JP2020167321A - Nitride semiconductor light-emitting device - Google Patents

Abstract

To provide a nitride semiconductor light-emitting device which is high in light output and low in drive voltage.SOLUTION: A nitride semiconductor device 10 comprises: an aluminum nitride (AlN) substrate 1; a first Group III nitride semiconductor layer 2 formed on the substrate; a Group III nitride semiconductor active layer 32 formed on the first Group III nitride semiconductor layer; and a second Group III nitride semiconductor layer 35 formed on the Group III nitride semiconductor active layer. The Group III nitride semiconductor active layer 35 has a multi-quantum well structure having well layers containing at least aluminum (Al) and gallium (Ga) and barrier layers formed by a Group III nitride semiconductor larger, in band gap energy, than the well layer, in which the well layers and the barrier layers are disposed alternately. In the Group III nitride semiconductor active layer 35, at least the barrier layers contain an n-type dopant at a concentration of 2.0×1018 cm-3 or more and 2.9×1019 cm-3 or less.SELECTED DRAWING: Figure 2

Description

The present invention relates to a nitride semiconductor light emitting device.

Nitride semiconductors AlN, GaN, InN, and their mixed crystals are attractive materials that can change the bandgap energy in various ways by changing the composition of group III elements (Al, Ga, In). is there. In particular, AlGaN is used in various devices such as AlGaN / GaN-based transistors and ultraviolet light receiving and emitting elements.
However, in the case of an ultraviolet light emitting device, AlGaN having a high bandgap energy and a high Al composition is used as the material of the light emitting layer, but the crystallinity deteriorates as the Al composition increases, so that the internal quantum efficiency decreases. In addition, since it is difficult to realize a p-type AlGaN having a sufficient hole concentration, carriers cannot be efficiently injected into the light emitting layer. From the above, it is currently extremely difficult to realize an ultraviolet light emitting element having sufficient luminous efficiency.

Patent Document 1 describes a light emitting device structure for the purpose of increasing the luminous efficiency of an ultraviolet light emitting device using a group III nitride semiconductor.
Specifically, the internal quantum is formed by forming a light emitting device structure using a so-called template in which an AlN layer is formed on a sapphire substrate and optimizing the thickness of the well layer of the multiple quantum well structure (MQW). We are trying to improve efficiency. Further, a final barrier layer is provided on the multiple quantum well structure, and an electron block layer (p-type or i-type AlN layer that serves as an energy barrier for electrons with respect to the final barrier layer) is provided on the final barrier layer, and the thickness thereof is increased. By optimizing the energy, the electron injection efficiency is improved.

Japanese Patent No. 5641173

However, even when the technique described in Patent Document 1 is used, the obtained optical output is still low.
When using an AlN template formed on a sapphire substrate, since the threading dislocation density of ^-2 10 ⁸ cm, it is difficult to obtain a sufficient internal quantum efficiency. In addition, we are trying to improve the internal quantum efficiency and carrier injection efficiency by optimizing the thickness of the well layer and final barrier layer of the multiple quantum well structure, but such measures alone are used in the nitride semiconductor laminated film. The distortion of the energy band caused by the internal electric field generated in the above and the influence of the spatial separation of electrons and holes cannot be sufficiently suppressed. As a result, sufficient light output cannot be obtained.

Further, it is generally known that the drive voltage is increased by increasing the potential barrier of the barrier layer or increasing the film thickness. Therefore, there is room for improvement in the nitride semiconductor light emitting device described in Patent Document 1 in that high power conversion efficiency (WPE) is realized by suppressing an increase in driving voltage.
An object of the present invention is to provide a nitride semiconductor light emitting device having a high light output and a low drive voltage.

In order to achieve the above object, one aspect of the present invention provides a nitride semiconductor light emitting device having the following configurations (1) to (3).
(1) Group III nitride semiconductor activity formed on the aluminum nitride (AlN) substrate, the first group III nitride semiconductor layer formed on the aluminum nitride substrate, and the first group III nitride semiconductor layer. It includes a layer and a second group III nitride semiconductor layer formed on the group III nitride semiconductor active layer.

(2) The group III nitride semiconductor active layer includes a well layer containing at least aluminum (Al) and gallium (Ga) and a barrier layer formed of a group III nitride semiconductor having a band gap energy larger than that of the well layer. It has a multiple quantum well structure with a plurality of alternating quantum wells.
(3) At least one layer of the plurality of barrier layers contains an n-type dopant at a concentration of 2.0 × 10 ¹⁸ cm ^-3 or more and 2.9 × 10 ¹⁹ cm ^-3 or less.

The nitride semiconductor light emitting element of the present invention can be expected to have a high light output and a low drive voltage.

It is a top view which shows the nitride semiconductor light emitting device of an embodiment. It is sectional drawing which shows the nitride semiconductor light emitting device of an embodiment, and shows the cross section AA of FIG. It is a top view which shows the state before the pad electrode and the insulating layer are formed in the nitride semiconductor light emitting device of FIG.

[One aspect of a nitride semiconductor light emitting device]
As described above, the nitride semiconductor light emitting device according to one aspect of the present invention includes an AlN substrate, a first group III nitride semiconductor layer, a group III nitride semiconductor active layer having a multiple quantum well structure, and a second group III nitride. At least one layer of a plurality of barrier layers having a nitride semiconductor layer and forming a multiple quantum well structure contains an n-type dopant at a concentration of 2.0 × 10 ¹⁸ cm ^-3 or more and 2.9 × 10 ¹⁹ cm ^-3 or less. Includes.
Another layer may be inserted between each of the above layers.
The nitride semiconductor light emitting device of one embodiment preferably has a composition gradient layer between the group III nitride semiconductor active layer and the second group III nitride semiconductor layer having a multiple quantum well structure.

<AlN board>
An AlN substrate is used as the substrate. By using the AlN substrate, the difference in lattice constant and the difference in coefficient of thermal expansion from the first group III nitride semiconductor layer formed on the substrate are reduced, so that through dislocations and cracks due to lattice relaxation are formed. The first group III nitride semiconductor layer can be grown without any need. In particular, by using a single crystal AlN substrate, it is possible to form a first group III nitride semiconductor layer having few crystal defects without being affected by grain boundaries, planar defects, and the like. By suppressing the propagation of threading dislocations into the first Group III nitride semiconductor to be laminated onto the substrate, from the viewpoint of enhancing the crystallinity, the threading dislocation density in the AlN substrate is preferably 10 ⁶ cm ^-2 or less, more preferably 10 ⁵ cm ^-2 or less, more preferably 10 ⁴ cm ^-2 or less. Impurities may be mixed in the AlN substrate. Further, the light extraction efficiency is improved by performing pattern processing on the surface of the AlN substrate or depositing the first group III nitride semiconductor layer in a state where the surface is masked with an insulating film such as SiO ₂ or SiN. ..

As the AlN substrate, for example, a substrate obtained by crystal-growth AlN to a thickness that can be used as a substrate can be used. As the growth surface, it is desirable to use the C surface because a flat growth surface can be realized relatively easily, but the growth surface is not limited to this. It is also possible to use an AlN substrate intentionally provided with an off angle.
By using the AlN substrate, the first group III nitride semiconductor layer having few crystal defects is formed on the substrate, so that the internal quantum efficiency of the nitride semiconductor light emitting device of one embodiment is increased and the driving voltage is decreased. Can be expected. In addition, it can be expected that a high value can be obtained for the rate of increase in optical output when the drive current is increased. Generally, in LEDs, it is observed that the light output is saturated when the amount of current is increased, and this is called the Drop phenomenon. By having the group III nitride semiconductor layer having few crystal defects formed on the AlN substrate, the nitride semiconductor light emitting element of one aspect can be expected to have an effect of suppressing the Drop phenomenon.

<Group III nitride semiconductor layer>
The first group III nitride semiconductor layer is a first conduction type group III nitride semiconductor layer. The material for forming the first group III nitride semiconductor layer is preferably a single crystal or a mixed crystal of AlN, GaN, or InN. These materials may contain Group V elements other than N such as P, As and Sb, and impurities such as C, H, F, O, Mg and Si, and the types of impurities are not limited to these. ..

When the first group III nitride semiconductor layer is used as an electron supply layer, the first conduction type is n-type. Specific examples of the first group III nitride semiconductor layer include n-Al _x Ga _(1-x) N (0 <x ≦ 1) in which Si is added as an n-type dopant. When a group III nitride semiconductor active layer that emits deep ultraviolet light of 300 nm or less is further laminated, the lattice constant difference from the substrate is reduced to reduce crystal defects, and in addition, the absorption of emitted ultraviolet light is suppressed. The Al composition is preferably high from the viewpoint of increasing the light extraction efficiency. On the other hand, as the Al composition increases, the contact resistance with the electrode increases, so that there is a suitable range for the Al composition. From the above viewpoint, the Al composition x of n—Al _x Ga _(1-x) N is preferably 0.40 ≦ x ≦ 0.90, and 0.50 ≦ x ≦ 0.80. More preferred.

When the first group III nitride semiconductor layer is relaxed with respect to the substrate, transmissive dislocations are formed in the film and propagate to the group III nitride semiconductor active layer (light emitting layer), resulting in a decrease in internal quantum efficiency. , Sufficient light output cannot be obtained. In addition, since the electron concentration and electron mobility in the film decrease, the sheet resistance of the first group III nitride semiconductor layer deteriorates and the drive voltage increases. Therefore, the lattice relaxation rate of the first group III nitride semiconductor layer is preferably 0% or more and 15% or less, more preferably 0% or more and 12% or less, and further preferably 0% or more and 10% or less. Is.

Means for lowering the lattice relaxation rate include reducing the difference in lattice constant from the substrate and setting the film thickness of the first group III nitride semiconductor layer to be equal to or lower than the critical film thickness. In one aspect of the nitride semiconductor light emitting device, the AlN substrate is used to reduce the difference in lattice constant with respect to the substrate of the first group III nitride semiconductor layer. Further, when the n-Al _x Ga _(1-x) N layer is laminated as the first group III nitride semiconductor layer directly on the AlN substrate or via the homoepitaxial layer, the above-mentioned Al composition range (0. In 40 ≦ x <0.90), the film thickness is preferably 1.5 μm or less, more preferably 1.3 μm or less, from the viewpoint of suppressing lattice relaxation. On the other hand, from the viewpoint of the elementization process process, the film thickness of the first group III nitride semiconductor is preferably 100 nm or more.

Therefore, the film thickness of the n-Al _x Ga _(1-x) N layer as the first group III nitride semiconductor is preferably 100 nm or more and 1.5 μm or less, and more preferably 100 nm or more and 1.3 μm or less. Is.
The first group III nitride semiconductor layer may be formed not directly on the AlN substrate but through a layer other than the first conduction type nitride semiconductor layer such as a buffer layer, and may be formed as a material for the buffer layer or as a material for the buffer layer. The film thickness is not particularly limited.

Further, the first group III nitride semiconductor layer can have a periodic structure (SPSL) of semiconductor layers having different lattice constants. One example is the periodic structure of Al _a Ga _(1-a) N / Al _b Ga _(1-b) N (a ≠ b). In the above structure, the two-dimensional electron (or hole) gas generated by the polarization effect caused by the strain in the film contributes to electrical conduction. Impurities may be added to SPSL.

<Group III nitride semiconductor active layer>
The group III nitride semiconductor active layer has a multiple quantum well structure (MQW) in which a plurality of well layers and barrier layers are alternately provided. The material forming the well layer is a group III nitride semiconductor containing at least Al and Ga, and the material forming the barrier layer is a group III nitride semiconductor having a bandgap energy larger than that of the well layer. At least one layer of the barrier layer contains an n-type dopant at a concentration of 2.0 × 10 ¹⁸ cm ^-3 or more and 2.9 × 10 ¹⁹ cm ^-3 or less. From the viewpoint of realizing higher light output and lower drive voltage, the concentration of the n-type dopant contained in the barrier layer is preferably 3.8 × 10 ¹⁸ cm ^-3 or more and 2.2 × 10 ¹⁹ cm ^-3 or less.

As an example of the multiple quantum well structure, an AlGaN layer is provided as a well layer, and an AlGaN layer or an AlN layer is provided as a barrier layer. The obtained emission wavelength can be adjusted by changing the Al composition and film thickness of the well layer and the barrier layer. Further, the well layer and the barrier layer may contain Group III elements other than Al and Ga such as In and B, and Group V elements other than N such as P, As and Sb.
As the n-type dopant contained in the barrier layer, C, Si, Ge, Sn, Te and the like can be used, but the n-type dopant is not particularly limited as long as it is an element having an action of generating free electrons when added to the mother crystal. ..

An example of the barrier layer is an AlGaN layer containing Si at a concentration of 2.0 × 10 ¹⁸ cm ^-3 or more and 2.9 × 10 ¹⁹ cm ^-3 or less.
In a multiple quantum well structure made of a group III nitride semiconductor on a sapphire substrate, the barrier layer generally contains no dopant or contains n-type dopant at a low concentration (10 ¹⁷ cm- ³ units). .. The reason is to avoid a decrease in crystallinity due to high-concentration doping, a decrease in internal quantum efficiency, and a decrease in electron injection efficiency.

On the other hand, according to the study by the present inventors, in the nitride semiconductor light emitting device of one aspect, an n-type dopant is 2.0 × 10 ¹⁸ cm in the barrier layer in the multiple quantum well structure formed on the AlN substrate. It was clarified that the light output can be improved at a high rate and the drive voltage can be reduced by adding the mixture at a high concentration of ^-3 or more and 2.9 × 10 ¹⁹ cm ^-3 or less. That is, according to the nitride semiconductor light emitting device of one aspect, both high light output and low drive voltage can be expected. The mechanism by which such an improvement in optical output and a reduction in drive voltage can be obtained is considered as follows.

When a group III nitride semiconductor active layer having a multiple quantum well structure (hereinafter, also simply referred to as “active layer”) is laminated on an AlN substrate, the active layer grows coherently with almost no release of lattice strain. An internal electric field due to the piezo effect is generated in the film, and the energy band is distorted, resulting in spatial separation of electrons and holes. When the n-type dopant is added to the barrier layer at a high concentration, the ionized dopant and the free electrons shield the internal electric field, so that an energy band screening effect is recognized. That is, the distortion of the energy band due to the internal electric field is suppressed, the spatial separation of electrons and holes can be reduced, and the internal quantum efficiency is improved. On the other hand, in the group III nitride semiconductor active layer having a multiple quantum well structure laminated on a sapphire substrate, strain is released by lattice relaxation, so that the above effect cannot be obtained.

Further, when the n-type dopant is added to the barrier layer at a high concentration, the Fermi energy of the barrier layer decreases, the potential energy for the electrons injected into the multiple quantum well decreases, and the potential energy for the injected holes increases. .. As a result, electrons are easily injected into the active layer, and are injected from the n-type layer (first group III nitride semiconductor layer) into the well layer near the p-type layer (second group III nitride semiconductor layer). The number of electrons increases.

On the other hand, for holes, since the potential energy increases, the confinement in the well layer near the p-type layer side is strengthened. As a result, the overlap integral of electrons and holes in the well layer near the p-type layer side increases, and the internal quantum efficiency increases. In addition, the drive voltage is reduced by increasing the electron injection efficiency and reducing the bulk resistance of the barrier layer by n-type doping of the barrier layer. As a result, the amount of heat generated when the amount of current is increased is reduced, so that the Drop phenomenon is suppressed.
The active layer may further contain impurities such as C, H, F, O, Mg and Si, and the types of impurities are not limited to these.

<Second group III nitride semiconductor layer>
The second group III nitride semiconductor layer is a second conduction type group III nitride semiconductor layer. That is, the conduction type of the second group III nitride semiconductor layer is different from the conduction type (first conduction type) of the first group III nitride semiconductor layer.
When the second group III nitride semiconductor layer is used as the hole supply layer, the second conduction type is p-type.
The material of the second group III nitride semiconductor layer is preferably any of AlN, GaN, InN simple substance and mixed crystal. Examples of the p-type III nitride semiconductor layer include a p-GaN layer and a p-AlGaN layer, and the p-GaN layer may be used from the viewpoint of improving the contact property with the second electrode layer. More preferred. That is, a specific example of a preferable p-type III nitride semiconductor layer can be represented by p—Al _w Ga _(1-w) N (0 ≦ w <1). Impurities such as C, H, F, O, Mg and Si may be contained, and the types of impurities are not limited to these.

The thickness of the second group III nitride semiconductor layer is preferably 5 nm or more and 100 nm or less from the viewpoint of suppressing the absorption of ultraviolet light in the active layer and reducing the risk of causing electrical defects. From the viewpoint of increasing the light output, it is more preferably 5 nm or more and 20 nm or less. By specifying this film thickness range, when the group III nitride semiconductor active layer is configured to emit ultraviolet rays, the emitted ultraviolet rays can be efficiently taken out from the light emitting element (with absorption and loss suppressed), and at the same time. It is possible to maintain a good contact state with the second electrode layer and suppress an increase in drive voltage and electrical defects.

Examples of the p-type dopant include Mg, Cd, Zn, Be and the like. When Mg is used as a p-type dopant, the doping concentration of Mg is preferably 5 × 10 ¹⁸ cm ^-3 or more from the viewpoint of increasing the electric conductivity and forming good contact with the second electrode. Further, from the viewpoint of improving the flatness of the surface of the p-GaN layer and improving the contact property with the second electrode layer, the doping concentration of Mg is 1 × 10 ²⁰ cm ^-3 or more and less than 8 × 10 ²⁰ cm ^-3 . It is preferably 2 × 10 ²⁰ cm ^-3 or more, and more preferably 6 × 10 ²⁰ cm ^-3 or less.

<Composition inclined layer>
It is preferable that a composition gradient layer is arranged between the active layer and the second group III nitride semiconductor layer. The material is Al _y Ga _(1-y) N (0.00 ≦ y ≦ 1.00), from the surface on the active layer side toward the surface on the second group III nitride semiconductor layer side. This is a layer in which the Al composition y is reduced.
The Al composition y of the composition gradient layer decreases from the surface on the active layer side toward the surface on the second group III nitride semiconductor layer side. The profile of the Al composition y may decrease continuously from the surface on the active layer side toward the surface on the side of the second group III nitride semiconductor layer, or may decrease intermittently. By "intermittently decreasing", it means that a portion having the same Al composition y in the film thickness direction of the composition gradient layer is included. That is, the composition gradient layer may include a portion in which the Al composition y does not decrease from the active layer side toward the second group III nitride semiconductor layer side, but does not include a portion in which the Al composition y increases. The composition gradient layer may include a portion having the same Al composition y, for example, with a thickness of several nm.

If the Al composition of the composition gradient layer on the surface on the active layer side is y1 and the Al composition of the composition gradient layer on the surface on the second group III nitride semiconductor layer side is y2, then y1> y2, but y1 and y2 The value is not particularly limited.
The active layer and the composition gradient layer may be in contact with each other, or another layer may be present between the active layer and the composition gradient layer. The composition gradient layer and the second group III nitride semiconductor layer may be in contact with each other, or another layer may be present between the composition gradient layer and the second group III nitride semiconductor layer.

Specifically, an electron block layer may be present between the active layer and the composition gradient layer. From the viewpoint of improving the effect of confining electrons in the active layer, the electron block layer is preferably formed of a material having a bandgap energy larger than that of the well layer and the barrier layer constituting the active layer. Impurities may be added to the electron block layer for conduction type control. An example of the electron block layer is an Al _z Ga _(1-z) N layer, and the Al composition z is preferably 0.70 or more and 1.00 or less, and more preferably 0.8 or more and 1.00 or more. It is as follows.

When the electron block layer is an Al _z Ga _(1-z) N layer, the Al composition y1 of the composition gradient layer and the Al composition z of the electron block layer may be the same or different. When y1 and z are different, either one may be larger. When the second group III nitride semiconductor layer is Al _w Ga _(1-w) N (0 ≦ w <1), the Al composition y2 of the composition gradient layer and the Al composition of the second group III nitride semiconductor layer w may be the same or different. When y2 and w are different, whichever is larger may be used.

From the viewpoint of reducing the potential barrier at the interface of each layer and improving the efficiency of hole injection from the composition gradient layer, the difference between the Al composition y1 of the composition gradient layer and the Al composition z of the electron block layer is 0.3. Hereinafter, the difference between the Al composition y2 of the composition gradient layer and the Al composition w of the second group III nitride semiconductor layer is preferably 0.4 or less.
The composition gradient layer may contain impurities such as C, H, F, O, Mg and Si.
Since the composition gradient layer has an action of generating holes by a polarization doping effect and efficiently injecting holes into the active layer, the group III nitride semiconductor active layer and the second group III nitride semiconductor layer are combined. By providing it in between, the luminous efficiency can be improved.

The film thickness of the composition gradient layer is preferably 5 nm or more and 110 nm or less, more preferably 15 nm or more and 90 nm or less, and further preferably 20 nm or more and 70 nm or less from the viewpoint of enhancing the luminous efficiency.
Instead of the composition gradient layer, a periodic structure (SPSL) of semiconductor layers having different lattice constants can be provided. One example is the periodic structure of Al _a Ga _(1-a) N / Al _b Ga _(1-b) N (a ≠ b). In the above structure, the two-dimensional hole gas generated by the polarization effect caused by the strain in the membrane is injected into the active layer. Impurities may be added to SPSL.

<Measurement method>
(Measurement of impurity concentration and dopant concentration)
The concentration of dopants and impurities contained in the substrate and each layer constituting the nitride semiconductor device of one aspect 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 being processed into a device, it can be performed in a state where the electrodes are removed by chemical etching or physical polishing. It is also possible to perform measurement by sputtering from the substrate side on which the electrode is not formed.
Specifically, SIMS measurement is performed under the measurement conditions provided by Evans Analytical Group (EAG). A cesium (Cs) ion beam having an energy of 14.5 keV is used for sputtering the sample at the time of measurement.

In the multiple quantum well structure (MQW), if there is a difference in the doping concentration between the well layer to be measured and the barrier layer, the measured concentration profile in the film thickness direction shows the vibration corresponding to the period of the well layer / barrier layer. When quantifying the concentration, the minimum or maximum value of the vibration is taken as the concentration value of the well layer or the barrier layer, respectively. Further, the Al composition of the multiple quantum well structure (MQW) is measured in advance by the composition evaluation method described later, and the higher Al composition is used as the barrier layer and the lower Al composition is used as the well layer, and the above SIMS measurement results are associated with each other. This quantifies the dopant concentrations in the well layer and the barrier layer.

(Measurement method of film thickness)
The film thickness of each layer constituting the nitride semiconductor light emitting device of one aspect is determined by cutting out a predetermined cross section perpendicular to the substrate, observing this cross section with a transmission electron microscope (TEM), and using the length measuring function of the TEM. Can be measured with. As a measuring method, first, a TEM is used to observe a cross section of the nitride semiconductor light emitting device perpendicular to the main surface of the substrate. Specifically, for example, the observation width is a range of 2 μm or more in a direction parallel to the main surface of the substrate in a TEM image showing a cross section perpendicular to the main surface of the substrate of the nitride semiconductor light emitting device. .. Since contrast is observed at the interface between the two layers having different compositions within this observation width range, the thickness up to this interface is observed in a continuous observation region having a width of 200 nm. The film thickness of each layer is obtained by calculating the average value of the thickness of each layer included in the observation region having a width of 200 nm from five points arbitrarily extracted from the observation width of 2 μm or more.

(Measurement method of lattice relaxation rate and Al composition of the first group III nitride semiconductor layer)
Examples of the method for measuring the Al composition x and the lattice relaxation rate of the first group III nitride semiconductor layer include reciprocal lattice mapping measurement (RSM) by the X-ray diffraction (XRD) method. Be done. Specifically, by analyzing the reciprocal lattice mapping data in the vicinity of the diffraction peak obtained by using the asymmetric surface as the diffraction surface, the lattice relaxation rate and the Al composition with respect to the substrate can be obtained. Examples of the diffraction plane include (10-15) plane and (20-24) plane.

(Measuring method of composition of each layer)
The composition of each layer constituting the nitride semiconductor light emitting device of one aspect can be measured by 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 the electron beam passes through the sample. Specifically, for example, in a sliced sample used for TEM observation or the like, the energy loss spectrum of the intensity of the transmission electron beam is measured and analyzed. Then, the composition can be obtained from the peak position by utilizing the fact that the peak position of the peak appearing in the vicinity of the energy loss amount of 20 eV changes according to the composition of each layer (for example, Al composition).

The Al composition of each layer is obtained by calculating the average value of the Al composition in the observation width of 200 nm from 5 points arbitrarily extracted from the observation region of 2 μm or more in the same manner as the above-mentioned film thickness calculation method by TEM observation. ..
The EDX measures and analyzes the characteristic X-rays generated by the electron beam in the sliced sample used in the above-mentioned TEM observation and the like. The Al composition of each layer is obtained by calculating the average value of the Al composition in the observation width of 200 nm from 5 points arbitrarily extracted from the observation region of 2 μm or more in the same manner as the above-mentioned film thickness calculation method by TEM observation.

In XPS, it is possible to evaluate in the depth direction by performing XPS measurement while performing sputter etching using an ion beam. Ar ⁺ is generally used for the ion beam, but other ion species such as Ar cluster ion may be used as long as the ions can be irradiated by the etching ion gun mounted on the XPS device. The XPS peak intensities of Al, Ga, and N are measured and analyzed to obtain the distribution of the Al composition of each layer in the depth direction. Instead of sputter etching, the nitride semiconductor light emitting device may be obliquely polished so that the cross section perpendicular to the main surface of the substrate is enlarged and exposed, and the exposed cross section may be measured by XPS.
The composition of each layer can be measured by measuring the cross section exposed by sputter etching or oblique polishing using not only XPS but also Aussie electron spectroscopy (AES). The composition of each layer can also be measured by SEM-EDX measurement on the cross section exposed by diagonal polishing.

<Ultraviolet light emitting module>
The light emitting device provided with the nitride semiconductor light emitting device according to one aspect of the present invention can be used as an ultraviolet light emitting module. The ultraviolet light emitting module can be applied to devices in the medical / life science field, the environmental field, the industrial / industrial field, the living / home appliance field, the agricultural field, and other fields, for example.

The light emitting device provided with the nitride semiconductor light emitting device according to one aspect of the present invention is a device for synthesizing / decomposing chemical substances, a device for sterilizing liquid / gas / solid (containers, foods, medical devices, etc.), and a device for cleaning semiconductors. , 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 / molding equipment for films, patterns, mockups, etc. , Applicable to measurement / inspection equipment for bills, scratches, blood, chemical substances, etc.

Examples of liquid sterilizers include automatic ice maker / ice tray and ice storage container / water tank for ice maker, freezer, ice maker, humidifier, dehumidifier, cold water tank / hot water tank / flow path of water server in refrigerator. Piping, stationary water purifiers, portable water purifiers, water dispensers, water dispensers, wastewater treatment equipment, disposers, toilet drain traps, washing machines, dialysis water sterilization modules, peritoneal dialysis connector sterilizers, disaster water storage systems, etc. This is not the case.
Examples of gas sterilizers include air purifiers, air conditioners, ceiling fans, floor and bedding vacuum cleaners, futon dryers, shoe dryers, washing machines, clothes dryers, indoor germicidal lamps, and storage ventilation. Systems, shoe boxes, tons, etc. can be mentioned, but not limited to this. Examples of solid sterilizers (including surface sterilizers) include vacuum packers, conveyor belts, hand tool sterilizers for medical / dental / barber / beauty salons, toothbrushes, toothbrush holders, chopstick boxes, cosmetic pouches, etc. Examples include, but are not limited to, drainage ditch lids, toilet bowl local cleaners, toilet bowl lids, etc.

[Embodiment]
Hereinafter, embodiments of the present invention will be described, 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 this limitation is not an essential requirement of the present invention.
In this embodiment, an example in which the nitride semiconductor light emitting device of one aspect of the present invention is applied to an ultraviolet light emitting device is described. Further, the conduction type of the first group III nitride semiconductor layer is n-type, and the conduction type of the second group III nitride semiconductor layer is p-type.

[overall structure]
First, the overall configuration of the ultraviolet light emitting device 10 of this embodiment will be described with reference to FIGS. 1 to 3.
As shown in FIGS. 1 and 2, the ultraviolet light emitting element 10 includes a substrate 1, an n-type group III nitride semiconductor layer (first group III nitride semiconductor layer) 2, a nitride semiconductor laminate 3, and the like. It has 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. The n-type group III nitride semiconductor layer 2 is formed on the substrate 1. The nitride semiconductor laminate 3 is a mesa portion formed on a part of the n-type III nitride semiconductor layer 2, and its side surface is a slope.

As shown in FIG. 2, the nitride semiconductor laminate 3 has an n-type group III nitride semiconductor layer 31, a group III nitride semiconductor active layer (light emitting layer) 32, an electron block layer 33, and a composition gradient from the substrate 1 side. The layer 34 and the p-type group III nitride semiconductor layer (second group III nitride semiconductor layer) 35 are formed in this order.
The nitride semiconductor laminate 3 has an n-type group III nitride semiconductor layer, a group III nitride semiconductor active layer, an electron block layer, a composition gradient layer, and a p-type group III nitride semiconductor layer on the substrate 1. , The laminate obtained by forming in this order is formed by removing the portion where the first electrode layer 4 is formed halfway in the thickness direction of the n-type Group III nitride semiconductor layer by mesa etching. ing. That is, the n-type III nitride semiconductor layer 31 of the nitride semiconductor laminate 3 is continuously formed on the n-type III nitride semiconductor layer 2.

The first electrode layer 4 is formed on the n-type group III nitride semiconductor layer 2 in a planar shape shown in FIG. 3, for example. The second electrode layer 5 is formed on the p-type III nitride semiconductor layer 35 in a 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, for example, an element that emits ultraviolet rays having a wavelength of 300 nm or less.

The substrate 1 is an AlN substrate, and the n-type group III nitride semiconductor layers 2 and 31 are n-Al _x Ga _(1-x) N (0.40 ≦ x ≦ 0.90) layers, and are group III nitrides. The semiconductor active layer 32 has a multiple quantum well structure (MQW) composed of a well layer made of AlGaN and a barrier layer made of AlGaN or AlN. The barrier layer contains Si (n-type dopant) at a concentration of 2.0 × 10 ¹⁸ cm ^-3 or more and 2.9 × 10 ¹⁹ cm ^-3 or less. The electron block layer 33 is an Al _z Ga _(1-z) N (0.80 ≦ z ≦ 1.00) layer, and its film thickness is 3 nm or more and 25 nm or less.

The composition gradient layer 34 is an Al _y Ga _(1-y) N (0.00 ≦ y ≦ 1.00) layer, and the surface where the Al composition y is in contact with the electron block layer 33 (the surface on the active layer 32 side). It continuously decreases from the surface toward the surface in contact with the p-type III nitride semiconductor layer 35 (the surface on the p-type III nitride semiconductor layer 35 side). The film thickness of the composition inclined layer 34 is 5 nm or more and 110 nm or less.
The p-type III nitride semiconductor layer 35 is a GaN layer containing Mg as an impurity in the range of 1 × 10 ²⁰ cm ^-3 or more and less than 8 × 10 ²⁰ cm ^-3 , and its film thickness is 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.
Examples of the material of the first pad electrode 6 and the second pad electrode 7 include Au, Al, Cu, Ag, W and the like, but Au having high conductivity is desirable.

The insulating layer 8 includes a portion not covered by the first electrode layer 4 of the n-type group III nitride semiconductor layer 2, a portion not covered by the second electrode layer 5 of the nitride semiconductor laminate 3, and a first portion. The portion of the electrode layer 4 not covered by the first pad electrode 6, the portion of the second electrode layer 5 not covered by the second pad electrode 7, and the lower portion of the first pad electrode 6 and the second pad electrode 7. It is formed on the side surface. The insulating layer 8 may cover a part of the upper part 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 layer.

[Action, effect]
By using an AlN substrate as the substrate 1, the ultraviolet light emitting device 10 of the embodiment can grow the n-type III nitride semiconductor 2 on the substrate 1 in a state where no through dislocation or cracks occur, so that a high light output can be obtained. .. Further, high-concentration n-type doping to the barrier layer provides high internal quantum efficiency and electron injection efficiency, so that the light output is high and the drive voltage is low.
Further, by having an Al _z Ga _(1-z) N (0.80 ≦ z ≦ 1.00) layer having a film thickness of 3 nm or more and 25 nm or less as the electron block layer, the inside of the group III nitride semiconductor active layer 32 The effect of confining electrons in the group is enhanced, and by having the composition gradient layer 34 having the above configuration, the hole injection efficiency into the group III nitride semiconductor active layer 32 is improved, so that high light emission efficiency can be obtained.

<Sample No.1-1 to No.1-6>
These samples are the ultraviolet light emitting elements 10 having the structure described in the embodiment, and have the following configurations.
The substrate 1 is an AlN substrate. The n-type III nitride semiconductor layer 2 and the n-type III nitride semiconductor layer 31 are n-type Al _0.7 Ga _0.3 N layers containing Si as impurities, and the n-type III group nitride semiconductor layer 2 and n-type. The total thickness of the group III nitride semiconductor layer 31 (that is, the thickness of the n-type group III nitride semiconductor layer between the substrate 1 and the group III nitride semiconductor active layer 32) is 500 nm. The group III nitride semiconductor active layer 32 alternately contains Al _0.51 Ga _0.49 N (well layer) having a thickness of 2.0 nm and Si-doped Al _0.78 Ga _0.22 N (barrier layer) having a thickness of 8.0 nm. It is a multi-quantum well structure with layers. The Si concentration of the barrier layer of each sample is as shown in Table 1. The electron block layer 33 is an Al _0.86 Ga _0.14 N layer and has a film thickness of 17 nm.

The composition gradient layer 34 is an Al _y Ga _(1-y) N layer, and the Al composition y is continuous from 0.86 to 0.25 from the electron block layer 33 toward the p-type III nitride semiconductor layer 35. It is a layer that changes in a target manner. The p-type III nitride semiconductor layer 35 is a p-type GaN layer containing 2.0 × 10 ²⁰ cm ^-3 as an impurity.
The first electrode layer 4 is Ti / Al / Ni / Au, and the second electrode layer 5 is Ni / Au. The first pad electrode 6 and the second pad electrode 7 have a laminated structure of Ti and Au.

Each element having the above configuration was manufactured by the following method.
First, by the MOCVD method, an n-type Al _0.70 Ga _0.30 N layer having a thickness of 500 nm containing Si as an impurity on the entire surface of the AlN substrate, the multiple quantum well structure, and an Al _0.86 Ga _0.14 N layer serving as an electron block layer 33, composition. A _y Ga _(1-y) N layer with a thickness of 28 nm to be the inclined layer 34 (Al composition y was continuously changed from 0.86 to 0.25), Mg as an impurity 2.0 × 10 ²⁰ A p-type GaN layer having a thickness of 10 nm including cm ^-3 was formed in this order. As a result, an object in which a laminate was formed on the AlN substrate 1 was obtained.

As raw materials, triethyl gallium (TEGa), trimethylaluminum (TMAl), ammonia (NH ₃ ), monosilane (SiH ₄ ), and biscyclopentadienyl magnesium (Cp ₂ Mg) were used. The Al composition of each layer was controlled by controlling the supply ratio of triethyl gallium (TEGa) and trimethylaluminum (TMAl), and the film thickness was controlled by changing the growth time. The doping concentration of Si was controlled by adjusting the supply amount of monosilane (SiH ₄ ).

During film formation, the substrate temperature is controlled to 1100 ° C. and the growth pressure is controlled to 50 hPa, and the raw material supply ratio (V / III ratio) of NH ₃ which is a group V raw material and group III raw materials (triethyl gallium, trimethylaluminum) is 3000. did. The Al composition of each layer was measured by XPS, and the Si concentration of the barrier layer was measured by quantitative analysis by SIMS.
Next, the nitride semiconductor laminate 3 shown in FIG. 2 was formed by etching the laminate on the AlN substrate 1 to remove a part of the in-plane at a predetermined depth. The etching depth is a depth at which a part of the n-type III nitride semiconductor layer 2 is removed, and a part of the n-type III nitride semiconductor layer 2 is exposed in a plan view by this etching. The portion that is not etched remains as the n-type III nitride semiconductor layer 31 of the nitride semiconductor laminate 3. As an etching method, dry etching was performed using an inductively coupled plasma type apparatus.

Next, after forming the insulating layer 8 on the entire surface of the substrate 1 in this state, a part of the insulating layer 8 in the plane is removed to expose a part of the n-type III nitride semiconductor layer 2. Etching with BHF was performed.
Next, the first electrode layer 4 was formed in the region of the n-type group III nitride semiconductor layer 2 that became an exposed surface in a plan view by the following method.
First, in this region, the titanium (Ti) layer, the aluminum (Al) layer, the nickel (Ni) layer, and the gold (Au) layer are arranged in this order in the order of 20 nm / 130 nm / in the planar shape of the first electrode layer 4 shown in FIG. A metal laminate was obtained by forming a thickness of 35 nm / 50 nm by a vapor deposition method. Next, the AlN substrate 1 in this state was placed in a heat treatment apparatus, and the metal laminate was heat-treated by an RTA (Rapid Thermal Annealing) method.

The heat treatment was carried out for 2 minutes by maintaining the temperature of the AlN substrate 1 at 850 ° C. and introducing nitrogen gas at 150 ° C. into the heat treatment apparatus. The temperature of nitrogen gas was adjusted by attaching a heater to the gas pipe.
Next, in order to expose a part of the p-type III nitride semiconductor layer 35 of the nitride semiconductor laminate 3 to the AlN substrate 1 after the first electrode layer 4 is formed, etching with BHF is performed. It was.

Next, the AlN substrate 1 in this state is placed in a vapor deposition apparatus, and nickel (in the plan shape of the second electrode layer 5 shown in FIG. 1) is placed on the p-type III nitride semiconductor layer 35 of the nitride semiconductor laminate 3. After forming the Ni) layer and the gold (Au) layer in this order, a known heat treatment was performed to form the second electrode layer 5.
Next, the insulating layer 8 is formed on the entire surface of the AlN substrate 1 in this state on which the second electrode layer 5 is formed, and then the first pad electrode 6 and the second pad electrode 7 are formed on the insulating layer 8. An opening was formed.
Next, the first pad electrode 6 and the second pad electrode 7 were formed of a laminated film of Ti and Au.

<Sample No.2-1 to No.2-11>
These samples do not have a composition gradient layer 34. Except for this point, it has the same structure as the ultraviolet light emitting device 10 of the embodiment. The Si concentration of the barrier layer of each sample is as shown in Table 1.
<Sample No.3-1 to No.3-7>
In these samples, the substrate is an AlN template using a sapphire substrate and does not have the composition gradient layer 34. Except for these points, it has the same structure as the ultraviolet light emitting device 10 of the embodiment. The film thickness of the AlN template layer was 1.5 μm, and the growth method used was a conventional technique with a reported example. During the film formation, the substrate temperature was controlled to 1120 ° C. and the growth pressure was controlled to 50 hPa, and the raw material supply ratio (V / III ratio) of NH _{3 as a} group V raw material and trimethylaluminum as a group III raw material was set to 1000. The Si concentration of the barrier layer of each sample is as shown in Table 1.

<Performance evaluation>
The light output and drive voltage of each of the obtained ultraviolet light emitting devices were examined. The light output was obtained by passing a current of 100 mA through each ultraviolet light emitting element, condensing the generated light with an integrating sphere, and then detecting it with a photodiode calibrated through a spectroscope. The drive voltage was obtained by passing a current of 100 mA through each ultraviolet light emitting element 10 and measuring the generated voltage value. In addition, the ultraviolet light emitting element of each sample obtained light emission having a peak wavelength in the vicinity of the wavelength of 270 nm.

Further, the power conversion efficiency was defined by dividing the optical output by the product of the current value (100 mA) and the drive voltage, and the relative value (value in which No. 2-7 was set to "1") was calculated. Further, the ratio (L500 / L100) of the optical output (L500) at a drive current of 500 mA to the optical output (L100) at a drive current of 100 mA was calculated. From this ratio, the drop suppressing effect can be verified.
These results are shown in Table 1 together with the configuration of each sample.

In the configuration column of Table 1, the essential requirements of one aspect of the present invention are "the substrate is an AlN substrate" and "the n-type dopant (Si) concentration of the barrier layer is 2.0 × 10 ¹⁸ cm ^-3 or more. The values that deviate from "2.9 x 10 ¹⁹ cm ^-3 or less" are underlined, and the preferred embodiment "n-type dopant (Si) concentration in the barrier layer is 3.8 x 10 ¹⁸ cm ^-3 or more and 2.2 x 10". Numerical values that deviate from " ¹⁹ cm ^-3 or less" and "having a composition gradient layer" are double underlined.
Further, in the performance column of Table 1, the numerical values deviating from "0.60 or more" corresponding to good power efficiency (relative value) are underlined.

As shown in Table 1, the elements No. 1-1 to No. 1-6 and No. 2-4 to No. 2-9 are "the substrate is an AlN substrate" and "n-type barrier layer". The dopant (Si) concentration satisfies "2.0 x 10 ¹⁸ cm ^-3 or more and 2.9 x 10 ¹⁹ cm ^-3 or less". Therefore, "0.60 or more" corresponding to good power efficiency (relative value) was obtained. On the other hand, at least "the substrate is an AlN substrate" and "the n-type dopant (Si) concentration of the barrier layer is 2.0 × 10 ¹⁸ cm ^-3 or more and 2.9 × 10 ¹⁹ cm ^-3 or less". The elements of No.2-1 to No.2-3, No.2-10, No.2-11, No.3-1 to No.3-7 that do not satisfy any of the above are power efficiency (relative value). Was 0.53 or less.

The elements No. 2-5 to No. 2-8 do not have a composition gradient layer, but "the n-type dopant (Si) concentration in the barrier layer is 3.8 x 10 ¹⁸ cm ^-3 or more. By satisfying "2 x 10 ¹⁹ cm ^-3 or less", "0.80 or more" corresponding to a more preferable power efficiency (relative value) was obtained.
Further, since the elements No. 1-1 to No. 1-6 have a composition gradient layer, "1.10 or more" corresponding to a more preferable power efficiency (relative value) was obtained. Among them, the elements No. 1-2 to No. 1-5 have "the n-type dopant (Si) concentration of the barrier layer is 3.8 x 10 ¹⁸ cm ^-3 or more and 2.2 x 10 ¹⁹ cm ^-3 or less. The power efficiency (relative value) was 1.28 or more, which was particularly high.

Further, the n-type dopant (Si) concentration of the barrier layer is the same in the range of "the n-type dopant (Si) concentration of the barrier layer is 2.0 × 10 ¹⁸ cm ^-3 or more and 2.9 × 10 ¹⁹ cm ^-3 or less". Comparing samples with different concentrations and different types of substrates and the presence or absence of a composition gradient layer, the drive voltage can be reduced by using an AlN substrate as the substrate, and the light output can be further improved by providing the composition gradient layer. I understand.

1 Substrate 2 n-type group III nitride semiconductor layer (first group III nitride semiconductor layer)
3 Nitride semiconductor laminate 31 n-type group III nitride semiconductor layer (first group III nitride semiconductor layer)
32 Group III nitride semiconductor active layer 33 Electron block layer 34 Composition gradient layer 35 p-type Group III nitride semiconductor layer (second group III nitride semiconductor layer)
4 1st electrode layer 5 2nd electrode layer 6 1st pad electrode 7 2nd pad electrode 8 Insulation layer 10 Ultraviolet light emitting element (nitride semiconductor light emitting element)

Claims (4)

Aluminum nitride (AlN) substrate and
The first group III nitride semiconductor layer formed on the aluminum nitride substrate and
The group III nitride semiconductor active layer formed on the first group III nitride semiconductor layer and
A second group III nitride semiconductor layer formed on the group III nitride semiconductor active layer and
With
The group III nitride semiconductor active layer alternates between a well layer containing at least aluminum (Al) and gallium (Ga) and a barrier layer formed of a group III nitride semiconductor having a band gap energy larger than that of the well layer. Has multiple quantum well structures
A nitride semiconductor light emitting device in which at least one layer of the plurality of barrier layers contains an n-type dopant at a concentration of 2.0 × 10 ¹⁸ cm ^-3 or more and 2.9 × 10 ¹⁹ cm ^-3 or less. The nitride semiconductor light emitting device according to claim 1, wherein the n-type dopant is at least one of C, Si, Ge, Sn, and Te. The nitride semiconductor light emitting device according to claim 1 or 2, wherein the concentration of the n-type dopant contained in the barrier layer is 3.8 × 10 ¹⁸ cm ^-3 or more and 2.2 × 10 ¹⁹ cm ^-3 or less. An Al _y Ga _(1-y) N (0.00 ≦ y ≦ 1.00) layer arranged between the group III nitride semiconductor active layer and the second group III nitride semiconductor layer. , Any of claims 1 to 3, further comprising a composition gradient layer in which the Al composition y decreases from the surface on the side of the group III nitride semiconductor active layer toward the surface on the side of the second group III nitride semiconductor layer. The nitride semiconductor light emitting device according to item 1.

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