JP7492885B2 - Ultraviolet light emitting device - Google Patents

Description

This disclosure relates to ultraviolet light emitting devices.

By controlling the band gap energy of the light-emitting layer, ultraviolet light-emitting elements can control the emission wavelength, and have a long life and high reliability. For this reason, ultraviolet light-emitting elements are used for a variety of purposes, including lighting, light sources for measuring instruments, and light sources for sterilization. A typical ultraviolet light-emitting element has a PIN structure on a substrate, with the light-emitting layer sandwiched between a p-type nitride semiconductor and an n-type nitride semiconductor.

In order to increase the light output (light emission intensity) of ultraviolet light-emitting elements, it is necessary to increase the light extraction efficiency. For this reason, methods of increasing the light extraction efficiency, for example, by combining a p-type AlGaN layer with high transmittance for ultraviolet light with a reflective electrode, are being considered (for example, Patent Document 1).

International Publication No. WO 2015/133000

However, further improvement in the light emission efficiency is required for ultraviolet light emitting devices. In the ultraviolet light emitting device in which p-type AlGaN and a reflective electrode are combined as described above, sufficient light emission intensity cannot be obtained because the device resistance deteriorates. In addition, when there is a difference in lattice constant between p-type AlGaN and the underlayer, it may not be possible to obtain a crystal with a good flat coverage. In that case, an increase in leakage current or a hopping phenomenon (discontinuous change) may occur in the IV characteristics.
That is, an object of the present disclosure is to provide an ultraviolet light emitting element that can achieve both light emission intensity and IV characteristics.

In order to solve the above-mentioned problems, an ultraviolet light-emitting element according to an embodiment of the present disclosure includes a substrate including a nitride semiconductor containing Al, a first conductivity type nitride semiconductor layer including Al and Ga disposed on the substrate, a light-emitting layer including a nitride semiconductor containing Al and Ga disposed on the first conductivity type nitride semiconductor layer, and a second conductivity type nitride semiconductor layer having a conductivity type different from that of the first conductivity type nitride semiconductor layer disposed on the light-emitting layer, wherein the second conductivity type nitride semiconductor layer contains Mg, a Mg concentration in the second conductivity type nitride semiconductor layer is equal to or greater than 8× ¹⁰ cm ⁻³ and equal to or less than 1× ¹⁰ cm ⁻³ , and a layer thickness is equal to or greater than 1 nm and equal to or less than 20 nm.

This disclosure makes it possible to provide an ultraviolet light-emitting element that can achieve both light emission intensity and I-V characteristics.

1 is a cross-sectional view showing a configuration example of an ultraviolet light emitting element according to a first embodiment of the present disclosure. FIG. 4 is a cross-sectional view showing another configuration example of the ultraviolet light emitting device according to the first embodiment of the present disclosure. FIG. 4 is a cross-sectional view showing a configuration example of an ultraviolet light emitting element according to a second embodiment of the present disclosure. FIG. 4 is a cross-sectional view showing a configuration example of an ultraviolet light emitting element according to a third embodiment of the present disclosure. FIG. 11 is a cross-sectional view showing a configuration example of an ultraviolet light emitting device according to a fourth embodiment of the present disclosure.

The ultraviolet light emitting device according to this embodiment will be described below through the embodiments, but the following embodiments do not limit the invention according to the claims. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

1. First Embodiment Hereinafter, an ultraviolet light emitting device 1 according to a first embodiment will be described with reference to FIG.
The ultraviolet light emitting element 1 is a semiconductor element capable of emitting ultraviolet light.

(1.1) Configuration of ultraviolet light emitting element An ultraviolet light emitting element 1 according to the first embodiment will be described with reference to Fig. 1. The ultraviolet light emitting element 1 includes a substrate 11 including a nitride semiconductor including Al, a first conductivity type nitride semiconductor layer 12 including Al and Ga arranged on the substrate 11, a light emitting layer 13 including a nitride semiconductor including Al and Ga arranged on the first conductivity type nitride semiconductor layer 12, and a second conductivity type nitride semiconductor layer 14 having a conductivity type different from that of the first conductivity type nitride semiconductor layer 12 arranged on the light emitting layer 13. The second conductivity type nitride semiconductor layer 14 includes Mg and has a layer thickness of 1 nm or more and 20 nm or less.

1, the ultraviolet light emitting element 1 is configured by sequentially stacking a substrate 11, a first conductivity type nitride semiconductor layer 12, a light emitting layer 13, and a second conductivity type nitride semiconductor layer 14. The ultraviolet light emitting element 1 also includes a first electrode 15A connected to the first conductivity type nitride semiconductor layer 12, and a second electrode 15B connected to the second conductivity type nitride semiconductor layer 14.
Each layer will be described in detail below.

(Substrate 11)
The substrate 11 includes a nitride semiconductor containing Al. The nitride semiconductor containing Al is, for example, AlN, that is, the substrate 11 is preferably an AlN single crystal substrate. The nitride semiconductor containing Al is not limited to AlN, and may be, for example, AlGaN. For example, when the substrate 11 is a nitride semiconductor single crystal substrate such as AlN or AlGaN, the difference in lattice constant with the nitride semiconductor layer formed on the upper side of the substrate 11 becomes small, and the nitride semiconductor layer is grown in a lattice-matched system, thereby reducing threading dislocations.

The term "comprises" in the expression "substrate 11...comprises a nitride semiconductor" means that the layer mainly contains a nitride semiconductor, but this expression also includes cases where other elements are included. Specifically, this expression also includes cases where the composition of this layer is slightly changed by adding small amounts of other elements (for example, a few percent or less of elements such as Ga (when Ga is not the main element), In, As, P, or Sb). In expressions regarding the composition of other layers, the word "comprises" has a similar meaning. Furthermore, the small amounts of elements contained are not limited to those mentioned above.

The substrate 11 may be doped with a donor impurity or an acceptor impurity to be n-type or p-type, or may be a mixed crystal of a nitride semiconductor such as AlN and sapphire ( _Al2O3 ), Si, SiC, _{MgO , Ga2O3 ,} ZnO, GaN, or InN.

For example, the substrate 11 preferably has a layer thickness of 100 μm or more and 600 μm or less.
The plane orientation may be c-plane (0001), a-plane (11-20), m-plane (10-10), etc., with the c-plane substrate 11 being more preferred.

(First Conductive Type Nitride Semiconductor Layer)
The first conductive type nitride semiconductor layer 12 is a layer of a nitride semiconductor containing Al and Ga. The first conductive type nitride semiconductor layer 12 is formed on the substrate 11. Here, for example, the word "on" in the expression "the first conductive type nitride semiconductor layer 12 is formed on the substrate 11" means that the first conductive type nitride semiconductor layer 12 is formed on the substrate 11. The above expression also includes a case where another layer is further present between the substrate 11 and the first conductive type nitride semiconductor layer 12. The word "on" has the same meaning in the relationship between other layers. For example, the case where the second conductive type nitride semiconductor layer 14 is formed on the light emitting layer 13 via an electron block layer (described in detail in a modified example) is also included in the expression "the second conductive type nitride semiconductor layer 14 is formed on the light emitting layer 13".

The first conductivity type nitride semiconductor layer 12 is formed of, for example, _{AzGa (1-z)} N (0<z<1). This makes it possible to enhance the crystallinity of the light emitting layer 13 and improve the light emission efficiency when a material corresponding to the band gap energy in the deep ultraviolet region is formed as the light emitting layer 13. From the viewpoint of realizing high light emission efficiency, the nitride semiconductor constituting the first conductivity type nitride semiconductor layer 12 is preferably a mixed crystal of AlN and GaN. Moreover, from the viewpoint of the transmittance of the light emitted from the light emitting layer 13, it is more preferable that the first conductivity type nitride semiconductor layer 12 is formed of _{AzGa (1-z)} N (0.6≦z≦0.9).

The first conductivity type nitride semiconductor layer 12 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, but the types of impurity elements are not limited to these.
Moreover, the first conductivity type nitride semiconductor layer 12 and the second conductivity type nitride semiconductor layer 14 are layers of nitride semiconductors having different conductivity types. In general, an n-type semiconductor has better crystallinity than a p-type semiconductor, and has less effect on the light emitting layer 13. For this reason, it is preferable that the first conductivity type nitride semiconductor layer 12 is an n-type, and the second conductivity type nitride semiconductor layer 14 is a p-type different from the first conductivity type nitride semiconductor layer 12.

From the viewpoints of relaxation and film resistance, the first conductivity type nitride semiconductor layer 12 preferably has a layer thickness of 200 nm or more and 800 nm or less, more preferably 300 nm or more and 750 nm or less, and even more preferably 300 nm or more and 500 nm or less.

(Light Emitting Layer)
The light emitting layer 13 is a layer of a nitride semiconductor containing Al and Ga. The light emitting layer 13 is formed on the first conductive type nitride semiconductor layer 12. The nitride semiconductor contained in the light emitting layer 13 is preferably a mixed crystal of AlN and GaN, for example, from the viewpoint of realizing high light emission efficiency. In addition to N, the light emitting layer 13 may contain impurities such as V group elements other than N, such as P, As, and Sb, and C, H, F, O, Mg, and Si, but the types of impurity elements are not limited to these. The light emitting layer 13 may have either a quantum well structure or a single layer structure. From the viewpoint of realizing high light emission efficiency, it is preferable that the light emitting layer 13 has at least one well structure.

If it is desired to set the emission wavelength of the ultraviolet light-emitting element 1 to a wavelength in the deep ultraviolet region (380 nm or less), the nitride semiconductor constituting the light-emitting layer 13 preferably contains Al, Ga, and N. From the viewpoint of increasing the light-emitting efficiency, it is preferable that the light-emitting layer 13 is a multiple quantum well structure (MQW) having a quantum well layer containing Al, Ga, and N, and an electron barrier layer containing AlN (neither of which is shown).

(Second Conductive Type Nitride Semiconductor Layer)
As described above, the second conductivity type nitride semiconductor layer 14 is a layer of a nitride semiconductor having a conductivity type different from that of the first conductivity type nitride semiconductor layer 12. The second conductivity type nitride semiconductor layer 14 is formed on the light emitting layer 13. The nitride semiconductor constituting the second conductivity type nitride semiconductor layer 14 is formed of, for example, GaN, AlN or InN, or a mixed crystal containing these.

The second conductivity type nitride semiconductor layer 14 may contain impurities such as Group V elements other than N, such as P, As, Sb, etc., and C, H, F, O, Mg, Si, Be, etc. However, as described above, it is preferable that the conductivity type of the first conductivity type nitride semiconductor layer 12 is n-type, and the conductivity type of the second conductivity type nitride semiconductor layer 14 is p-type.
From the viewpoint of versatility of the raw material gas, the impurity contained in the second conductivity type nitride semiconductor layer 14 is preferably Mg. From the viewpoint of improving the IV characteristics (coverage), the concentration of Mg contained is preferably 8×10 ¹⁹ cm ^-3 or more and 1×10 ²¹ cm ^-3 or less. From the viewpoint of the threshold voltage, the concentration of Mg contained is more preferably 5×10 ²⁰ cm ^-3 or more and 1×10 ²¹ cm ^-3 or less.

The second conductive type nitride semiconductor layer 14 has a thickness of 1 nm or more and 20 nm or less. The thinner the second conductive type nitride semiconductor layer 14, the higher the emission intensity of the ultraviolet light emitting element 1, and the thicker the layer, the lower the emission intensity of the ultraviolet light emitting element 1. On the other hand, the thinner the second conductive type nitride semiconductor layer 14, the higher the threshold voltage of the ultraviolet light emitting element 1, and the thicker the layer, the lower the threshold voltage of the ultraviolet light emitting element 1. Therefore, when the thickness of the second conductive type nitride semiconductor layer 14 is 1 nm or more and 20 nm or less, the threshold voltage does not become too high while maintaining a high emission intensity, and the device characteristics of the ultraviolet light emitting element 1 are improved overall.

From the viewpoint of improving the IV characteristics (electrical characteristics) of the ultraviolet light-emitting element 1, the coverage of the second conductivity type nitride semiconductor layer 14 with respect to the layer immediately below is preferably 80% or more and 100% or less, and more preferably 90% or more and 100% or less.
From the viewpoint of increasing the carrier injection efficiency of the ultraviolet light emitting device 1 as a p-type semiconductor, the second conductivity type nitride semiconductor layer 14 is preferably formed of AlGaN (0≦x≦0.2).
From the viewpoint of increasing the carrier injection efficiency, the second conductive type nitride semiconductor layer 14 is preferably lattice-matched with the layer immediately below. More specifically, the lattice constant in the in-plane direction parallel to the upper surface of the substrate 11 is preferably consistent with the lattice constant in the same direction of the layer immediately below. At the same time, it is preferable that atoms (e.g., Mg) contained as impurities are substituted for the lattices of Ga or Al and N.
This makes it possible to obtain the second conductivity type nitride semiconductor layer 14 having high carrier injection efficiency and excellent IV characteristics.

(electrode)
The ultraviolet light emitting element 1 may further include an electrode. The electrode may be at least one of an n-type electrode and a p-type electrode.
The n-type electrode is formed of a metal such as Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zr, or a mixed crystal thereof, or a conductive oxide such as ITO or Ga ₂ O _3. The n-type electrode is preferably formed so as to contact the layer having n-type conductivity of the first conductivity type nitride semiconductor layer 12 and the second conductivity type nitride semiconductor layer 14. In FIG. 1, the first electrode 15A connected to the first conductivity type nitride semiconductor layer 12 is the n-type electrode.

The p-type electrode is formed of a metal such as Ni, Au, Pt, Ag, Rh, Pd, Pt, Cu, Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Ir, Zr, or a mixed crystal thereof, or a conductive oxide such as ITO or Ga ₂ O _3. The p-type electrode is preferably formed so as to contact the layer having p-type conductivity among the first conductivity type nitride semiconductor layer 12 and the second conductivity type nitride semiconductor layer 14. In FIG. 1, the second electrode 15B connected to the second conductivity type nitride semiconductor layer 14 is the p-type electrode.

(1.2) Modified Examples of Ultraviolet Light-Emitting Elements The ultraviolet light-emitting element 1A described above may include an AlN layer 16 provided between the substrate 11 and the first conductivity type nitride semiconductor layer 12, or an electron blocking layer 17 provided between the light-emitting layer 13 and the second conductivity type nitride semiconductor layer 14.
The AlN layer 16 and the electron blocking layer 17 will be described in detail below with reference to FIG.

(AlN layer)
The AlN layer 16 is an example of an underlayer, and is formed on the entire surface of the substrate 11. That is, the AlN layer 16 is provided between the substrate 11 and the first conductivity type nitride semiconductor layer 12.
The AlN layer 16 can recover pits and polishing marks formed on the surface of the substrate. In addition, the difference in lattice constant and thermal expansion coefficient between the AlN layer 16 and the first conductive type nitride semiconductor layer 12 is small, and a nitride semiconductor layer with few defects can be grown on the AlN layer 16. In addition, the AlN layer 16 can grow the first conductive type nitride semiconductor layer 12 under compressive stress, and can suppress the occurrence of cracks in the first conductive type nitride semiconductor layer 12. Therefore, even if the substrate 11 is formed of a nitride semiconductor such as AlN or AlGaN, a nitride semiconductor layer with few defects can be grown above the substrate 11 via the AlN layer 16.

The AlN layer 16 may contain impurities such as C, B, O, H, Si, Fe, and Mg.
When AlN is used as the material for forming the substrate 11, the AlN layer 16 and the substrate 11 are formed of the same material, and therefore the boundary between the AlN layer 16 and the substrate 11 becomes unclear. When the substrate 11 is formed of AlN, the substrate 11 may be regarded as constituting the substrate 11 and the AlN layer 16.

The AlN layer 16 has a thickness of, for example, several μm. Specifically, the thickness of the AlN layer 16 is preferably greater than 10 nm and less than 1 μm. If the thickness of the AlN layer 16 is greater than 10 nm, the recovery of the substrate surface advances and the crystallinity of the AlN increases. Also, from the viewpoint of raw material costs, it is preferable that the thickness of the AlN layer 16 is less than 1 μm.

(Electron Blocking Layer)
The electron blocking layer 17 has a function of preventing electrons injected from the first conductive type nitride semiconductor layer 12 from being unable to recombine in the light emitting layer 13 and from overflowing to the second conductive type nitride semiconductor layer 14. For example, the electron blocking layer 17 is formed of GaN, AlN, or InN, or a mixed crystal containing them. However, the electron blocking layer 17 is not limited to being formed of these materials as long as it can prevent the overflow of electrons. In addition, the electron blocking layer 17 may contain impurities such as V group elements other than N, such as P, As, and Sb, and C, H, F, O, Mg, Si, and Be. From the viewpoint of lattice matching and the electron blocking effect, the electron blocking layer 17 is preferably formed of AlGaN having a band gap larger than that of the light emitting layer 13.

(1.3) Method for Manufacturing Ultraviolet Light Emitting Device The ultraviolet light emitting device 1 of this embodiment is manufactured through the process of forming each layer on the substrate 11.
The substrate 11 is formed by a general substrate growth method such as a vapor phase growth method, such as a sublimation method or a hydride vapor phase epitaxy (HVPE) method, or a liquid phase growth method.

The process of forming each layer on the substrate 11 can be performed by, for example, molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), metal organic chemical vapor deposition (MOCVD), or the like.
Here, among the layers formed on the substrate 11, the nitride semiconductor layer can be formed using an Al raw material containing, for example, trimethylaluminum (TMAl), a Ga raw material containing, for example, trimethylgallium (TMGa) or triethylgallium (TEGa), or an N raw material containing, for example, ammonia (NH ₃ ).

The ultraviolet light emitting element 1 is manufactured through a process of removing unnecessary portions of each layer formed on the substrate 11 by etching (see FIG. 1). This process can be performed by, for example, inductively coupled plasma (ICP) etching or the like.
The ultraviolet light emitting element 1 can be manufactured through a process of forming electrodes (first electrode 15A, second electrode 15B). This process can be performed by various methods, such as evaporating metal by electron beam deposition (EB) method.
Here, the ultraviolet light emitting element is manufactured by dividing the substrate 11 on which each layer has been formed through the above-mentioned steps into individual pieces by dicing.

The electrodes such as the first electrode 15A and the second electrode 15B are formed by resistance heating deposition, electron gun deposition, sputtering, or the like, but are not limited to these methods. The electrodes may be formed in a single layer or in multiple layers. In addition, the electrodes may be heat-treated in an oxygen, nitrogen, or air atmosphere after the layers are formed.

(1.4) Method for Measuring Physical Properties of Ultraviolet Light Emitting Device The physical properties of the ultraviolet light emitting device 1 described above can be measured as follows.

(Measurement of Coverage of Second Conductive Type Nitride Semiconductor Layer)
The coverage of the second conductivity type nitride semiconductor layer 14 is measured using, for example, a scanning electron microscope (FE-SEM "SU9000" manufactured by Hitachi High-Technologies Corporation) and image processing software (image analysis software "A-zo-kun" (registered trademark) manufactured by Asahi Kasei Engineering Co., Ltd.). More specifically, during SEM measurement, any three points within the surface where the surface of the second conductivity type nitride semiconductor layer 14 is exposed are measured within a measurement range of 50 μm × 50 μm. At this time, a backscattered electron image is obtained by the lower detector, thereby obtaining an SEM image in which the contrast due to composition differences is clear.

The obtained SEM image is processed using image processing software. Areas in the SEM image whose brightness is 50% or more lower than the average brightness are determined to be areas that are not covered by the second conductivity type nitride semiconductor layer 14 with respect to the light emitting layer 13 directly below. The coverage rate is the proportion of the covered area, and the average value of any three points is determined to be the coverage rate of the second conductivity type nitride semiconductor layer 14.

(Measurement of impurity and doping concentrations)
The concentrations of dopants and impurities contained in the substrate 11 and each layer constituting the ultraviolet light emitting device 1 can be measured by secondary ion mass spectrometry (SIMS).
When the concentration of dopants and impurities contained in each layer is measured by SIMS after processing into a device, the measurement can be performed in a state where the electrodes are removed by chemical etching or physical polishing. The concentration of dopants and impurities contained in each layer can also be measured by sputtering from the substrate 11 side where no electrodes are formed.
Specifically, the SIMS measurement is performed under the measurement conditions provided by Evans Analytical Group (EAG), Inc. A cesium (Cs) ion beam having an energy of 14.5 keV is used for sputtering the sample during the measurement.

(Method of measuring layer thickness)
The thickness of each layer constituting the ultraviolet light emitting element 1 can be measured by cutting out a predetermined cross section perpendicular to the substrate 11, observing this cross section with a transmission electron microscope (TEM), and using the length measurement function of the TEM. As a measurement method, first, a cross section perpendicular to the main surface of the substrate 11 of the ultraviolet light emitting element is observed using a TEM. Specifically, for example, the observation width is set to a range of 2 μm or more in a direction parallel to the main surface of the substrate 11 in a TEM image showing a cross section perpendicular to the main surface of the substrate 11 of the ultraviolet light emitting element 1. Since contrast is observed at the interface between two layers with different compositions within this observation width range, the thickness up to this interface is observed in a continuous observation region with a width of 200 nm. The average value of the thickness of each layer included in this 200 nm wide observation region can be calculated from five points arbitrarily extracted from the above-mentioned observation width of 2 μm or more to obtain the layer thickness of each layer.

(Method of Measuring Al Composition of First Conductive Type Nitride Semiconductor Layer)
The Al composition of the first conductivity type nitride semiconductor layer 12 can be measured by reciprocal space mapping (RSM) using an X-ray diffraction (XRD) method. Specifically, the lattice relaxation rate relative to the underlayer and the Al composition can be obtained by analyzing reciprocal space mapping data in the vicinity of a diffraction peak obtained using an asymmetric plane as a diffraction plane. Examples of the diffraction plane include the (10-15) plane and the (30-24) plane.

(Method of measuring the Al composition of the light-emitting layer and the electron-blocking layer)
It can be measured by X-ray Photoelectron Spectroscopy (XPS), Energy Dispersive X-ray Spectroscopy (EDX), and Electron Energy-Loss Spectroscopy (EELS).
In EELS, the composition of a sample is analyzed by measuring the energy lost when an electron beam passes through the sample. Specifically, for example, the energy loss spectrum of the intensity of a transmitted electron beam is measured and analyzed for a thin-sectioned sample used in TEM observation. The position of a peak that appears near the energy loss of 20 eV changes depending on the composition of each layer, and the composition can be determined from the peak position.
In the same manner as in the method of calculating layer thickness by TEM observation described above, the Al composition of each layer is obtained by calculating the average value of the Al composition in an observation width of 200 nm from five points arbitrarily selected from an observation region of 2 μm or more.

In EDX, characteristic X-rays generated by an electron beam in a thin-sectioned sample used in the above-mentioned TEM observation are measured and analyzed. In the same manner as the layer thickness calculation method using TEM observation described above, the Al composition of each layer is obtained by calculating the average value of the Al composition in an observation width of 200 nm from five points arbitrarily selected from an observation area of 2 μm or more.

In XPS, the evaluation in the depth direction is possible by performing XPS measurement while performing sputter etching using an ion beam. Although Ar+ is generally used as the ion beam, other ion species such as Ar cluster ions may be used as long as they can be irradiated by an etching ion gun mounted on the XPS device. The XPS peak intensities of Al, Ga, and N are measured and analyzed to obtain the depth direction distribution of the Al composition of each layer. Instead of sputter etching, the ultraviolet light emitting element may be obliquely polished so that a cross section perpendicular to the main surface of the substrate 11 is enlarged and exposed, and the exposed cross section may be measured by XPS.
The composition of each layer can be measured not only by XPS but also by Auger Electron Spectroscopy (AES). In this case, the composition can be measured by performing Auger Electron Spectroscopy on a cross section exposed by sputter etching or oblique polishing. The composition of each layer can also be measured by SEM-EDX measurement of a cross section exposed by oblique polishing.

(Measurement of atomic arrangement of nitride semiconductor layer)
The atomic arrangement of each layer constituting the ultraviolet light emitting device 1 can be measured by high-resolution TEM measurement. The sample exfoliated to 50 nm during the above-mentioned TEM measurement is subjected to high-resolution TEM measurement using an atomic resolution analytical electron microscope (manufactured by JEOL Ltd., "JEM-ARM200F" (acceleration voltage: 200 kV). The atomic arrangement is a state in which group III atoms and nitrogen atoms are alternately stacked, and if it is observed that the group III atoms are vertically arranged (i.e., there is no misalignment), it can be determined that lattice matching is achieved. In addition, it can be observed that Mg is substituted into the lattice as an impurity because there are no unbonded atoms between the atoms.

(1.5) Application Fields of Ultraviolet Light Emitting Device The ultraviolet light emitting device according to this embodiment can be applied to devices in, for example, the medical and life science fields, the environmental field, the industrial and manufacturing fields, the lifestyle and home appliances fields, the agricultural field, and other fields. The ultraviolet light emitting device can be applied to a drug or chemical synthesis and decomposition device, a liquid, gas, and solid (container, food, medical equipment, etc.) sterilization device, a semiconductor cleaning device, a surface modification device for film, glass, metal, etc., an exposure device for manufacturing semiconductors, FPDs, PCBs, and other electronic products, a printing and coating device, an adhesive and sealing device, a transfer and molding device for film, pattern, mockup, etc., and a measurement and inspection device for banknotes, wounds, blood, chemical substances, etc.

Examples of liquid sterilization devices include, but are not limited to, automatic ice makers, ice trays and ice storage containers in refrigerators, water tanks for ice makers, freezers, ice makers, humidifiers, dehumidifiers, cold water tanks, hot water tanks and flow pipes of water servers, freestanding water purifiers, portable water purifiers, water supply units, hot water heaters, wastewater treatment devices, garbage disposers, toilet drain traps, washing machines, dialysis water sterilization modules, peritoneal dialysis connector sterilizers, disaster water storage systems, etc.

Examples of gas sterilization devices include, but are not limited to, air purifiers, air conditioners, ceiling fans, floor or bedding vacuum cleaners, futon dryers, shoe dryers, washing machines, clothes dryers, indoor germicidal lamps, storage ventilation systems, shoe boxes, chests of drawers, etc.

Examples of solid sterilization equipment (including surface sterilization equipment) include, but are not limited to, vacuum packing machines, belt conveyors, hand tool sterilization equipment for medical/dental use, barber shops/beauty salons, toothbrushes, toothbrush holders, chopstick cases, cosmetic pouches, drain covers, toilet spot cleaners, toilet lids, etc.

(1.6) Effects of First Embodiment The ultraviolet light emitting device according to the first embodiment has the following effects (1) to (7).
(1) The ultraviolet light emitting element 1 includes a second conductivity type nitride semiconductor layer 14 containing Mg, the Mg concentration in the second conductivity type nitride semiconductor layer 14 being equal to or greater than 8×10 ¹⁹ cm ⁻³ and equal to or less than 1×10 ²¹ cm ⁻³ , and the layer thickness being equal to or greater than 1 nm and equal to or less than 20 nm.
As a result, the ultraviolet light emitting element 1 has improved emission intensity, reduced contact resistance with the electrodes, and improved carrier injection efficiency into the light emitting layer, resulting in improved IV characteristics.

(2) In the ultraviolet light-emitting element 1, the coverage of the second conductivity type nitride semiconductor layer 14 with respect to the layer immediately below is preferably 80% or more and 100% or less.
In this case, the contact area between the layer immediately below and the second conductivity type nitride semiconductor layer 14 is increased, and the area where the electrode (second electrode 15B) directly contacts the layer immediately below is reduced, thereby suppressing leakage current and discontinuous changes, and improving the IV characteristics.

(3) In the ultraviolet light emitting element 1, the second conductivity type nitride semiconductor layer is preferably formed of Al _x Ga.sub. _(1-x) N (0≦x≦0.2).
This improves the carrier injection efficiency of the p-type semiconductor, leading to a further increase in emission intensity, improved IV characteristics, and a lowered threshold voltage.

(4) The ultraviolet light emitting element 1 includes a substrate 11 made of AlN single crystal.
This reduces the difference in lattice constant with the nitride semiconductor layer formed on the upper side of the substrate 11, making it possible to grow a nitride semiconductor layer with fewer defects on the substrate 11, thereby improving the emission intensity and other device characteristics.

(5) The ultraviolet light emitting element 1 includes a first conductivity type nitride semiconductor layer 12 having a thickness of 300 nm or more and 750 nm or less.
This causes the first conductivity type nitride semiconductor layer 12 to be lattice relaxed relative to the underlayer, reducing the film resistance, improving the emission intensity, and also improving other device characteristics.

(6) The ultraviolet light emitting element 1 includes a first conductivity type nitride semiconductor layer 12 made of _{AlzGa (1-z)} N (0.6≦z≦0.9).
This improves the transmittance of the first conductivity type nitride semiconductor layer 12 for the light emitted from the light emitting layer 13, improving the emission intensity and also improving other device characteristics.

2. Second embodiment With reference to Fig. 3, an ultraviolet light emitting element 2 according to a second embodiment will be described. As shown in Fig. 3, the ultraviolet light emitting element 2 includes a substrate 11, a first conductive type nitride semiconductor layer 12, a light emitting layer 13, and a second conductive type nitride semiconductor layer 14. The ultraviolet light emitting element 2 also includes a first electrode 15A connected to the first conductive type nitride semiconductor layer 12, a second electrode 15B connected to the second conductive type nitride semiconductor layer 14, and a reflective electrode 25C provided on the second conductive type nitride semiconductor layer 14. That is, the ultraviolet light emitting element 2 differs from the ultraviolet light emitting element 1 according to the first embodiment in that the ultraviolet light emitting element 2 includes a reflective electrode 25C on the second conductive type nitride semiconductor layer 14.

The reflective electrode 25C will be described in detail below. Note that the substrate 11, the first conductivity type nitride semiconductor layer 12, the light emitting layer 13, the second conductivity type nitride semiconductor layer 14, the first electrode 15A, and the second electrode 15B have the same configuration as the ultraviolet light emitting element 1, and therefore their description will be omitted.

(2.1) Configuration of UV light emitting element (reflection electrode)
The reflective electrode 25C has a function of reflecting light emitted from the light emitting layer 13 toward the second conductivity type nitride semiconductor layer 14 toward the light emitting layer 13, thereby improving the emission intensity of the ultraviolet light emitting element 2. From the viewpoint of improving the emission intensity, the reflective electrode 25C preferably has an ultraviolet light reflectance of 90% or more. More specifically, the ultraviolet light reflectance of the reflective electrode 25C at a peak wavelength of 265 nm measured by a reflection/transmission measuring device is preferably 90% or more.

The reflective electrode 25C is formed of, for example, Al, Ti, Pt, Rd, Ru, Ni, or Rh, but is not limited to these as long as the reflectance is satisfied. The reflective electrode 25C may be used as the second electrode 15B, which is a p-type electrode, or may be provided on the second electrode 15B. Furthermore, the reflective electrode 25C may be provided at a location on the second conductivity type nitride semiconductor layer 14 other than the second electrode 15B.
Also, a semiconductor multi-layer distributed Bragg reflector in which oxides such as _HfO2 and _SiO2 are laminated may be used for the reflecting electrode.
The reflective electrode 25C may be a single layer, or may be a laminate of multiple layers.

(2.2) Method for Manufacturing Ultraviolet Light Emitting Device The reflective electrode 25C is preferably formed by resistance heating deposition, electron gun deposition, sputtering, or the like, but is not limited to these methods.

(2.3) Effects of the Second Embodiment The ultraviolet light emitting element 2 according to the second embodiment has the following effects in addition to the effects described in the first embodiment.
(7) The ultraviolet light emitting element 2 includes a reflective electrode 25C provided on the second conductivity type nitride semiconductor layer 14 .
As a result, the ultraviolet light-emitting element 2 can reflect light emitted from the light-emitting layer 13 toward the second conductivity type nitride semiconductor layer 14 toward the light-emitting layer 13, thereby improving the light emission intensity of the ultraviolet light-emitting element 2.

3. Third embodiment The ultraviolet light emitting element 3 according to the second embodiment will be described with reference to FIG. 4. As shown in FIG. 4, the ultraviolet light emitting element 3 includes a substrate 11, a first conductive type nitride semiconductor layer 12, a light emitting layer 13, a composition gradient layer 38, and a second conductive type nitride semiconductor layer 14. The ultraviolet light emitting element 3 also includes a first electrode 15A connected to the first conductive type nitride semiconductor layer 12, and a second electrode 15B connected to the second conductive type nitride semiconductor layer 14. That is, the ultraviolet light emitting element 3 differs from the ultraviolet light emitting element 1 according to the first embodiment in that the ultraviolet light emitting element 3 includes a composition gradient layer 38 between the light emitting layer 13 and the second conductive type nitride semiconductor layer 14, in which the Al composition decreases in the direction away from the substrate 11.

The composition gradient layer 38 will be described in detail below. Note that the substrate 11, the first conductivity type nitride semiconductor layer 12, the light emitting layer 13, the second conductivity type nitride semiconductor layer 14, the first electrode 15A, and the second electrode 15B have the same configuration as the ultraviolet light emitting element 1, and therefore their description will be omitted.

(3.1) Structure of UV light emitting device (composition gradient layer)
The composition gradient layer 38 is provided between the light emitting layer 13 and the second conductive type nitride semiconductor layer 14. The composition gradient layer 38 is formed of Al _y Ga _(1-y) N (0.1≦y≦1.0), and the Al composition y decreases in the direction away from the substrate 11. The Al composition y of the composition gradient layer 38 decreases in the direction away from the substrate 11. The profile (gradient) of the Al composition y in the composition gradient layer 38 may decrease continuously or may decrease intermittently. Here, "intermittently decreasing" means that the composition gradient layer 38 includes a portion in which the Al composition y is the same. In other words, the composition gradient layer 38 may include a portion in which the Al composition y does not decrease in the direction away from the substrate 11, but does not include a portion in which the Al composition y increases.

The composition gradient layer 38 has the effect of generating holes by a polarization doping effect and efficiently injecting the holes into the active layer in the light emitting layer 13. Therefore, by providing the composition gradient layer 38 between the light emitting layer 13 and the second conductivity type nitride semiconductor layer 14, the light emitting efficiency of the ultraviolet light emitting element 3 can be improved.
From the viewpoint of increasing the light emission efficiency, the thickness of the composition gradient layer 38 is preferably 15 nm or more and 60 nm or less, and more preferably 20 nm or more and 50 nm or less.

The compositionally graded layer 38 may be in contact with the light emitting layer 13, or another layer may be present between the compositionally graded layer 38 and the light emitting layer 13. The compositionally graded layer 38 may be in contact with the second conductivity type nitride semiconductor layer 14, or another layer may be present between the compositionally graded layer 38 and the second conductivity type nitride semiconductor layer 14.
Specifically, the electron blocking layer 17 may be present between the light emitting layer 13 and the compositionally graded layer 38 .

Here, in the ultraviolet light-emitting device 3 according to this embodiment, the coverage of the second conductivity type nitride semiconductor layer 14 indicates the coverage of the composition gradient layer 38.

(3.2) Method for Manufacturing Ultraviolet Light Emitting Device Like the other layers, the compositionally graded layer 38 can be formed by any of the molecular beam epitaxy (MBE) method, the hydride vapor phase epitaxy (HVPE) method, the metalorganic chemical vapor deposition (MOCVD) method, etc. Also, like the other layers, the waveguide layer 49 can be formed by using an Al source containing, for example, trimethylaluminum (TMAl), a Ga source containing, for example, trimethylgallium (TMGa) or triethylgallium (TEGa), or an N source containing, for example, ammonia (NH ₃ ).

(3.3) Measurement method for physical properties of ultraviolet light emitting device (measurement method for Al composition of composition gradient layer)
The Al composition of the compositionally graded layer 38 can be measured by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS), or the like, in the same manner as the light emitting layer 13 and the electron blocking layer 17 .

(3.4) Modifications of Ultraviolet Light Emitting Element The ultraviolet light emitting element 3 according to this embodiment may have a configuration (not shown) including the reflective electrode 25C described in the second embodiment.

(3.5) Effects of the Third Embodiment The ultraviolet light emitting device according to the third embodiment has the following effects in addition to the effects described in the first or second embodiment.
(8) The ultraviolet light emitting element 3 includes a compositionally graded layer 38 provided between the light emitting layer 13 and the second conductivity type nitride semiconductor layer 14 .
This allows the ultraviolet light emitting element 3 to efficiently inject holes into the active layer in the light emitting layer 13, thereby improving the light emitting efficiency of the ultraviolet light emitting element 3.

4. Fourth embodiment With reference to FIG. 5, an ultraviolet light emitting element 4 according to a fourth embodiment will be described. As shown in FIG. 5, the second ultraviolet light emitting element includes a substrate 11, a first conductive type nitride semiconductor layer 12, a first waveguide layer 49A, a light emitting layer 13, a second waveguide layer 49B, and a second conductive type nitride semiconductor layer 14. The ultraviolet light emitting element 4 also includes a first electrode 15A connected to the first conductive type nitride semiconductor layer 12 and a second electrode 15B connected to the second conductive type nitride semiconductor layer 14. That is, the ultraviolet light emitting element 4 differs from the ultraviolet light emitting element 1 according to the first embodiment in that it includes a waveguide layer 49 (a first waveguide layer 49A and a second waveguide layer 49B) above and below the light emitting layer 13.

The waveguide layer 49 will be described in detail below. Note that the substrate 11, the first conductivity type nitride semiconductor layer 12, the light emitting layer 13, the second conductivity type nitride semiconductor layer 14, the first electrode 15A, and the second electrode 15B have the same configuration as the ultraviolet light emitting element 1, and therefore their description will be omitted.

(4.1) Structure of UV light emitting element (waveguide layer)
The waveguide layer 49 has a function of confining light from the light emitting layer 13. From the viewpoint of confining light more efficiently, the waveguide layer 49 is preferably disposed above and below the light emitting layer 13 as a first waveguide layer 49A and a second waveguide layer 49B, as shown in FIG.
The waveguide layer 49 is preferably formed of, for example, Al _p Ga _(1-p) N (0.5≦p≦z). From the viewpoint of light confinement, the Al composition p of the waveguide layer 49 is preferably lower than the Al composition z of the first conductivity type nitride semiconductor layer 12.
The waveguide layer 49 may contain impurities such as P, As, Sb, and other group V elements other than N, as well as C, H, F, O, Mg, Si, and Be.

(4.2) Method of Manufacturing Ultraviolet Light Emitting Device The waveguide layer 49, like the other layers, can be formed by any one of molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), metalorganic chemical vapor deposition (MOCVD), etc. Also, like the other layers, the waveguide layer 49 can be formed using an Al source containing, for example, trimethylaluminum (TMAl), a Ga source containing, for example, trimethylgallium (TMGa) or triethylgallium (TEGa), or an N source containing, for example, ammonia (NH ₃ ).

(4.3) Measurement method for physical properties of ultraviolet light emitting device (measurement method for Al composition of waveguide layer)
The Al composition of the waveguide layer 49, like the light emitting layer 13 and the electron blocking layer 17, can be measured by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS), or the like.

(4.4) Modified Examples of Ultraviolet Light-Emitting Device The ultraviolet light-emitting device 4 according to this embodiment may include at least one of the reflective electrode 25C described in the second embodiment and the compositionally graded layer 38 described in the third embodiment, in addition to the waveguide layers 49 provided above and below the light-emitting layer 13. In this case, the compositionally graded layer 38 may be provided between the second waveguide layer 49B provided above the light-emitting layer 13 and the second conductivity-type nitride semiconductor layer 14.

The ultraviolet light-emitting element 4 according to this embodiment may also include a compositionally graded layer 38, similar to the ultraviolet light-emitting element 2 according to the fourth embodiment, in addition to the waveguide layers 49 (first waveguide layer 49A and second waveguide layer 49B) provided above and below the light-emitting layer 13. In this case, the compositionally graded layer 38 may be provided between the second waveguide layer 49B provided above the light-emitting layer 13 and the second conductivity type nitride semiconductor layer 14.

(4.5) Effects of the Fourth Embodiment The ultraviolet light emitting device according to the fourth embodiment has the following effects in addition to any of the effects described in the first to third embodiments.
(9) The ultraviolet light emitting element 4 includes the waveguide layers 49 (the first waveguide layer 49A and the second waveguide layer 49B) provided above and below the light emitting layer 13, respectively.
This allows the ultraviolet light emitting element 4 to efficiently guide the light from the light emitting layer 13 to the outside, thereby improving the light emitting efficiency of the ultraviolet light emitting element 4.

[Example 1]
Annealing was performed on a c-plane AlN single crystal substrate having a thickness of 550 μm using a metal-organic chemical vapor deposition (MOCVD) apparatus. The annealing was performed in two sets, each set consisting of a 5-minute treatment in an _NH3 atmosphere and a 5-minute treatment in an _H2 atmosphere at 1300° C.
Next, an AlN layer, which is a homoepitaxial layer, was formed at 1200° C. to a thickness of 500 nm. At this time, the ratio (V/III ratio) of the supply rate of the group III element source gas to the supply rate of the nitrogen source gas was set to 50. The degree of vacuum was set to 50 mbar. The growth rate of the AlN layer was 0.5 um/hr. Trimethylaluminum (TMAl) was used as the Al source. Ammonia (NH ₃ ) was used as the N source.

A first conductivity type nitride semiconductor layer, which is an n-type AlGaN layer, was formed on the AlN layer formed as described above. The first conductivity type nitride semiconductor layer was an n-type AlGaN layer (Al: 70%, i.e., _{an Al0.70Ga0.30N} layer) using Si as a dopant impurity.
The first conductive type nitride semiconductor layer was formed to a thickness of 500 nm under the conditions of a temperature of 1080° C., a degree of vacuum of 50 mbar, and a V/III ratio of 4000. The growth rate of the first conductive type nitride semiconductor layer was 0.5 μm/hr. Trimethylaluminum (TMAl) was used as the Al raw material. Triethylgallium (TEGa) was used as the Ga raw material. Ammonia (NH ₃ ) was used as the N raw material. Monosilane (SiH ₄ ) was used as the Si raw material.

Subsequently, a light emitting layer was formed on the first conductive type nitride semiconductor layer. The light emitting layer was formed by depositing a film so as to have a multiple quantum well structure in which quantum well layers and barrier layers were stacked five times. Here, the quantum well layer was an AlGaN layer (Al: 52%, i.e., an Al _0.52 Ga _0.48 N layer) having a thickness of 3.0 nm. The barrier layer was an AlGaN layer (Al: 75%, i.e., an Al _0.75 Ga _0.25 N layer) having a thickness of 6.0 nm.
The light-emitting layer was formed under conditions of a vacuum degree of 50 mbar and a V/III ratio of 4000. The growth rate of the quantum well layer was 0.18 μm/hr, and the growth rate of the barrier layer was 0.15 μm/hr.

Subsequently, an electron blocking layer was formed on the light emitting layer, where the electron blocking layer was an AlGaN layer (Al: 85%, i.e., an Al _0.85 Ga _0.15 N layer) having a thickness of 15 nm.
The electron blocking layer was formed under conditions where the degree of vacuum was set to 50 mbar and the V/III ratio was set to 4000. The growth rate of the electron blocking layer at this time was 0.12 μm/hr.

Next, a compositionally graded layer was formed on the electron blocking layer, where the compositionally graded layer was an AlGaN layer having a thickness of 35 nm and an Al composition uniformly graded from 75% to 30% (Al: 75→30%, i.e. _{, an Al0.75Ga0.25N} → _Al0.3Ga0.7N layer).
The composition gradient layer was formed under the conditions of a degree of vacuum of 50 mbar and a V/III ratio of 4,000.

Next, a second conductivity type nitride semiconductor layer was formed on the composition gradient layer. Here, the second conductivity type nitride layer was a p-type GaN layer (i.e., Al: 0%) having a thickness of 10 nm and using Mg as a dopant impurity.
The second conductive type nitride semiconductor layer was formed under conditions of a temperature of 920° C., a degree of vacuum of 150 mbar, and a V/III ratio of 3650. The growth rate of the second conductive type nitride semiconductor layer at this time was 0.2 μm/hr.
In this manner, a nitride semiconductor laminate was formed on the AlN substrate.

When the nitride semiconductor laminate formed as described above was observed using a SEM, the surface coverage of the second conductivity type nitride semiconductor layer with respect to the compositionally graded layer immediately below was 85%. The Mg concentration of the second conductivity type nitride semiconductor layer obtained by SIMS measurement was 1×10 ²⁰ cm ^-3 . Finally, when a cross-sectional TEM measurement was performed using a high-resolution TEM, it was found that the compositionally graded layer and the second conductivity type nitride semiconductor layer were lattice-matched.

The obtained nitride semiconductor laminate was dry etched from the second conductive type nitride semiconductor layer side to expose a part of the first conductive type nitride semiconductor layer, which is an n-type AlGaN layer. An alloy electrode (corresponding to an n-type electrode) containing Ti, Al, Ni and Au was formed on the exposed first conductive type nitride semiconductor layer. Also, an alloy electrode (corresponding to a p-type electrode) containing Ni and Au was formed on the second conductive type nitride semiconductor layer, which is a p-type GaN layer. The AlN substrate was ground from the back side to a thickness of 100 μm, and then the nitride semiconductor laminate was divided into individual pieces of ultraviolet light emitting elements by dicing.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.8 V and the emission intensity at a peak wavelength of 265 nm was 44 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 2]
An ultraviolet light-emitting element was formed in the same manner as in Example 1, except that the formation conditions of the second conductive type nitride semiconductor layer were changed so that the Mg concentration contained in the second conductive type nitride semiconductor layer was ^adjusted to 3× ¹⁰ cm and the surface coverage of the composition gradient layer was adjusted to 90%.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.6 V and the emission intensity at a peak wavelength of 265 nm was 45 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 3]
An ultraviolet light emitting element was formed in the same manner as in Example 1, except that the formation conditions of the second conductive type nitride semiconductor layer were changed so that the Mg concentration contained in the second conductive type nitride semiconductor layer was ^adjusted to 7× ¹⁰ cm and the surface coverage of the composition gradient layer was adjusted to 100%.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.2 V and the emission intensity at a peak wavelength of 265 nm was 50 mW. Furthermore, no leakage or discontinuous change points were observed in the IV characteristics.

[Example 4]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that the formation conditions of the second conductive type nitride semiconductor layer were changed so that the Mg concentration contained in the second conductive type nitride semiconductor layer was adjusted to 9×10 ²⁰ cm ⁻³ .
The obtained ultraviolet light emitting device was evaluated by driving it with a current of 500 mA, and the driving voltage was 6.3 V and the emission intensity at a peak wavelength of 265 nm was 49 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 5]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that the conditions for forming the second conductive type nitride semiconductor layer were changed so that the thickness of the second conductive type nitride semiconductor layer was adjusted to 2 nm.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.7 V and the emission intensity at a peak wavelength of 265 nm was 60 mW. Furthermore, no leakage or discontinuous change points were observed in the IV characteristics.

[Example 6]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that the conditions for forming the second conductive type nitride semiconductor layer were changed so that the thickness of the second conductive type nitride semiconductor layer was adjusted to 18 nm.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.0 V and the emission intensity at a peak wavelength of 265 nm was 43 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 7]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that the conditions for forming the second conductive type nitride semiconductor layer were changed so that the second conductive type nitride semiconductor layer was a p-type Al _0.1 Ga _0.9 N layer.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.6 V and the emission intensity at a peak wavelength of 265 nm was 48 mW. Furthermore, no leakage or discontinuous change points were observed in the IV characteristics.

[Example 8]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that the conditions for forming the second conductive type nitride semiconductor layer were changed so that the second conductive type nitride semiconductor layer was a p-type Al _0.8 Ga _0.92 N layer.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 7.0 V and the emission intensity at a peak wavelength of 265 nm was 40 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 9]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that the conditions for forming the second conductive type nitride semiconductor layer were changed so that the second conductive type nitride semiconductor layer was a p-type Al _0.25 Ga _0.75 N layer.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 8.1 V and the emission intensity at a peak wavelength of 265 nm was 35 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 10]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that no composition gradient layer was provided.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 7.5 V and the emission intensity at a peak wavelength of 265 nm was 36 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 11]
An ultraviolet light emitting device was produced in the same manner as in Example 3, except that the composition gradient layer was formed so that the layer thickness of the composition gradient layer was 10 nm.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 7.3 V and the emission intensity at a peak wavelength of 265 nm was 37 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 12]
An ultraviolet light emitting device was produced in the same manner as in Example 3, except that the composition gradient layer was formed so that the layer thickness on the composition gradient layer was 20 nm.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.4 V and the emission intensity at a peak wavelength of 265 nm was 45 mW. Furthermore, no leakage or discontinuous change points were observed in the IV characteristics.

[Example 13]
An ultraviolet light emitting device was produced in the same manner as in Example 3, except that the composition gradient layer was formed so that the layer thickness on the composition gradient layer was 50 nm.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.1 V and the emission intensity at a peak wavelength of 265 nm was 43 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 14]
An ultraviolet light emitting device was produced in the same manner as in Example 3, except that the composition gradient layer was formed so that the layer thickness on the composition gradient layer was 70 nm.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.3 V and the emission intensity at a peak wavelength of 265 nm was 39 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 15]
An ultraviolet light emitting device was produced in the same manner as in Example 3, except that the first conductivity type nitride semiconductor layer was formed so that the layer thickness of the first conductivity type nitride semiconductor layer was 200 nm.
The obtained ultraviolet light emitting device was evaluated by driving it with a current of 500 mA, and the driving voltage was 7.4 V and the emission intensity at a peak wavelength of 265 nm was 50 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 16]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that the first conductivity type nitride semiconductor layer was formed so that the layer thickness of the first conductivity type nitride semiconductor layer was 300 nm.
The obtained ultraviolet light emitting device was evaluated by driving it with a current of 500 mA, and the driving voltage was 6.9 V and the emission intensity at a peak wavelength of 265 nm was 50 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Example 17]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that the first conductivity type nitride semiconductor layer was formed so that the layer thickness of the first conductivity type nitride semiconductor layer was 800 nm.
The obtained ultraviolet light emitting device was evaluated by driving it with a current of 500 mA, and the driving voltage was 5.9 V and the emission intensity at a peak wavelength of 265 nm was 38 mW. Furthermore, no leakage or discontinuous change points were observed in the IV characteristics.

[Example 18]
An ultraviolet light emitting element was formed in the same manner as in Example 3, except that a 100 nm thick Al metal was deposited as a reflective electrode on the second conductivity type nitride semiconductor layer.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.5 V and the emission intensity at a peak wavelength of 265 nm was 105 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Comparative Example 1]
An ultraviolet light emitting element was formed in the same manner as in Example 3, except that the formation conditions of the second conductive type nitride semiconductor layer were changed so that the Mg concentration contained in the second conductive type nitride semiconductor layer was adjusted to 6×10 ¹⁹ cm ⁻³ and the surface coverage of the composition gradient layer was adjusted to 78%.
Cross-sectional TEM measurement using a high-resolution TEM revealed that the compositionally graded layer and the second conductivity type nitride semiconductor layer were lattice-matched.
The obtained ultraviolet light emitting device was evaluated by driving it with a current of 500 mA, and the driving voltage was 7.0 V and the emission intensity at a peak wavelength of 265 nm was 39 mW. In addition, leakage and discontinuous change points were observed in the IV characteristics.

[Comparative Example 2]
An ultraviolet light-emitting element was formed in the same manner as in Example 3, except that the formation conditions of the second conductive type nitride semiconductor layer were changed so that the Mg concentration contained in the second conductive type nitride semiconductor layer was adjusted to 3× ¹⁰ cm ⁻³ and the surface coverage was adjusted to 100%.
The obtained ultraviolet light emitting device was evaluated by driving it with a current of 500 mA, and the driving voltage was 7.2 V and the emission intensity at a peak wavelength of 265 nm was 41 mW. In addition, leakage was observed in the IV characteristics.

[Comparative Example 3]
An ultraviolet light emitting device was manufactured in the same manner as in Example 3, except that the second conductive type nitride semiconductor layer was not formed.
When a current of 500 mA was applied to the obtained ultraviolet light emitting device for evaluation, no light was emitted and no diode characteristics were confirmed.

[Comparative Example 4]
An ultraviolet light emitting device was formed in the same manner as in Example 3, except that the thickness of the second conductivity type nitride semiconductor layer was adjusted to 25 nm.
The obtained ultraviolet light emitting device was evaluated by driving it at a current of 500 mA, and the driving voltage was 6.0 V and the emission intensity at a peak wavelength of 265 nm was 32 mW. Furthermore, no leakage or discontinuous change point was observed in the IV characteristics.

[Comparative Example 5]
An ultraviolet light-emitting element was formed in the same manner as in Example 31, except that the substrate was a sapphire substrate, and the formation conditions of the second conductivity type nitride semiconductor layer were changed so that the surface coverage of the second conductivity type nitride semiconductor layer with respect to the composition gradient layer was adjusted to be 65%.
The obtained ultraviolet light emitting device was evaluated by driving it with a current of 500 mA, and the driving voltage was 7.3 V and the emission intensity at a peak wavelength of 265 nm was 22 mW. In addition, leakage and discontinuous change points were observed in the IV characteristics.

The configuration and evaluation of each example and comparative example are shown in Table 1 below.

As shown in Table 1, each of the ultraviolet light-emitting devices of Examples 1 to 19, in which the substrate is a nitride semiconductor containing Al, the second conductivity type nitride semiconductor layer contains Mg, the Mg concentration in the second conductivity type nitride semiconductor layer is 8×10 ¹⁹ cm ^-3 or more and 1×10 ²¹ cm ^-3 or less, and the layer thickness is 1 nm or more and 20 nm or less, was found to achieve both light emission intensity and I-V characteristics compared to Comparative Examples 1 and 2 in which the Mg concentration is outside the above-mentioned range, Comparative Example 3 which does not include a second conductivity type nitride semiconductor layer, Comparative Example 4 in which the film thickness is outside the above-mentioned range, and Comparative Example 5 in which the substrate is not a nitride semiconductor.

From Examples 1 to 4, it was confirmed that when the coverage of the second conductivity type nitride semiconductor layer with respect to the layer directly below is 80% or more and 100% or less, the emission intensity is improved and the threshold voltage is also reduced regardless of the Mg concentration.

From Examples 3 and 10, it was confirmed that when a composition gradient layer is provided between the light emitting layer and the second conductivity type nitride semiconductor layer, the light emission intensity is improved and the threshold voltage is reduced compared to when no composition gradient layer is provided.
Moreover, it was confirmed from Examples 3 and 11 to 14 that when the layer thickness of the composition gradient layer was 15 nm or more and 60 nm or less, the emission intensity was improved and the threshold voltage was reduced.

From Example 3 and Comparative Example 5, it was found that the substrate is preferably formed of a nitride semiconductor containing Al, such as AlN.
Moreover, from Example 3 and Examples 15 to 17, it was found that the substrate having a layer thickness of 300 nm or more and 750 nm or less is preferable because it can achieve both higher emission intensity and lower threshold voltage.

Furthermore, from Examples 3 and 18, it was found that by providing an electrode (reflective electrode) with an ultraviolet light reflectance of 90% or more on the second conductivity type nitride semiconductor layer, the emission intensity is significantly improved and the threshold voltage is further reduced.

The scope of the present disclosure is not limited to the exemplary embodiments shown and described, but includes all embodiments that achieve equivalent effects to those intended by the present disclosure. Furthermore, the scope of the present disclosure is not limited to the combination of inventive features defined by the claims, but may be defined by any desired combination of specific features among all the respective disclosed features.

Reference Signs List 1, 2, 3, 4 UV light emitting element 11 Substrate 12 First conductivity type nitride semiconductor layer 13 Light emitting layer 14 Second conductivity type nitride semiconductor layer 15A First electrode 15B Second electrode 16 AlN layer 17 Electron blocking layer 25C Reflecting electrode 38 Compositionally graded layer 49 Waveguide layer 49A First waveguide layer 49B Second waveguide layer

Claims (8)

A substrate including a nitride semiconductor including Al;
a first conductivity type nitride semiconductor layer disposed on the substrate and containing Al and Ga;
a light emitting layer disposed on the first conductivity type nitride semiconductor layer and including a nitride semiconductor including Al and Ga;
a second conductivity type nitride semiconductor layer disposed on the light emitting layer and having a conductivity type different from that of the first conductivity type nitride semiconductor layer;
Equipped with
the second conductivity type nitride semiconductor layer contains Mg, a concentration of the Mg in the second conductivity type nitride semiconductor layer is equal to or greater than 8×10 ¹⁹ cm ⁻³ and equal to or less than 1×10 ²¹ cm ⁻³ , and a thickness of the second conductivity type nitride semiconductor layer is equal to or greater than 1 nm and equal to or less than 20 nm;
The lattice constant of the second conductive type nitride semiconductor layer in the in-plane a-axis direction is matched with that of the layer immediately below, and Mg is substituted into the lattice.
Ultraviolet light emitting element. The ultraviolet light emitting element according to claim 1 , wherein the coverage of the second conductive type nitride semiconductor layer with respect to the layer immediately below is 80% or more and 100% or less. 3. The ultraviolet light emitting element according to claim 1, wherein the second conductive type nitride semiconductor layer is made of Al _x Ga.sub. _(1-x) N (0.ltoreq.x.ltoreq.0.2). a composition gradient layer provided between the light emitting layer and the second conductivity type nitride semiconductor layer,
4. The ultraviolet light emitting element according to claim 1, wherein the composition gradient layer is formed of _{Al.sub.yGa.sub} . _(1-y) N (0.1.ltoreq.y.ltoreq.1.0), with the Al composition y decreasing in the direction away from the substrate. 5. The ultraviolet light emitting element according to claim 4, wherein the film thickness of the composition gradient layer is 15 nm or more and 60 nm or less. the substrate is an AlN single crystal substrate,
6. The ultraviolet light emitting element according to claim 1, wherein the first conductive type nitride semiconductor layer is made of _{Al.sub.zGa.sub} . _(1-z) N (0.6.ltoreq.z.ltoreq.0.9) and has a thickness of 300 nm or more and 750 nm or less. the first conductive type nitride semiconductor layer is made of Al _z Ga _(1-z) N (0.6≦z≦0.9);
7. The ultraviolet light emitting element according to claim 1, further comprising a waveguide layer made of _{Al.sub.pGa.sub} . _(1-p) N (0.5.ltoreq.p.ltoreq.z) above and below the light emitting layer. The ultraviolet light emitting element according to claim 1 , further comprising a reflective electrode disposed on the second conductivity type nitride semiconductor layer and having an ultraviolet light reflectance of 90% or more.

Images