Semiconductor ultraviolet light emitting element

Abstract

The present invention has: a substrate that is composed of single crystal AlN; an n-type cladding layer that is an n-type AlXGa1-XN layer formed on the substrate; a quantum well active layer that is formed on the n-type cladding layer; an electron blocking layer that is formed on the quantum well active layer; and a p-type AlGaN layer that is an Al composition gradient layer which is formed on the electron blocking layer, and in which the Al composition at the terminal end is smaller than the Al composition at the starting end, the Al composition at the terminal end being more than 0.80 but not more than 0.90.

Description

Ultraviolet semiconductor light emitting element

[0001] The present invention relates to an ultraviolet semiconductor light-emitting device, and more particularly to a nitride semiconductor light-emitting device that emits deep ultraviolet light.

[0002] In recent years, research and development has been progressing on semiconductor light-emitting elements that emit light in the deep ultraviolet region as light sources that have the effect of inactivating bacteria and viruses and sterilizing them. Furthermore, semiconductor light-emitting elements that emit deep ultraviolet light have attracted attention as light sources for resin curing and inspection.

[0003] Deep-ultraviolet semiconductor light-emitting devices have been known that employ a compositionally graded layer in which the Al composition of the p-type semiconductor layer is graded to improve light output. This is because the polarization doping effect increases the number of holes generated and increases light output. For example, Patent Document 1 discloses a two-stage compositionally graded structure consisting of a first compositionally graded layer in which the Al composition of the p-AlGaN layer monotonically decreases from 80% to 40% toward the p-electrode, and a second compositionally graded layer in which the Al composition monotonically decreases from 40% to 0%. Patent Document 2 also discloses a two-stage compositionally graded structure consisting of a first compositionally graded layer in which the AlN molar fraction of the p-AlGaN layer decreases from 90% to 60% away from the active layer, and a second compositionally graded layer in which the AlN molar fraction decreases from 60% to 0% toward the active layer. Patent Document 3 discloses an ultraviolet light-emitting device in which the Al composition (%) in the first compositionally graded layer changes by 20% to 50% per 1 nm of thickness, and the Al composition (%) in the second compositionally graded layer changes by 9% to 20% per 1 nm of thickness.

[0004] Japanese Patent No. 7228176 Japanese Patent Application Laid-Open No. 2023-121445 Japanese Patent No. 7405554

[0005] In conventional technology, when a compositionally graded structure is used in the p-cladding layer, the Al composition at the end is minimized. This is because a high Al composition reduces the amount of holes generated. Furthermore, when polarization doping is used to grade the composition, the greater the gradient, i.e., the greater the composition difference, the greater the theoretical amount of holes generated. However, in conventional technology, a low Al composition on the p-contact layer side (end) increases the hole concentration while lowering the energy required to generate nitrogen vacancies, thereby increasing the amount of nitrogen vacancies generated. Therefore, while increased injection efficiency improves initial output, the increased amount of nitrogen vacancies increases the amount of nitrogen vacancies diffusing into the active layer during current application, thereby shortening device life. Therefore, it has been difficult to achieve high reliability (long life) while maintaining high output characteristics. This application addresses the problem of increased nitrogen vacancy generation causing device degradation, and aims to provide an ultraviolet semiconductor light-emitting device that combines high output characteristics with high reliability (long life).

[0006] An ultraviolet semiconductor light emitting device according to one embodiment of the present invention comprises a substrate made of single crystal AlN; and an n-type AlN layer formed on the substrate. Ｘ Ga １－Ｘ The p-type AlGaN layer has an n-type cladding layer which is an N layer, a quantum well active layer formed on the n-type cladding layer, an electron blocking layer formed on the quantum well active layer, and an Al composition gradient layer formed on the electron blocking layer, the Al composition at the end being smaller than the Al composition at the beginning, the Al composition at the end being greater than 0.80 and not greater than 0.90.

[0007] 1 is a cross-sectional view schematically showing the structure of an ultraviolet semiconductor light-emitting device according to a first embodiment of the present invention; FIG. 2 is a diagram showing a band diagram of the ultraviolet light-emitting device of the first embodiment; FIG. 3 is a table showing the configuration of each layer of the ultraviolet light-emitting device; FIG. 4 is a diagram showing the forward voltage Vf, initial output, output maintenance ratio, and Fermi level for a comparative example and an example; FIG. 5 is a diagram plotting the output maintenance ratio (%) for each of the comparative example and the example; FIG. 6 is a diagram plotting the optical output ratio versus the thickness of the p-type cladding layer; FIG. 7 is a diagram plotting the degradation rate after 100 hours versus the thickness of the p-type cladding layer; FIG. 8 is a diagram schematically showing band diagrams of an active layer, an electron blocking layer, and a p-type cladding layer; FIG. 9 is a diagram showing a simulation result of the quasi-Fermi energy dEf versus the thickness of the p-type cladding layer; FIG. 10 is a diagram showing a band diagram of an ultraviolet light-emitting device of a second embodiment; FIG. 11 is a diagram showing the relationship between the quantum well layer thickness and the ground levels of electrons and holes; and FIG. 12 is a diagram schematically showing a band diagram of an active layer. 1A and 1B are diagrams schematically showing wave functions of electrons and holes when the thickness of a quantum well layer is small and large.

[0008] In the following, preferred embodiments of the present invention will be described, but these may be modified and combined as appropriate. In the following description and accompanying drawings, substantially the same or equivalent parts are designated by the same reference numerals.

[0009] 1 is a cross-sectional view schematically illustrating the structure of an ultraviolet semiconductor light-emitting element 10 according to a first embodiment of the present invention. The ultraviolet semiconductor light-emitting element (hereinafter also simply referred to as an ultraviolet light-emitting element) 10 is an ultraviolet light-emitting diode, and can be manufactured by, for example, but not limited to, a metal-organic chemical vapor deposition (MOCVD) method.

[0010] The ultraviolet light-emitting element 10 has an n-type cladding layer 12 (n-type AlGaN layer), an active layer 13, an electron blocking layer 14 (p-type AlN layer), a p-type cladding layer 15 (p-type AlGaN layer), and a p-type contact layer 16 (p-type GaN layer) stacked in this order on a substrate 11 by epitaxial growth.

[0011] 2 is a schematic diagram showing a band diagram of the ultraviolet light-emitting element 10. The ultraviolet light-emitting element 10 will be described in more detail below with reference to FIGS.

[0012] The substrate 11 has a dislocation density of 10 ８ cm －２ The AlGaN-based semiconductor material constituting the ultraviolet light-emitting device of the present invention has a dislocation density of 10 or less, as described in, for example, OPTICS EXPRESS Vol. 25 No. 16 A639 (2017). ７ ~10 ８ cm －２ It is known that the luminous efficiency drops sharply when the dislocation density exceeds 10. Therefore, the lower the dislocation density of the single crystal AlN substrate, the more preferable it is. ６ cm －２ More preferably, 10 ４ cm －２ By using the AlN substrate 11 with such a low dislocation density, the dislocation density in the active layer 13 (described later) can be reduced to 10% or less without reducing the light emission efficiency. ７ cm －２ The AlN substrate 11 can be an AlN template in which AlN is grown on a substrate such as sapphire.

[0013] The growth plane (surface) of the AlN substrate 11 of the present invention is not particularly limited and can be a C-plane, M-plane, or other growth plane, but is preferably the C-plane, which is commonly used as a growth plane for AlGaN-based materials. Furthermore, when the C-plane is used as the crystal growth plane, the AlN substrate 11 is preferably an OFF substrate that is slightly inclined from the C-plane, for purposes such as improving the smoothness of the AlGaN layer grown on the substrate. The inclination angle from the C-plane is not particularly limited and may be determined as appropriate so as to obtain a smooth AlGaN layer, but is typically selected in the range of 0.1 to 1.0°. The inclination direction from the C-plane is also not particularly limited and may be selected as appropriate, such as the A-axis direction or the M-axis direction, but it is preferable to select the M-axis direction, which increases the linearity of the step edges.

[0014] Furthermore, since a large surface roughness of the AlN substrate 11 can cause abnormal growth of the AlGaN layer grown on the substrate, the surface roughness (RSM) is preferably 1.0 nm or less, and more preferably 0.5 nm or less. In order to obtain such a smooth surface or to remove damaged layers formed on the substrate surface during the substrate manufacturing process, the substrate surface is preferably subjected to chemical mechanical polishing (CMP).

[0015] Furthermore, if the absorption coefficient of the substrate for the ultraviolet light emitted from the active layer is large, the total amount of ultraviolet light that can be extracted to the outside may decrease, which may lead to a decrease in light emission efficiency. Therefore, the absorption coefficient of the AlN substrate or the AlN layer of the AlN template is preferably 20 cm －１ It is preferably 10 cm or less. －１ Less than 10cm －１ By setting the thickness as below, even if the thickness of the AlN substrate 11 is 100 μm, for example, it is possible to ensure an in-line transmittance of 90% or more.

[0016] n-type cladding layer 12 (n-type Al Ｘ Ga １－Ｘ The n-type cladding layer 12 and the n-type AlGaN layer 13 are n-type conductive layers doped with silicon (Si). In an ultraviolet semiconductor light-emitting element, ultraviolet light emitted from the light-emitting layer is usually emitted to the outside after passing through the n-type cladding layer 12 and the substrate 11. As the Al composition (aluminum composition) of the n-type AlGaN layer increases, the band gap of the n-type AlGaN layer increases, and accordingly, ultraviolet light of a shorter wavelength can be transmitted. Therefore, the Al composition of the n-type AlGaN layer can be appropriately determined so as to obtain sufficient transmittance for the desired ultraviolet light emission wavelength.

[0017] The n-type cladding layer 12 may be formed of a plurality of layers with different Al compositions (X), and may be a compositionally graded layer in which the Al composition is graded in the stacking direction. For example, the first n-type cladding layer 12A (n-type Al Ｘ１ Ga １－Ｘ１ N layer) and the second n-type cladding layer 12B (n-type Al Ｘ２ Ga １－Ｘ２ N layers).

[0018] The first n-type cladding layer 12A is a compositionally graded layer in which the Al composition X1 decreases from 1.0 to 0.75 in the stacking direction (growth direction), and the second n-type cladding layer 12B is a compositionally graded layer in which the Al composition X2 decreases from 0.75 to 0.70. It is preferable that the Al compositions at the interfaces of the first n-type cladding layer 12A and the second n-type cladding layer 12B are equal.

[0019] Furthermore, the thickness of the n-type cladding layer 12 is not particularly limited and may be determined appropriately. However, if the thickness of the n-type cladding layer 12 is too large, lattice relaxation occurs between the AlN substrate 11 and the n-type cladding layer 12, making dislocations more likely to occur. Therefore, it is preferable to set the total thickness of the n-type cladding layer 12 in the range of 0.5 to 2.0 μm.

[0020] For example, the n-type cladding layer 12 may be the first n-type cladding layer 12A (n-type Al Ｘ１ Ga １－Ｘ１ N layer) and the second n-type cladding layer 12B (n-type Al Ｘ２ Ga １－Ｘ２ In the case of a laminated structure consisting of a first n-type cladding layer 12A and a second n-type cladding layer 12B, the first n-type cladding layer 12A may have a thickness of 200 nm and the second n-type cladding layer 12B may have a thickness of 1000 nm. Naturally, the thicknesses of the first and second n-type cladding layers 12A and 12B are not limited to the values ​​shown in the example, and may be appropriately determined so that the total thickness is 2.0 μm or less.

[0021] The concentration of Si doped into the n-type cladding layer 12 may be determined appropriately so as to obtain the desired n-type conductivity. However, from the viewpoint of reducing the resistance value of the n-type cladding layer 12, it is preferable to set the concentration of Si to 1×10 １８ ~1 x 10 ２０ cm －３ It is preferable that the １８ ~5 x 10 １９ cm －３ It is preferable that:

[0022] The Si doping concentration may be constant in the layer thickness direction in the n-type cladding layer 12, or may be modulated doping in which the Si concentration varies in the layer thickness direction. The Si concentration and the Mg (magnesium) concentration described later can be measured by known secondary ion mass spectrometry (SIMS) analysis. The Si concentration and Mg concentration in this application are measured using AlN and AlGaN layers, respectively, for the AlN layer, the AlGaN layer, and the GaN layer. ０．６５ Ga ０．３５ Quantitative values ​​using standard samples of N and GaN are used.

[0023] The active layer 13 (ACT) is Al Ｗ１ Ga １－Ｗ１ A plurality of barrier layers 13B made of N layers and Al Ｗ２ Ga １－Ｗ２ It has a multi-quantum well (MQW) structure made up of a plurality of quantum well layers 13A each made up of an N layer. Note that the active layer 13 is not limited to a multi-quantum well active layer, and may be a single quantum well (SQW) active layer.

[0024] The peak emission wavelength of the active layer 13 is in the range of 200 to 360 nm. The wavelength of light emitted from the active layer 13 is determined by the Al composition and thickness of the quantum well layers 13A and the barrier layers 13B, and therefore the Al composition and thickness of the quantum well layers 13A and the barrier layers 13B can be appropriately determined so as to obtain a desired emission wavelength within the above wavelength range.

[0025] It is preferable that the emission peak wavelength of the active layer 13 is in the UVC region, which is a short wavelength region of deep ultraviolet, and particularly in the range of 200-280 nm, which has an excellent bactericidal effect.

[0026] The quantum well layer 13A and the barrier layer 13B may be n-type layers doped with Si. The quantum well layer 13A and the barrier layer 13B may be Si-doped layers, or the quantum well layer 13A may be doped with Si, or the barrier layer 13B may be doped with Si. The doping concentration of Si is not particularly limited, but may be 1×10 １７ ~5 x 10 １８ cm－３ The range is preferred.

[0027] An electron blocking layer (EBL) 14 on the active layer 13 prevents electrons injected into the active layer 13 from passing through a p-type cladding layer (p-type Al Ｙ Ga １－Ｙ N layer) 15. Ｚ Ga １－Ｚ It is preferable that the p-type cladding layer 15 has a larger band gap than the active layer 13 and the p-type cladding layer 15 .

[0028] As the emission wavelength becomes shorter, the Al composition of the AlGaN layer epitaxially grown on the substrate 11 becomes higher. When the emission wavelength is shorter than 270 nm, the Al composition Z of the electron blocking layer 14 is preferably 0.95≦Z≦1.0 in order to fully exhibit the function as an electron blocking layer. Ｚ Ga １－Ｚ AlN (Z=1) is used as the N layer.

[0029] 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 function as an electron blocking layer.

[0030] Examples of p-type dopant materials that can be used in the electron blocking layer 14 include magnesium (Mg), zinc (Zn), beryllium (Be), and carbon (C). 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 examples of the present invention described below.

[0031] The p-type dopant material may be doped uniformly in the stacking direction of the electron blocking layer 14, or the concentration of the dopant material may be varied in the stacking direction. For example, a stacked structure may be formed from an undoped AlN layer (Z=1) and a p-type AlN layer doped with Mg (magnesium) from the side in contact with the active layer. The p-type dopant concentration in the electron blocking layer 14 is set to 1.0×10 from the viewpoint of obtaining the function as an electron blocking layer and increasing the efficiency of carrier injection into the light-emitting layer. １９ ~8.0 x 10 １９ cm －３ is preferably 3.0 × 10 １９ ~5.0 x 10 １９ cm －３ , particularly preferably 3.0 × 10 １９ ~4.0 x 10 １９ cm －３ is.

[0032] The electron blocking layer 14 preferably has a thickness in the range of 4 to 10 nm. If the thickness is less than 4 nm, the effect as an electron blocking layer is reduced due to the tunneling effect, and if the thickness is 10 nm or more, the efficiency of hole injection decreases.

[0033] The p-type cladding layer 15 is formed on the electron blocking layer 14 and is made of p-type Al doped with Mg. Ｙ Ga １－Ｙ The p-type cladding layer 15 is an N layer and functions as a cladding layer. The aluminum composition (Al composition) Y of the p-type cladding layer 15 is formed as a composition gradient layer in which the aluminum composition Y decreases with increasing distance from the electron blocking layer 14, i.e., toward the p-type contact layer 16.

[0034] Specifically, the Al composition Y of the p-type cladding layer 15 decreases from an Al composition Y1 on the side in contact with the electron blocking layer 14 (starting end) to an Al composition Y2 on the side in contact with the p-type contact layer 16 (ending end) (Y1>Y2). By forming the p-type cladding layer 15 as a compositionally graded layer, a polarization doping effect is obtained within the p-type cladding layer 15, making it easier to obtain a higher hole concentration and, as a result, increasing the efficiency of hole injection into the active layer 13. Note that the Al composition Y1 at the interface with the electron blocking layer 14 is preferably equal to or less than the Al composition Z of the electron blocking layer 14. For example, when the emission wavelength is 270 nm or shorter, the Al composition Y1 on the side in contact with the electron blocking layer 14 is preferably 0.95 to 1.0.

[0035] Furthermore, the Al composition Y2 on the opposite side (termination) in contact with the p-type contact layer 16 is preferably greater than 0.80, more preferably 0.83 or greater, and even more preferably 0.85 or greater. By adopting such a structure, not only can the polarization doping effect described above be enhanced and transparency at the emission wavelength be maintained, resulting in high luminous efficiency, but also the generation of nitrogen vacancies in the p-type cladding layer 15 can be suppressed, thereby suppressing device degradation. The above-mentioned materials can be used as the p-type dopant material without any restrictions, but it is preferable to use Mg, as in the electron blocking layer 14.

[0036] The thickness of the p-type cladding layer 15 is preferably 40 to 120 nm from the viewpoints of suppressing carrier overflow and improving resistance. In the embodiment of the present invention, a compositionally graded layer is employed in which the Al composition Y decreases in the growth direction from the Al composition of the electron blocking layer 14.

[0037] The p-type cladding layer 15 (p-type Al Ｙ Ga １－Ｙ The p-type cladding layer 15 is co-doped with p-type impurities that act as acceptors and n-type impurities that act as donors. It is preferable that the p-type cladding layer 15 is co-doped, but this is not limitative.

[0038] Examples of p-type impurities that can be used to dope the p-type cladding layer 15 include magnesium (Mg), zinc (Zn), beryllium (Be), and carbon (C). Of these, it is preferable to use Mg, which is commonly used as a p-type dopant material for AlGaN semiconductors. Examples of n-type impurities that can be used include silicon (Si), germanium (Ge), selenium (Se), sulfur (S), and oxygen (O). Of these, it is preferable to use silicon (Si), which is commonly used as an n-type dopant material.

[0039] In the p-type cladding layer 15 of this embodiment, the ratio (Nd / Na) of the n-type impurity concentration (Nd) to the p-type impurity concentration in the p-type cladding layer 15 satisfies the following formula: 0.009≦(Nd / Na)≦0.80

[0040] The amount of p-type impurity doped into the p-type cladding layer 15 is 1×10 １７ ~5.0 x 10 １９ cm －３ Furthermore, as theoretically shown in J. Appl. Phys., Vol. 95, No. 8, 15 April (2004), it is believed that the amount of nitrogen defects, which are considered to be a cause of degradation, increases with the amount of p-type impurities in the p-type cladding layer 15. Therefore, when the amount of p-type impurities is 1.2 × 10 ２０ cm －３ If the nitrogen vacancy amount exceeds 100%, the amount of nitrogen vacancies formed in the early stage becomes too large, making it difficult to obtain a high power retention rate.

[0041] Furthermore, when the p-type impurity concentration is low, in the case of a composition gradient layer in which the Al composition Y is gradient, the mobility of minority carriers (electrons) increases, resulting in a decrease in output and making it difficult to obtain high luminous efficiency. Therefore, the p-type impurity concentration can be appropriately determined within the above range, taking into consideration such a trade-off. However, in order to obtain a higher output maintenance rate and high output, the amount of p-type impurity doped into the p-type cladding layer 15 should be 5.0×10 １８ cm －３ ~5.0 x 10 １９ cm －３ It is preferable that the ratio is 1.0 × 10 １９ ~4.0 x 10 １９ cm－３ With these amounts of n-type impurities, an ultraviolet light emitting device 10 with high luminous efficiency can be obtained.

[0042] The concentrations of the p-type impurities and n-type impurities co-doped into the p-type cladding layer 15 may be constant within the layer, or may vary in concentration in the stacking direction. For example, the side in contact with the electron blocking layer 14 may be a co-doped layer, and the remaining p-type cladding layer 15 may be a layer that is not doped with n-type impurities.

[0043] A p-type contact layer 16 (p-type GaN layer) doped with a p-type dopant may be formed on the p-type cladding layer 15 in order to reduce the contact resistance with the electrode. The above-mentioned known p-type dopant materials can be used as the p-type dopant material, but for the same reason, it is preferable to use Mg. The Mg doping concentration in the p-type contact layer 16 is not particularly limited, but in order to reduce the resistance value in the p-type GaN layer and the contact resistance, it is preferable to use a concentration of 1×10 １８ ~2 x 10 ２０ cm －３ The thickness of the p-type contact layer 16 is not particularly limited, and may be determined appropriately within the range of 5 to 500 nm. The p-type contact layer 16 is not limited to a p-type GaN layer. For example, a p-type Al layer having a small Al composition and a small In composition may be used. ｘ Ga １－ｘ－ｙ In ｙ It is also possible to use N layers (0≦x≦0.3, 0≦y≦0.3).

[0044] All of the AlGaN layers 12, 13, 14, and 15 except for the p-type contact layer 16 are grown in a state of lattice matching with the AlN substrate 11, and therefore have a low dislocation density equivalent to that of the AlN substrate 11. ５ cm －２ The dislocation density is as follows:

[0045] Although the present specification describes a case where the ultraviolet light emitting element 10 of the present invention is a light emitting diode, it may also be configured as a semiconductor laser element (LD: Laser Diode).

[0046] Next, a method for manufacturing the ultraviolet light-emitting device 10 having the above-described structure will be described. The ultraviolet light-emitting device 10 of the present invention can be manufactured by known crystal growth methods such as metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Among these, MOCVD is preferred because it has high productivity and is widely used industrially. Known source gases can be used as the group III (Al, Ga) source gas and group V (N) source gas used in the present invention without any particular restrictions.

[0047] For example, the Group III source gas may be trimethylaluminum, triethylaluminum, trimethylgallium, triethylgallium, etc. The Group V source gas is usually ammonia.

[0048] Furthermore, known materials can be used without any restrictions as dopant source gases for Mg and Si, and examples that can be used include biscyclopentadienyl magnesium, monosilane, and tetraethylsilane.

[0049] The above-mentioned source gases are supplied onto the substrate 11 together with a carrier gas such as hydrogen and / or nitrogen, thereby growing the element layers of the ultraviolet light emitting element 10 .

[0050] The supply ratio of the Group III source gas to the Group V source gas (V / III ratio) may be determined appropriately so as to obtain desired characteristics, but is preferably set within the range of 500 to 10,000.

[0051] Furthermore, the growth temperature of the element layers constituting the ultraviolet light-emitting device 10 is not particularly limited and may be determined appropriately so as to obtain the desired characteristics of each layer and the characteristics of the ultraviolet light-emitting device 10. However, growth at a temperature of 1000 to 1200°C is preferable, and 1000 to 1150°C is more preferable.

[0052] (2) Characteristics of the Ultraviolet Semiconductor Light-Emitting Device (Al Composition Y2 at the End of the p-Type Cladding Layer) The present invention will be specifically described below using an example in which an ultraviolet light-emitting device 10 with an emission wavelength of 265 nm was fabricated, but the present invention is not limited to the example. (a) Device Structure FIG. 3 is a table showing the configuration of each layer of the fabricated ultraviolet light-emitting device 10 of the first embodiment. Samples were fabricated in which the Al composition Y2 at the end of the p-type cladding layer 15, which is a composition-graded layer, was varied. The Al composition of the p-type cladding layer 15 was graded so that it monotonically (linearly) decreased from the side in contact with the electron blocking layer 14 (starting end) to the side in contact with the p-type contact layer 16 (end end).

[0053] More specifically, sample lots (Comparative Example (CX1), Example EX1, Example EX2, and Example EX3, respectively) were fabricated in which the Al composition Y1 of the p-type cladding layer 15 on the side in contact with the electron blocking layer 14 (starting end) was 0.98 (98%), and the Al composition Y2 on the side in contact with the p-type contact layer 16 (terminating end) was 0.80, 0.83, 0.85, and 0.90 (80%, 83%, 85%, and 90%). In addition, a plurality of samples were fabricated for each lot of the Comparative Example (CX1) and Examples (EX1 to EX3), and the device characteristics were evaluated.

[0054] The configuration of the other semiconductor layers of the ultraviolet light emitting element 10 is as described above. Ｘ１ Ga １－Ｘ１ N layer) and the second n-type cladding layer 12B (n-type Al Ｘ２ Ga １－Ｘ２ N layer) is a composition gradient layer, and Si is 1.0 × 10 １９ cm －３ It is doped at a concentration of

[0055] The active layer 13 is a multiple quantum well (MQW) active layer consisting of three quantum well layers 13A, and the compositions and thicknesses of the quantum well layers 13A and the barrier layers 13B are adjusted so that the emission wavelength is 265 nm. The quantum well layers 13A and the barrier layers 13B each contain 1.0×10 Si. １８ cm －３The electron blocking layer 14 is formed as an AlN layer (thickness: 9 nm) doped with Mg at a concentration of 4.0×10 １９ cm －３ It is doped at a concentration of

[0056] The p-type cladding layer 15 is formed as a co-doped layer. Specifically, the p-type cladding layer 15 contains 4.0×10 Mg, which is a p-type impurity. １９ cm －３ The n-type impurity Si is doped at a concentration of 3.5×10 １８ cm －３ The p-type cladding layer 15 is doped at a concentration of 60 nm.

[0057] On the p-type cladding layer 15, Mg is 3.0 to 5.0 × 10 １９ cm －３ A p-type contact layer 16 (p-type GaN layer, layer thickness: 270 nm) doped at a concentration of 0.1% is formed on the semiconductor substrate 10 .

[0058] (b) Element Characteristics FIG. 4 is a diagram showing the forward voltage Vf (V), initial output (mW), output maintenance rate after 250 hours (%, @50 mW equivalent), and Fermi level (calculated value) for the above-mentioned comparative example (CX1) and examples (EX1 to EX3). The output maintenance rate (%) was calculated by conducting a life test at a driving current of 440 mA (room temperature) and measuring the output of the ultraviolet light-emitting device 10 in one lot at the initial stage and after 250 hours. Therefore, the variation in the plot corresponds to the output variation within each lot.

[0059] FIG. 5 is a plot of the output maintenance rate (%) after 250 hours versus the initial output (mW) for each of the comparative example (CX1) and examples (EX1-EX3). The comparative example (CX1), in which the Al composition Y2 at the termination of the p-type cladding layer 15 was 0.80, had an output maintenance rate (%) of 85%. However, compared to the comparative example (CX1), the examples (EX1-EX3), in which the Al composition Y2 exceeded 0.80, had an output maintenance rate (%) of 90% or more, a significant improvement. Furthermore, when the Al composition Y2 was 0.85 (EX2), the output maintenance rate (%) improved to 97%. That is, a 15% improvement in the output maintenance rate was observed at 50 mW. On the other hand, when the Al composition Y2 was 0.90 (EX3), a decrease in the initial output was observed.

[0060] The above results show that the device life can be improved by setting the Al composition Y2 at the termination of the p-type cladding layer 15 to a value exceeding 0.80. The Al composition Y2 at the termination of the p-type cladding layer 15 is preferably greater than 0.80 and equal to or less than 0.90, and more preferably equal to or greater than 0.83 and less than 0.90 (0.83≦Y2<0.90).

[0061] (3) Characteristics of Ultraviolet Semiconductor Light-Emitting Device (Thickness of p-Type Cladding Layer) Fig. 6 is a plot of the optical output ratio versus the thickness (nm) of the p-type cladding layer 15. Fig. 7 is a plot of the deterioration rate after 100 hours versus the thickness of the p-type cladding layer 15. The graph shows the measurement results for an ultraviolet light-emitting device 10 having a p-type cladding layer 15 with an Al composition Y1 of 0.98 at the starting end and an Al composition Y2 of 0.80 at the end.

[0062] 6, the thinner the p-type cladding layer 15 (compositionally graded layer), the more improved the optical output. This is thought to indicate that the hole injection efficiency is increased by the polarization doping effect.

[0063] 7, it was found that reliability (device life) was improved by increasing the thickness of the p-type cladding layer 15. The thickness of the p-type cladding layer 15 is preferably 45 nm or more and 80 nm or less, and more preferably 50 nm or more and 70 nm or less.

[0064] (4) Considerations regarding p-type cladding layer and element degradation (a) Mechanism of element degradation The external quantum efficiency (EQE) ηext of a semiconductor light-emitting element is expressed by the following formula (1) using the internal quantum efficiency ηint, injection efficiency ηinj, and light extraction efficiency ηextract: ηext = ηint + ηinj + ηextract (1) Since the light extraction efficiency ηextract can be ignored as a factor in reducing reliability, it is thought that the rate of increase in the non-radiative recombination probability in the internal quantum efficiency ηint and the rate of deterioration of the injection efficiency ηinj determine the slope of reliability due to current flow.

[0065] In other words, the main factors that reduce reliability are presumed to be a decrease in the recombination probability due to the diffusion of nitrogen defects in the p-type cladding layer into the active layer (i.e., deterioration of the active layer), and a decrease in the efficiency of hole injection from the p-type cladding layer (i.e., an increase in the amount of nitrogen defects in the p-type cladding layer (a decrease in injection efficiency)). Therefore, it is clear that the p-type cladding layer, which is related to these two factors, has a significant impact on reliability, and in particular, controlling the amount of nitrogen vacancies in the p-type cladding layer is extremely important for improving reliability.

[0066] Nitrogen vacancies, which act as donors, are generated in the p-type cladding layer to maintain electrical neutrality within the crystal. These nitrogen vacancies diffuse into the active layer during current flow, resulting in a decrease in the output maintenance rate. Furthermore, ultraviolet semiconductor light-emitting devices have a significantly lower light extraction efficiency than, for example, blue LEDs, so their light output is increased by passing a large current through them. A large current value also means that the semiconductor light-emitting device generates a large amount of heat. Therefore, thermal damage to the light-emitting layer due to heat generation is likely to increase non-radiative recombination centers, i.e., nitrogen defects, significantly reducing the device's lifespan.

[0067] 8A is a diagram schematically showing a band diagram of the active layer 13 (ACT in the figure), the electron blocking layer 14 (EB), and the p-type cladding layer 15 (p-CLAD). It shows that when the Al composition Y2 at the end of the p-type cladding layer 15 is increased, for example, from 0.80 to 0.85, the hole concentration decreases, and therefore the difference (dEf) between the valence band edge of the p-type cladding layer 15 and the quasi-Fermi level Fv increases.

[0068] 8B is a graph showing the simulation results, with the horizontal axis representing the thickness (nm) of the p-type cladding layer 15 and the vertical axis representing the quasi-Fermi energy dEf (eV) measured from the band edge, where the calculated values ​​are calculated using the Al composition Y2 at the termination of the p-type cladding layer 15 as a parameter.

[0069] As explained with reference to Figure 5, increasing the Al composition Y2 at the end of the p-type cladding layer 15 improved the output maintenance ratio. The generation energy of point defects (nitrogen vacancies) in AlGaN depends on the quasi-Fermi level, and the nitrogen vacancy concentration decreases as the quasi-Fermi energy dEf measured from the band edge increases. That is, increasing the Al composition Y2 increases the generation energy, thereby reducing the amount of nitrogen vacancies generated and, ultimately, the amount of nitrogen vacancies diffusing into the active layer 13.

[0070] 8B , compared to the Al composition Y2=0.80 (Comparative Example CX1) indicated by the dashed circle, the theoretical differences (Δ) in quasi-Fermi energy dEf for Y2=0.85 (Example EX2) and Y2=0.90 (Example EX3) are 0.020 eV and 0.078 eV, respectively. It is believed that increasing the Al composition Y2 at the termination reduces the amount of nitrogen vacancies. Therefore, it is believed that the diffusion of nitrogen vacancies into the active layer 13 is suppressed, which suppresses deterioration of the ultraviolet light-emitting device 10 and improves reliability. However, if the Al composition Y2 at the termination is too large, the hole concentration decreases, resulting in a decrease in light output.

[0071] (c) Co-doping Concentration Figure 9 shows the simulation results of the quasi-Fermi energy dEf (eV) versus the thickness (nm) of the p-type cladding layer 15, showing calculated values ​​with the co-doped Si concentration as a parameter. Note that the calculated values ​​are fixed at an Al composition Y2 of 0.85 at the end of the p-type cladding layer 15. The dashed line (wo CoSi) indicates the case where no co-doping is performed.

[0072] It can be seen that the quasi-Fermi energy dEf increases by increasing the co-doping concentration Nd. For example, when the co-doping concentration Nd is 3.0×10 １８ cm －３ From 4.0 x 10 １８ cm －３ From this simulation result, the co-doping concentration Nd is 2.0 × 10 １８ cm －３ Above 4.0 x 10 １８ cm －３ Preferably, it is 3.0 × 10 or less. １８ cm －３ Above 4.0 x 10 １８ cm －３ It is even more preferable that:

[0073] [Second Embodiment] (1) Structure of Ultraviolet Semiconductor Light-Emitting Device An ultraviolet light-emitting device 30 of the second embodiment differs from the ultraviolet light-emitting device 10 of the first embodiment in that the active layer 13 has one main quantum well layer and multiple sub-quantum well layers. The other configurations are the same as those of the ultraviolet light-emitting device 10 of the first embodiment. That is, the configuration of the p-type cladding layer 15, which is a composition-graded layer, is also the same as that of the ultraviolet light-emitting device 10 of the first embodiment.

[0074] 10 is a schematic diagram showing a band diagram of the ultraviolet light emitting device 30 of the second embodiment. The active layer 13 of the ultraviolet light emitting device 30 is Al Ｗ１ Ga １－Ｗ１ A plurality of barrier layers 13B made of N layers and Al Ｗ２ Ga １－Ｗ２ It has a multiple quantum well (MQW) structure consisting of three quantum well layers QS1, QS2, and QM each consisting of N layers.

[0075] The active layer 13 has one main quantum well layer QM and two sub-quantum well layers QS1 and QS2, with the sub-quantum well layers QS1 and QS2 and the main quantum well layer QM being provided in this order from the substrate 11 side.

[0076] The active layer 13 also has a first barrier layer 13B1 (layer thickness: TB1) provided on the n-type cladding layer 12, a second barrier layer 13B2 (layer thickness: TB2) provided between the first sub-quantum well layer QS1 and the second sub-quantum well layer QS2, a third barrier layer 13B3 (layer thickness: TB3) provided between the second sub-quantum well layer QS2 and the main quantum well layer QM, and a final barrier layer 13L (layer thickness: TL) provided between the main quantum well layer QM and the electron blocking layer 14.

[0077] The first sub-quantum well layer QS1 and the second sub-quantum well layer QS2 have the same crystal composition and layer thickness TS. Furthermore, the first sub-quantum well layer QS1, the second sub-quantum well layer QS2, and the main quantum well layer QM have the same crystal composition (Al composition: W2). Here, in this specification, "same" in terms of crystal composition, layer thickness, etc. includes "substantially same" and refers to the degree of sameness obtained in the crystal growth of the semiconductor layer.

[0078] The barrier layers 13B, i.e., the first barrier layer 13B1, the second barrier layer 13B2, the third barrier layer 13B3, and the final barrier layer 13L, have the same crystal composition (Al composition: W1). When there is no need to distinguish between the multiple barrier layers, they will be collectively referred to as the barrier layer 13B.

[0079] It is preferable that the thickness of the barrier layer between each sub-quantum well layer (TB2 in the above case) is the same as the thickness of the barrier layer between the sub-quantum well layer closest to the main quantum well layer QM (TB3 in the above case).

[0080] The main quantum well layer QM is the quantum well layer closest to the electron blocking layer 14. The two sub-quantum well layers QS1 and QS2 have the same layer thickness TS, and the main quantum well layer QM has a layer thickness TM (TS<TM) greater than that of the sub-quantum well layers QS1 and QS2.

[0081] Although the example shows the case where two sub-quantum well layers are provided, it is sufficient to provide at least one sub-quantum well layer QS1, QS2, ..., QSn (n is an integer of 1 or more). When multiple sub-quantum well layers are provided, the multiple sub-quantum well layers have the same crystal composition and layer thickness. When multiple sub-quantum well layers are not particularly distinguished from one another, they will be collectively referred to as sub-quantum well layer QS.

[0082] The emission peak wavelength of the active layer 13 is in the range of 200 to 360 nm. The wavelength of light emitted from the active layer 13 is determined by the Al composition and layer thickness of the main quantum well layer QM, and therefore the Al composition and layer thickness can be appropriately determined so as to obtain a desired emission wavelength within the above wavelength range.

[0083] It is preferable that the emission peak wavelength of the active layer 13 is in the UVC region, which is a short wavelength region of deep ultraviolet, and particularly in the range of 200-280 nm, which has an excellent bactericidal effect.

[0084] The main quantum well layer QM, the sub-quantum well layers QS1 and QS2, and the barrier layer 13B may be n-type layers doped with Si. The main quantum well layer QM, the sub-quantum well layers QS1 and QS2, and the barrier layer 13B may be Si-doped layers, or either the main quantum well layer QM or the sub-quantum well layers QS1 and QS2 may be doped with Si, or only the barrier layer 13B may be doped with Si. The doping concentration of Si is not particularly limited, but may be 1×10 １７ ~5 x 10 １８ cm －３ The range is preferred.

[0085] The final barrier layer 13L may contain a p-type dopant such as Mg. The Mg contained in the final barrier layer 13L may be obtained by diffusion doping from the electron blocking layer 14, which will be described later, or may be obtained by intentionally doping with Mg.

[0086] (2) Consideration of Device Characteristics (a) Improvement of External Differential Efficiency (EQE) and Light Output First, in ultraviolet semiconductor light-emitting devices, the hole concentration in the p-type semiconductor layer is generally low and the mobility of holes is lower than that of electrons, resulting in low external quantum efficiency and light output.

[0087] In the ultraviolet light-emitting device 30 of this embodiment, the efficiency of hole injection from the p-type cladding layer 15 to the active layer 13 via the electron blocking layer 14 is increased. That is, because the thickness of the main quantum well layer QM closest to the electron blocking layer 14 is increased, the efficiency of hole carrier capture from the electron blocking layer 14 by the main quantum well layer QM is increased, and the number of carriers contributing to light emission is increased, which increases the recombination probability and increases the external differential efficiency (EQE) and light output.

[0088] 11 is a schematic diagram showing the relationship between the quantum well layer thickness Lz and the ground levels of electrons and holes. As shown in the figure, increasing the quantum well layer thickness Lz brings the ground levels of electrons and holes (quantum levels En1 and Eh1) closer to the band edges of the quantum well, i.e., the conduction band Ec and the valence band Ev, respectively, making radiative recombination easier and increasing the light output.

[0089] (b) Light emission from the main quantum well layer QM is dominant Fig. 12 is a schematic diagram showing a band diagram of the active layer 13. The reason why light emission from the main quantum well layer QM is dominant in the ultraviolet light-emitting device 30 of this embodiment will be described below.

[0090] 12, since the mobility of holes is lower than that of electrons, it is thought that the main quantum well layer QM closest to the electron blocking layer 14 captures most of the holes and confines them within the main quantum well layer QM. Therefore, it is thought that light emission by the main quantum well layer QM becomes dominant.

[0091] In fact, when the EL (electroluminescence) spectrum of a device in which the thickness TM of the main quantum well layer QM is twice the thickness TS of the sub-quantum well layers QS1 and QS2 was examined, the wavelength shift amount compared to a comparative device having the same structure except for the fact that it is composed of three quantum well layers each having the same thickness as the sub-quantum well layer QS matched the wavelength shift amount obtained as a result of simulation for the increase in the thickness of the main quantum well layer QM relative to the sub-quantum well layer QS.

[0092] Furthermore, the EL spectrum showed no increase in the full width at half maximum (FWHM), and in fact the FWHM was reduced compared to the comparative example. Therefore, it is presumed that the light emission from the sub-quantum well layers QS1 and QS2 is very small. The reason for the reduced full width at half maximum compared to the comparative example is thought to be that the thicker main quantum well layer QM has a lower rate of change in the ground state relative to thickness fluctuations.

[0093] (c) Function of the Sub-Quantum Well Layer The following describes the function of the sub-quantum well layer QS provided in the active layer 13. Referring again to Fig. 12, when the thickness TB3 of the third barrier layer 13B3 between the main quantum well layer QM and the sub-quantum well layer QS2 adjacent to the main quantum well layer QM is small (for example, several nm), electrons confined in the sub-quantum well layer QS2 tunnel due to the tunnel effect and are injected into the main quantum well layer QM, which has a low ground level.

[0094] It is believed that electron tunneling has the effect of confining and injecting carriers into the main quantum well layer QM, which has a thick well layer, and that electron tunneling can reduce electron overflow, further increasing the probability of radiative recombination and increasing output power.

[0095] On the other hand, in the valence band, the hole concentration is low and the hole mobility is small, so most of the holes are captured in the main quantum well layer QM. Therefore, the sub-quantum well layer QS has the function of enhancing the confinement of carriers in the main quantum well layer QM.

[0096] The thickness TB3 of the third barrier layer 13B3 is preferably 10 nm or less, more preferably 7 nm or less, and even more preferably 6 nm or less, from the viewpoint of electron tunneling.

[0097] (d) Thickness of the Main Quantum Well Layer FIG. 13 is a diagram showing the wave functions of electrons and holes when the quantum well layer thickness Lz is small (left side of the diagram) and large (right side of the diagram).

[0098] The difference in crystal composition between the quantum well layer and the barrier layer causes lattice distortion between the quantum well layer and the barrier layer, resulting in a piezoelectric field. As a result, the wave functions of electrons and holes in the quantum well layer are misaligned, reducing their overlap, which in turn reduces the probability of radiative transition and the luminous efficiency. Increasing the thickness Lz of the quantum well layer reduces the overlap of the wave functions, resulting in a decrease in the luminous efficiency.

[0099] Furthermore, when an AlGaN layer is stacked on an AlN substrate, the difference in lattice constant causes tension and compressive strain in the underlying layer. As the AlGaN layer is made thicker, lattice relaxation occurs when it reaches a critical thickness, causing threading dislocations in the AlGaN layer, reducing the internal quantum efficiency and light-emitting efficiency.

[0100] Using band gap modeling software for semiconductor devices ("SiLENSe"), simulations were performed by varying the layer thicknesses of the sub-quantum well layers QS1, QS2 and the main quantum well layer QM based on the composition and layer thickness of the AlGaN stack.

[0101] According to this simulation, it was found that the internal quantum efficiency increases with increasing quantum well layer thickness, and that an effective thickness is at least 1.2 times the thickness of the sub-quantum well layer QS2. Furthermore, in order to fully and effectively exert the above-mentioned electron tunneling effect from the sub-quantum well layer QS2 to the main quantum well layer QM and the electron confinement effect, the thickness TM of the main quantum well layer QM is preferably at least 1.5 times, and even more preferably at least twice the thickness TS of the sub-quantum well layer QS (sub-quantum well layer QS2) adjacent to the main quantum well layer QM.

[0102] On the other hand, the thickness TM of the main quantum well layer QM is preferably 16 nm or less, which is the critical thickness when an AlGaN layer is stacked on an AlN substrate, and more preferably 10 nm or less, which is the range in which the overlap integral of electrons and holes takes a significant value, based on a simulation of the overlap integral of electrons and holes.

[0103] (e) Thickness of the Final Barrier Layer The inventors of the present application have conducted studies and experiments on the diffusion of p-type impurities (Mg) from the p-type cladding layer 15 into the active layer 13. １８ cm －３ If the concentration is greater than this, that is, if a large number of nitrogen defects are present in the main quantum well layer QM, the device life will be reduced.

[0104] As a result of extensive research, it has been found that the Mg concentration entering the main quantum well layer QM from the p-type cladding layer 15 through the electron blocking layer 14 is 1×10 １８ cm －３ Furthermore, when the layer thickness TL of the final barrier layer 13L is 12 nm or more, the Mg concentration in the main quantum well layer QM can be suppressed to 1×10 １７ cm －３ It was found that this could be reduced to the following:

[0105] On the other hand, increasing the layer thickness TL of the final barrier layer 13L may result in a decrease in light output. This is because the hole injection efficiency decreases as the layer thickness TL of the final barrier layer 13L increases. However, no significant decrease in light output was observed when the layer thickness TL was 30 nm or less. Note that the layer thickness TL is more preferably 27 nm or less, and even more preferably 21 nm or less.

[0106] (3) Consideration of Device Lifetime As described above, nitrogen vacancies diffuse from the p-type cladding layer to the active layer, reducing the probability of non-radiative recombination in the quantum well layer and thereby impairing reliability. However, with the ultraviolet light-emitting device 30 of this embodiment, the thickness of the main quantum well layer, which is primarily responsible for light emission, is thick, reducing the injected carrier density per volume, thereby suppressing the proliferation of nitrogen vacancies in the quantum well. Therefore, it is possible to provide an ultraviolet light-emitting device that suppresses device degradation and has an excellent device lifetime.

[0107] Furthermore, the increase in non-radiative recombination centers due to heat generation in the active layer, i.e., the increase in nitrogen vacancies, can be suppressed, thereby suppressing deterioration of the element and providing an ultraviolet light-emitting element with an excellent element life.

[0108] As described above in detail, according to the present invention, it is possible to provide an ultraviolet semiconductor light-emitting element that can suppress element degradation and achieves both high output characteristics and high reliability (long life).

[0109] The present invention is not limited to the above-described embodiments and may be modified and applied without departing from the scope of the present disclosure. For example, the above-described embodiments have been described with reference to an Al composition gradient in a p-type cladding layer (AlGaN layer), but the present disclosure can be applied to a p-type semiconductor layer that increases the hole generation concentration by the polarization doping effect. Furthermore, while the p-type cladding layer has been described with reference to a composition gradient layer in which the Al composition monotonically decreases from the beginning to the end, the p-type cladding layer may include a layer in which the Al composition is constant or increases in part. For example, a composition gradient layer in which the Al composition decreases stepwise may also be employed.

[0110] 10: UV light emitting element 11: Substrate 12: n-type cladding layer 13: Active layer 13A: Quantum well layer 13B: Barrier layer 13L: Final barrier layer 14: Electron blocking layer 15: P-type cladding layer 16: P-type contact layer QM: Main quantum well layer QS1, QS2: Sub-quantum well layers

Claims

1. A substrate made of single crystal AlN, and an n-type AlN layer formed on the substrate. Ｘ Ga １－Ｘ an n-type cladding layer which is an N layer; a quantum well active layer formed on the n-type cladding layer; an electron blocking layer formed on the quantum well active layer; and a p-type AlGaN layer formed on the electron blocking layer, the p-type AlGaN layer being an Al composition gradient layer having an Al composition at a terminal end that is smaller than an Al composition at a starting end, the Al composition at the terminal end being greater than 0.80 and not greater than 0.

90.

2. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the thickness of the p-type AlGaN layer formed on the electron blocking layer is 45 nm or more and 80 nm or less.

3. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the thickness of the p-type AlGaN layer formed on the electron blocking layer is 50 nm or more and 70 nm or less.

4. The p-type AlGaN layer formed on the electron blocking layer is co-doped with n-type impurities, and the concentration of the n-type impurities is 2.0×10 １８ cm －３ Above 4.0 x 10 １８ cm －３ 2. The ultraviolet semiconductor light-emitting device according to claim 1, wherein:

5. The p-type impurity concentration of the p-type AlGaN layer is 5.0×10 １８ cm －３ ~5.0 x 10 １９ cm －３ 5. The ultraviolet semiconductor light-emitting element according to claim 4, wherein the wavelength of said ultraviolet semiconductor light-emitting element is within the range of 1000 nm to 1500 nm.

6. The ultraviolet semiconductor light-emitting element according to claim 5, wherein the ratio (Nd / Na) of the n-type impurity concentration (Nd) to the p-type impurity concentration (Na) in the p-type AlGaN layer satisfies 0.009≦(Nd / Na)≦0.

80.

7. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the Al composition of the termination of the p-type AlGaN layer formed on the electron blocking layer is 0.83 or more and less than 0.

90.

8. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the p-type AlGaN layer formed on the electron blocking layer has an Al composition that monotonically decreases with increasing distance from the electron blocking layer.

9. The ultraviolet semiconductor light emitting element according to claim 1, further comprising a p-type contact layer formed on said p-type AlGaN layer formed on said electron blocking layer.

10. The ultraviolet semiconductor light-emitting element according to claim 1, wherein the quantum well active layer comprises barrier layers having the same crystal composition as each other, at least one sub-quantum well layer separated from the other by the barrier layers, and a main quantum well layer which is the quantum well layer closest to the electron blocking layer, wherein the at least one sub-quantum well layer has the same crystal composition and thickness, and the at least one sub-quantum well layer and the main quantum well layer have the same crystal composition, and the main quantum well layer has a thickness that is 1.2 times or more that of the at least one sub-quantum well layer.

11. The ultraviolet semiconductor light emitting device according to claim 10, wherein the thickness of said main quantum well layer is 16 nm or less.

12. The ultraviolet semiconductor light-emitting device according to claim 10, wherein the main quantum well layer has a thickness at least 1.5 times that of the sub-quantum well layer.

13. The ultraviolet semiconductor light-emitting device according to claim 10, wherein the thickness of the barrier layer between the main quantum well layer and the sub-quantum well layer adjacent to the main quantum well layer is 7 nm or less.

14. The ultraviolet semiconductor light-emitting device according to claim 10, wherein the final barrier layer between the main quantum well layer and the electron blocking layer has a thickness in the range of 9 to 27 nm.

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