is covered with a nitride semiconductor of Al x Ga 1-x N is constructed, the intermediate layer 22 with a nitride semiconductor of Al r Ga 1-r N constructed, wherein the aluminium composition r is the same as the aluminium composition x of the trough layer 142.

[0026] According to the electron blocking structure layer 20 of form 1 described above, the intermediate layer 22 can reduce the charge applied to the second electron blocking layer 23 and can consequently mitigate the decrease in lifetime attributable to the deterioration of the second electron blocking layer 23. Electron blocking structure layer 20 of form 2

[0027] Fig. Figure 3 shows the band structure of the electron blocking structure layer 20 of form 2.

[0028] In the electron blocking structure layer 20 of form 2, the band gap of the intermediate layer 22 is set to a larger band gap than the band gap of the trough layer 142, but to a smaller band gap than the barrier layer 141 or 143. For example, if the trough layer 142 is covered with a nitride semiconductor of Al x Ga 1-x N is constructed and the barrier layers 141 and 143 are made of a nitride semiconductor of Al y Ga 1-y N are constructed, the intermediate layer 22 with a nitride semiconductor of Al r Ga 1-r N constructed, wherein the aluminium composition r is greater than the aluminium composition x of the trough layer 142, but less than the aluminium composition y of the barrier layer 141 or 143.

[0029] The electron blocking structure layer 20 of form 2 described above can further reduce the decrease in the carrier injection efficiency into the trough layer 142, as well as the charge or load applied to the second electron blocking layer 23, as in the case of form 1.

[0030] In each of the electron blocking structure layers 20 of forms 1 and 2 described above, the band gap of the intermediate layer 22 was set to be smaller than the band gap of the barrier layer 141 or 143. P-type defects in the electron blocking structure layer 20

[0031] The electron blocking layer 20 contains p-type defects as required because it is a layer located between the active layer 14 and the p-side layer 15. However, in this embodiment, it is preferred that at least one of the first electron blocking layer 21, the intermediate layer 22, and the second electron blocking layer 23 be an undoped layer. The inclusion of at least one undoped layer in the electron blocking layer 20 can reduce the p-type defects that have diffused into the well layer 142, thereby mitigating the reduction in lifetime.

[0032] In this embodiment, it is preferred to make the first electron-blocking layer 21 an undoped layer. Having the first electron-blocking layer 21 undoped can further reduce the p-type defects that have diffused into the well layer 142, thus mitigating the lifetime reduction more effectively.

[0033] An undoped layer here refers to a layer formed without doping with p-type or n-type impurities, while allowing the layer to grow, for example, when the layer is grown by metal-organic evaporation, the supply of starting material gas for the impurities is stopped, and means that the impurity concentration is at most 1 x 10-16, i.e., the layer effectively contains no impurities.

[0034] The electron blocking structure layer 20, structured as above, can reduce the load or charge applied to the electron blocking structure layer 20 and can thus reduce the deterioration of the electron blocking structure layer 20 compared to the one in Fig. 5 electron blocking structure shown, which is structured with two layers, namely the first electron blocking layer 21 and the second electron blocking layer 23, without an intermediate layer 22.

[0035] Accordingly, the light-emitting nitride semiconductor element with the electron-blocking structure layer 20 of this embodiment can extend the lifetime or durability compared to the nitride semiconductor with a two-layer electron-blocking structure, which is composed of the first electron-blocking layer 21 and the second electron-blocking layer 23.

[0036] The electron-blocking structure layer 20, structured as above, can reduce the emission intensity compared to the light-emitting nitride semiconductor element of the in Fig. 4 shown reference example increase where the first electron blocking layer 21 and the second electron blocking layer 23 have the same band gap.

[0037] In other words, the light-emitting nitride semiconductor element with the electron-blocking structure layer 20 of this embodiment can increase both the emission intensity and the lifetime.

[0038] In the light-emitting nitride semiconductor element of this embodiment, the intermediate layer 22 has a band gap that is the same as or higher than that of the trough layer 142. Accordingly, when the emission wavelength is set to 250 nm or higher, the band gaps between the intermediate layer 22 and both the first electron-blocking layer 21 and the second electron-blocking layer 23 can be set with a large difference. As a result, the load-reducing effect of the intermediate layer 22 will become evident. Furthermore, when the emission wavelength is set to 310 nm or lower, the electric field resulting from the electron-blocking effect of the first electron-blocking layer 21 and the second electron-blocking layer 23 is further intensified. Thus, the intermediate layer can exert the load-reducing effect more effectively.Accordingly, in the light-emitting nitride semiconductor element of this embodiment, adjusting the emission wavelength to a range between 250 nm and 310 nm can more effectively reduce the deterioration of the electron blocking structure layer 20.

[0039] In the light-emitting nitride semiconductor element of this embodiment, the first electron-blocking layer 21 is arranged in contact with the trough layer 142. Arranging the first electron-blocking layer 21 in contact with the trough layer 142 in this way can effectively reduce electron overflow.

[0040] However, in the light-emitting nitride semiconductor element of this embodiment, a barrier layer can be arranged as the outermost layer of the p-layer side of the active layer 14 in order to place the first electron-blocking layer 21 in contact with the barrier layer. Arranging the first electron-blocking layer 21 in contact with the outermost barrier layer in this way can effectively reduce the diffusion of p-type defects into the trough layer 142.

[0041] In the light-emitting nitride semiconductor element of this embodiment, a further layer can be arranged, as necessary, between the active layer 14 and the first electron-blocking layer 21.

[0042] The components other than the electron blocking structure layer 20 in this embodiment will be explained in detail below. Substrate 10

[0043] For substrate 10, for example, a sapphire substrate can be used. A sapphire substrate is transparent to deep ultraviolet light and is suitable as a substrate for a light-emitting nitride semiconductor element that emits deep ultraviolet light. A sapphire substrate with a c-plane, preferably a plane inclined to the c-plane in a direction of an a-axis or an m-axis at an angle in the range between 0.2 and 2 degrees, as the upper surface, is suitable for nitride semiconductor growth. Buffer layer 11

[0044] The buffer layer 11, for example, is made of aluminum nitride (AlN) grown on the upper surface of the substrate 10 and buffers the lattice mismatch between the sapphire and the nitride semiconductor layer grown on it. Aluminum nitride has an extremely wide band gap and is transparent to deep ultraviolet light, making it suitable in cases where deep ultraviolet light is extracted from the sapphire substrate side. During the early stages of the growth of the buffer layer 11, numerous defects attributable to the lattice mismatch with the sapphire substrate and the difference in their thermal expansion coefficients can occasionally be introduced. Accordingly, the buffer layer 11 is preferably built up to a certain thickness, for example, to a thickness of at least 2 µm.There is no specific upper limit for the thickness of buffer layer 11, but it is set to a maximum of 4 µm to avoid reducing productivity. Buffer layer 11 (aluminum nitride) is preferably a single crystal. Single-crystal aluminum nitride has increased c-axis orientation, which improves the crystal orientation of the active layer 14, thereby increasing the emission efficiency. Superlattice layer 12

[0045] Formed by alternating stacking of a first layer with a small lattice constant and a second layer with a larger lattice constant than that of the first layer into a periodic multilayer structure, the superlattice layer 12 buffers the stress that will be applied to the upper layers formed on it. This can prevent fractures or cracks from occurring in the first composition gradient layer 131. The superlattice layer 12 is formed, for example, by alternating stacking of an aluminum nitride (AlN) layer and an aluminum gallium nitride (AlGaN) layer.

[0046] The bottom layer of the superlattice layer 12 in contact with the buffer layer 11 can be either an aluminum nitride (AlN) layer or an aluminum gallium nitride (AlGaN) layer. The top layer of the superlattice layer 12 in contact with the first composition gradient layer 131 can also be either an aluminum nitride (AlN) layer or an aluminum gallium nitride (AlGaN) layer. The aluminum nitride layer and / or the aluminum gallium nitride layer can also be doped with an additive element, such as n-type defects, according to a given objective. In the case of extracting deep ultraviolet light from the side of the sapphire substrate 10, the composition of the aluminum gallium nitride layer is adjusted to have a larger band gap than the photon energy of the deep ultraviolet light emitted by the active layer. N-side layer 13

[0047] The n-sided layer 13 contains a first composition gradient layer 131 and a second composition gradient layer 132. First composition gradient layer 131

[0048] The first composition gradient layer 131 is a layer whose composition changes continuously from the side of the superlattice layer to the side of the second composition gradient layer, i.e., in the upward direction, and is formed, for example, in contact with the upper surface of the superlattice layer. The first composition gradient layer 131 can improve the crystallinity of the second composition gradient layer 132, which is formed in contact with the first composition gradient layer 131.

[0049] The first composition gradient layer 131, for example, is made of undoped aluminum gallium nitride, and the aluminum content mAl1 in the aluminum gallium nitride (Al mAl1 Ga1-mAl1 N) decreases successively or gradually in the upward direction. Furthermore, the aluminum content mAl1 is defined by the ratio of aluminum to the sum of gallium and aluminum in the aluminum gallium nitride. Additionally, in the case of deep ultraviolet light extraction from the side of the sapphire substrate 10, the minimum value of mAl1 is set such that the first composition gradient layer is transparent as a whole to deep ultraviolet light emitted from the active layer. Second composition gradient layer 132

[0050] The second composition gradient layer 132 is a layer whose composition changes continuously from the side of the first composition gradient layer 131 to the side of the active layer 14, and is formed in contact with the upper surface of the first composition gradient layer 131. The second composition gradient layer 132 is, for example, made of an aluminum gallium nitride doped with n-type defects, and the aluminum content mAl2 in the aluminum gallium nitride (Al mAl2 Ga 1-mAl2N decreases successively or gradually in the upward direction. Such a second composition gradient layer 132 can form the region where the n-electrode is located deep within an Al composition. Forming a deep Al composition can better activate the n-type defects, thereby reducing the contact resistance between the n-electrode and the second composition gradient layer 132. Here, the n-type defects are, for example, silicon.

[0051] Furthermore, in the case of extraction of deep ultraviolet light from the side of the sapphire substrate 10, the minimum value of mAl2 is set such that the second composition gradient layer as a whole is transparent to the deep ultraviolet light emitted by the active layer. Relationship between the first and second composition gradient layers

[0052] The compositions of the first composition gradient layer 131 and the second composition gradient layer 132, as well as how the compositions change, can be set independently of each other. However, the aluminum content mAl1 at the upper surface of the first composition gradient layer 131 (hereinafter referred to as mAl1u) is preferably equal to or greater than the aluminum content mAl2 at the lower surface of the second composition gradient layer 132 (hereinafter referred to as mAl2b), since this will impart a compressive stress to the entire second composition gradient layer 132, which reduces the occurrence of fractures or cracks. It is preferred to adjust the values ​​to satisfy the relationship mAl1u > mAl2b, while keeping the difference between mAl1u and mAl2b relatively small, as this prevents the occurrence of defects at the interface caused by lattice mismatch.The preferred range of the mAl1u / mAl2b ratio is 1.00 or higher, but 1.02 or lower. Furthermore, the first composition gradient layer 131 and the second composition gradient layer 132 form the n-side layer 13, and the n-electrode 31 is formed in contact with the upper surface of the first composition gradient layer 131. Active layer 14

[0053] The active layer 14, formed on the upper surface of the second composition gradient layer 132, has a trough layer 142 and barrier layers 141 and 143, which are, for example, made of a group III nitride semiconductor that emits deep ultraviolet light. The trough layer 142 and the barrier layers 141 and 143 can, for example, be composed of a group III nitride described by the general formula In a Al b Ga 1-a-bN (0 ≤ a ≤ 0.1, 0.4 ≤ b ≤ 1.0, a + b ≤ 1.0) is represented. In the case of reaching a light-emitting nitride semiconductor element, which in this embodiment emits deep ultraviolet light with a peak wavelength of 280 nm, for example a nitride semiconductor with three elements of Al x Ga 1-x N is selected to build the tub layer 142, and can have the aluminum composition x of 0.45, i.e., a nitride semiconductor Al 0,45 Ga 0,55 N, can be used. In this case, for example, a nitride semiconductor with three elements of Al can be used. y Ga 1-y N is selected to build the barrier layers 141 and 143, and can have an aluminum composition y of 0.56, i.e., a nitride semiconductor Al 0,56 Ga 0,44 N, can be used. Fig.Figure 1 represents an active layer 14 containing two trough layers 142 and two barrier layers 141 and 143. Specifically, barrier layer 141 is in contact with the second composition gradient layer 132, trough layer 142, barrier layer 143, and trough layer 142, stacked in that order towards the electron-blocking structure layer 20. Furthermore, in the active layer 14, the thickness of each layer is set, for example, to 50 nm for barrier layer 141, 2.5 nm for barrier layer 143, and 4.4 nm for trough layer 142. Fig.In the active layer 14, the barrier layer 141 is arranged as the bottom layer, such that the barrier layer 141 is in contact with the second composition gradient layer 132, and the trough layer 142 is formed as the top layer, such that the trough layer 142 is in contact with the first electron blocking layer 21. The active layer 14 constructed in this way can reduce electron overflow and promote the diffusion of holes into the active layer 14.

[0054] However, in this embodiment, the trough layers and the barrier layers in the active layer 14 can be arranged in different ways according to a given objective, and, for example, a barrier layer can alternatively be designed as the top layer and a trough layer can be designed as the bottom layer.

[0055] In this embodiment, the active layer 14 is not limited to a multi-quantum well structure containing two or more trough layers, but can be a single quantum well structure, or it is not necessary for it to be a quantum well structure. P-side layer 15

[0056] The p-side layer 15 contains a p-side cladding layer 151, a p-side composition gradient cladding layer 152, a p-side low-concentration doped layer 153, and a p-side contact layer 154, which are constructed such that their band gaps decrease stepwise or gradually become smaller as they move further away from the electron-blocking structure layer 20. This construction, designed to increase the band gap towards the electron-blocking structure layer 20, allows holes in the p-side low-concentration doped layer 153 to migrate through the p-side composition gradient cladding layer 152, thereby efficiently supplying the holes to the active layer. P-side mantle layer 151

[0057] The p-side cladding layer 151 is designed to have a smaller band gap than that of the second electron-blocking layer 23 and is, for example, made of an AlGaN layer with a smaller aluminum content than that of the second electron-blocking layer 23. If the second electron-blocking layer 23 is covered with a nitride semiconductor with an aluminum content of, for example, 0.78%, Al 0,78 Ga 0,22 N, builds up, the p-side cladding layer 151 can be coated with a nitride semiconductor, for example Al 0,63 Ga 0,37 N, be structured. P-side composition gradient mantle layer 152

[0058] The p-side composition gradient cladding layer 152 is built up with a progressively smaller band gap as it moves further away from the p-side cladding layer 151. For example, if the p-side cladding layer 151 is coated with a nitride semiconductor Al 0,63 Ga 0,37N is built up and the p-side low-concentration doped layer 153 is built up with GaN, the p-side composition gradient mantle layer 152 can be built up with Al mAl3 Ga 1-mAl3 N is constructed, which means the aluminum content mAl3 of Al 0,63 Ga 0,37 N is successively reduced to GaN. This allows the holes in the p-side low-concentration doped layer 153 to migrate through the p-side composition gradient mantle layer 152, effectively directing the holes to the active layer. P-side low-concentration doped layer 153

[0059] The p-side low-concentration doped layer 153, for example, is made of GaN and plays a role in the lateral diffusion of the current. P-side contact layer 154

[0060] The p-side contact layer 154, for example, is made of GaN and contains p-type defects in a higher concentration than that of the p-side, low-concentration doped layer 153. Magnesium is preferably used for the p-type defects.

[0061] The invention will be explained in more detail below using examples. Example 1

[0062] A buffer layer 11 made of aluminium nitride with a thickness of 3.5 µm was formed on the upper surface of the c-plane of a sapphire substrate 10 with a diameter of 7.62 cm (3 inches).

[0063] Next, the substrate 10 formed with the buffer layer 11 on it was placed in a reaction vessel and a buffer layer of single-crystal aluminum nitride was allowed to grow to a thickness of about 0.1 µm using ammonia and trimethylaluminium (TMA) as starting material gases.

[0064] Continuing, an Al 0,7 Ga 0,3 An N-layer (layer a) with a thickness of approximately 27.0 nm was formed using ammonia, TMA, and trimethylgallium (TMG). Then, using ammonia and TMA, with a suspension of TMG introduction, an AlN-layer (layer b) with a thickness of approximately 10.2 nm was formed. Layers a and b were each formed alternately 30 times to form a superlattice layer 12.

[0065] Then a first composition gradient layer 131, which was undoped and whose mAl1 had successively decreased, was created by Al 0,7 Ga 0,3 N to Al 0,56 Ga 0,44 N in the upward direction, formed using ammonia, TMA and TMG to a thickness of 500 nm.

[0066] Continuing, a second composition gradient layer 131, which was doped with silicon and whose mAl2 had successively decreased, was formed by Al0,56 Ga 0,44 N to Al 0,45 Ga 0,55 N in the upward direction, formed using ammonia, TMA, TMG and monosilane to a thickness of 2500 nm.

[0067] After the formation of the second composition gradient layer 132, all gases were stopped and the conditions inside the reaction vessel were adjusted to 970°C and 26.7 kPa (200 Torr). Following these adjustments, a silicon-doped aluminum alloy was added. 0,56 Ga 0,44 N-barrier layer 141 formed using ammonia, TMA, triethylgallium (TEG) and monosilane in a thickness of about 50.0 nm.

[0068] Then an Al 0,45 Ga 0,55 N-cavity layer 142 was formed using ammonia, TMA, and TEG to a thickness of approximately 4.4 nm upon exposure to the introduction of monosilane. Continuing, a silicon-doped Al 0,56 Ga 0,44An N-barrier layer 143 was formed using ammonia, TMA, TEG, and monosilane in a thickness of approximately 2.5 nm. Next, an Al 0,45 Ga 0,55 N-pan layer 142 formed using ammonia, TMA and TEG under exposure to the introduction of monosilane in a thickness of about 4.4 nm.

[0069] The active layer 14 was formed as above.

[0070] After the active layer had formed, all gases were stopped and the conditions inside the reaction vessel were adjusted to 870°C and 13.3 kPa. Following this adjustment, a first electron-blocking layer 21, approximately 1.0 nm thick, was formed using ammonia and TMA and made of aluminum nitride doped with a p-type defect (Mg).

[0071] Next, an Al 0,45 Ga 0,55N-interlayer 22 formed using ammonia, TMA and TMG (or TEG) in a thickness of about 1.0 nm.

[0072] Continuing, a second Al doped with a p-type defect site was created using ammonia, TMA and TMG (or TEG). 0,78 Ga 0,22 N-electron blocking layer 23 formed in a thickness of about 4.0 nm.

[0073] Continuing, a magnesium-doped p-sided Al was synthesized using ammonia, TMA, TMG (or (TEG) and bis(cyclopentadienyl)magnesium (Cp2Mg; magnesocene). 0,63 Ga 0,37 N-cladding layer 151 formed with a thickness of approximately 78.0 nm.

[0074] Then a p-sided composition gradient layer 152, doped with magnesium and whose mAl3 successively decomposed from Al 0,6 Ga 0,4 Nzu GaN becomes smaller in the upward direction, formed using ammonia, TMA, TNG (or TEG) and Cp2Mg in a thickness of 23.0 nm.

[0075] Continuing, a p-side layer 153 doped with low concentration magnesium-doped gallium nitride was formed using ammonia, TMG and Cp2Mg to a thickness of about 300.0 nm.

[0076] Then, using ammonia, TMG and Cp2Mg, a magnesium-doped p-sided gallium nitride contact layer 154 was formed with a thickness of about 15 nm. Exposing an n-sided layer

[0077] A mask was formed exclusively of the specified areas so that only these areas were etched. After forming the mask, the semiconductor stack was placed in a dry etching fixture, and etching was performed from the p-side layer down to approximately 0.8 µm to expose the n-side layer in a specified area. After etching, the semiconductor stack was removed from the dry etching fixture, and the mask was removed. N-electrode training

[0078] A mask was designed such that sputtering was performed only on the exposed electrode formation area on the n-side layer. The semiconductor stack was then placed in a sputtering device and sputtering was performed using a titanium-aluminum alloy. Subsequently, the mask was removed from the exposed area on the top surface of the n-side layer. At this point, the layer was in the state where multiple semiconductor stacks had been formed on a wafer, and the individual semiconductor stacks shared the n-side layer. P-electrode training

[0079] A mask was designed such that sputtering was performed only on the top surface of the p-side contact layer 154. The semiconductor stacks were then placed in a sputtering apparatus and sputtering was performed using ITO. Subsequently, the mask was removed from a p-electrode on the top surface of the p-side contact layer 154. Separation into pieces or individual parts

[0080] Following the steps described above, the wafer was divided into individual elements. The light-emitting nitride semiconductor element of Example 1, which emits deep ultraviolet light of a wavelength of 280 nm, was fabricated as described above. Lifetime testing

[0081] A lifetime test was performed before cutting into pieces on the wafer on which the semiconductor stacks were formed under the same conditions as those of Example 1 above, and on a reference wafer prepared under the same conditions as those of Example 1 above, except that the second electron blocking layer 23 was formed on the first electron blocking layer 21 without forming an intermediate layer 22, by continuous use while 150 mA at room temperature is applied to measure the optical output (20 mA) at one-minute intervals. Fig. Figure 6 shows the results. Furthermore, it shows Fig. 6 the changes over time assuming the initial output is 100%.

[0082] The light-emitting nitride semiconductor elements of Example 1 retained 85% of their light-emitting nitride semiconductor elements even after 800 minutes, compared to the light-emitting nitride semiconductor elements of the comparison example, which retained less than 60%, demonstrating an improved lifetime for the light-emitting nitride semiconductor elements of Example 1. Example 2

[0083] Six light-emitting nitride semiconductor elements (sample nos. 1 - 6) were prepared as an example in a similar way to the light-emitting nitride semiconductor elements of Example 1, except for changing the thickness of the first electron blocking layer 21, and their emission output maintenance rates after a certain time period were evaluated.

[0084] Maintenance rates were determined by performing an accelerated test. Specifically, after subjecting each light-emitting element to an accelerated test by running an electric current of 150 mA for 700 minutes, the optical output was measured at 20 mA, and the output maintenance rates were calculated before and after the test.

[0085] Fig. Figure 7 shows the results.

[0086] The thickness of the first electron blocking layer 21 in each of the light-emitting nitride semiconductor elements is shown in Table 1 below. Table 1 Sample no. Thickness of the first electron blocking layer 21 (nm) 1 0.5 2 0.9 3 1.3 4 1.3 5 1.3 6 2.3

[0087] From the in Fig.The results shown in Figure 7 confirmed that when the thickness of the first electron blocking layer 21 made of AlN is in the range of 0.9 to 2.0 nm, high levels of electron blocking effects can be achieved, and when the thickness is in the range of 0.9 to 1.3 nm, an even higher level of electron blocking effect can be achieved.

[0088] When the first electron-blocking layer 21 (AlN layer) becomes as thin as approximately 0.5 nm, a decrease in the maintenance rate was observed, which is believed to be attributable to a decrease in the electron-blocking effect. When the first electron-blocking layer 21 (AlN layer) becomes as thick as approximately 2.3 nm, a decrease in the maintenance rate was observed. This is believed to be because the distortion occurring in the band gap of the trough layer becomes larger, thereby allowing the concentration of an electric field. That is, the band gap becomes small, allowing most of the radiative recombination to occur in this small band gap portion, which consequently allows the electric field to concentrate and reduces the lifetime of the elements. NAMES OF REFERENCE MARKS 10 substrate 11 Buffer layer 12 superlattice layers 13 n-sided layer 14 active layers 15 p-sided layer 20 electron blocking structure layer 21 first electron blocking layer 22 Intermediate shift 23 second electron blocking layer 131 first composition gradient layer 132 second composition gradient layer 141, 143 Blocking layer 142 Tub layer 151 p-sided mantle layer 152 p-sided composition gradient mantle layer 153 p-sided low-concentration doped layer 154 p-sided contact layer

Claims

[1] Light-emitting nitride semiconductor element comprising: an n-sided layer (13), a p-sided layer (15), and an active layer (14) arranged between the n-sided layer (13) and the p-sided layer (15), comprising a trough layer (142) containing Al, Ga and N, and a barrier layer (141, 143) containing Al, Ga and N, the Al content being higher than that of the trough layer (142), wherein the light-emitting nitride semiconductor element has an electron-blocking structure layer (20) between the active layer (14) and the p-side layer (15), and the electron-blocking structure layer (20) contains the following a first electron blocking layer (21) located between the p-side layer (15) and the active layer (14) is arranged and has a band gap that is larger than that of the barrier layer (141, 143), a second electron blocking layer (23) which is arranged between the p-side layer (15) and the first electron blocking layer (21) and has a band gap, which is larger than that of the barrier layer (141, 143), but smaller than that of the first electron blocking layer (21), and an intermediate layer (22) located between the first electron blocking layer (21) and the second electron blocking layer (23) is arranged and has a band gap that is smaller than that of the second electron blocking layer (23), and where the band gap of the intermediate layer (22) is smaller than the band gap of the barrier layer (141, 143). [2] Light-emitting nitride semiconductor element according to claim 1, wherein the first electron-blocking layer (21) is in contact with the trough layer (142). [3] Light-emitting nitride semiconductor element according to one of claims 1 to 2, wherein the thickness of the first electron blocking layer (21) is smaller than the thickness of the second electron blocking layer (23). [4] Light-emitting nitride semiconductor element according to one of claims 1 to 3, wherein the thickness of the intermediate layer (22) is smaller than the thickness of the second electron-blocking layer (23). [5] Light-emitting nitride semiconductor element according to any one of claims 1 to 4, wherein the intermediate layer (22) has a band gap that is the same as, or larger than, that of the trough layer (142). [6] Light-emitting nitride semiconductor element according to claim 5, having an emission peak wavelength in a range between 250 and 310 nm. [7] Light-emitting nitride semiconductor element according to claim 6, wherein the first electron-blocking layer (21) is made of AIN. [8] Light-emitting nitride semiconductor element according to claim 6 or 7, wherein the intermediate layer (22) is made of AIGaN. [9] Light-emitting nitride semiconductor element according to one of claims 6 to 8, wherein the second electron-blocking layer (23) is made of AIGaN. [10] Light-emitting nitride semiconductor element according to any one of claims 7 to 9, wherein the thickness of the first electron blocking layer (21) is 0.9 to 2.0 nm. [11] Light-emitting nitride semiconductor element according to any one of claims 1 to 10, wherein the first electron-blocking layer (21) is an undoped layer, which effectively contains no p-type defects.

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