Semiconductor light-emitting element

Abstract

The semiconductor light-emitting element has: an n-type semiconductor layer having an AlGaN or AlInGaN composition; an active layer containing a semiconductor based on AlGaN or a semiconductor based on AlInGaN and formed on the n-type semiconductor layer; a p-type semiconductor layer having an AlN, AlGaN or AlInGaN composition and formed on the active layer; and a p electrode formed on the p-type semiconductor layer, wherein the p-type semiconductor layer has a contact layer formed on the p electrode, the contact layer includes an AlGaN layer or an AlInGaN layer having a band gap decreasing toward the interface with the p electrode, and the contact layer has a tunneling contact layer in contact with the p electrode, and the tunneling contact layer is connected to the p electrode through a tunneling junction.

Description

Technical Field

[0001] The present invention relates to a semiconductor light-emitting element, and more particularly to a semiconductor light-emitting element that emits light in the ultraviolet region. Background Art

[0002] Recently, as a new light source having a bactericidal effect on air and water, semiconductor light-emitting elements having an emission wavelength band in the ultraviolet region (for example, a peak wavelength of 200 nm to 365 nm), particularly in the deep ultraviolet region (for example, a peak wavelength of 200 nm to 300 nm), have been attracting attention. For example, Non-Patent Document 1 discloses a light-emitting element including a p-type contact layer having an AlGaN composition and having an emission wavelength in the deep ultraviolet region.

[0003] Non-Patent Document 1: Applied Physics Express 9, 012102 (2016). Summary of the Invention

[0004] Problems to be Solved by the Invention

[0005] A semiconductor light-emitting element having an emission wavelength in the ultraviolet region has a problem in high output power. In particular, for an ultraviolet light-emitting element formed of a nitride semiconductor, one of the problems is that light emitted from the active layer is not easily extracted to the outside, that is, the light extraction efficiency is low.

[0006] The present invention has been made in view of the above points, and an object of the present invention is to provide a semiconductor light-emitting element having high output power in the ultraviolet region.

[0007] Solutions to the Problems

[0008] The semiconductor light-emitting element according to the present invention includes an n-type semiconductor layer, an active layer, a p-type semiconductor layer, and a p electrode. The n-type semiconductor layer has a composition of AlGaN or AlInGaN. The active layer is formed on the n-type semiconductor layer. The active layer contains an AlGaN semiconductor or an AlInGaN semiconductor. The p-type semiconductor layer is formed on the active layer. The p-type semiconductor layer has a composition of AlN, AlGaN, or AlInGaN. The p electrode is formed on the p-type semiconductor layer. The p-type semiconductor layer includes a contact layer formed on the p electrode. The contact layer includes an AlGaN layer or an AlInGaN layer whose band gap decreases toward the interface with the p electrode. The contact layer includes a tunneling contact layer in contact with the p electrode. The tunneling contact layer is connected to the p electrode through a tunneling junction. Brief Description of the Drawings

[0009] Figure 1It is a top view of the semiconductor light-emitting element according to Embodiment 1.

[0010] Figure 2 It is a cross-sectional view of the semiconductor light-emitting element according to Embodiment 1.

[0011] Figure 3 It is an energy band diagram of the semiconductor light-emitting element according to Embodiment 1.

[0012] Figure 4 It is a diagram illustrating the energy band structure of the semiconductor light-emitting element according to Embodiment 1.

[0013] Figure 5 It is a diagram illustrating the energy band structure of the semiconductor light-emitting element according to the comparative example.

[0014] Figure 6A It is a diagram illustrating the measurement results of the contact resistance between the p-AlGaN layer having a constant composition and the p-electrode and the contact resistance between the p-AlGaN layer having different compositions and the p-electrode in the p-AlGaN layer.

[0015] Figure 6B It is a diagram illustrating the configuration of the test structure including the p-AlGaN layer having a constant composition in the p-AlGaN layer.

[0016] Figure 6C It is a diagram illustrating the configuration of the test structure including the p-AlGaN layer having different compositions in the p-AlGaN layer.

[0017] Figure 7 It is a diagram illustrating the optical path in the light-emitting element according to Embodiment 1.

[0018] Figure 8 It is an energy band diagram of the semiconductor light-emitting element according to Embodiment 2.

[0019] Figure 9 It is an energy band diagram of the semiconductor light-emitting element according to Embodiment 3. Detailed Embodiments

[0020] Embodiments of the present invention will be described in detail below.

[0021] Embodiment 1

[0022] Figure 1 It is a top view of the semiconductor light-emitting element (hereinafter simply referred to as the light-emitting element) 10 according to Embodiment 1. Figure 2 It is a cross-sectional view of the light-emitting element 10 and is a cross-sectional view taken along Figure 1 the V-V line in Figure 3 It is an energy band diagram of the light-emitting element 10. Reference will be made toFigures 1 to 3 Describe the configuration of the light-emitting element 10.

[0023] First, the structure of the light-emitting element 10 will be described. In this embodiment, the light-emitting element 10 includes a nitride semiconductor having an emission wavelength band in the ultraviolet region (for example, in the range of 200 nm to 365 nm).

[0024] The light-emitting element 10 includes an n-type semiconductor layer 12, an active layer 13, an electron blocking layer 14, and a p-type semiconductor layer 15 formed on a growth substrate 11. The n-type semiconductor layer 12 to the p-type semiconductor layer 15 serve as light-emitting structure layers in the light-emitting element 10. The light-emitting element 10 includes an n electrode 16 and a p electrode 17 connected to the n-type semiconductor layer 12 and the p-type semiconductor layer 15, respectively.

[0025] First, in this embodiment, the growth substrate 11 is formed of an AlN substrate, a GaN substrate, a sapphire substrate, a SiC substrate, a Si substrate, or the like. In this embodiment, the growth substrate 11 is a single crystal AlN substrate. That is, each of the n-type semiconductor layer 12 to the p-type semiconductor layer 15 is a semiconductor layer epitaxially grown on the single crystal AlN substrate serving as the growth substrate 11.

[0026] Note that in consideration of the growth of the n-type semiconductor layer 12 to the p-type semiconductor layer 15 having high crystal quality, the growth substrate 11 preferably has a relatively low dislocation density. For example, the dislocation density of the growth substrate 11 is preferably 10 8 cm -2 or less, and more preferably 10 7 cm -2 or less. Note that the dislocation density can be measured using known methods, for example, by measuring the number of dislocations in a transmission electron microscope image and by measuring the number of etch pits after immersing the etch pits in a heated acid mixture solution.

[0027] In this embodiment, the growth substrate 11 has a +C plane as a crystal growth surface. Therefore, in this embodiment, the n-type semiconductor layer 12 to the p-type semiconductor layer 15 are semiconductor layers grown on the C plane of the AlN substrate. However, the crystal growth surface of the growth substrate 11 is not limited to the case of the +C plane, and for example, a plane inclined (deviated) from the C plane can be used as the crystal growth surface. When a plane inclined from the +C plane is used as the crystal growth surface, the inclination angle (deviation angle) is preferably in the range of 0.1° to 0.5°, and more preferably in the range of 0.3° to 0.4°. The crystal growth surface of the growth substrate 11 can be an M plane or an A plane.

[0028] The thickness of the growth substrate 11 can be determined by considering, for example, the reduction in the absorption coefficient of light, ease of processing (yield), etc. The thickness of the growth substrate 11 is preferably in the range of 50 μm to 1000 μm, for example.

[0029] A buffer layer (not illustrated) may be provided between the growth substrate 11 and the n-type semiconductor layer 12. When the buffer layer is provided, for example, the buffer layer preferably includes a single AlN layer, has a superlattice structure including an AlN layer and an AlGaN layer, has a superlattice structure including AlGaN layers having mutually different compositions, has a structure in which a gradient is provided to the composition such that the Al composition decreases toward the n-type semiconductor layer 12, or has a structure in which these structures are combined. The buffer layer preferably includes, for example, an AlGaN layer having a higher Al composition than the n-type semiconductor layer 12. However, the buffer layer only needs to have a composition that contributes to improving the yield during the crystal growth process of the n-type semiconductor layer 12 to the p-type semiconductor layer 15.

[0030] When the buffer layer is provided, for example, in consideration of productivity, the buffer layer preferably has a layer thickness in the range of 1 nm to 10000 nm, and more preferably has a layer thickness in the range of 10 nm to 5000 nm.

[0031] Reference Figure 2 and Figure 3 , the n-type semiconductor layer 12 is formed on the growth substrate 11. The n-type semiconductor layer 12 has a function of injecting electrons into the active layer 13.

[0032] The n-type semiconductor layer 12 has a composition of Al s Ga 1-s N (0 < s < 1, preferably x2 ≤ s ≤ 0.9, where x2 is the Al composition of the barrier layer 13B of the active layer 13 described later). In this embodiment, the n-type semiconductor layer 12 has a composition of Al 0.7 Ga 0.3 N. The n-type semiconductor layer 12 preferably has a layer thickness of 100 nm or more, for example. The n-type semiconductor layer 12 preferably has a low dislocation density.

[0033] The n-type semiconductor layer 12 may contain In. For example, the n-type semiconductor layer 12 may have a composition of AlInGaN. Also in this case, preferably, the bandgap of the n-type semiconductor layer 12 is, for example, equal to or greater than the bandgap of the barrier layer 13B of the active layer 13 and equal to or less than the bandgap of Al 0.9 Ga 0.1 N. That is, the n-type semiconductor layer 12 only needs to have a composition of AlGaN or AlInGaN.

[0034] The n-type semiconductor layer 12 contains Si or the like as an n-type dopant and has an n-type conductivity type. The dopant concentration of the n-type semiconductor layer 12 is not particularly limited and can be appropriately determined depending on the purpose. For example, when considering providing high conductivity, the dopant concentration of the n-type semiconductor layer 12 is preferably in the range of 1×10 15 cm -3 to 5×10 19 cm -3 .

[0035] The n-type semiconductor layer 12 may include a plurality of semiconductor layers exhibiting an n-type conductivity type. In this case, each semiconductor layer preferably has a dopant concentration in the range of 1×10 15 cm -3 to 5×10 19 cm -3 .

[0036] The active layer 13 is formed on the n-type semiconductor layer 12 and has a bandgap equal to or less than that of the n-type semiconductor layer 12. The active layer 13 serves as a light-emitting layer. In this embodiment, the active layer 13 has a crystal composition containing an AlGaN semiconductor. Specifically, the active layer 13 has a composition of Al x Ga 1-x N (0 < x < 1). The active layer 13 emits light in the ultraviolet region. Preferably, the active layer 13 is directly formed on the n-type semiconductor layer 12, that is, in contact with the n-type semiconductor layer 12.

[0037] The active layer 13 may contain In. For example, the active layer 13 may have a composition of AlInGaN. Also in this case, the active layer 13 only needs to have a bandgap greater than that of GaN and less than that of AlN, for example. That is, the active layer 13 only needs to have a crystal composition containing an AlGaN semiconductor or an AlInGaN semiconductor.

[0038] In this embodiment, as Figure 3 illustrated, the active layer 13 has a multi-quantum well (MQW) structure. In this embodiment, the active layer 13 includes: a plurality of well layers 13A, each well layer having a composition of Al x1 Ga 1-x1 N; and a plurality of barrier layers 13B, each barrier layer having a composition of Al x2 Ga 1-x2 N and having a larger bandgap than the well layer 13A.

[0039] In this embodiment, each well layer 13A has a composition of Al 0.5 Ga 0.5 N, and each barrier layer 13B has a composition of Al 0.65 Ga0.35 The components of N. For example, each well layer 13A has a layer thickness in the range of 3 nm to 6 nm, and each barrier layer 13B has a layer thickness in the range of 3 nm to 10 nm.

[0040] In this embodiment, the Al component x2 of the barrier layer 13B is less than the Al component s of the n-type semiconductor layer 12. Therefore, the barrier layer 13B has a smaller bandgap than the n-type semiconductor layer 12. Thus, in this embodiment, an energy level difference of the bandgap is provided between the n-type semiconductor layer 12 and the barrier layer 13B.

[0041] The configuration of the active layer 13 is not limited to this. For example, the active layer 13 is not limited to having a multi-quantum well structure. For example, the active layer 13 may have a single quantum well structure or may be formed of a single layer.

[0042] The electron blocking layer 14 is formed on the active layer 13 and has a larger bandgap than the active layer 13. The electron blocking layer 14 has Al t Ga 1-t The component of N (s < t ≤ 1). The electron blocking layer 14 serves as a layer that suppresses electrons injected into the active layer 13 from overflowing into the p-type semiconductor layer 15.

[0043] In this embodiment, the electron blocking layer 14 has a larger bandgap than the n-type semiconductor layer 12. In this embodiment, the electron blocking layer 14 has the same component as that of the growth substrate 11, that is, the component of AlN (corresponding to the case of t = 1). For example, the electron blocking layer 14 has a layer thickness in the range of 1 nm to 50 nm.

[0044] In this embodiment, the electron blocking layer 14 contains Mg or the like as a p-type dopant and has a p-type conductivity type. However, the electron blocking layer 14 does not need to contain a p-type dopant or may partially contain a p-type dopant. The electron blocking layer 14 does not need to be provided. That is, the p-type semiconductor layer 15 may be provided on the active layer 13.

[0045] The bandgap of the p-type semiconductor layer 15 is equal to or less than the bandgap of the electron blocking layer 14. The p-type semiconductor layer 15 contains Mg or the like as a p-type dopant and has a p-type conductivity type. The p-type semiconductor layer 15 and the n-type semiconductor layer 12 together serve as a cover layer. In this embodiment, the p-type semiconductor layer 15 includes a p-type cover layer 15A and a contact layer 15B. The p-type cover layer 15A serves as a cover layer. The contact layer 15B is provided at the interface with the p-electrode 17 to contact the p-electrode 17 and form an electrical connection with the p-electrode 17.

[0046] The p-type cover layer 15A has Al u Ga 1-uComponent of N(x2 < u < 1, u ≤ t). That is, the bandgap of the p-type cover layer 15A is equal to or less than the bandgap of the electron blocking layer 14. For example, the p-type cover layer 15A has a composition of Al 0.8 Ga 0.2 N. For example, the layer thickness of the p-type cover layer 15A is in the range of 1 nm to 100 nm.

[0047] The contact layer 15B has the following composition: wherein, the bandgap monotonically decreases from the interface with the p-type cover layer 15A toward the interface with the p-electrode 17 along the direction perpendicular to the p-type semiconductor layer 15 (the layer thickness direction of each layer). In this embodiment, the composition of the contact layer 15B is within Al y Ga 1-y N (0 ≤ y ≤ 1), and is configured to have an Al composition y that continuously decreases toward the p-electrode 17.

[0048] The Al composition y of the contact layer 15B is preferably configured to gradually decrease toward the p-electrode 17 within a range greater than the Al composition x1 of the well layer 13A of the active layer 13. That is, the Al composition of the contact layer 15B preferably changes within the range of x1 ≤ y ≤ 1.

[0049] Therefore, the contact layer 15B generally has a larger bandgap compared to the well layer 13A. When the Al composition y of the contact layer 15B changes within the range of x1 ≤ y ≤ 1, semi-transparency can be provided to the wavelength of the light emitted from the active layer 13. Therefore, the light extraction efficiency of the light-emitting element 10 is improved.

[0050] In this embodiment, the contact layer 15B has a composition of Al 0.8 Ga 0.2 N (the same composition as the p-type cover layer 15A) at the interface with the p-type cover layer 15A. The contact layer 15B has a composition of Al 0.6 Ga 0.4 N at the interface with the p-electrode 17. That is, in the contact layer 15B, the Al composition y decreases from 0.8 to 0.6 toward the p-electrode 17. For example, the contact layer 15B has a layer thickness of 20 nm to 60 nm.

[0051] For example, the change rate (decrease rate) of the Al composition y in the contact layer 15B is within 0.0008 nm -1 to 0.05 nm -1 The range of the change rate of the Al composition y in the contact layer 15B is preferably from 0.0008 nm -1 to 0.035 nm -1 and more preferably from 0.003 nm -1 to 0.03 nm -1range.

[0052] The Al component y of the contact layer 15B only needs to monotonically decrease toward the p electrode 17. For example, the Al component y of the contact layer 15B is not limited to the case where it continuously (linearly) decreases as exemplified by Figure 3 Rather, it can decrease in stages (in a step pattern), for example. That is, for example, the contact layer 15B is an AlGaN layer having a component in which the Al component y continuously or stepwise decreases from the interface with the p-type cover layer 15A toward the interface with the p electrode 17.

[0053] In this embodiment, the respective bandgaps of the n-type semiconductor layer 12, the active layer 13, the electron blocking layer 14, and the p-type semiconductor layer 15 (the p-type cover layer 15A and the contact layer 15B) have the relationship as Figure 3 exemplified.

[0054] Reference Figure 1 and Figure 2 , the n electrode 16 is formed on the n-type semiconductor layer 12, and the p electrode 17 is formed on the p-type semiconductor layer 15 (the contact layer 15B). For example, the n electrode 16 includes a laminate of a Ti layer, an Al layer, and an Au layer.

[0055] Exemplary configurations of the p electrode 17 include: a laminate of a Ni layer and an Au layer, a laminate of a Pt layer or a Pd layer and an Au layer, etc. Another exemplary configuration of the p electrode 17 includes a configuration in which a metal oxide such as ITO is very thin and then a material such as Al that is reflective to ultraviolet light is laminated. Note that the p electrode 17 is preferably formed of a material such as an Rh layer or an Ru layer, which can form a satisfactory ohmic contact with the p-type nitride semiconductor and has high reflectivity to ultraviolet light.

[0056] In this embodiment, a concave portion (mesa structure portion) having a comb shape in a top view is formed on the surface of the p-type semiconductor layer 15. The concave portion penetrates the p-type semiconductor layer 15, the electron blocking layer 14, and the active layer 13 to reach the n-type semiconductor layer 12. The n electrode 16 is formed in a comb shape on the surface of the n-type semiconductor layer 12 exposed at the bottom of the concave portion.

[0057] The p electrode 17 is formed in a laminated manner and in a comb shape on the surface of the p-type semiconductor layer 15 where the concave portion is not provided, and is arranged such that the comb teeth of the p electrode 17 mesh with the comb teeth of the n electrode 16 in a top view.

[0058] In this embodiment, the light-emitting element 10 includes a support substrate (mounting substrate) 18 that supports the light-emitting structure layer from the p-type semiconductor layer 15 side. The support substrate 18 includes a substrate body 18A and an n-side pad electrode 18B and a p-side pad electrode 18C formed on the substrate body 18A. The n-side pad electrode 18B and the p-side pad electrode 18C are connected to the n electrode 16 and the p electrode 17, respectively. In this embodiment, the light-emitting element 10 includes a connection electrode 19 that connects the n-side pad electrode 18B and the n electrode 16. In other words, the light-emitting element 10 is mounted by flip-chip mounting.

[0059] The above-described configurations of the n-type semiconductor layer 12 to the p-type semiconductor layer 15 and the configurations of the n electrode 16 and the p electrode 17 are merely examples. For example, the n electrode 16 only needs to be in contact with the n-type semiconductor layer 12, and the p electrode 17 only needs to be in contact with the p-type semiconductor layer 15. That is to say, for example, the configurations (electrode shapes, etc.) of the light-emitting element 10 illustrated in Figure 1 and Figure 2 are merely examples. The support device or mounting device of the light-emitting element 10 is not limited thereto.

[0060] Figure 4 is a diagram schematically illustrating the energy band structure of the valence band near the contact layer 15B and the p electrode 17 of the light-emitting element 10. The contact layer 15B will be described in detail with reference to Figure 4 in detail. Figure 4 The dotted line in

[0061] indicates the Fermi level.

[0062] In this embodiment, the contact layer 15B includes an AlGaN layer in which the Al component y gradually decreases toward the interface with the p electrode 17. The inventors of the present application have found that the contact layer 15B formed as such a composition gradient layer improves the electrode performance. This is considered because the depletion layer DL formed in the contact layer 15B is significantly thinned, so that the tunneling effect is easily generated at the interface with the p electrode 17.

[0063] More specifically, AlGaN is a semiconductor material that has a larger bandgap as the Al component increases and generally has a poor activation rate of electron holes. Therefore, when AlGaN is formed as the contact layer 15B, a high energy barrier (Schottky barrier) is formed at the interface with the p electrode 17, and a thick depletion layer is easily formed. Therefore, it is difficult to form an ohmic contact with the p electrode 17. Figure 4As illustrated, it is considered that the energy band of the contact layer 15B is significantly bent near the interface with the p electrode 17.

[0064] Therefore, it is considered that the depletion layer DL formed in the contact layer 15B is significantly thinned, and due to the tunneling effect, electron-hole pairs can easily cross the Schottky barrier. As a result, an ohmic contact is formed between the contact layer 15B and the p electrode 17. That is, the contact layer 15B includes a tunneling contact layer 15BT that is connected to the p electrode 17 through a tunneling junction near the interface with the p electrode 17.

[0065] The tunneling contact layer 15BT is a region where the tunneling effect occurs with a high probability in the contact layer 15B, and its layer thickness changes depending on the material of the p electrode 17, the composition of the contact layer 15B, the value of the current to be applied, etc. However, as described above, providing a gradient to the composition of the contact layer 15B can form the tunneling contact layer 15BT suitable for stably forming an ohmic contact.

[0066] Figure 5 FIG. is a diagram illustrating the energy band structure of the light-emitting element 100 according to the comparative example. The light-emitting element 100 according to the comparative example includes a contact layer 101 formed of an AlGaN layer having a constant Al composition y (for example, y = 0.6) instead of the contact layer 15B.

[0067] The energy barrier between the contact layer 101 and the p electrode 17 formed thereon has approximately the same height as that formed on the contact layer 15B in this embodiment. At the same time, the depletion layer DL1 formed in the contact layer 101 is significantly thicker than the depletion layer DL formed in the contact layer 15B. This is caused by the significantly low activation rate of electron-hole pairs in the contact layer 101. Therefore, it is expected that it is difficult to form a satisfactory ohmic contact between the contact layer 101 and the p electrode 17.

[0068] Figure 6A FIG. is a diagram illustrating the measurement results of the contact resistance between a p-AlGaN layer having different compositions in the p-AlGaN layer similar to the contact layer 15B in this embodiment and a metal layer formed of the same material as the p electrode 17, and the contact resistance between a p-AlGaN layer having a constant composition in the p-AlGaN layer similar to the contact layer 101 in the comparative example and the metal layer. Figure 6A FIG. is a diagram illustrating the contact resistance value between a p-AlGaN layer having different compositions in the p-AlGaN layer when the constant resistance value between a p-AlGaN layer having a constant composition in the p-AlGaN layer and the metal layer is assumed to be 1.

[0069] To obtain Figure 6A the results, prepared as Figure 6B and Figure 6CThe illustrated test structures T1 and T2. Figure 6B FIG. is a diagram illustrating the configuration of a test structure T1 including a p-AlGaN layer TL3 having components similar to those of the contact layer 101 in the comparative example. Figure 6C FIG. is a diagram illustrating the configuration of a test structure T2 including a p-AlGaN layer TL4 having components similar to those of the contact layer 15B in this embodiment.

[0070] First, as Figure 6B illustrated, the test structure T1 has a structure in which an AlN buffer layer TL2 and a p-AlGaN layer TL3 are grown on an AlN substrate TL1, and a Ni layer and an Au layer as a metal layer TE are formed on the p-AlGaN layer TL3. As Figure 6C illustrated, the test structure T2 has a structure in which an AlN buffer layer TL2 and a p-AlGaN layer TL4 are grown on an AlN substrate TL1, and a metal layer TE is formed on the p-AlGaN layer TL4.

[0071] Figure 6A FIG. is a diagram illustrating the measurement results of the contact resistance between the p-AlGaN layer TL3 and the metal layer TE in the test structure T1 and the contact resistance between the p-AlGaN layer TL4 and the metal layer TE in the test structure T2.

[0072] As Figure 6A illustrated, it is seen that the contact resistance with the metal layer TE is significantly reduced in the p-AlGaN layer TL4 having different components in the layer compared to the p-AlGaN layer TL3 having constant components.

[0073] Therefore, it is seen that a satisfactory ohmic contact can be formed between the p-AlGaN layer TL4 having different components in the p-AlGaN layer and the metal layer TE. That is, it is seen that a satisfactory ohmic contact can be formed between the contact layer 15B and the p electrode 17. Therefore, forming the contact layer 15B allows for providing a light-emitting element 10 including, for example, a contact layer 15B having a low driving voltage and high translucency.

[0074] When considering increasing the activation rate of electron holes, it is preferable to form the contact layer 15B in a strained state. That is, it is preferable to epitaxially grow the n-type semiconductor layer 12 to the p-type semiconductor layer 15 as the light-emitting structure layer in a pseudomorphic state. Therefore, like this embodiment, the n-type semiconductor layer 12, the active layer 13, the electron blocking layer 14, and the p-type semiconductor layer 15 are preferably semiconductor layers epitaxially grown on a single-crystal AlN substrate as the growth substrate 11. Similarly, the contact layer 15B preferably has a composition of AlInGaN.

[0075] When considering forming a more satisfactory ohmic contact, the contact layer 15B (especially near the interface in contact with the p-electrode 17) preferably has a relatively high dopant concentration. For example, the contact layer 15B preferably has a dopant concentration equal to or greater than that of the p-type cover layer 15A. The contact layer 15B preferably has a higher dopant concentration compared to the p-type cover layer 15A.

[0076] For example, the contact layer 15B preferably has the highest dopant concentration near the interface with the p-electrode 17, that is, in the tunneling contact layer 15BT. In this case, the contact layer 15B does not need to have a high dopant concentration as a whole compared to the p-type cover layer 15A.

[0077] For example, in a region excluding the tunneling contact layer 15BT, such as a region on the p-type cover layer 15A side with respect to the tunneling contact layer 15BT, the contact layer 15B may have a dopant concentration equal to or less than that of the p-type cover layer 15A or a dopant concentration less than that of the p-type cover layer 15A. That is, for example, the contact layer 15B preferably has a higher dopant concentration near the interface with the p-electrode 17 compared to the p-type cover layer 15A.

[0078] Figure 7 is a diagram schematically illustrating the route of light in the light-emitting element 10. Note that for convenience of explanation, Figure 7 only the route of light (reference numeral EL) emitted from the active layer 13 toward the p-type semiconductor layer 15 in the light-emitting element 10 is illustrated.

[0079] In this embodiment, the contact layer 15B has an AlGaN composition. The contact layer 15B has a larger Al composition than the well layer 13A of the active layer 13. Therefore, the contact layer 15B has a larger bandgap compared to the well layer 13A.

[0080] Therefore, the contact layer 15B hardly absorbs the light (reference numeral EL) emitted from the active layer 13, that is, the light having a wavelength corresponding to the bandgap of the well layer 13A (in this embodiment, light in the deep ultraviolet region). Therefore, most of the light (reference numeral EL) is transmitted through the contact layer 15B. In this embodiment, the n-type semiconductor layer 12, the electron blocking layer 14, and the entire p-type semiconductor layer 15 are semi-transparent to the light emitted from the active layer 13.

[0081] In this embodiment, the p-electrode 17 is reflective to the light emitted from the active layer 13. Therefore, the light (reference numeral EL) transmitted through the p-type semiconductor layer 15 is reflected by the p-electrode 17 toward the n-type semiconductor layer 12 (growth substrate 11). In this embodiment, the growth substrate 11 has an AlN composition. Therefore, most of the light emitted from the active layer 13 is extracted to the outside via the growth substrate 11 and is not absorbed in the light-emitting element 10.

[0082] That is to say, in this embodiment, the surface of the growth substrate 11 on the opposite side of the n-type semiconductor layer 12 serves as the light extraction surface of the light-emitting element 10. Then, light is efficiently emitted from the light extraction surface.

[0083] As described above, in this embodiment, the light-emitting element 10 includes a contact layer 15B having a gradually changing composition at the interface with the p-electrode 17. Therefore, a satisfactory ohmic contact can be formed between the light-emitting structure layer and the electrode. Thus, a contact layer 15B that can reduce the increase in the driving voltage and has low light absorption can be formed.

[0084] Since the p-electrode 17 is reflective, the light emitted from the active layer 13 can be efficiently extracted from the light extraction surface. Therefore, a light-emitting element 10 with high luminous efficiency and light extraction efficiency can be provided.

[0085] In this embodiment, the p-electrode 17 preferably forms a satisfactory ohmic contact and preferably has high reflectivity. Therefore, for example, only the relationship between the bandgap energy corresponding to the emission wavelength of the light-emitting element 10, the work function, and the reflectivity to the emission wavelength needs to be considered when selecting the material of the p-electrode 17.

[0086] For example, as described above, the material of the p-electrode 17 includes Rh or Ru. The material of the p-electrode 17 is not limited to a material that is reflective to the emission wavelength and can be an electrode material that is extremely thin to ensure translucency. For example, the p-electrode 17 can be a laminate in which a layer such as Al with high reflectivity is laminated on a metal oxide such as ITO and metals such as Ni and Au.

[0087] Therefore, in this embodiment, the light-emitting element 10 has an emission wavelength band in the ultraviolet region, and the p-type semiconductor layer 15 includes a contact layer 15B having a gradually changing composition at the interface with the p-electrode 17. The p-electrode 17 is reflective to light in the ultraviolet region. Therefore, a light-emitting element 10 with high output power can be provided.

[0088] Embodiment 2

[0089] Figure 8 is the energy band diagram of the light-emitting element 20 according to Embodiment 2. The light-emitting element 20 has a configuration similar to that of the light-emitting element 10 except for the configuration of the p-type semiconductor layer 21. The p-type semiconductor layer 21 has a configuration similar to that of the p-type semiconductor layer 15 except for the configuration of the contact layer 21A.

[0090] In this embodiment, the decreasing rate of the Al component y in the contact layer 21A gradually decreases from the interface with the p-type cover layer 15A. The contact layer 21A has a component on the p-type cover layer 15A side and a component on the p-electrode 17 side similar to those of the contact layer 15B. Therefore, the contact layer 21A exhibits a band gap as Figure 8 illustrated.

[0091] Similar to this embodiment, it is also possible to increase the activation rate of electron holes in the contact layer 21A by decreasing the Al component y while gradually decreasing the decreasing rate toward the interface with the p-electrode 17. Therefore, a satisfactory ohmic contact can be formed with the p-electrode 17. Therefore, a light-emitting element 20 with high output power can be provided.

[0092] Embodiment 3

[0093] Figure 9 is the energy band diagram of the light-emitting element 30 according to Embodiment 3. The light-emitting element 30 has a configuration similar to that of the light-emitting element 10 except for the configuration of the p-type semiconductor layer 31. The p-type semiconductor layer 31 has a configuration similar to that of the p-type semiconductor layer 15 except for the configuration of the contact layer 31A.

[0094] In this embodiment, the decreasing rate of the Al component y in the contact layer 31A gradually increases from the interface with the p-type cover layer 15A. The contact layer 31A has a component on the p-type cover layer 15A side and a component on the p-electrode 17 side similar to those of the contact layer 15B. Therefore, the contact layer 31A exhibits a band gap as Figure 9 illustrated.

[0095] Similar to this embodiment, it is also possible to increase the activation rate of electron holes in the contact layer 31A by decreasing the Al component y while increasing the increasing rate toward the interface with the p-electrode 17. Therefore, a satisfactory ohmic contact can be formed with the p-electrode 17. Therefore, a light-emitting element 30 with high output power can be provided.

[0096] Similar to the contact layers 21A and 31A in Embodiments 2 and 3 described above, the contact layer 15B can be configured such that the Al component y gradually decreases while the change rate changes. Similarly, in the contact layer 15B configured in this way, a tunneling contact layer similar to the tunneling contact layer 15BT is formed at the interface with the p-electrode 17. Therefore, a satisfactory ohmic contact is formed between the p-type semiconductor layers 15, 21, and 31 and the p-electrode. Therefore, light-emitting elements 10, 20, and 30 with high output power can be provided.

[0097] The above-described embodiments are merely examples. For example, the contact layers 15B, 21A, and 31A generally have a large Al composition y (i.e., bandgap) compared to the well layer 13A of the active layer 13. However, the contact layers 15B, 21A, or 31A may have an Al composition y that is partially less than the Al composition y of the well layer 13A. For example, the contact layer 15B may have a small Al composition y near the interface with the p electrode 17 compared to the well layer 13A. Also in this case, most of the light is transmitted through the contact layer 15B, thereby allowing for a high light extraction efficiency to be provided.

[0098] In the above description, the case where the p-type semiconductor layers 15, 21, and 31 each include an AlGaN layer having a constant Al composition u as the p-type cover layer 15A has been described. However, the configuration of the p-type semiconductor layers 15, 21, or 31 is not limited thereto.

[0099] For example, the p-type cover layer 15A may be a composition gradient layer having an AlGaN composition in which the Al composition u gradually changes. In this case, the entire p-type semiconductor layer 15 serves as both a cover layer and a contact layer. That is, the p-type semiconductor layer 15 does not need to include the p-type cover layer 15A that only serves as a cover layer.

[0100] When forming the p-type cover layer 15A, the Al composition y of the contact layer 15B, 21A, or 31A preferably changes at a larger rate of change compared to the Al composition u of the p-type cover layer 15A in consideration of the formation of an ohmic contact.

[0101] In other words, the p-type semiconductor layers 15, 21, or 31 only need to include the contact layers 15B, 21A, or 31A formed on the p electrode 17 and including an AlGaN layer having an Al composition y that gradually decreases toward the interface with the p electrode 17. Similar to the contact layer 15B, the contact layers 15B, 21A, and 31A only need to include the tunneling contact layer 15BT that contacts the p electrode 17 and is connected to the p electrode through a tunneling junction.

[0102] When considering stably improving the luminous efficiency, it is preferable to provide the p-type cover layer 15A. For example, it is preferable that the p-type semiconductor layers 15, 21, or 31 are formed on the active layer 13 side with respect to the contact layers 15B, 21A, or 31A and include the p-type cover layer 15A having a higher dopant concentration compared to the contact layers 15B, 21A, or 31A.

[0103] However, the contact layers 15B, 21A, or 31A may have a high dopant concentration compared to the p-type capping layer 15A at the interface with the p electrode 17. In this case, the dopant concentration of the contact layers 15B, 21A, or 31A may vary within their respective layers (e.g., toward the p-type capping layer 15A) and become equal to or less than the dopant concentration of the p-type capping layer 15A, or less than the dopant concentration of the p-type capping layer 15A in regions other than near the interface with the p electrode 17. Therefore, in the case where the dopant concentration varies within the layer, the p-type capping layer 15A preferably has a higher dopant concentration compared to the contact layers 15B, 21A, or 31A.

[0104] In the above description, the case where the p-type semiconductor layers 15, 21, and 31 are formed on the electron blocking layer 14 has been described. However, the p-type semiconductor layers 15, 21, or 31 may be formed on the active layer 13 without providing the electron blocking layer 14, i.e., a semiconductor layer serving as an electron blocking layer. In this case, for example, the p-type semiconductor layers 15, 21, or 31 do not need to contain Ga near the interface with the active layer 13. That is, the p-type semiconductor layers 15, 21, or 31 may partially have an AlN component.

[0105] In the above description, the case where the contact layers 15B, 21A, and 31A have an AlGaN component has been described. However, the components of the contact layers 15B, 21A, and 31A are not limited thereto.

[0106] For example, the contact layers 15B, 21A, and 31A only need to have a component whose bandgap gradually decreases toward the interface with the p electrode 17. For example, the contact layers 15B, 21A, or 31A only need to have an AlInGaN component. That is, the contact layers 15B, 21A, or 31A may contain In. In this case, it is only necessary to adjust the bandgap in the contact layers 15B, 21A, or 31A by adjusting the components of Al and In.

[0107] For example, the contact layers 15B, 21A, or 31A may contain AlInGaN in which the Al component gradually decreases and the In component gradually increases toward the interface with the p electrode 17. For example, the contact layers 15B, 21A, or 31A may contain AlInGaN in which the Al component gradually decreases toward the interface with the p electrode 17.

[0108] In the case where the contact layers 15B, 21A, and 31A are configured in this way, an increase in the activation rate of electron holes and a thinning of the depletion layer can be expected. Therefore, a satisfactory ohmic contact allows for efficient current injection, thereby allowing for the provision of the light-emitting elements 10, 20, or 30 with high luminous efficiency.

[0109] In other words, the p-type semiconductor layers 15, 21, or 31 only need to have a composition of AlN, AlGaN, or AlInGaN. Then, the contact layers 15B, 21A, or 31A only need to have a composition of AlGaN or AlInGaN.

[0110] In the above description, a case where the p-electrode 17 is reflective to light emitted from the active layer 13 is described. However, the configuration of the p-electrode 17 is not limited thereto. It is only necessary to form the p-electrode 17 on the p-type semiconductor layers 15, 21, or 31. Then, the p-type semiconductor layers 15, 21, and 31 only need to include the contact layers 15B, 21A, and 31A that are in contact with the p-electrode 17. Therefore, a satisfactory ohmic contact is formed between the p-type semiconductor layers 15, 21, and 31 and the p-electrode 17, and the p-electrode 17 is reflective to light emitted from the active layer 13, thereby allowing for providing a high luminous efficiency.

[0111] As described above, for example, the light-emitting element 10 includes: an n-type semiconductor layer 12 having an AlGaN composition; an active layer 13 formed on the n-type semiconductor layer 12 and containing an AlGaN semiconductor; a p-type semiconductor layer 15 formed on the active layer 13 and having a composition of AlN, AlGaN, or AlInGaN; and a p-electrode 17 formed on the p-type semiconductor layer 15.

[0112] The p-type semiconductor layer 15 includes a contact layer 15B that is formed on the p-electrode 17 and contains an AlGaN layer or an AlInGaN layer whose bandgap decreases toward the interface with the p-electrode 17. The contact layer 15B includes a tunneling contact layer 15BT that is in contact with the p-electrode 17 and is connected to the p-electrode 17 through a tunneling junction. The light-emitting elements 20 and 30 include contact layers 21A and 31A similar to the contact layer 15B. Therefore, it is possible to provide the light-emitting elements 10, 20, and 30 having a high output power in the ultraviolet region.

[0113] List of Reference Numerals

[0114] 10, 20, 30 Semiconductor light-emitting element

[0115] 12 n-type semiconductor layer

[0116] 13 Active layer

[0117] 15, 21, 31 p-type semiconductor layer

[0118] 15B, 21A, 31A Contact layer

Claims

1. A semiconductor light-emitting element, the semiconductor light-emitting element comprising: An n-type semiconductor layer having a composition of AlGaN or AlInGaN; An active layer formed on the n-type semiconductor layer, the active layer containing an AlGaN semiconductor or an AlInGaN semiconductor; A p-type semiconductor layer formed on the active layer, the p-type semiconductor layer having a composition of AlN, AlGaN or AlInGaN; And A p electrode formed on the p-type semiconductor layer, wherein The p-type semiconductor layer includes a contact layer formed on the p electrode, the contact layer being composed of an AlGaN layer in which the Al component decreases toward the interface between the p-type semiconductor layer and the p electrode such that the band gap of the contact layer decreases toward the interface between the p-type semiconductor layer and the p electrode, and The contact layer includes a tunneling contact layer composed of an AlGaN layer in direct contact with the p electrode, and the tunneling contact layer is connected to the p electrode through a tunneling junction, The active layer has a quantum well structure including a well layer and a barrier layer, and the barrier layer has a larger band gap than the well layer, The well layer is composed of an AlGaN layer with a composition of Al x1 Ga 1-x1 N, and the Al composition of the well layer is in the range equal to 0.5 or less than the Al composition of the barrier layer, and The band gap of the contact layer varies within a band gap range greater than the band gap of the well layer, Wherein the reduction rate of the Al component in the contact layer decreases or increases toward the interface between the p-type semiconductor layer and the p electrode.

2. The semiconductor light-emitting element according to claim 1, wherein The p electrode is reflective to light emitted from the active layer.

3. The semiconductor light-emitting element according to claim 1, wherein The p-type semiconductor layer has a larger band gap than the well layer.

4. The semiconductor light-emitting element according to claim 1, wherein The n-type semiconductor layer, the active layer and the p-type semiconductor layer are semiconductor layers epitaxially grown on a single crystal AlN substrate.

5. The semiconductor light-emitting element according to claim 1, wherein The p-type semiconductor layer includes a p-type cover layer formed on the active layer side with respect to the contact layer, and The contact layer has a higher dopant concentration than the p-type cover layer near the interface between the p-type semiconductor layer and the p electrode.

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