JP7405554B2 - UV light emitting element - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act applies Posted on September 4, 2020 https://meeting. jsap. or. jp/program (Proceedings of the 80th Japan Society of Applied Physics Autumn Academic Conference, pp. 13-060) [Publications, etc.] September 18-21, 2020 80th Japan Society of Applied Physics Autumn Academic Conference Hokkaido University Sapporo campus

The present invention relates to an ultraviolet light emitting device.

Ultraviolet light-emitting elements can control the emission wavelength by controlling the bandgap energy of the light-emitting layer, and have a long life and high reliability. Therefore, they are used for various purposes such as lighting, light sources for measuring instruments, and light sources for sterilization. A typical ultraviolet light emitting element has a PIN structure in which a light emitting layer is sandwiched between a p-type nitride semiconductor and an n-type nitride semiconductor on a substrate.
In order to increase the light emission output (light emission intensity) of a light emitting element, it is important to improve the light emission efficiency. For example, Patent Document 1 describes a technology that improves luminous efficiency by forming a quantum barrier layer between an active layer (emitting layer) and a p-type nitride semiconductor layer and adjusting the band gap energy of each layer. Disclosed. Furthermore, for example, Non-Patent Document 1 describes that by shortening the lifetime of carriers in the active layer, the non-radiative recombination rate is reduced and the luminous efficiency is improved.

Japanese Patent Application Publication No. 2010-114403

J. Soc. Mat. Sci., Japan), Vol.50, No.4, pp. 372-375, Apr. 2001

Ultraviolet light emitting elements are required to have even higher emission intensity.
An object of the present invention is to provide an ultraviolet light emitting element that can be expected to achieve higher emission intensity.

In order to solve the above problems, the ultraviolet light emitting element of the first aspect of the present invention has the following configurations (1) and (2).
(1) A substrate, a first conductivity type nitride semiconductor layer formed on the substrate and containing Al and Ga, and a nitride semiconductor layer formed on the first conductivity type nitride semiconductor layer and containing Al and Ga. The device includes a light emitting layer made of a material, and a second conductivity type nitride semiconductor layer formed on the light emitting layer and having a conductivity different from that of the first conductivity type nitride semiconductor layer. The light emitting layer includes at least one quantum well layer and a quantum barrier layer sandwiching the quantum well layer.
(2) In the quantum barrier layer, the Al composition (%) of the nitride semiconductor in the material is nonuniform in the depth direction, and the amount of change in the Al composition (%) in the depth direction is 5% or more and 12%. It is as follows.

The ultraviolet light emitting device according to the second aspect of the present invention has the above configuration (1) and the following configuration (3).
(3) In the quantum barrier layer, the Al composition (%) of the nitride semiconductor in the material is nonuniform in a plane perpendicular to the depth direction, and the amount of change in the Al composition (%) in the plane is 5. % or more and 16% or less.

The ultraviolet light emitting device according to the third aspect of the present invention has the above configuration (1) and the following configurations (4) and (5).
(4) A first compositionally graded layer in which the Al composition (%) of the nitride semiconductor in the material changes is present at the boundary where the quantum well layer switches to the quantum barrier layer toward the second conductivity type nitride semiconductor layer. , a second compositionally graded layer in which the Al composition (%) of the nitride semiconductor in the material changes is present at the boundary where the quantum barrier layer switches to the quantum well layer toward the second conductivity type nitride semiconductor layer.
(5) The amount of change in the Al composition (%) in the first graded composition layer is 20% or more and not more than 50% per 1 nm of thickness, and the amount of change in the Al composition (%) in the second graded composition layer is: It is 9% or more and 20% or less per 1 nm of thickness.

According to the ultraviolet light-emitting elements of the first, second, and third aspects of the present invention, higher emission intensity can be expected.

FIG. 1 is a plan view showing a nitride semiconductor light emitting device according to an embodiment. 2 is a cross-sectional view showing a nitride semiconductor light emitting device according to an embodiment, and shows a cross section taken along the line AA in FIG. 1. FIG. FIG. 2 is a plan view showing the nitride semiconductor light emitting device of FIG. 1 in a state before a pad electrode and an insulating layer are formed.

[Ultraviolet light emitting device of the first to third embodiments]
As described above, the ultraviolet light emitting device of the first to third aspects of the present invention includes a substrate, a first conductivity type nitride semiconductor layer formed on the substrate and containing Al and Ga, and a first conductivity type nitride semiconductor layer formed on the substrate. A light emitting layer formed on the semiconductor layer and made of a material containing a nitride semiconductor containing Al and Ga; and a second conductive layer formed on the light emitting layer and having a conductivity different from that of the first conductivity type nitride semiconductor layer. type nitride semiconductor layer.

<Configuration>
(substrate)
Preferably, the substrate is made of a material containing a nitride semiconductor containing Al. A nitride semiconductor containing Al is, for example, AlN. The nitride semiconductor containing Al is not limited to AlN, and may be, for example, AlGaN. Here, the word "contains" in the expression "the substrate...contains a nitride semiconductor" means that the layer mainly contains a nitride semiconductor, but this expression also includes cases where other elements are included. It will be done. Specifically, slight changes are made to the composition of this layer by adding a small amount of other elements (for example, Ga (if Ga is not the main element), In, As, P, or Sb, etc., in a few percent or less). This expression also includes the addition of . In expressing the composition of other layers, the word "contains" has the same meaning. Moreover, the small amount of elements contained is not limited to the above.

Further, the substrate includes not only a single crystal substrate but also an AlN thin film and an AlGaN thin film formed on a different type of substrate.
For example, if a nitride semiconductor single crystal substrate such as AlN or AlGaN or a substrate (template substrate) in which a nitride semiconductor layer such as AlN or AlGaN is formed on a different substrate is used, the nitride formed on the upper side of the substrate The difference in lattice constant from the semiconductor layer becomes small, and threading dislocations can be reduced by growing the nitride semiconductor layer in a lattice-matched system.

The substrate may also be doped n-type or p-type with donor or acceptor impurities. Further, the substrate may be a mixed crystal of a nitride semiconductor such as AlN and sapphire (Al ₂ O ₃ ), Si, SiC, MgO, Ga ₂ O ₃ , ZnO, GaN, or InN.
As a method for manufacturing the substrate, general substrate growth methods such as a vapor phase growth method such as a sublimation method or an HVPE method, and a liquid phase growth method can be applied. Further, the thickness of the substrate may be, for example, 100 μm or more and 600 μm or less. Further, examples of the plane orientation include c-plane (0001), a-plane {11-20}, m-plane {10-10}, etc., but a c-plane substrate is more preferable.

(First conductivity type nitride semiconductor layer)
The first conductivity type nitride semiconductor layer is a nitride semiconductor layer that has conductivity and contains Al and Ga. A first conductivity type nitride semiconductor layer is formed on the substrate. Here, for example, the word "on" in the expression "the first conductivity type nitride semiconductor layer is formed on the substrate" means that the first conductivity type nitride semiconductor layer is formed on the substrate. However, this expression also includes the case where another layer is further present between the substrate and the first conductivity type nitride semiconductor layer. Regarding the relationships between other layers, the word "above" has the same meaning. For example, a case where a second conductivity type nitride semiconductor layer is formed on the light emitting layer via an electron blocking layer is also included in the expression "the second conductivity type nitride semiconductor layer is formed on the light emitting layer". .

The nitride semiconductor included in the first conductivity type nitride semiconductor layer is, for example, Al _x Ga _(1-x) N (0<x<1). By using Al _x Ga _(1-x) N (0<x<1), when forming a light-emitting layer using a material that corresponds to the band gap energy in the deep ultraviolet region, its crystallinity can be improved and the light-emitting efficiency can be improved. It becomes possible to improve the performance. From the viewpoint of achieving high luminous efficiency, the nitride semiconductor included in the first conductivity type nitride semiconductor layer is preferably a mixed crystal of AlN and GaN. In addition to N, impurities such as group V elements other than N such as P, As, and Sb, C, H, F, O, Mg, and Si are mixed into the nitride semiconductor of the first conductivity type nitride semiconductor layer. However, the types of impurity elements are not limited to this. Furthermore, from the viewpoint of lattice matching with the substrate and light extraction from the light emitting layer, Al _x Ga _(1-x) N (0.6<x<1) is more preferable.

Further, the first conductivity type nitride semiconductor layer and the second conductivity type nitride semiconductor layer are nitride semiconductor layers having mutually different conductivities. Generally, an n-type semiconductor has better crystallinity than a p-type semiconductor, and has less influence on the light emitting layer. Therefore, it is preferable that the first conductivity type nitride semiconductor layer is n-type and the second conductivity type nitride semiconductor layer is p-type.
Further, it is preferable that the first conductivity type nitride semiconductor layer has a lattice relaxation rate of 0% or more and less than 5%. Thereby, the occurrence of lattice defects such as misfit dislocations at the interface between the substrate and the first conductivity type nitride semiconductor layer can be suppressed, and high luminous intensity can be obtained without being affected by dislocations.

Further, the first conductivity type nitride semiconductor layer preferably has a thickness of 300 nm or more and 1000 nm or less. When the thickness of the first conductivity type nitride semiconductor layer is 1000 nm or less, the lattice relaxation rate can be easily controlled to less than 5%. Further, when the thickness of the first conductivity type nitride semiconductor layer is set to 300 nm or more, the driving voltage is easily reduced. Therefore, by setting the film thickness of the first conductivity type nitride semiconductor layer within the above range, it is possible to obtain an ultraviolet light emitting element with increased emission intensity while suppressing an increase in lattice relaxation rate.

(Light emitting layer)
The light emitting layer is a layer formed of a material containing a nitride semiconductor containing Al and Ga. The light emitting layer is formed on the first conductivity type nitride semiconductor layer.
The light emitting layer includes at least one quantum well layer and a quantum barrier layer sandwiching the quantum well layer.
In the ultraviolet light emitting device of the first aspect, the quantum barrier layer has a non-uniform Al composition (%) of the nitride semiconductor in the material in the depth direction, and an amount of change in the Al composition (%) in the depth direction. is 5% or more and 12% or less. In this way, the presence of a portion in which there is little Al in the quantum barrier layer, that is, where Ga is localized, shortens the carrier lifetime and allows carriers to bond preferentially. As a result, the ultraviolet light emitting device of the first embodiment becomes less susceptible to the effects of threading dislocations and impurities that exist in small numbers, and the emission intensity increases.

In the ultraviolet light emitting device of the second aspect, the quantum barrier layer has a non-uniform Al composition (%) of the nitride semiconductor in the material in a plane perpendicular to the depth direction, and the Al composition (%) in this plane is non-uniform. %) is 5% or more and 16% or less. In this way, the presence of a portion in which there is little Al in the quantum barrier layer, that is, where Ga is localized, shortens the carrier lifetime and allows carriers to bond preferentially. As a result, the ultraviolet light-emitting device of the second embodiment becomes less susceptible to the effects of threading dislocations and impurities that exist in small numbers, and the emission intensity increases.
Note that, from the viewpoint of the wavelength half-width, the amount of change in the Al composition (%) in the depth direction and in the plane is preferably distributed in a range of 6% or more and 10% or less. This makes it possible to achieve both a narrow wavelength half-width and high emission intensity.

In the ultraviolet light emitting device of the third aspect, the Al composition (%) of the nitride semiconductor in the material changes at the boundary where the quantum well layer switches to the quantum barrier layer toward the second conductivity type nitride semiconductor layer. A second composition in which a compositionally graded layer exists and the Al composition (%) of the nitride semiconductor in the material changes at the boundary where the quantum barrier layer switches to the quantum well layer toward the second conductivity type nitride semiconductor layer. Graded layers are present. The amount of change in the Al composition (%) in the first compositionally graded layer is 20% or more and 50% or less per 1 nm of thickness, and the amount of change in the Al composition (%) in the second compositionally graded layer is 20% or more and 50% or less per 1 nm of thickness. The thickness is 9% or more and 20% or less per 1 nm.

When the amount of change in the Al composition (%) in the first compositionally graded layer is 20% or more and 50% or less per 1 nm of thickness, the half width of the emission wavelength can be narrowed.
By setting the amount of change in the Al composition (%) in the second compositionally graded layer to 9% or more and 20% or less per 1 nm of thickness, uneven distribution of carriers due to internal electrolysis (Stark effect) can be alleviated. .

Further, the quantum well layer is a nitride semiconductor layer containing Al and Ga, and the central value X (%) of the Al composition is 30% or more and 75% or less, and the central value Y (%) of the Al composition in the quantum barrier layer is 30% or more and 75% or less. It is preferable that X+10%≦Y≦100% be satisfied. This is the Al composition of the quantum well layer necessary for emitting ultraviolet light, and satisfying this relationship is necessary for lattice-matched epitaxial growth on a substrate containing Al. Further, from the viewpoint of carrier confinement, the quantum barrier layer is required to have a sufficiently higher Al composition than the quantum well layer.

Further, the thickness of the quantum well layer is preferably 1 nm or more and 5 nm or less, and the thickness of the quantum barrier layer is preferably 6 nm or more and 15 nm or less. This is because there is a relationship between the width of the quantum well required for confining carriers and the thickness of the quantum barrier layer; when the quantum barrier layer is thin, the amount of carrier recombination decreases, and the quantum well layer As the film thickness increases, the recombination rate of carriers decreases. Further, if the quantum barrier layer is thin, the carrier confinement function is reduced, and if the quantum barrier layer is thick, the longitudinal resistance of the entire stack increases.

The nitride semiconductor included in the light emitting layer is preferably a mixed crystal of AlN or GaN, for example, from the viewpoint of achieving high luminous efficiency. In addition to N, the light-emitting layer may contain impurities such as group V elements other than N such as P, As, and Sb, and impurities such as C, H, F, O, Mg, and Si. However, this is not the case.
Furthermore, if the emission wavelength of the light emitting element is desired to be in the deep ultraviolet region (280 nm or less), the nitride semiconductor included in the light emitting layer preferably contains Al, Ga, and N. Further, from the viewpoint of increasing luminous efficiency, the light emitting layer preferably has a multiple quantum well structure (MQW) having a quantum well layer containing Al, Ga, and N and a quantum barrier layer containing AlN.

(Second conductivity type nitride semiconductor layer)
The second conductivity type nitride semiconductor layer is a nitride semiconductor layer having conductivity different from that of the first conductivity type nitride semiconductor layer. A second conductivity type nitride semiconductor layer is formed on the light emitting layer. The nitride semiconductor included in the second conductivity type nitride semiconductor layer is, for example, GaN, AlN, InN, and a mixed crystal containing them. In addition to N, the nitride semiconductor of the second conductivity type nitride semiconductor layer contains impurities such as group V elements other than N such as P, As, and Sb, and C, H, F, O, Mg, Si, and Be. may be mixed in. However, as described above, it is preferable that the first conductivity type nitride semiconductor layer has n-type conductivity and the second conductivity type nitride semiconductor layer has p-type conductivity.

(electrode)
The ultraviolet light emitting element can further include an electrode. The electrode can be at least one of an n-type electrode and a p-type electrode.
Materials for the n-type electrode include metals such as Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, and Zr. , a mixed crystal thereof, or a conductive oxide such as ITO or Ga ₂ O ₃ can be used. Moreover, it is preferable that the n-type electrode be formed so as to be in contact with a layer having n-type conductivity among the first conductivity type nitride semiconductor layer and the second conductivity type nitride semiconductor layer.

Materials for the p-type electrode include Ni, Au, Pt, Ag, Rh, Pd, Pt, Cu, Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Ir, Zr, etc. metals, mixed crystals thereof, or conductive oxides such as ITO or Ga ₂ O ₃ can be used. Further, the p-type electrode is preferably formed so as to be in contact with a layer having p-type conductivity among the first conductivity type nitride semiconductor layer and the second conductivity type nitride semiconductor layer.
Examples of the method for forming the electrodes include resistance heating evaporation, electron gun evaporation, sputtering, and the like, but the method is not limited to these. The electrode can be a single layer. Also, the electrodes may be laminated. Further, the electrode may be subjected to heat treatment in an oxygen, nitrogen, or air atmosphere after forming the layer.

(UV light emitting module)
A light emitting device including the ultraviolet light emitting element of the first to third aspects can be used as an ultraviolet light emitting module. The ultraviolet light emitting module can be applied to devices in, for example, the medical/life science field, the environmental field, the industry/industrial field, the lifestyle/home appliance field, the agricultural field, and other fields. The light-emitting device equipped with the ultraviolet light-emitting element of the first to third aspects includes a synthesis/decomposition device for drugs or chemical substances, a sterilization device for liquids, gases, and solids (containers, foods, medical equipment, etc.), a cleaning device for semiconductors, etc. Surface modification equipment for films, glass, metals, etc.; exposure equipment for manufacturing semiconductors, FPDs, PCBs, and other electronic products; printing/coating equipment; adhesion/sealing equipment; transfer/forming equipment for films, patterns, mockups, etc.; It can be applied to measurement and inspection devices for banknotes, scratches, blood, chemical substances, etc.

Examples of liquid sterilizers include automatic ice makers, ice trays and ice storage containers in refrigerators, water supply tanks for ice makers, cold water tanks, hot water tanks, and flow channels for freezers, ice makers, humidifiers, dehumidifiers, and water servers. Piping, stationary water purifiers, portable water purifiers, water heaters, water heaters, wastewater treatment equipment, disposers, toilet drain traps, washing machines, dialysis water sterilization modules, peritoneal dialysis connector sterilizers, disaster water storage systems, etc. It can be mentioned, but it is not limited to this.
Examples of gas sterilizers include air purifiers, air conditioners, ceiling fans, floor or bedding vacuum cleaners, futon dryers, shoe dryers, washing machines, clothes dryers, indoor germicidal lights, and storage ventilation. Examples include, but are not limited to, systems, shoe boxes, chests of drawers, etc.

Examples of solid sterilizers (including surface sterilizers) include vacuum packers, belt conveyors, hand tool sterilizers for medical, dental, barber and beauty salons, toothbrushes, toothbrush holders, chopstick cases, cosmetic pouches, Examples include, but are not limited to, drain covers, toilet bowl washers, toilet bowl lids, etc.

<Manufacturing method>
The ultraviolet light-emitting elements of the first to third aspects are manufactured through a process of forming each layer on a substrate. This process may be performed using, for example, molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), or metal organic chemical vapor deposition (MOCVD). Deposition method etc. be able to.

Here, among the layers formed on the substrate, the nitride semiconductor layer is made of an Al raw material containing, for example, trimethylaluminum (TMAl), a Ga raw material containing, for example, trimethyl gallium (TMGa) or triethyl gallium (TEGa), for example, ammonia, etc. It can be formed using an N raw material containing (NH ₃ ).
Ultraviolet light emitting elements are manufactured through a process of removing unnecessary portions of each layer formed on a substrate by etching. This step can be performed, for example, by inductively coupled plasma (ICP) etching.

Further, the ultraviolet light emitting device can be manufactured through a step of forming electrodes. This step can be performed by various methods, such as depositing metal by electron beam evaporation (EB) or resistance heating.
Here, the ultraviolet light emitting device is manufactured by dividing the substrate on which each layer is formed through the above steps into individual pieces by dicing. For products such as ultraviolet light emitting devices that may include a stochastic element in the manufacturing process, evaluation of intermediate products (evaluation process) is generally included in the manufacturing process. That is, the characteristics of the final product can be predicted based on the evaluation results of intermediates in the evaluation process, and intermediates can be sorted, processes can be selected or added, and so on.
Intermediates in the manufacture of ultraviolet light emitting devices include, for example, a substrate and a nitride semiconductor laminate before forming electrodes.

The composition of Al is determined by atomic mapping in the nitride semiconductor stack. Atom probe tomography (APT) is used for this atomic mapping. In APT analysis of a nitride semiconductor stack, a multilayer film part to be observed is processed into a needle-shaped sample with a radius of curvature of about 100 nm at the tip using focused ion beam (FIB) processing (manufactured by Toshiba). When electrodes are formed on the surface of the semiconductor multilayer film, the electrodes are attached, and when there are no electrodes, a protective film of W is formed and processed into a needle-shaped sample by FIB. The semiconductor stack is processed so that the vertical direction of the film is aligned with the axial direction of the needle-shaped sample, and the multilayer film portion of the MQW or the like to be observed is located near the tip of the needle.

A voltage pulse is applied to this needle-shaped sample, or a voltage pulse is applied and a laser pulse is irradiated to the observation site, and the ion species emitted from the tip of the needle-shaped sample are subjected to mass spectrometry to identify the ion species. By identifying the position within the needle-like sample using a two-dimensional detector, a three-dimensional distribution of atoms within the needle-like sample is obtained.
As described above, mapping of atoms in the film is observed in the depth direction and in a plane perpendicular to the depth direction using APT. In addition, atomic diffusion at the interface is evaluated by extracting the line profile in the depth direction of the cross section.

A method for measuring the central value X of the Al composition using this APT method will be described below. In a plane perpendicular to the depth direction of the formed needle-shaped sample, 25% of the area at the center of the sample is removed, and the average value of the mapping in the remaining area is taken as the central value X of the Al composition. The maximum and minimum compositions are calculated using the maximum and minimum compositions within the above region. When performing calculation within this plane, calculation is performed at a depth at least 1 nm away from the vicinity of the interface. In addition, in the depth direction, a region corresponding to 25% of the center of the sample is similarly removed, and in the case of a laminate, a 1 nm region near the interface is removed, and the average value of the mapping in the remaining region is removed. Let be the center value X of the Al composition. The maximum and minimum compositions are calculated using the maximum and minimum compositions within the above region.

Next, a method for measuring the diffusion length of Al at the interface will be described below. A line profile is formed from the measurement results of the APT method in the depth direction. This line profile is determined as an average value of the in-plane composition of the film at a certain depth, excluding a region including a 50% measurement error at the center of the sample. In this line profile, there are two regions: a flat region with a certain Al composition value (N%) (the region of composition variation within 10% is considered flat) and a flat region with a composition different from a certain value (M%). At the boundary between , the film thickness of the portion where the difference in Al composition between the two regions is 90% to 10% is defined as the diffusion length of the Al composition. Then, by dividing the difference between the composition N and the composition M by this diffusion length, the amount of change (%/nm) in the Al composition per 1 nm of the composition gradient layer is calculated. Furthermore, if there are multiple boundaries in the MQW structure, etc., the average value of the multiple boundaries is included in the calculation.

[Embodiment]
Embodiments of the present invention will be described below, but the present invention is not limited to the embodiments shown below. In the embodiments shown below, technically preferable limitations are made for carrying out the present invention, but these limitations are not essential to the present invention.
This embodiment describes an example in which a nitride semiconductor light-emitting element of one embodiment of the present invention is applied to an ultraviolet light-emitting element. Further, the conductivity type of the first conductivity type nitride semiconductor layer is n type, and the conductivity type of the second conductivity type nitride semiconductor layer is p type.

<Overall configuration>
First, the overall configuration of the ultraviolet light emitting device 10 of this embodiment will be explained using FIGS. 1 to 3.
As shown in FIGS. 1 and 2, the ultraviolet light emitting device 10 includes an AlN substrate 1, an n-type group III nitride semiconductor layer (first conductivity type nitride semiconductor layer) 2, and a nitride semiconductor stack 3. , a first electrode layer 4 , a second electrode layer 5 , a first pad electrode 6 , a second pad electrode 7 , and an insulating layer 8 . An n-type group III nitride semiconductor layer 2 is formed on an AlN substrate 1. The nitride semiconductor stack 3 is a mesa portion formed on a portion of the n-type group III nitride semiconductor layer 2, and has sloped side surfaces.

As shown in FIG. 2, the nitride semiconductor stack 3 includes, from the AlN substrate 1 side, an n-type group III nitride semiconductor layer (first conductivity type nitride semiconductor layer) 31, a multiple quantum well layer (light emitting layer) 32 , an electron block layer 33, and a p-type group III nitride semiconductor layer (second conductivity type nitride semiconductor layer) 35 are formed in this order.
The nitride semiconductor stack 3 includes an n-type group III nitride semiconductor layer, a multiple quantum well layer, an electron block layer, and a p-type group III nitride semiconductor layer formed in this order on the AlN substrate 1. The obtained laminate is formed by mesa etching to remove the portion where the first electrode layer 4 is to be formed halfway in the thickness direction of the n-type group III nitride semiconductor layer. That is, the n-type group III nitride semiconductor layer 31 of the nitride semiconductor stack 3 is formed continuously on the n-type group III nitride semiconductor layer 2.

The first electrode layer 4 is formed on the n-type Group III nitride semiconductor layer 2 in the planar shape shown in FIG. 3, for example. The second electrode layer 5 is formed on the p-type Group III nitride semiconductor layer 35 in the planar shape shown in FIG. 3, for example. The first pad electrode 6 is formed on the first electrode layer 4 in the same planar shape as the first electrode layer 4. The second pad electrode 7 is formed on the second electrode layer 5 in the same planar shape as the second electrode layer 5.
The ultraviolet light emitting element 10 is an element that emits ultraviolet light having a wavelength of 230 to 300 nm.

The n-type group III nitride semiconductor layers 2 and 31 formed on the AlN substrate 1 are n-Al _x Ga _(1-x) N (0.40≦x≦0.90) layers.
The multiple quantum well layer (light emitting layer) 32 is a plurality of quantum well layers formed of AlGaN and quantum barrier layers formed of AlGaN or AlN, which are stacked alternately. The Al composition (%) of the quantum barrier layer is nonuniform in the depth direction and in the plane perpendicular to the depth direction, and the amount of change in the depth direction is 5% to 12%, and the variation in the depth direction is 5% to 12%. The amount of change within the plane is 5% or more and 16% or less.

A first composition gradient layer in which the Al composition (%) changes exists at the boundary where the quantum well layer switches to the quantum barrier layer toward the p-type group III nitride semiconductor layer 35, and the p-type group III nitride semiconductor layer 35 At the boundary where the quantum barrier layer switches to the quantum well layer toward the top, there is a second compositionally graded layer in which the Al composition (%) changes. The amount of change in Al composition (%) in the first compositionally graded layer is 20% or more and 50% or less per 1 nm of thickness, and the amount of change in Al composition (%) in the second compositionally graded layer is 9% per 1 nm of thickness. % or more and 20% or less.
The electron block layer 33 is an Al _x Ga _(1-x) N (0.70≦x≦1.00) layer.

The p-type group III nitride semiconductor layer 35 is a GaN layer containing Mg as an impurity in a range of 4×10 ¹⁹ cm ^-3 or more and less than 8×10 ²⁰ cm ^-3 , and has a film thickness of 5 nm or more and 100 nm or less. .
The first electrode layer 4 is formed of an alloy layer of a material containing Al and Ni.
The second electrode layer 5 is an alloy layer of Ni and Au.
Materials for the first pad electrode 6 and the second pad electrode 7 include, for example, Au, Al, Cu, Ag, W, etc., but Au, which has high conductivity, is desirable.

The insulating layer 8 includes a portion of the n-type group III nitride semiconductor layer 2 that is not covered with the first electrode layer 4, a portion of the nitride semiconductor stack 3 that is not covered with the second electrode layer 5, and a portion of the n-type group III nitride semiconductor layer 2 that is not covered with the second electrode layer 5, and A portion of the electrode layer 4 that is not covered with the first pad electrode 6, a portion of the second electrode layer 5 that is not covered with the second pad electrode 7, and a lower portion of the first pad electrode 6 and the second pad electrode 7. formed on the sides. The insulating layer 8 may partially cover the upper portions of the first pad electrode 6 and the second pad electrode 7. Examples of the insulating layer 8 include oxides and nitrides such as SiN, SiO ₂ , SiON, Al ₂ O ₃ and ZrO layers.

<Action, effect>
In the ultraviolet light emitting device 10 of the embodiment, the amount of change in the Al composition (%) of the quantum barrier layer in the depth direction is 5% or more and 12% or less, and the amount of change in the plane perpendicular to the depth direction is 5% or more. 16% or less, the amount of change in Al composition (%) in the first compositionally graded layer is 20% or more and 50% or less per 1 nm of thickness, and the amount of change in Al composition (%) in the second compositionally graded layer is 20% or more and 50% or less per 1 nm of thickness. A high luminescence intensity can be obtained by setting the thickness to 9% or more and 20% or less per 1 nm.
In addition, by using the AlN substrate 1 as the substrate, the n-type group III nitride semiconductor layer 2 and the multiple quantum well layer (light emitting layer) 32 with few crystal defects are formed on the substrate, resulting in high optical output and a narrow semicircular half. An emission spectrum with a range of values is obtained.

<Formation of AlN layer>
First, a c-plane AlN substrate with a thickness of 550 μm was annealed at 1300° C. using a metal organic chemical vapor deposition (MOCVD) apparatus. Next, an AlN layer, which is a homoepitaxial layer, was formed to a thickness of 500 nm. The film forming conditions were a temperature of 1200° C., a V/III ratio of 50, a degree of vacuum of 50 hPa, and a growth rate of 0.5 μm/hr. Further, trimethylaluminum (TMAl) was used as an Al raw material, and ammonia (NH ₃ ) was used as a N raw material.

<Formation of first conductivity type nitride semiconductor layer>
On the AlN layer, an n-type AlGaN layer (Al composition: 70%) containing Si as a dopant was formed with a thickness of 500 nm as a first conductivity type nitride semiconductor layer. The film forming conditions were a temperature of 1030° C., a vacuum degree of 100 hPa, a V/III ratio of 4000, and a growth rate of 0.5 μm/hr. Further, trimethylaluminum (TMAl) was used as an Al raw material, triethyl gallium (TEGa) was used as a Ga raw material, ammonia (NH ₃ ) was used as a N raw material, and monosilane (SiH ₄ ) was used as a Si raw material.

<Formation of light emitting layer>
A multi-quantum well layer having five quantum well layers and five quantum barrier layers alternately was formed as a light emitting layer on the n-type AlGaN layer. As the quantum well layer, an AlGaN (Al composition: 52%) layer, that is, an Al _0.52 Ga _0.48 N layer was formed. As the quantum barrier layer, an AlGaN (Al composition: 75%) layer, that is, an Al _0.75 Ga _0.25 N layer was formed. The film forming conditions were a vacuum degree of 50 hPa, a V/III ratio of 4000, a quantum well layer growth rate of 0.18 μm/hr, and a quantum barrier layer growth rate of 0.15 μm/hr. Note that the growth temperature was changed to the values shown in Table 1 for each sample.

For samples No. 1 to 33, a step of forming a quantum well layer with a thickness of 3 nm with an Al composition of 52%, a step of forming a first composition gradient layer in which the Al composition changes from 52% to 75%, and a step of forming a quantum well layer with a thickness of 3 nm with an Al composition of 52%. The step of forming a 75% quantum barrier layer with a thickness of 6 nm and the step of forming a second compositionally graded layer in which the Al composition changes from 75% to 52% were repeated five times in this order. The formation process of the first compositionally graded layer and the second compositionally graded layer of each sample was performed so that the film thicknesses had the respective values shown in Table 1.

In sample No. 34, the light-emitting layer was formed in the same manner as in sample No. 3, except that the step of forming the first compositionally gradient layer was not performed. In sample No. 35, the light-emitting layer was formed in the same manner as in sample No. 3, except that the step of forming the second compositionally gradient layer was not performed. In sample No. 36, the light-emitting layer was formed in the same manner as in No. 3 except that the step of forming the first and second compositionally gradient layers was not performed. In sample No. 37, the light-emitting layer was formed in the same manner as in No. 30, except that the step of forming the first and second compositionally gradient layers was not performed. In sample No. 38, the light-emitting layer was formed in the same manner as in No. 31 except that the step of forming the first and second compositionally gradient layers was not performed.

<Formation of electronic block layer>
An AlGaN (Al composition: 85%) layer was formed as an electron blocking layer on the light emitting layer.
<Formation of second conductivity type nitride semiconductor layer>
A p-type GaN layer was formed as a second conductivity type nitride semiconductor layer on the electron block layer.
As described above, a nitride semiconductor stack including a first conductivity type nitride semiconductor layer, a light emitting layer, an electron block layer, and a second conductivity type nitride semiconductor layer was formed on the AlN substrate.

<Measurement of Al composition change amount of quantum barrier layer and composition gradient layer>
The formed nitride semiconductor stack is processed into a measurement sample with a diameter of 30 nm (acicular sample with a radius of curvature of about 100 nm at the tip) and observed using APT to obtain an atomic mapping image in the depth direction. An atomic mapping image was obtained in a plane perpendicular to the depth direction. Table 1 shows the amount of change in the Al composition of the quantum barrier layer found from each atomic mapping image obtained.
In addition, line profiles were analyzed from the obtained atomic mapping images in the depth direction, and the amount of change in the Al composition of the first compositionally graded layer and the second compositionally graded layer was investigated. These results are also shown in Table 1.

Note that in samples No. 34 and No. 36 to 38 in which the step of forming the first compositionally gradient layer was not performed, the first compositionally gradient layer having a resolution higher than the resolution of APT measurement (0.3 nm) was not observed. Similarly, in samples Nos. 35 to 38 in which the step of forming the second compositionally graded layer was not performed, the second compositionally graded layer with a resolution higher than the APT measurement resolution (0.3 nm) was not observed. In other words, if the composition gradient layer formation step was not performed, even if the composition gradient layer had been formed, its film thickness would have been less than 0.3 nm, so the column for film thickness in Table 1 would be "<0. 3" and "0.0" was displayed in the column for the amount of change in Al composition of the compositionally graded layer.

<Production of ultraviolet light emitting device>
By performing dry etching on the nitride semiconductor stack, a part of the n-type AlGaN layer is exposed, and an alloy electrode (n-type electrode) containing Ti, Al, Ni, and Au is placed on the exposed n-type AlGaN layer. ) was formed. Further, an alloy electrode (corresponding to a p-type electrode) containing Ni and Au was formed on the p-type GaN layer (second conductivity type nitride semiconductor layer).
Next, the AlN substrate was ground to a thickness of 100 μm, and then divided into individual pieces of ultraviolet light emitting elements by dicing. The obtained ultraviolet light emitting device was driven with a current of 100 mA, and the emission intensity at a peak wavelength of 265 nm was measured. The results are also shown in Table 1.

As can be seen from Table 1, "the amount of change in the Al composition (%) of the quantum barrier layer in the depth direction is 5% or more and 12% or less" and "the amount of change in the plane perpendicular to the depth direction". is 5% or more and 16% or less," and ``the amount of change in Al composition (%) in the first compositionally graded layer is 20% or more and 50% or less per 1 nm of thickness, and the second compositionally graded layer is A high emission intensity (35 mW or more) can be obtained by satisfying the requirement that the amount of change in Al composition (%) is 9% or more and 20% or less per 1 nm of thickness.

1 Substrate 2 N-type group III nitride semiconductor layer (first conductivity type nitride semiconductor layer)
3 Nitride semiconductor laminate 31 N-type group III nitride semiconductor layer (first conductivity type nitride semiconductor layer)
32 Multiple quantum well layer (light emitting layer)
33 Electron block layer 35 P-type group III nitride semiconductor layer (second conductivity type nitride semiconductor layer)
4 First electrode layer 5 Second electrode layer 6 First pad electrode 7 Second pad electrode 8 Insulating layer 10 Ultraviolet light emitting element

Claims (6)

A substrate and
a first conductivity type nitride semiconductor layer formed on the substrate and containing Al and Ga;
a light emitting layer formed on the first conductivity type nitride semiconductor layer and made of a material containing a nitride semiconductor containing Al and Ga;
a second conductivity type nitride semiconductor layer formed on the light emitting layer and having a different conductivity from the first conductivity type nitride semiconductor layer;
The light emitting layer includes at least one quantum well layer and a quantum barrier layer sandwiching the quantum well layer,
In the quantum barrier layer , the Al composition (%) of the nitride semiconductor in the material is nonuniform in the depth direction, and the amount of change in the Al composition (%) in the depth direction is 5% or more and 12%. The following is
The Al composition (%) of the nitride semiconductor in the material is nonuniform in a plane perpendicular to the depth direction, and the amount of change in the Al composition (%) in the plane is 5% or more and 16% or less. ,
A first composition gradient layer in which the Al composition (%) of the nitride semiconductor in the material changes is present at the boundary where the quantum well layer switches to the quantum barrier layer toward the second conductivity type nitride semiconductor layer. , a second compositionally graded layer in which the Al composition (%) of the nitride semiconductor in the material changes is present at the boundary where the quantum barrier layer switches to the quantum well layer toward the second conductivity type nitride semiconductor layer; death,
The amount of change in the Al composition (%) in the first compositionally graded layer is 20% or more and 50% or less per 1 nm of thickness, and the amount of change in the Al composition (%) in the second compositionally graded layer is 20% or more and 50% or less per 1 nm of thickness. An ultraviolet light-emitting element having a UV radiation of 9% or more and 20% or less per 1 nm . 2. The ultraviolet light emitting device according to claim 1 , wherein the substrate is an AlN substrate, and the dislocation density of the substrate is 1×10 ⁵ cm ⁻² or less. 3. The ultraviolet light emitting device according to claim 1, wherein the first conductivity type nitride semiconductor layer has a lattice relaxation rate of 0% or more and less than 5%. The ultraviolet light emitting device according to any one of claims 1 to 3, wherein the first conductivity type nitride semiconductor layer has a thickness of 300 nm or more and 750 nm or less. The quantum well layer is a nitride semiconductor layer containing Al and Ga and having an Al composition center value X (%) of 30% or more and 75% or less,
The ultraviolet rays according to any one of claims 1 to 4 , wherein the quantum barrier layer is a nitride semiconductor layer containing Al and Ga and having a central value Y (%) of Al composition satisfying X+10%≦Y≦100%. Light emitting element. The thickness of the quantum well layer is 1 nm or more and 5 nm or less,
The ultraviolet light emitting device according to any one of claims 1 to 5 , wherein the quantum barrier layer has a thickness of 6 nm or more and 15 nm or less.

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