Ultraviolet semiconductor light-emitting element, and method for manufacturing ultraviolet semiconductor light-emitting element

Abstract

The present invention is formed of an AlGaN-based semiconductor in which an n-type semiconductor layer, an active layer, an electron blocking layer, a p-type semiconductor layer, and a p-electrode are sequentially laminated. The p-type semiconductor layer has: a first composition gradient layer which is formed on the electron blocking layer and in which the Al composition reduces toward the p-electrode; and a second composition gradient layer which is formed on the first composition gradient layer and in which the Al composition reduces toward the p-electrode. The interface between the first composition gradient layer and the second composition gradient layer has a heterojunction in which the Al composition is discontinuous. At the interface, the Al composition of the first composition gradient layer is more than the Al composition of the second composition gradient layer.

Description

UV semiconductor light-emitting device and method for manufacturing a UV semiconductor light-emitting device

[0001] The present invention relates to an ultraviolet semiconductor light-emitting element and a method for manufacturing an ultraviolet semiconductor light-emitting element, and more particularly to a nitride semiconductor light-emitting element that emits deep ultraviolet light and a method for manufacturing the same.

[0002] In recent years, AlGaN-based semiconductor light-emitting devices with emission wavelengths in the deep ultraviolet region have attracted attention as light sources that have the effect of inactivating bacteria and viruses and providing sterilization. However, there is a need for even higher output power of these light-emitting devices.

[0003] For example, Patent Document 1 discloses a semiconductor light-emitting element having a single composition gradient contact layer made of an AlGaN layer or an AlInGaN layer, where the Al composition decreases towards the p-electrode.

[0004] Furthermore, Patent Document 2 discloses a group III nitride semiconductor light-emitting device having a first p-type contact layer in contact with a p-type electron blocking layer and co-doped with Mg and Si, and a second p-type contact layer in contact with the p-side electrode and doped with Mg.

[0005] Furthermore, Patent Document 3 discloses a deep ultraviolet light-emitting device having a p-type contact layer with a superlattice structure in which AlGaN layers having different Al composition ratios are alternately stacked.

[0006] Non-patent document 1 describes a theoretical model of an AlGaN / GaN light-emitting diode.

[0007] Japanese Patent Publication No. 2020-064955, Japanese Patent Publication No. 6908422, Japanese Patent Publication No. 6849641

[0008] Shah et al., J. Appl. Phys., Vol. 94, No. 4, 15 August 2003

[0009] Although it has been taught that the contact resistance can be lowered compared to the case where Al is constant by making the p-type AlGaN contact layer a compositionally graded layer (for example, Patent Document 1), this was insufficient in terms of conductivity such as ohmic characteristics and resistivity in AlGaN-based semiconductor light-emitting devices with an emission wavelength band in the deep ultraviolet region.

[0010] The present invention has been made in view of the above-mentioned problems, and aims to provide an ultraviolet semiconductor light-emitting element and a method for manufacturing an ultraviolet semiconductor light-emitting element that have excellent ohmic characteristics of a p-contact layer, reduced turn-on voltage, and excellent element characteristics such as high efficiency and high output, while utilizing the effect of polarization doping.

[0011] An ultraviolet semiconductor light-emitting element according to one embodiment of the present invention is an ultraviolet semiconductor light-emitting element made of an AlGaN-based semiconductor in which an n-type semiconductor layer, an active layer, an electron blocking layer, a p-type semiconductor layer and a p-electrode are sequentially stacked, wherein the p-type semiconductor layer has a first composition gradient layer formed on the electron blocking layer and in which the Al composition decreases toward the p-electrode, and a second composition gradient layer formed on the first composition gradient layer and in which the Al composition decreases toward the p-electrode, the interface between the first composition gradient layer and the second composition gradient layer has a heterojunction in which the Al composition is discontinuous, and at the interface, the Al composition of the first composition gradient layer is greater than the Al composition of the second composition gradient layer.

[0012] A method for manufacturing an ultraviolet semiconductor light-emitting element according to another embodiment of the present invention is a method for manufacturing an ultraviolet semiconductor light-emitting element made of an AlGaN-based semiconductor in which an n-type semiconductor layer, an active layer, an electron blocking layer, a p-type semiconductor layer, and a p-electrode are sequentially stacked, wherein the p-type semiconductor layer has a first composition gradient layer formed on the electron blocking layer, the Al composition of which decreases toward the p-electrode, and a second composition gradient layer formed on the first composition gradient layer, the Al composition of which decreases toward the p-electrode, the interface between the first composition gradient layer and the second composition gradient layer has a heterojunction in which the Al composition is discontinuous, the Al composition of the first composition gradient layer is greater than the Al composition of the second composition gradient layer at the interface, and the growth of the second composition gradient layer is carried out while gradually decreasing the growth temperature, and is terminated when the growth temperature is reduced by 20°C or more.

[0013] This is a schematic cross-sectional view showing the structure of an ultraviolet semiconductor light-emitting element according to one embodiment of the present invention. This is a band diagram of the ultraviolet semiconductor light-emitting element. This is a diagram showing the semiconductor layer structure of the ultraviolet semiconductor light-emitting element. This is a schematic band diagram showing the configuration of the p-type AlGaN layer of (A) Example 1, (B) Comparative Example 1, and (C) Comparative Example 2. This is a diagram showing the simulation results of the upper end of the valence band near the interface with the p-electrode for three cases of the p-type AlGaN layer (A) to (C). This is a diagram showing the definition of the barrier Vb at the contact between the p-type AlGaN layer and the p-electrode. This is a diagram plotting the relationship between the normalized contact resistance and the normalized barrier when the p-contact layer is p-GaN, a single-composition gradient layer, and a double-composition gradient layer. This is a diagram showing the equivalent circuit of the light-emitting diode. This is a diagram schematically showing the heterobarrier between the p-AlGaN layer and the p-AlGaN contact in the case of (i) p-GaN contact and (ii) p-AlGaN contact (Example 1). This figure shows the I-V characteristics for a p-AlGaN contact where the p-AlGaN layer is a double-composition gradient layer (Example 1) and for a p-GaN contact. This figure shows the profiles of Al composition, growth temperature TG, Mg concentration, and Si concentration during the growth of the p-type AlGaN layer. p-type Al in contact with the p electrode Ｙ２ Ga １－Ｙ２ This figure shows the relationship between the amount of Mg per unit layer thickness and the barrier Vb at the surface of the N layer. This is the band diagram of the p-type AlGaN layer of Modification 1. This is the band diagram of the p-type AlGaN layer of Modification 2. This is the band diagram of the p-type AlGaN layer of Modification 3. This is the band diagram of the p-type AlGaN layer of Modification 4. This is the band diagram of the p-type AlGaN layer of Modification 5. This is the band diagram of the p-type AlGaN layer 15 of Modification 6. This is the band diagram of the p-type AlGaN layer 15 of Modification 7.

[0014] Preferred embodiments of the present invention will be described below, but these may be modified and combined as appropriate. In the following description and accompanying drawings, substantially identical or equivalent parts will be denoted by the same reference numerals.

[0015] (1) Structure of the ultraviolet semiconductor light-emitting element Figure 1 is a schematic cross-sectional view showing the structure of the ultraviolet semiconductor light-emitting element 10, which is Example 1 of one embodiment of the present invention. The ultraviolet semiconductor light-emitting element 10 is an ultraviolet light-emitting diode (ultraviolet LED) and is made of an AlGaN-based semiconductor.

[0016] More specifically, the ultraviolet semiconductor light-emitting element 10 is formed by sequentially stacking an n-type semiconductor layer, an n-type AlGaN layer 12, an active layer 13, an electron blocking layer 14 (EBL: Electron Blocking Layer), and a p-type semiconductor layer, a p-type AlGaN layer 15, on a substrate 11 by epitaxial growth.

[0017] The electron block layer 14 has a first electron block layer 14A made of an AlGaN layer and a second electron block layer 14B made of an AlGaN layer formed on the first electron block layer 14A.

[0018] The p-type AlGaN layer 15 consists of a first p-type semiconductor layer, which is a p-type AlGaN layer 15A, and a second p-type semiconductor layer, which is a p-type AlGaN layer 15B, formed on the p-type AlGaN layer 15A. The p-type AlGaN layer 15 functions as a p-type contact layer. In the following, "p-type" may be written as "p-".

[0019] Furthermore, a p-electrode 21 is formed on the p-type AlGaN layer 15, making ohmic contact with the p-type AlGaN layer 15. Also, an n-electrode 23 is formed, making ohmic contact with the n-type AlGaN layer 12. The n-electrode 23 is formed on the n-type AlGaN layer 12, which is exposed by etching the growth layer from the p-type AlGaN layer 15 side.

[0020] In the following description, the case in which the ultraviolet semiconductor light-emitting element 10 consists of an AlN layer and an AlGaN layer will be explained, but it may also have a layer made of an AlInGaN semiconductor. Figures 2A and 2B show the band diagram and semiconductor layer structure of the ultraviolet semiconductor light-emitting element 10, respectively. The ultraviolet semiconductor light-emitting element 10 will be described in detail below with reference to Figures 1, 2A, and 2B.

[0021] The substrate 11 is preferably a substrate with a low dislocation density such that the dislocation density in the active layer is low, but it is not particularly limited. The dislocation density in the active layer is preferably 10 ９ cm －２ or less, preferably 10 ８ cm －２ or less, and materials that can be lowered as described above are preferable. An AlN template substrate with an AlN film laminated on a sapphire substrate or a single crystal AlN substrate can be used.

[0022] Also, from the viewpoint of reducing the dislocation density in the active layer, it is preferable to use a single crystal AlN substrate as the substrate 11. The dislocation density of the single crystal AlN substrate is preferably 10 ８ cm －２ or less, more preferably 10 ６ cm －２ or less, and most preferably 10 ４ cm －２ or less. By using an AlN substrate with a lower dislocation density, 10 ６ cm －２ or less, and further 10 ４ cm －２ or less, it is possible to prevent a reduction in the light emission efficiency in the active layer due to dislocations, and further, it is possible to prevent problems such as diffusion of impurities through dislocations generated when the ultraviolet light emitting device is energized and an increase in leakage current.

[0023] In the ultraviolet semiconductor light emitting device 10 of this embodiment, an AlN substrate with a dislocation density of 10 ４ cm －２ was used.

[0024] Also, the surface roughness (RMS) of the single crystal AlN substrate 11 is preferably 1.0 nm or less, more preferably 0.5 nm or less, for the same reason as the above-described AlN template substrate. Naturally, in the case of the AlN substrate, the surface may be polished by a known polishing method such as chemical mechanical polishing.

[0025] Further, if the absorption coefficient of the substrate for the ultraviolet light emitted from the active layer is large, there is a concern that the total amount of ultraviolet light that can be extracted to the outside will decrease, leading to a decrease in the light emission efficiency. Therefore, the absorption coefficient of the AlN layer of the AlN substrate and the AlN template is preferably 20 cm－１ The following, and more preferably 10 cm －１ The following: 10 cm －１ By doing the following, for example, even if the thickness of the AlN substrate 11 is 100 μm, a linear transmittance of 90% or more can be secured.

[0026] The n-type AlGaN layer 12 is a Si (silicon) doped n-type conductive layer. The Al composition of the n-type AlGaN layer can be appropriately determined to obtain sufficient transmittance for the desired ultraviolet light emission wavelength. In the ultraviolet semiconductor light-emitting element 10, ultraviolet light emitted from the active layer 13 is transmitted through the n-type AlGaN layer 12 and the substrate 11 and emitted to the outside. Furthermore, as the Al composition of the n-type AlGaN layer increases, the band gap of the n-type AlGaN layer increases, and accordingly, shorter wavelength ultraviolet light can be transmitted.

[0027] Furthermore, the n-type AlGaN layer 12 may be formed from multiple layers with different Al compositions, and it can also be a composition gradient layer in which the Al composition is gradient in the stacking direction. In the ultraviolet semiconductor light-emitting element 10, the n-type AlGaN layer 12 is a first n-type Al Ｘ１ Ga １－Ｘ１ N-layer 12A and second n-type Al Ｘ２ Ga １－Ｘ２ It consists of N layers 12B. The first n-type Al Ｘ１ Ga １－Ｘ１ The N layer 12A is a composition gradient layer in which the Al composition X1 decreases from 1.0 to 0.75 in the stacking direction, and the second n-type Al Ｘ２ Ga １－Ｘ２ The N layer 12B can be a composition gradient layer in which the Al composition X2 decreases from 0.75 to 0.70.

[0028] The thickness of the n-type AlGaN layer 12 is not particularly limited and can be determined as appropriate. When a single-crystal AlN substrate is used for the substrate 11, the thickness of the n-type AlGaN layer 12 is preferably 0.5 μm or more and 2 μm or less. From the viewpoint of lowering the resistance value of the n-type AlGaN layer 12, a thicker layer of the n-type AlGaN layer 12 is preferable. However, when an AlN substrate is used for the substrate 11, if the thickness of the n-type AlGaN layer 12 becomes too thick, the n-type AlGaN layer will undergo lattice relaxation and dislocations will be more likely to occur.

[0029] For example, the n-type AlGaN layer 12 is made of the first n-type Al Ｘ１ Ga １－Ｘ１ N-layer 12A and second n-type Al Ｘ２ Ga １－Ｘ２ When formed as a laminated structure consisting of N layer 12B, the first n-type Al Ｘ１ Ga １－Ｘ１ The N layer 12A has a layer thickness of 200 nm, and the second n-type Al Ｘ２ Ga １－Ｘ２ The N layer 12B can have a layer thickness of 1000 nm. Furthermore, these first n-type Al Ｘ１ Ga １－Ｘ１ N-layer 12A and second n-type Al Ｘ２ Ga １－Ｘ２ The thickness of the N layer 12B is not limited to the example given, and can be appropriately determined so that the total layer thickness is 2.0 μm or less.

[0030] Furthermore, the Si concentration used to dope the n-type AlGaN layer 12 can be appropriately determined to obtain the desired n-type conductivity, but from the viewpoint of lowering the resistance of the n-type AlGaN layer 12, 1 × 10 １８ ~1 x 10 ２０ cm －３ Preferably, 5 × 10 １８ ~5 x 10 １９ cm －３ It is preferable that this is the case. Furthermore, the Si doping concentration may be constant in the thickness direction within the n-type AlGaN layer 12, or it may be modulated doping with different Si concentrations in the thickness direction.

[0031] The Si concentration and the Mg concentration, as described later, can be measured by known Secondary Ion Mass Spectrometry (SIMS) analysis. In addition, the Si concentration and Mg concentration measurements in this application are for the AlN layer, AlGaN layer, and GaN layer, respectively, using AlN and Al ０．６５ Ga ０．３５ Quantitative values ​​using standard samples of N and GaN are employed.

[0032] The active layer 13 (ACT) is Al Ａ１ Ga １－Ａ１ A barrier layer consisting of N layers and Al Ａ２ Ga １－Ａ２ This quantum well structure is composed of a quantum well layer made up of N layers. The emission peak wavelength of the active layer 13 is in the range of 210 to 300 nm. The wavelength of light emitted from the active layer 13 is determined by the Al composition and thickness of the well layer, so the Al composition and thickness can be appropriately determined so that a desired emission wavelength is obtained within the above wavelength range. In this embodiment, the emission wavelength of the active layer 13 is 265 nm.

[0033] The lower the Al composition of the p-type AlGaN contact layer, the lower the Schottky barrier, making it easier to achieve ohmic contact. Since the present invention provides a method for achieving ohmic contact even in a p-type AlGaN layer with a high Al composition, it can be an effective means of increasing the power output of ultraviolet semiconductor light-emitting devices having an emission wavelength of 300 nm or less, more preferably 285 nm or less, and more preferably 270 nm or less.

[0034] For example, the thickness of the quantum well layer can be set in the range of 2 to 10 nm, and the Al composition (aluminum composition) can be determined to obtain the desired emission wavelength. Similarly, the Al composition and thickness of the barrier layer are not particularly limited, but for example, the Al composition can be set in the range of A2 < A1 ≤ 1.0 and the thickness in the range of 2 to 15 nm.

[0035] Furthermore, the quantum well layer and barrier layer can also be Si-doped n-type layers. Both the quantum well layer and barrier layer may be Si-doped layers, or only the quantum well layer or only the barrier layer may be Si-doped. The Si concentration to be doped is not particularly limited, but is 1 × 10⁻⁶. １７ ~5 x 10 １８ cm －３ A range of [this] is preferred.

[0036] Furthermore, the number of quantum well layers is not particularly limited; it may be a multiple quantum well (MQW) structure with multiple quantum well layers, or a single quantum well (SQW). The number of quantum well layers is preferably determined appropriately within the range of 1 to 5.

[0037] The electron blocking layer 14 is provided adjacent to the active layer 13, and is made of Al Ｘ３ Ga １－Ｘ３ It is an N layer. The electron blocking layer 14 has the function of suppressing the overflow of electrons injected into the active layer 13 to the p-type AlGaN layer 15. Therefore, the electron blocking layer 14 is the active layer 13 and the p-type Al (described later). Ｙ１ Ga １－Ｙ１ Having a larger band gap than the N layer 15A, the Al composition X3 of the electron blocking layer 14 is determined to be in the range of 0.8 < X3 ≤ 1.0.

[0038] As the emission wavelength is shortened, the Al composition of the AlGaN layer epitaxially grown on the substrate 11 increases. When the emission wavelength is shorter than 270 nm, it is preferable that the Al composition X3 is 0.9 ≤ X3 ≤ 1.0 in order to fully exhibit its function as an electron blocking layer. In the ultraviolet semiconductor light-emitting element 10, AlN (X3 = 1) is used as the electron blocking layer 14.

[0039] Furthermore, the electron blocking layer 14 may be an undoped layer or may be doped with a p-type dopant, as long as it can perform its function as an electron blocking layer. As the p-type dopant material in the electron blocking layer 14, Mg (magnesium), Zn (zinc), Be (beryllium), C (carbon), etc., can be used. In particular, it is preferable to use Mg, which is commonly used as a p-type dopant material for AlGaN layers, and Mg is also used in the ultraviolet semiconductor light-emitting element 10.

[0040] The p-type dopant material may be uniformly doped in the stacking direction of the electron blocking layer 14, or the concentration of the dopant material can be varied in the stacking direction. In this embodiment, the stacked structure consists of a first electron blocking layer 14A (undoped AlN layer, thickness: 1 nm) and a second electron blocking layer 14B (Mg-doped p-type AlN layer, thickness: 8 nm) from the side in contact with the active layer 13.

[0041] The p-type dopant concentration in the electron block layer 14 is not particularly limited, but in order to obtain the function of an electron block layer, it should be 5 × 10⁻⁶. １８ ~1 x 10 ２０ cm －３ It is preferable that this is the case, and from the viewpoint of increasing the efficiency of carrier injection into the active layer, 1 × 10 １９ ~8 x 10 １９ cm －３ It is particularly preferable that this be the case.

[0042] The electron blocking layer 14 of the present invention does not contain an n-type dopant, or it contains a p-type Al as described later. Ｙ１ Ga １－Ｙ１ The n-type dopant can be included at a concentration less than that of the n-type dopant contained in the N layer 15A. Specifically, the concentration of n-type impurities in the electron block layer 14 is 1 × 10⁻⁶. １８ cm －３ The following is preferable:

[0043] According to the inventors' findings, p-type Al Ｙ１ Ga １－Ｙ１It is known that dopant diffusion occurs between adjacent electron block layers 14 during the growth of the N layer 15A. Therefore, the n-type dopant concentration in the electron block layer 14 is p-type Al Ｙ１ Ga １－Ｙ１ If it is higher than that of the N layer 15A, p-type Al from the electron block layer 14 Ｙ１ Ga １－Ｙ１ n-type dopant diffuses into layer N 15A, p-type Al Ｙ１ Ga １－Ｙ１ Precise control of the n-type dopant concentration in the N layer 15A can be difficult. This diffusion of p-type Al can also be a factor. Ｙ１ Ga １－Ｙ１ In order to prevent changes in the concentration of the n-type dopant in the N layer 15A, at least the n-type dopant in the electron block layer 14 is p-type Al Ｙ１ Ga １－Ｙ１ It is preferable that the concentration is less than the concentration of the n-type dopant contained in the N layer 15A.

[0044] Furthermore, the thickness of the electron blocking layer 14 is determined by its function as an electron blocking layer, and the p-type Al Ｙ１ Ga １－Ｙ１ The thickness of the N layer 15A should be appropriately determined to efficiently implant holes into the active layer, but a range of 1 to 30 nm is preferred. If the layer thickness is less than 1 nm, electrons tunnel, reducing its function as an electron blocking layer. On the other hand, if the layer thickness exceeds 30 nm, p-type Al... Ｙ１ Ga １－Ｙ１ Holes are less likely to be injected into the active layer from the N layer 15A. Taking these factors into consideration, the thickness of the electron blocking layer 14 is preferably 2 to 20 nm, and more preferably 5 to 15 nm.

[0045] Furthermore, as described above, the Mg doping in the electron blocking layer 14 can be differentiated in concentration depending on the stacking direction. For example, a first electron blocking layer 14A, which is an undoped AlN layer, can be stacked with a thickness of 1 to 5 nm on the side in contact with the active layer 13, and a second electron blocking layer 14B, which is a Mg-doped p-type AlN layer, can be stacked with a thickness of 5 to 15 nm. In this case, the Mg doping concentration can be 5 × 10⁻⁶, as described above. １８ ~1 x 10 ２０ cm －３It is preferably, 1×10 １９ ~8×10 １９ cm －３ and particularly preferably so.

[0046] In this embodiment, the electron blocking layer 14 is made of an AlN layer, and a p-type Al Ｙ１ Ga １－Ｙ１ N layer 15A and a p-type Al Ｙ２ Ga １－Ｙ２ N layer 15B, a p-type AlGaN layer 15 is formed.

[0047] The p-type Al Ｙ１ Ga １－Ｙ１ N layer 15A is a composition gradient layer (first composition gradient layer) in which the Al composition Y1 decreases in the stacking direction, and has a layer thickness of 60 nm. Specifically, the Al composition Y1 of the p-type Al Ｙ１ Ga １－Ｙ１ N layer 15A continuously and linearly decreases from the Al composition 1.0 (starting Al composition Y1B) of the electron blocking layer 14 (AlN layer) to the Al composition Y1E = 0.71 at the end. The p-type Al Ｙ１ Ga １－Ｙ１ The composition gradient rate RY1, which is the gradient rate of the Al composition of the N layer 15A in the stacking direction, is RY1 = (1.0 - 0.71) / 60 nm = 0.48% / nm.

[0048] Also, the p-type Al Ｙ２ Ga １－Ｙ２ N layer 15B is a composition gradient layer (second composition gradient layer) in which the Al composition Y2 decreases in the stacking direction, and has a layer thickness of 12 nm. Specifically, the Al composition Y2 of the p-type Al Ｙ２ Ga １－Ｙ２ N layer 15B continuously and linearly decreases from the Al composition Y2B = 0.64 at the start to the Al composition Y2E = 0.58 at the end. The p-type Al Ｙ２ Ga １－Ｙ２ The composition gradient rate RY2 of the N layer 15B is RY2 = (0.64 - 0.58) / 12 nm = 0.50% / nm, and the relationship of the composition gradient rates is 0 < RY1 < RY2.

[0049] The p-type Al Ｙ１ Ga １－Ｙ１ N layer 15A and the p-type Al Ｙ２ Ga１－Ｙ２ By making the N layer 15B a compositionally graded layer, a polarization doping effect can be obtained within the p-type AlGaN layer 15, making it easier to obtain a higher hole concentration, and as a result, the efficiency of hole injection into the active layer is increased.

[0050] Note that p-type Al Ｙ２ Ga １－Ｙ２ The compositional gradient RY2 of layer N 15B is p-type Al Ｙ１ Ga １－Ｙ１ It is preferable that the compositional gradient of the N layer 15A is greater than RY1 (RY1 < RY2), but it is not limited to this.

[0051] Here, p-type Al Ｙ２ Ga １－Ｙ２ The Al composition Y2B at the beginning of the N layer 15B is p-type Al. Ｙ１ Ga １－Ｙ１ The Al composition at the end of the N layer 15A is smaller than Y1E (Y1E > Y2B). In other words, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ The interface of layer N 15B is a heterointerface with a discontinuous Al composition. That is, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ The Al composition difference ΔY at the interface of layer N 15B is ΔY = Y1E - Y2B > 0. The Al composition difference ΔY is ΔY ≥ 0.01 (1% or more), and more preferably ΔY ≥ 0.02 (2% or more). Here, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ Between layers N and 15B, there is an Al Ｙ３ Ga １－Ｙ３ An N layer may be present, but it is preferable that ΔY2 = Y1E - Y3 ≥ 0, and its thickness is preferably in the range of 0.25 to 50 nm.

[0052] By making the p-type AlGaN layer 15 a compositionally graded layer, a polarization doping effect can be obtained within the p-type AlGaN layer 15, making it easier to obtain a higher hole concentration, and as a result, the hole injection efficiency into the active layer is increased. For example, when the emission wavelength is 270 nm or less, the p-type Al on the side in contact with the electron blocking layer 14 Ｙ１ Ga １－Ｙ１ The Al composition Y1 (=Y1B) of the N layer 15A is preferably 0.85 to 1.0, and more preferably 0.89 to 1.0. In this case, the relationship X3 ≥ Y1 is satisfied.

[0053] p-type Al opposite to the p-type AlGaN layer 15 Ｙ２ Ga １－Ｙ２ The Al composition Y2 (=Y2E) in the surface layer of the N layer 15B (i.e., the side in contact with the p electrode 21) is preferably in the range A2 to 0.85, and more preferably A1 to 0.85, which exceeds the Al composition of the well layer, from the viewpoint of maintaining transparency.

[0054] By adopting such a structure, a high polarization doping effect can be utilized, and the p-contact layer becomes transparent to the emission wavelength, resulting in a p-contact layer with excellent ohmic characteristics, a reduced turn-on voltage, and a highly efficient and high-power ultraviolet semiconductor light-emitting element.

[0055] Furthermore, the thickness of the p-type AlGaN layer 15, that is, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ The total thickness of the N layer 15B is not particularly limited, but can be appropriately determined within the range of 1 to 150 nm. From the viewpoint of polarization doping efficiency, a thinner layer or a higher gradient is preferable. On the other hand, if the layer thickness is thick, the transparency decreases (light absorption loss increases). From this viewpoint and from the viewpoint of practical productivity, the thickness of the p-type AlGaN layer 15 is preferably 2 to 120 nm, and particularly preferably 5 to 100 nm. Ｙ２ Ga １－Ｙ２ The thickness of the N layer 15B is preferably in the range of 1.0 to 20 nm, and more preferably in the range of 3.0 to 15 nm. Here, p-type Al Ｙ２ Ga １－Ｙ２The surface layer of N layer 15B is Al Ｙ４ Ga １－Ｙ４ An N layer may be present, but it is preferable that ΔY3 = Y2E - Y4 ≥ 0, and its thickness is preferably in the range of 0.25 to 50 nm.

[0056] Since the p-type AlGaN layer 15 is grown in a state of pseudo-lattice matching with the AlN substrate 11, it has a low dislocation density equivalent to that of the AlN substrate 11. Specifically, 10 ５ cm －２ The following dislocation densities are observed. It is preferable that the relaxation rate of the pseudo-lattice-matched semiconductor layer be 20% or less.

[0057] The p-type AlGaN layer 15 functions as a p-type contact layer. In the ultraviolet semiconductor light-emitting element 10 of this embodiment, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ In the N layer 15B, p-type impurities that act as acceptors and n-type impurities that act as donors are co-doped, but p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A is undoped Al Ｙ１ Ga １－Ｙ１ It may also be an N layer, p-type Al Ｙ２ Ga １－Ｙ２ The N-layer 15B does not need to be co-doped with n-type impurities.

[0058] Specifically, p-type Al Ｙ１ Ga １－Ｙ１ In layer N 15A, magnesium (Mg) is doped as a p-type impurity, and the amount of p-type impurity is 2.0 × 10⁻⁶. １９ cm －３ Furthermore, silicon (Si) was doped as an n-type impurity, and the amount of n-type impurity was 3.0 × 10⁻⁶. １８ cm －３ That was the case.

[0059] Also, p-type Al Ｙ２ Ga １－Ｙ２ The N layer 15B is doped with Mg as a p-type impurity, and the amount of p-type impurity at the start of growth is 2.0 × 10⁻⁶. １９ cm －３Therefore, at the end of the process, with a layer thickness of 3 to 15 nm, the result is 3.0 × 10⁻⁶. １９ ~2.0 x 10 ２０ cm －３ It was increased to that extent. In addition, Si (silicon) was doped as an n-type impurity, and the amount of n-type impurity was p-type Al Ｙ１ Ga １－Ｙ１ Same as N-layer 15A: 3.0 × 10 １８ cm －３ That was the case.

[0060] The p-type impurities used to dope the p-type AlGaN layer 15 include Mg, Zn (zinc), Be (beryllium), C (carbon), etc. Among these, it is preferable to use Mg, which is commonly used as a p-type dopant material for AlGaN semiconductors. The n-type impurities include Si, Ge (germanium), Se (selenium), S (sulfur), O (oxygen), etc. Among these, it is preferable to use Si, which is commonly used as an n-type dopant material.

[0061] Furthermore, the amount of p-type impurities doped into the p-type AlGaN layer 15A is 1 × 10⁻⁶ １７ ~1.2 x 10 ２０ cm －３ It is preferable that this is the case. Furthermore, as theoretically shown in J. Appl. Phys., Vol. 95, No. 8, 15 April (2004), it is thought that the amount of nitrogen vacancies, which are considered to be a factor in degradation, increases with the amount of p-type impurities in the p-type AlGaN layer 15A. Therefore, the amount of p-type impurities in the p-type AlGaN layer 15A is 1.2 × 10⁻⁶. ２０ cm －３ If this value is exceeded, the amount of nitrogen vacancies formed initially becomes too large, making it difficult to achieve a high power maintenance rate.

[0062] Furthermore, the p-type AlGaN layer 15A may be undoped, so the range is 0 to 5 × 10⁻⁶. １９ cm －３ Preferably, it is within the range of 0 to 4 × 10 １９ cm －３ It is within the range.

[0063] In the ultraviolet semiconductor light-emitting element 10 of this embodiment, p-type Al Ｙ１ Ga１－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ Each of the N layers 15B is a composition gradient layer in which the Al compositions Y1 and Y2 continuously decrease in the stacking direction, that is, in the direction away from the interface with the adjacent electron block layer 14 (EBL) (towards the p electrode 21).

[0064] p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ The N layer 15B is preferably a composition gradient layer in which the Al compositions Y1 and Y2 decrease linearly, but is not limited to this, and may also be a composition gradient layer in which the Al composition decreases with a curvilinear profile.

[0065] Furthermore, the p-type and n-type impurities doped into the p-type AlGaN layer 15 may have a constant concentration within the layer, or a concentration difference may be provided in the stacking direction. However, in order to drive with higher output and lower voltage, p-type Al Ｙ１ Ga １－Ｙ１ It is preferable that the interface between the N layer 15A and the electron blocking layer 14 contains no or very low concentrations of n-type impurities.

[0066] Furthermore, when AlN is used as the substrate 11, all layers of the n-type AlGaN layer 12, active layer 13, electron blocking layer 14, and p-type AlGaN layer 15 are crystallized in a state of pseudo-lattice matching with the AlN substrate 11, and therefore have a low dislocation density equivalent to that of the AlN substrate 11. Specifically, 10 ５ cm －２ The following dislocation densities are observed. It is preferable that the relaxation rate of the pseudo-lattice-matched semiconductor layer be 20% or less.

[0067] Although a semiconductor light-emitting element 10 comprising a substrate 11 has been described, the substrate 11 can be removed as needed or optionally provided, depending on the properties of the substrate used. In this case as well, the active layer 13, the electron blocking layer 14, and the p-type AlGaN layer 15 are pseudo-lattice-matched with respect to the n-type AlGaN layer 12, and it is preferable that the relaxation rate is within 20%.

[0068] As an ultraviolet semiconductor light-emitting element 10, a semiconductor light-emitting element with a substrate 11 on the n-type AlGaN layer 12 side has been described, but a support substrate other than the substrate 11 used for epitaxial growth is p-type Al Ｙ２ Ga １－Ｙ２ It can also be provided on the N layer 15B side. In this case, the material of the support substrate is not particularly limited, and can be polycrystalline AlN, Si, Al ２ O ３ Any known material used as a support substrate for light-emitting elements, such as Cu or CuW, can be used without limitation.

[0069] Although the case where the ultraviolet semiconductor light-emitting element 10 is a light-emitting diode (LED) has been described, it may also be configured as a semiconductor laser element (LD: Laser Diode).

[0070] (2) Characteristics of the double-composition gradient structure contact layer Figure 3A is a band diagram schematically showing the configuration of the p-type AlGaN layer 15 for (A) Example 1 (EX1), (B) Comparative Example 1 (CX1), and (C) Comparative Example 2 (CX2). Figure 3B is a figure showing the simulation results of the upper end of the valence band near the interface with the p electrode for the three cases (A) to (C) of the p-type AlGaN layer 15.

[0071] Figure 4 shows the definition of the barrier (or p-barrier) Vb at the contact between the p-type AlGaN layer 15 and the p-electrode 21. The barrier Vb is defined as the voltage value Vb at the intersection of the tangent line of the I-V (current-voltage) curve and the V-axis (X-axis). The simulation software used was SiLENSe (Simulator for Light Emitters based on Nitride Semiconductor) (registered trademark). Furthermore, since calculations with metal electrodes were not possible, the value of p-In was assumed to be 0.5 eV, which is the barrier between p-GaN and the metal electrode in the case of a metal electrode. ０．５５ Ga ０．４５ The simulation was performed assuming N was the electrode EL.

[0072] (A) Example 1 (EX1) is a p-type Al layer in which the Al composition Y decreases from 1.0 to 0.80 in the direction of electrode EL.Ｙ１ Ga １－Ｙ１ N layer 15A and p-type Al, which is a composition gradient layer in which the Al composition Y decreases from 0.70 to 0.60. Ｙ２ Ga １－Ｙ２ It consists of an N layer 15B. Also, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ The Al composition at the interface of layer N 15B is discontinuous.

[0073] (B) Comparative Example 1 (CX1) is a p-type Al layer in which the Al composition Y decreases from 1.0 to 0.80. Ｙ１ Ga １－Ｙ１ N layer 15A and p-type Al with Al composition Y constant at 0.60 Ｙ２ Ga １－Ｙ２ It consists of an N layer 15B. Also, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ The Al composition at the interface of layer N 15B is discontinuous.

[0074] (C) Comparative Example 2 (CX2) is a single composition gradient layer in which the p-type AlGaN layer 15 decreases from 1.0 to 0.60.

[0075] As described above, in (A) Example 1, (B) Comparative Example 1 and (C) Comparative Example 2, the electrode EL (p-In) of the p-type AlGaN layer 15 ０．５５ Ga ０．４５ The simulation was performed with the Al composition Y on the side in contact with N fixed at 0.60.

[0076] In Figure 3B, the depletion layer width WD is the thickness at which the energy from the interface with the electrode EL becomes nearly constant. A narrower depletion layer width WD is preferable because it facilitates the tunneling effect and lowers resistance.

[0077] In Example (A) 1 and Comparative Example (B), the p-type AlGaN layer 15 consists of two layers with a compositional gradient, and the depletion layer width WD is narrower than in Comparative Example (C) 2. Furthermore, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２In Example (A), where both N layers 15B are compositionally graded layers, the depletion layer width WD (=WD(A)) is narrower and more effective than in Comparative Example (B) 1. Furthermore, in Example (A), it can be seen that the depletion layer width WD(A) is much closer to the depletion layer width of GaN than in Comparative Example (B) 1 and Comparative Example (C) 2.

[0078] Figure 5 plots the relationship between normalized contact resistance and normalized barrier when the p-contact layer is p-type GaN, a single-composition gradient layer, and a double-composition gradient layer.

[0079] By using a p-type AlGaN layer with a double-composition gradient as the p-contact layer, we obtained results that closely approximated the electrode characteristics of a p-type GaN contact layer.

[0080] Figure 6A shows the equivalent circuit of a light-emitting diode. As described in the paper by Sher et al. (Non-Patent Literature 1), the heterojunction Jup (unipolar junction) between the p-GaN contact layer and the p-AlGaN layer acts as a parasitic diode and parasitic resistance, increasing the ideal factor of the diode, which is a factor that increases the turn-on voltage of the light-emitting diode.

[0081] Figure 6B schematically shows the heterobarrier (valence band) HB between the p-AlGaN layer and the p-AlGaN layer in the case of (i) p-GaN contact and (ii) p-AlGaN contact (Example 1, EX1). Specifically, in the case of (i) p-GaN contact where the p-GaN layer is in contact with the p-electrode, the heterobarrier HB between the second p-AlGaN layer and the p-AlGaN layer is larger than the heterobarrier HB between the first p-AlGaN layer and the second p-AlGaN layer in the case of (ii) p-AlGaN contact where the p-AlGaN layer is in contact with the p-electrode.

[0082] Figure 7 shows the I-V (current-voltage) characteristics for a p-AlGaN contact where the p-type AlGaN layer 15 is a double-composition gradient layer (Example 1, EX1) and for a p-GaN contact. Specifically, it shows the normalized forward current IF with respect to the normalized operating voltage Vop.

[0083] Therefore, in the case of (ii) the p-AlGaN contact (Example 1), the ideal factor becomes smaller, and the forward voltage (Vf) can be reduced by the amount of the reduction in the turn-on voltage.

[0084] As explained above, by using a p-type AlGaN contact layer which is a double-composition gradient layer, it is possible to provide an ultraviolet semiconductor light-emitting element that has a p-contact layer with excellent ohmic characteristics, a reduced turn-on voltage, and excellent device characteristics such as high efficiency and high output.

[0085] (3) Method for Manufacturing an Ultraviolet Semiconductor Light-Emitting Device The method for manufacturing the ultraviolet semiconductor light-emitting device 10 made of an AlGaN-based semiconductor layer of this embodiment will be described below. The ultraviolet semiconductor light-emitting device 10 of the present invention can be manufactured by the MOCVD method, which has high productivity and is widely used industrially. There are no particular restrictions on the use of known source gases for the Group III (Al, Ga) source gas and Group V (N) source gas.

[0086] Furthermore, the Mg and Si dopant source gases can be any known material without restriction, such as biscyclopentadienylmagnesium (Cp2Mg) and monosilane (SiH2Mg). ４ ), tetraethylsilane, etc. can be used.

[0087] The above raw material gases are converted to hydrogen (H ２ ) and / or nitrogen (N ２ The element layer of the ultraviolet semiconductor light-emitting element 10 is grown by supplying it onto the substrate 11 along with a carrier gas such as ).

[0088] Furthermore, the growth temperature of the element layers constituting the ultraviolet semiconductor light-emitting element 10 is not limited except as specifically specified, and can be appropriately determined to obtain the desired characteristics of each layer and the characteristics of the ultraviolet semiconductor light-emitting element 10. However, it is preferable to grow them at 1000 to 1200°C, and more preferably at 1000 to 1150°C.

[0089] (3.1) Growth of each semiconductor layer The substrate on which the semiconductor layer of the ultraviolet semiconductor light-emitting element 10 is grown has a dislocation density of 10 ４ cm －２The following AlN substrate was used as substrate 11. Referring again to Figure 2B, the first n-type Al was processed on this substrate 11 using a MOCVD apparatus. Ｘ１ Ga １－Ｘ１ N layer 12A (layer thickness: 200 nm), second n-type Al Ｘ２ Ga １－Ｘ２ An N-layer 12B (1000 nm) was grown on the substrate 11 and the first n-type Al Ｘ１ Ga １－Ｘ１ An AlGaN buffer layer may be present between the N layers 12A (the Al composition is the first n-type Al Ｘ１ Ga １－Ｘ１ (Al composition of N layer 12A is greater than or equal to 100%).

[0090] First n-type Al Ｘ１ Ga １－Ｘ１ N layer 12A and second n-type Al Ｘ２ Ga １－Ｘ２ The N layer 12B is a composition gradient layer, and the first n-type Al Ｘ１ Ga １－Ｘ１ In the N layer 12A, the Al composition (X1) decreases from 1.0 to 0.75 from the side in contact with the AlN layer, forming a second n-type Al Ｘ２ Ga １－Ｘ２ In layer N12B, the first n-type Al Ｘ１ Ga １－Ｘ１ From the side in contact with the N-type AlGaN layer 12A, the Al composition (X2) decreases from 0.75 to 0.70. Also, the Si concentration in the n-type AlGaN layer 12 is 1 × 10⁻⁶. １９ cm －３ It was controlled to achieve that.

[0091] Next, n-type Al ０．５９ Ga ０．４１ A barrier layer made of N (thickness: 7 nm) and Al ０．５ Ga ０．５ An active layer 13 with a triple quantum well layer structure, consisting of a quantum well layer made of N (4 nm), was grown. The Si concentration of the barrier layer was 1 × 10⁻⁶. １８ cm －３ It was controlled to achieve that.

[0092] Next, an electron blocking layer 14 made of AlN (9 nm) was grown. The first electron blocking layer 14A on the side in contact with the barrier layer of the active layer 13 was made of an undoped AlN layer (1 nm), and the remaining second electron blocking layer 14B was made of 4 × 10⁻¹⁶ １９ cm －３ The p-type AlN layer was made Mg-doped. Note that Si was not doped into the first electron blocking layer 14A and the Mg-doped second electron blocking layer 14B.

[0093] Next, a p-type AlGaN layer 15 was grown. The p-type AlGaN layer 15 is made of p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ１ Ga １－Ｙ１ p-type Al grown on N layer 15A Ｙ２ Ga １－Ｙ２ It consists of N layer 15B.

[0094] (3.2) The growth diagram 8 of the p-type AlGaN layer shows the profiles of the Al composition, growth temperature TG, Mg concentration and Si concentration during the growth of the p-type AlGaN layer 15. Specifically, the p-type AlGaN layer is co-doped with Mg and Si from the interface with the substrate 11 (AlN). Ｙ１ Ga １－Ｙ１ N-layer 15A was grown. Next, p-type Al Ｙ２ Ga １－Ｙ２ Layer N-15B was grown.

[0095] Growth temperature TG is 1030°C, Mg concentration is 2 × 10⁻⁶ １９ cm －３ Growth was carried out at a constant concentration, and in the final growth temperature reduction region Rg of the p-type AlGaN layer 15, the growth temperature TG was gradually reduced by 40°C from the previous 1030°C (reduced growth temperature = 990°C), and the Mg flow rate was increased to increase the Mg concentration. Ｙ２ Ga １－Ｙ２ The Mg concentration on the surface of layer N 15B is 4 × 10 １９ cm －３ That was the case.

[0096] Furthermore, the Si concentration remained consistently 3 × 10⁻⁶ from the start to the end of growth. １８ cm －３ That is, p-type Al Ｙ１ Ga１－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ N layer 15B was grown as a layer co-doped with Mg and Si.

[0097] Furthermore, the Mg concentration in the layer in which the Mg concentration was increased was 3 × 10⁻⁶. １９ ~2 x 10 ２０ cm －３ It is preferable that this is the case. Furthermore, the thickness of the growth temperature reduction region Rg is preferably in the range of 1.0 to 20 nm, and more preferably in the range of 3 to 15 nm.

[0098] Figure 9 shows p-type Al in contact with p-electrode 21. Ｙ２ Ga １－Ｙ２ The amount of Mg per unit layer thickness (1 nm) in the surface layer of N layer 15B (cm －３ This shows the relationship between the amount of Mg (cm³ / nm) and the barrier Vb (V). In other words, the horizontal axis represents the increased amount of Mg (cm³) in the surface portion. －３ This is the value (or average value) obtained by dividing the value by the thickness (nm) of the region where the amount of Mg has been increased.

[0099] In Figure 9, "No Si co-doping" indicates that Si co-doping was not performed only in the surface layer portion (layer thickness 12 nm).

[0100] In the following explanation, the surface layer portion in which the Mg content is increased or which is Si co-doped will be defined as the "surface contact layer SC".

[0101] As shown in Figure 8, in the growth temperature reduction region Rg, the growth temperature TG is gradually reduced to 40°C to complete the growth, and p-type Al Ｙ２ Ga １－Ｙ２ As shown in Figure 9, by performing Si co-doping in the N layer 15B, good ohmic contact can be achieved even with a relatively low amount of Mg. Ｙ２ Ga １－Ｙ２ It is effective if n-type impurities are co-doped in at least the surface contact layer SC of the N layer 15B.

[0102] More specifically, the amount of Mg (acceptor amount) in the surface contact layer SC is 8 × 10 １８ cm－３ If the value is greater than or equal to / nm, the barrier Vb (V) will be 0.1 (V) or less, resulting in good contact performance. Here, the acceptor amount is defined as the amount of Mg per unit layer thickness. The barrier Vb is defined as the voltage value Vb at the intersection of the tangent line of the I-V curve and the V axis (X axis) (Figure 4).

[0103] This is thought to be because lowering the growth temperature TG suppresses the generation of point defects, reduces the Schottky barrier, and improves the ohmic properties. The preferred temperature range for reducing the growth temperature TG is 20 to 60°C.

[0104] Also, p-type Al Ｙ２ Ga １－Ｙ２ p-type Al at the interface on the side of the N layer 15B that is in contact with the p electrode 21 Ｙ２ Ga １－Ｙ２ The donor concentration (Si concentration) in the surface layer of N-layer 15B above 1 nm is 1 × 10⁻¹⁰ １７ ~2 x 10 ２０ cm －３ It is preferable that this is the case. In this case, p-type Al Ｙ２ Ga １－Ｙ２ The surface contact layer SC (for example, 8 nm thick) of the N layer 15B may be Mg-Si co-doped, while other regions may be doped only with the p-dopant Mg. Ｙ２ Ga １－Ｙ２ The N layer 15B has the advantages of low contact resistance, enabling good ohmic contact, and low resistivity (series resistance).

[0105] (4) Modified example figure 10A is a band diagram of the p-type AlGaN layer 15 of Modified Example 1. In the p-type AlGaN layer 15, Ｙ２ Ga １－Ｙ２ The compositional gradient RY2 of layer N 15B is p-type Al Ｙ１ Ga １－Ｙ１ The fact that the compositional gradient of layer N 15A is greater than RY1 (RY1 < RY2) is the same as in Example 1 described above.

[0106] Also, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga１－Ｙ２ The fact that the Al composition is discontinuous at the interface of the N layer 15B is also the same as in Example 1 described above.

[0107] On the other hand, in the p-type AlGaN layer 15 of Example 1, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ We have described the case where the N layer 15B is co-doped with p-type and n-type impurities, but in this modified example 1, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ Layer N 15B is an uncodoped layer. That is, p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ The N layer 15B is doped with p-type impurities, but not with n-type impurities.

[0108] Figure 10B is a band diagram of the p-type AlGaN layer 15 of Modification 2. In the p-type AlGaN layer 15 of Modification 2, p-type Al Ｙ２ Ga １－Ｙ２ The compositional gradient RY2 of layer N 15B is p-type Al Ｙ１ Ga １－Ｙ１ This differs from Example 1 described above in that the compositional gradient of the N layer 15A is smaller than RY1 (RY1 > RY2 > 0).

[0109] p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ The other configurations are the same as in Example 1 described above, including the fact that the Al composition is discontinuous at the interface of the N layer 15B.

[0110] Figure 11A is the band diagram of the p-type AlGaN layer 15 of the modified example 3.

[0111] In the p-type AlGaN layer 15 of the modified example 3, p-type Al Ｙ１ Ga １－Ｙ１ This embodiment differs from the first embodiment described above in that the N layer 15A has a superlattice structure (SL).

[0112] In the modified example 3, the Al composition of multiple ultrathin films constituting the superlattice structure is modulated in the stacking direction, the Al composition decreases continuously in the stacking direction, and a composition gradient layer is formed in which the effective Al composition (Y1eff, shown by the dashed line) is sloped.

[0113] Note that p-type Al Ｙ１ Ga １－Ｙ１ N-layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ It is sufficient if at least one of the N layer 15B has a superlattice structure.

[0114] Figure 11B is a band diagram of the p-type AlGaN layer 15 of the modified example 4.

[0115] The p-type AlGaN layer 15 in modified example 4 is made of p-type Al Ｙ１ Ga １－Ｙ１ This differs from Example 1 described above in that the N layer 15A has a certain Al composition Y1. Ｙ２ Ga １－Ｙ２ The N layer 15B is configured as a composition gradient layer, the same as in Example 1.

[0116] Figure 11C is a band diagram of the p-type AlGaN layer 15 of Modification 5.

[0117] In the p-type AlGaN layer 15 of modified example 5, p-type Al Ｙ１ Ga １－Ｙ１ The N layer 15A is configured as a composition gradient layer, the same as in Example 1, but p-type Al Ｙ２ Ga １－Ｙ２ This example differs from Example 1 in that the N layer 15B has a constant Al composition Y2.

[0118] Figure 12A is a band diagram of the p-type AlGaN layer 15 of Modification 6. In the p-type AlGaN layer 15 of Modification 6, p-type Al Ｙ１ Ga １－Ｙ１ N layer 15A and p-type Al Ｙ２ Ga １－Ｙ２ A connecting layer 15C is provided between the N layer 15B and the p-type Al having Al composition Y4. Ｙ４ Ga １－Ｙ４ It is an N layer. The connecting layer 15C is an undoped AlＹ４ Ga １－Ｙ４ It can be an N-layer, or it can be co-doped.

[0119] Figure 12B is a band diagram of the p-type AlGaN layer 15 of Modification 7. In the p-type AlGaN layer 15 of Modification 7, p-type Al Ｙ２ Ga １－Ｙ２ A final contact layer 15D is provided as a surface layer on the N layer 15B. The final contact layer 15D is p-type Al having Al composition Y5. Ｙ５ Ga １－Ｙ５ It is an N-layer.

[0120] In the modified examples 6 and 7, both the connecting layer 15C and the final contact layer 15D may be provided, or only one of them may be provided. Furthermore, the Al compositions Y4 and Y5 may be constant, or they may be composition gradient layers in which the Al composition changes in the layer thickness direction. If the Al composition changes, it is preferable that the Al composition decreases in the lamination direction.

[0121] The same effects as in Example 1 can be obtained in the above-described modifications 1 to 7.

[0122] As described in detail above, this disclosure provides an ultraviolet semiconductor light-emitting element that has a high polarization doping effect, a p-contact layer with excellent ohmic characteristics, a reduced turn-on voltage, and excellent device characteristics such as high efficiency and high output.

[0123] 10: Ultraviolet semiconductor light-emitting element 11: Substrate 12: n-type AlGaN layer 12A: First n-type AlGaN layer 12B: Second n-type AlGaN layer 13: Active layer 14: Electron blocking layer 14A: First electron blocking layer 14B: Second electron blocking layer 15: p-type AlGaN layer 15A: p-type AlY1Ga1-Y1N layer 15A: p-type Al Ｙ１ Ga １－Ｙ１ N layer 15B: p-type Al Ｙ２ Ga １－Ｙ２N-layer 21: p-electrode 23: n-electrode RY1, RY2: composition tilt SC: surface component layer Vb: barrier WD: depletion layer width X1, X2, X3: Al composition Y1, Y2: Al composition ΔY: Al composition difference

Claims

1. An ultraviolet semiconductor light-emitting element made of an AlGaN-based semiconductor in which an n-type semiconductor layer, an active layer, an electron blocking layer, a p-type semiconductor layer, and a p-electrode are sequentially stacked, wherein the p-type semiconductor layer has a first composition gradient layer formed on the electron blocking layer, the Al composition of which decreases toward the p-electrode, and a second composition gradient layer formed on the first composition gradient layer, the Al composition of which decreases toward the p-electrode, the interface between the first composition gradient layer and the second composition gradient layer has a heterojunction in which the Al composition is discontinuous, and at the interface, the Al composition of the first composition gradient layer is greater than the Al composition of the second composition gradient layer.

2. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the composition gradient ratio RY2 of the second composition gradient layer is greater than the composition gradient ratio RY1 of the first composition gradient layer.

3. The ultraviolet semiconductor light-emitting element according to claim 1, characterized in that the Al composition of the first composition gradient layer and the second composition gradient layer decreases continuously toward the p electrode.

4. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the Al composition on the side of the first composition gradient layer in contact with the electron block layer is in the range of 0.8 to 1.0, the Al composition on the side of the second composition gradient layer in contact with the p electrode is in the range of 0.85 to 0.85 of the Al composition of the well layer in the active layer, and the difference in Al composition at the interface between the first composition gradient layer and the second composition gradient layer is 0.01 or more.

5. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the second composition gradient layer has a layer thickness of 1 nm or more and 20 nm or less.

6. The acceptor quantity at the surface of the interface where the second composition gradient layer contacts the p electrode is 8 × 10 １８ cm －３ The ultraviolet semiconductor light-emitting element according to claim 1, wherein the element is ≥ / nm.

7. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the first composition gradient layer and the second composition gradient layer are co-doped with p-type impurities and n-type impurities.

8. The donor concentration in the surface layer of 1 nm or more of the interface where the second composition gradient layer contacts the p electrode is 1 × 10⁻¹⁰ １７ ~2 x 10 ２０ cm －３ The ultraviolet semiconductor light-emitting element according to claim 7.

9. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the active layer, the electron blocking layer, and the p-type semiconductor layer are pseudo-lattice-matched with respect to the n-type semiconductor layer with a relaxation rate of 20% or less.

10. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the n-type semiconductor layer, the active layer, the electron blocking layer, and the p-type semiconductor layer are laminated on an AlN substrate, and the n-type semiconductor layer, the active layer, the electron blocking layer, and the p-type semiconductor layer are pseudo-lattice-matched with respect to the AlN substrate with a relaxation rate of 20% or less.

11. The ultraviolet semiconductor light-emitting element according to any one of claims 1 to 10, wherein the emission wavelength of the active layer is in the range of 210 to 300 nm.

12. The ultraviolet semiconductor light-emitting element according to claim 1, wherein p-type impurities and n-type impurities are co-doped in the surface contact layer, which is the surface layer portion of the second composition gradient layer.

13. The ultraviolet semiconductor light-emitting element according to claim 1, further comprising a connecting layer made of an AlGaN layer, provided between the first composition gradient layer and the second composition gradient layer.

14. The ultraviolet semiconductor light-emitting element according to claim 1, which is provided on the second composition gradient layer and has a final contact layer made of an AlGaN layer.

15. A method for manufacturing an ultraviolet semiconductor light-emitting element comprising an AlGaN-based semiconductor in which an n-type semiconductor layer, an active layer, an electron blocking layer, a p-type semiconductor layer, and a p-electrode are sequentially stacked, wherein the p-type semiconductor layer comprises a first composition gradient layer formed on the electron blocking layer, the Al composition of which decreases toward the p-electrode, and a second composition gradient layer formed on the first composition gradient layer, the Al composition of which decreases toward the p-electrode, the interface between the first composition gradient layer and the second composition gradient layer has a heterojunction in which the Al composition is discontinuous, the Al composition of the first composition gradient layer is greater than the Al composition of the second composition gradient layer at the interface, and the growth of the second composition gradient layer is carried out while gradually decreasing the growth temperature, and terminated when the growth temperature is reduced by 20°C or more.

16. The method for manufacturing an ultraviolet semiconductor light-emitting element according to claim 15, wherein p-type impurities and n-type impurities are co-doped in the surface contact layer, which is at least the surface layer portion of the second composition gradient layer.

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