Patent Application
20220190202
The present patent application claims the priority of Japanese patent application No. 2020/208841 filed on Dec. 16, 2020, and the entire contents of Japanese patent application No. 2020/208841 are hereby incorporated by reference.
The present invention relates to a light-emitting element and a method for manufacturing the light-emitting element.
A technique is known which uses a degenerately doped gallium nitride layer as a tunnel junction in a light-emitting diode (LED) (see, e.g., Patent Literature 1). “Degenerately doped” mentioned above is considered to mean that the Fermi level and the conduction band are overlapped (or degenerated) due to doping a dopant at a high concentration. Semiconductors in which the Fermi level and the conduction band are degenerated usually behave similarly to a metal and have reduced electrical resistance. In addition, since such semiconductors behave similarly to a metal, there is no temperature dependency in electrical resistance. Therefore, LEDs using a degenerately doped gallium nitride layer as the tunnel junction can be expected to be driven in a wide temperature range.
Concerning n-type AlGaN including a group IV element such as Si as a dopant, according as a concentration of the group IV element is increased, electric resistance decreases in the same manner as that of general semiconductors up to a certain concentration, but starts to increase conversely once exceeding the certain concentration. For this reason, even by the known usual method where the concentration of the group IV element is simply increased, it is not possible to effectively reduce the electric resistance.
It is an object of the invention to provide a light-emitting element which includes an n-type contact layer including an AlGaN with a group IV element dopant such that the electrical resistance can be effectively reduced by the degeneracy of the Fermi level and the conduction band, as well as a method for manufacturing the light-emitting element.
According to an aspect of the invention, a light-emitting element as set forth in (1) to (5) below and a method for manufacturing a light-emitting element as set forth in (6) and (7) below are provided.
(1) A light-emitting element, comprising:
According to an aspect of the invention, a light-emitting element can be provided which includes an n-type contact layer including an AlGaN with a group IV element dopant such that the electrical resistance can be effectively reduced by the degeneracy of the Fermi level and the conduction band, as well as a method for manufacturing the light-emitting element.
“On (preposition indicating position)” in the configuration of the light-emitting element 1 is “on” when the light-emitting element 1 is placed in a direction as shown in
The substrate 10 is a growth substrate formed of sapphire. A thickness of the substrate 10 is, e.g., 900 μm. In addition to sapphire, it is possible to use AlN, Si, SiC, ZnO, etc., as a material of the substrate 10.
The buffer layer 11 has a structure in which, e.g., three layers; a nucleation layer, a low-temperature buffer layer and a high-temperature buffer layer, are sequentially stacked. The nucleation layer is a layer that is formed of non-doped AlN grown at a low temperature and is a nucleus of crystal growth. A thickness of the nucleation layer is, e.g., 10 nm. The low-temperature buffer layer is a layer that is formed of non-doped AlN grown at a higher temperature than the nucleation layer. A thickness of the low-temperature buffer layer is, e.g., 0.3 μm. The high-temperature buffer layer is a layer that is formed of non-doped AlN grown at a higher temperature than the low-temperature buffer layer. A thickness of the high-temperature buffer layer is, e.g., 2.7 μm. A threading dislocation density in AlN is reduced by providing such a buffer layer 11.
The light-emitting layer 13 is a layer stacked on the n-type contact layer 12. The light-emitting layer 13 is formed of AlGaN and preferably has a multiple quantum well (MQW) structure. An Al composition x of the light-emitting layer 13 (an Al composition x of well layers when having the MQW structure) is set according to a desired emission wavelength and is set to, e.g., 0.35 to 0.45 when the emission wavelength is about 280 nm. The Al composition x here is a proportion of an Al content when the total of a Ga content and the Al content is defined as 1, and is expressed as AlxGa1-xN (0≤x≤1) in an ideal composition of AlGaN.
The light-emitting layer 13 has, e.g., a MQW structure having two well layers, i.e., a structure in which a first barrier layer, a first well layer, a second barrier layer, a second well layer and a third barrier layer are stacked in this order. The first well layer and the second well layer are formed of n-type AlGaN. The first barrier layer, the second barrier layer and the third barrier layer are formed of n-type AlGaN with a higher Al composition (including the Al composition x of 1, i.e., AlN) than the first well layer and the second well layer.
As an example, an Al composition x, a thickness and a concentration of Si as a dopant for each of the first well layer and the second well layer are 0.4, 2.4 nm and 9×1018/cm3. An Al composition x, a thickness and a concentration of Si as a dopant for each of the first barrier layer and the second barrier layer are 0.55, 19 nm and 9×1018/cm3. An Al composition x, a thickness and a concentration of Si as a dopant for the third barrier layer are 0.55, 4 nm and 5×1018/cm3.
The n-type contact layer 12 is formed of n-type AlGaN including a group IV element, such as Si or Ge, as a donor. The lower limit of the Al composition x of the n-type contact layer 12 is set as a lower limit of a range in which absorption of light emitted from the light-emitting layer 13 can be suppressed. Absorption of light emitted from the light-emitting layer 13 by the n-type contact layer 12 can be effectively suppressed when the Al composition x of the n-type contact layer 12 is not less than 0.1 greater than the Al composition x of AlGaN constituting the light-emitting layer 13 (the Al composition x of the well layers when the light-emitting layer 13 has the MQW structure), and the absorption can be suppressed more effectively when the Al composition x of the n-type contact layer 12 is not less than 0.15 greater than the Al composition x of AlGaN constituting the light-emitting layer 13. Thus, the Al composition x of the n-type contact layer 12 is preferably not less than 0.1, more preferably not less than 0.15, greater than the Al composition x of the light-emitting layer 13.
In case that the Al composition x of the light-emitting layer 13 is, e.g., 0.35 to 0.45, light with a wavelength of about 280 nm is emitted, the absorption can be effectively suppressed when the Al composition x of the n-type contact layer 12 is not less than 0.5, and the absorption can be suppressed more effectively when the Al composition x of the n-type contact layer 12 is not less than 0.55.
Meanwhile, the upper limit of the Al composition x of the n-type contact layer 12 can be set as an upper limit of a range in which an increase in electrical resistance with an increase in the Al composition x can be suppressed. When the Al composition x is increased, electrical resistance of AlGaN is substantially constant up to the Al composition x of 0.7 and starts to increase when exceeding 0.7. For this reason, the Al composition x of the n-type contact layer 12 is preferably set to not more than 0.7.
Thus, as a preferable example when the light-emitting element 1 is an ultraviolet light-emitting element, the Al composition x of the n-type contact layer 12 is within the range of not less than 0.5 and not more than 0.7. In this case, the n-type contact layer 12 ideally has a composition expressed by AlxGa1-xN (0.5≤x≤0.7).
The n-type contact layer 12 also has an effective donor concentration Nd−Na at which a Fermi level and a conduction band are degenerated. Here, Nd is a donor concentration and Na is an acceptor concentration. When Nd−Na is not more than 4.0×1019 cm3, self-compensation due to complex defects of group III vacancies and group IV elements hardly occurs and a value of Nd−Na is thus substantially equal to a value obtained by subtracting a concentration of an element acting as an acceptor from a concentration of a group IV element acting as a donor in the n-type contact layer 12. An impurity concentration can be measured by secondary ion mass spectrometry (SIMS).
In AlGaN, group IV elements serve as a donor when entered the Ga or Al site, and serve as an acceptor when entered the N site. In AlGaN, C has a property of easily entering the N site and the group IV elements other than C have a property of easily entering the Ga or Al site. Therefore, usually, the group IV element serving as an acceptor in AlGaN is mainly C. In addition, not only C intentionally added as a dopant but also, e.g., C included in a group III raw material used in MOCVD, etc., are incorporated into AlGaN. Therefore, regardless of the type of group IV element used as a donor, the acceptor concentration Na is usually substantially equal to a concentration of C, and if incorporation of C into the n-type contact layer 12 can be suppressed by adjusting the growth temperature, etc., the effective donor concentration Nd−Na becomes substantially equal to a concentration of the group IV element.
According to Non-Patent Literature “A. Wolos et al., “Properties of metal-insulator transition and electron spin relaxation in GaN:Si”, PHYSICAL REVIEW B 83, 165206 (2011)”, the Fermi level and the conduction band are degenerated in GaN including Si as a dopant when the Si concentration is not less than 1.6×1018 cm−3. In this regard, it is considered that this Si concentration condition can be generalized as the effective donor concentration Nd−Na condition, including the case where GaN includes an acceptor. That is, the lower limit of the Si concentration at which degeneracy occurs in AlGaN with the Al composition x of 0 (GaN) is 1.6×1018 cm3.
The present inventors also derived that the lower limit of the effective donor concentration Nd−Na at which degeneracy occurs in AlGaN with the Al composition x of 0.62 (Al0.62Ga0.38N) is 9.5×1018 cm−3. A method for this derivation will be described later.
Furthermore, the present inventors derived that the lower limit of the effective donor concentration Nd−Na at which degeneracy occurs in AlGaN with the Al composition of 0.5 (Al0.5Ga0.5N) is 7.6×1018 cm−3, and the lower limit of the effective donor concentration Nd-Na at which degeneracy occurs in AlGaN with the Al composition of 1 (AlN) is 1.6×1019 cm−3. A method for this derivation will be described later.
In AlGaN including a group IV element as a donor, the Fermi level and the conduction band usually can be made degenerate by increasing the effective donor concentration Nd−Na. However, when the effective donor concentration Nd−Na exceeds 4.0×1019 cm−3, the complex defects of group III vacancies and group IV elements are formed, self-compensation occurs and electrical resistance is thus not effectively reduced.
The details of the complex defects of group III vacancies and group IV elements have not yet been revealed, but one possible likelihood is that when the group IV element does not enter the group III vacancy generated in the process of growing AlGaN and stays at another location, the group IV element cannot behave as a donor (cannot emit electrons) and one to three holes are emitted depending on the state.
The Fermi level and the conduction band of the n-type contact layer 12 become degenerate when the effective donor concentration Nd−Na is above the approximate curve in
In the present embodiment, the n-type contact layer 12 can have electrical resistivity of not more than 5×10−2 Ω·cm by respectively setting, e.g., the concentration of the group IV element in the n-type contact layer 12 within a range of not less than 5×1018 cm−3 and not more than 4×1019 cm−3, the growth temperature of the n-type contact layer 12 within a range of not less than 850° C. and not more than 1100° C., and the V/III ratio of the source gas of the n-type contact layer 12 (described later) within a range of not less than 1000 and not more than 3200. In addition, it is considered that the lower limit of electrical resistivity of the n-type contact layer 12 under these conditions of the concentration of the group IV element, the growth temperature and the V/III ratio of the source gas of the n-type contact layer 12 is about 1×10−3 Ω·cm. The thickness of the n-type contact layer 12 is, e.g., 500 to 3000 nm.
The electron blocking layer 14 is formed of p-type AlGaN with a higher Al composition x than the third barrier layer. Diffusion of electrons to the p-type contact layer 15 side is suppressed by the electron blocking layer 14. An Al composition x, a thickness and a concentration of Mg as a dopant for the electron blocking layer 14 are, e.g., respectively 0.8, 25 nm and 5×1019/cm3.
The p-type contact layer 15 has a structure in which a first p-type contact layer and a second p-type contact layer are sequentially stacked. The first p-type contact layer and the second p-type contact layer are formed of p-type GaN. A thickness and a concentration of Mg as a dopant for the first p-type contact layer are, e.g., respectively 700 nm and 2×1019/cm3. Meanwhile, a thickness and a concentration of Mg as a dopant for the second p-type contact layer are, e.g., respectively 60 nm and 1×1020/cm3.
A trench is provided in a part of a region on a surface of the p-type contact layer 15. The trench penetrates the p-type contact layer 15 and the light-emitting layer 13 and reaches the n-type contact layer 12, and the n-electrode 18 is connected to a surface of the n-type contact layer 12 exposed by the trench.
The transparent electrode 16 is formed of, e.g., a conductive oxide transparent to visible light, such as IZO, ITO, ICO, ZnO. When light emitted from the light-emitting layer 13 is ultraviolet light (light of not more than 365 nm), a large amount of the light is absorbed by the p-type contact layer 15 formed of GaN and does not pass through the p-electrode 17, hence, reflected light from the p-electrode 17 is not obtained. In this regard, however, when the thin p-type contact layer 15 formed of GaN or the p-type contact layer 15 formed of AlGaN is used and the thin transparent electrode 16 or the transparent electrode 16 formed of a material transparent to ultraviolet light is used, absorption of the ultraviolet light thereby can be suppressed and light output can thus be significantly increased. The p-electrode 17 is formed of, e.g., Ni/Au. The n-electrode 18 is formed of, e.g., Ti/Al/Ni, V/Al/Ni, or V/Al/Ru, etc.
The light-emitting element 1 may be of a face-up type. In addition, the characteristic configuration of the light-emitting element 1 such as the n-type contact layer 12 can be applied to light-emitting elements other than LED, such as laser diode.
Next, an example of a method for manufacturing the light-emitting element 1 in the embodiment of the invention will be described. When forming each layer of the light-emitting element 1 by a vapor-phase growth method, a Ga source gas, an Al source gas and an N source gas used are, e.g., respectively trimethylgallium, trimethylaluminum and ammonia. Meanwhile, an n-type dopant source gas and a p-type dopant source gas used are, e.g., respectively a silane gas, which is a Si source gas, and a bis (cyclopentadienyl) magnesium gas, which is a Mg source gas. In addition, a carrier gas used is, e.g., a hydrogen gas or a nitrogen gas. The growth temperature of each layer in the present embodiment is temperature of a heater of a film formation apparatus and a surface temperature of the substrate 10 is about 100° C. lower than the temperature of the heater.
Firstly, the substrate 10 is prepared and the buffer layer 11 is formed thereon. When forming the buffer layer 11, the nucleation layer formed of AlN is firstly formed by sputtering. The growth temperature is, e.g., 880° C. Next, the low-temperature buffer layer and the high-temperature buffer layer, which are formed of AlN, are sequentially formed on the nucleation layer by the MOCVD method. The growth conditions for the low-temperature buffer layer are, e.g., a growth temperature of 1090° C. and a growth pressure of 50 mbar. The growth conditions for the high-temperature buffer layer are, e.g., a growth temperature of 1270° C. and a growth pressure of 50 mbar.
Next, the n-type contact layer 12 formed of AlGaN including a group IV element such as Si is formed on the buffer layer 11 by the MOCVD method. When forming the n-type contact layer 12, the V/III ratio of the source gas of the n-type contact layer 12 is set within a range of not less than 1000 and not more than 3200 to reduce electrical resistance of the n-type contact layer 12. The V/III ratio here means a ratio of the number of group III element atoms (Ag, Al) and the number of group V element atoms (N) in the source gas.
In addition, when forming the n-type contact layer 12, the growth temperature of the n-type contact layer 12 is preferably set to not more than 1150° C. An increase in electrical resistance with an increase in the growth temperature can be suppressed by setting the growth temperature to not more than 1150° C. It is considered that this is because evaporation of the group III elements, particularly Ga which easily evaporates, is suppressed, the excessive generation of group III vacancies is thus suppressed, and an increase in electrical resistance due to the influence of the complex defects of group III vacancies and group IV elements is thereby suppressed.
When forming the n-type contact layer 12, the growth temperature of the n-type contact layer 12 is also preferably set to not less than 850° C. When the growth temperature is less than 850° C., ammonia, which is a source of the group V element N, is less likely to be decomposed, hence, a supply amount of ammonia needs to be increased and the V/III ratio needs to be set abnormally high. In addition, when the growth temperature is low, there may arise a problem that C from the group III source material is incorporated. Therefore, the growth temperature is preferably set to a temperature at which this problem can be avoided, e.g., not less than 850° C.
Meanwhile, the growth pressure of the n-type contact layer 12 is set to, e.g., 20 to 200 mbar.
Next, the light-emitting layer 13 is formed on the n-type contact layer 12 by the MOCVD method. The light-emitting layer 13 is formed by stacking the first barrier layer, the first well layer, the second barrier layer, the second well layer and the third barrier layer in this order. The growth conditions for the light-emitting layer 13 are, e.g., a growth temperature of 975° C. and a growth pressure of 400 mbar.
Next, the electron blocking layer 14 is formed on the light-emitting layer 13 by the MOCVD method. The growth conditions for the electron blocking layer 14 are, e.g., a growth temperature of 1025° C. and a growth pressure of 50 mbar.
Next, the p-type contact layer 15 is formed on the electron blocking layer 14 by the MOCVD method. The p-type contact layer 15 is formed by stacking the first p-type contact layer and the second p-type contact layer in this order. The growth conditions for the first p-type contact layer are, e.g., a growth temperature of 1,050° C. and a growth pressure of 200 mbar. The growth conditions for the second p-type contact layer are, e.g., a growth temperature of 1050° C. and a growth pressure of 100 mbar.
Next, a predetermined region on the surface of the p-type contact layer 15 is dry etched and a trench with a depth reaching the n-type contact layer 12 is thereby formed.
Next, the transparent electrode 16 is formed on the p-type contact layer 15. Next, the p-electrode 17 is formed on the transparent electrode 16 and the n-electrode 18 is formed on the n-type contact layer 12 exposed on the bottom surface of the trench. The transparent electrode 16, the p-electrode 17 and the n-electrode 18 are formed by sputtering or vapor deposition, etc.
In the embodiment of the invention, it is possible to obtain the n-type contact layer that is formed of AlGaN and has electrical resistance effectively reduced by suppressing the amount of the complex defects of group III vacancies and group IV elements and causing degeneracy of the Fermi level and the conduction band. Output of the light-emitting element relative to the forward current can be increased by reducing electrical resistance of the n-type contact layer. In addition, since the electrical resistance of the n-type contact layer in which the Fermi level and the conduction band are degenerated does not have temperature dependency, the light-emitting element can be driven in a wide temperature range.
Next, evaluation results of characteristics of the n-type contact layer 12 in the embodiment of the invention will be described. In Example 1, the n-type contact layers 12 were formed on the substrates 10 via the buffer layers 11 under various conditions (described later), and these n-type contact layers 12 were evaluated. The configurations and the growth conditions for the substrate 10, the buffer layer 11 and the n-type contact layer 12 in Example 1 are shown n Table 1 below. Si was used as an n-type dopant in the n-type contact layer 12.
In Example 1, the electrical resistivity, the carrier concentration and the mobility for the n-type contact layer 12 were measured by Hall effect measurement and the Si concentration was measured by secondary ion mass spectrometry (SIMS).
According to
In n-type AlGaN in which the Fermi level and the conduction band are degenerated, there is almost no temperature dependency in the carrier concentration. According to
On the other hand, the n-type contact layer 12 having the Si concentration of 4.30×1019 cm−3 has temperature dependency in the carrier concentration and it can thus be determined that the Fermi level and the conduction band are not degenerate. It is considered that the reason why degeneracy is not observed in the n-type contact layer 12 having the Si concentration of 4.30×1019 cm−3 even though the Si concentration is high enough is that the Fermi level is reduced due to compensation of electrons and the degeneracy is lifted.
According to
In addition, according to
Meanwhile, a peak at a photon energy of around 3.2 eV is due to light emission caused by C on the group V site, and a peak at a photon energy of around 4.9 eV is due to light emission corresponding to the band gap.
Since the electrical resistance of the n-type contact layer 12 increases with an increase in the number of the complex defects of group III vacancies and Si, the result obtained from
Since the electrical resistance of the n-type contact layer 12 increases with an increase in the number of the complex defects of group III vacancies and Si, the result obtained from
Since the electrical resistance of the n-type contact layer 12 increases with an increase in the number of the complex defects of group III vacancies and Si, the result obtained from
Although Si was used as the n-type dopant in the n-type contact layer 12 for each evaluation in Example 1, similar evaluation results are obtained also when using a group IV element other than Si, such as Ge.
Described next is the derivation method to derive that the lower limit of the effective donor concentration Nd−Na at which degeneracy occurs in AlGaN with the Al composition x of 0.62 (Al0.62Ga0.38N) is 9.5×1018 cm−3, which is mentioned above in the embodiment of the invention.
In Example 2, plural Al0.62Ga0.38N having different Si concentrations (samples #1 to #9) were made, and Si and C concentrations, electrical resistivity, electron concentration and effective donor concentration Nd−Na were measured for each sample.
Here, the Si and C concentrations in each sample were measured by SIMS. The electrical resistivity and the electron concentration were estimated from the results of van-der-Pauw method and Hall effect measurement in the temperature range of 30 to 300K. The effective donor concentration Nd−Na was estimated from the result of electrochemical capacitance-voltage (C-V) measurement using a 0.1 mol/l NaOH solution as an electrolyte. Static relative permittivity of Al0.62Ga0.38N was presumed to be 8.66 by linear interpolation between 8.9, which is relative permittivity of GaN, and 8.5, which is relative permittivity of AlN.
Measurement results of the samples #1 to #9 are shown in Table 6 below. n300K and ρ300K in Table 6 are respectively the electron concentration and the electrical resistivity at 300K. n300K and ρ300K of the samples #1, #2 and #4 were directly measured by a Hall device, and n300K and ρ300K of the samples #3, #5, #7, #8 and #9 were estimated from calibration samples produced under the same growth conditions.
In Table 6, the samples #1 and #2 doped with a low concentration of Si are classified into Group A, the samples #3 to #7 doped with a medium concentration of Si are classified into Group B, and the samples #8 and #9 doped with a high concentration of Si are classified into Group C.
The sample #2 had a low effective donor concentration relative to the Si concentration and a high electrical resistance. It is considered that this is because the concentration of C, which acts as an acceptor and compensates free electrons, on the N site is high. In the sample #2, the C concentration was 5.9×1018 cm−3 and was comparable to the Si concentration of 6.5×1018 cm−3, as shown in Table 6. In addition, the effective donor concentration Nd−Na in the sample #2 was 9.3×1017 cm−3 and was close to 6.0×1017 cm−3 which is a value obtained by subtracting the C concentration from the Si concentration. This shows that most of electrons in the sample #2 were trapped by C on the N site, resulting in an increase in electrical resistance.
In contrast to this, the effective donor concentration (Nd−Na) in the samples #8 and #9 in Group C, which were doped with a high concentration of Si, was two orders of magnitude lower than the Si concentration. This is because electrons in the samples #8 and #9 were significantly compensated by the complex defects of group III vacancies and group IV elements.
A multiplicative inverse of electrical resistivity ρ (i.e., conductivity) is fitted by a double exponential function that takes into account two activation energies E1 and E2 (E1>E2), as shown in the equation 1 below. The fitting parameters E1 and E2 respectively correspond to a thermal activation energy from the singly occupied donor state to the conduction band and a thermal activation energy from the doubly occupied donor state to the conduction band. Two pre-exponential factors C1 and C2 in the equation 1 are also fitting parameters that respectively correspond to amplitudes of conduction caused by electrons forming a singly-occupied donor band and a doubly-occupied donor band.
Here, E1 is expressed as a function of the effective donor concentration Nd−Na, as shown in the equations 2 and 3 below. Ed,0 included in the equation 2 is an ionization energy when the effective donor concentration is 0, and f(K) included in the equation 3 is existence probability of another donor in the vicinity of the ionization donor and is a geometric factor including a compensation ratio K. In addition, a is the overlap of the Coulomb potentials between the ionization donors.
As shown in
In addition, the effective donor concentration Nd−Na at the point on the approximate straight line shown in
Described next is the derivation method to derive that the lower limit of the effective donor concentration Nd−Na at which degeneracy occurs in AlGaN with the Al composition x of 0.5 (Al0.5Ga0.5N) is 7.6×1018 cm−3, and the lower limit of the effective donor concentration Nd−Na at which degeneracy occurs in AlGaN with the Al composition x of 1 (AlN) is 1.6×1019 cm−3, which is mentioned above in the embodiment of the invention.
In this derivation method, firstly, under the assumption that Ed,0 and a in AlGaN have a linear relationship with the Al composition x, values of Ed,0 and values of a for AlGaN with the Al compositions x of 0.5 and 1 are calculated based on the value of Ed,0 (62 meV) and the value of a (2.9×10−5 meVcm) for the AlGaN with the Al composition x of 0.62 obtained in Example 2 described above and the value of Ed,0 (27.0 meV) and the value of a (2.3×10−5 meVcm) for AlGaN with the Al composition x of 0 (GaN) disclosed in Non-Patent Literature “A. Wolos et al., “Properties of metal-insulator transition and electron spin relaxation in GaN:Si”, PHYSICAL REVIEW B 83, 165206 (2011)”.
Then, using the values of Ed,0 and the values of a for AlGaN with the Al compositions x of 0.5 and 1, Nd−Na when E1=0, i.e., the lower limits of the effective donor concentration Nd−Na at which degeneracy occurs were calculated to be 7.6×1018 cm−3 and 1.6×1019 cm−3 from the equation 2.
Although the embodiment and Examples of the invention have been described, the invention is not limited to the embodiment and Examples, and the various kinds of modifications can be implemented without departing from the gist of the invention. In addition, the constituent elements in the embodiment can be arbitrarily combined without departing from the gist of the invention.
In addition, the embodiment and Examples described above do not limit the invention according to claims. Further, please note that not all combinations of the features described in the embodiment and Examples are necessary to solve the problem of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-208841 | Dec 2020 | JP | national |
