Method for manufacturing nitride semiconductor ultraviolet light emitting device, and nitride semiconductor ultraviolet light emitting device

Abstract

A method for producing a nitride semiconductor ultraviolet light emitting element which is provided with a light emitting element structure part that is formed of an AlGaN-based semiconductor having a wurtzite structure, wherein: an n-type layer of an n-type AlGaN-based semiconductor is epitaxially grown on a base part that comprises a sapphire substrate; an active layer having a quantum well structure which comprises a well layer that is configured from an AlGaN-based semiconductor is epitaxially grown on the n-type layer; a p-type layer of a p-type AlGaN-based semiconductor is epitaxially grown on the active layer; the target value Xwt of the AlN molar fraction during the epitaxial growth of the well layer is set within the range of (n – 0.24) / 12 ≤ Xwt ≤ (n + 0.24) / 12 (wherein n is 3, 4, 5 or 6); and while forming a Ga enriched well region, in which the AlN molar fraction is lower than the average AlN molar fraction Xwa, and an Al enriched well region, in which the AlN molar fraction is higher than the average AlN molar fraction Xwa, within the well layer, a metastable AlGaN region which is composed of AlnGa12-nN12 having an AlGaN composition ratio that is expressed by an integral ratio is grown in the Ga enriched well region or in the Al enriched well region.

Description

[Technical field]

[0001] The present invention relates to a nitride semiconductor ultraviolet light emitting device having a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in the vertical direction, and to a method for manufacturing the same. [Background technology]

[0002] Generally, there are many nitride semiconductor light emitting devices that have a light emitting device structure formed of multiple nitride semiconductor layers by epitaxial growth on a substrate such as sapphire. 1-x-y Ga x In y It is expressed as N(0≦x≦1, 0≦y≦1, 0≦x+y≦1).

[0003] The light-emitting element structure of the light-emitting diode has a double heterostructure in which an active layer made of a nitride semiconductor layer is sandwiched between two cladding layers, an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. When the active layer is an AlGaN-based semiconductor, the band gap energy can be adjusted within a range in which the band gap energies that GaN and AlN can take (approximately 3.4 eV and approximately 6.2 eV) are the lower and upper limits, respectively, by adjusting the AlN mole fraction (also called the Al composition ratio), and an ultraviolet light-emitting element with an emission wavelength of approximately 200 nm to approximately 365 nm is obtained. Specifically, by passing a forward current from the p-type nitride semiconductor layer to the n-type nitride semiconductor layer, light is emitted in the active layer according to the above-mentioned band gap energy due to recombination of carriers (electrons and holes). In order to supply the forward current from the outside, a p-electrode is provided on the p-type nitride semiconductor layer, and an n-electrode is provided on the n-type nitride semiconductor layer.

[0004] When the active layer is an AlGaN-based semiconductor, the n-type nitride semiconductor layer and the p-type nitride semiconductor layer sandwiching the active layer are composed of AlGaN-based semiconductors with a higher AlN molar fraction than the active layer. However, since it is difficult for a p-type nitride semiconductor layer with a high AlN molar fraction to form a good ohmic contact with a p-electrode, it is generally practiced to form a p-type contact layer that can make a good ohmic contact with a p-electrode made of a p-type AlGaN-based semiconductor (specifically, p-GaN) with a low AlN molar fraction on the top layer of the p-type nitride semiconductor layer. Since the AlN molar fraction of this p-type contact layer is smaller than that of the AlGaN-based semiconductor constituting the active layer, ultraviolet light emitted from the active layer toward the p-type nitride semiconductor layer side is absorbed by the p-type contact layer and cannot be effectively extracted outside the element. For this reason, a typical ultraviolet light-emitting diode with an active layer of an AlGaN-based semiconductor adopts an element structure as shown in FIG. 19, and effectively extracts ultraviolet light emitted from the active layer toward the n-type nitride semiconductor layer side to the outside of the element (for example, see Patent Documents 1 and 2 below).

[0005] As shown in FIG. 19, a typical ultraviolet light-emitting diode is configured by depositing an AlGaN semiconductor layer 101 (e.g., an AlN layer) on a substrate 100 such as a sapphire substrate, and then depositing an n-type AlGaN semiconductor layer 103, an active layer 104, a p-type AlGaN semiconductor layer 105, and a p-type contact layer 106 in that order on a template 102, and then etching away parts of the active layer 104, the p-type AlGaN semiconductor layer 105, and the p-type contact layer 106 until the n-type AlGaN semiconductor layer 103 is exposed, and then forming an n-electrode 107 on the exposed surface of the n-type AlGaN semiconductor layer 103 and a p-electrode 108 on the surface of the p-type contact layer 106.

[0006] In order to increase the light emission efficiency (internal quantum efficiency) due to carrier recombination in the active layer, the active layer is formed into a multiple quantum well structure, an electron blocking layer is provided on the active layer, and so on.

[0007] On the other hand, it has been reported that composition modulation occurs due to Ga segregation (segregation associated with mass transfer of Ga) in a cladding layer composed of an n-type AlGaN-based semiconductor layer, forming a layered region with a locally low AlN mole fraction that extends in an oblique direction relative to the cladding layer surface (see, for example, Patent Document 3, Non-Patent Documents 1 and 2 below). Since an AlGaN-based semiconductor layer with a locally low AlN mole fraction also has a locally small band gap energy, Patent Document 3 reports that carriers in the cladding layer are easily localized in the layered region, which can provide a low-resistance current path for the active layer and improve the light-emitting efficiency of ultraviolet light-emitting diodes.

[0008] Furthermore, Non-Patent Document 1 reports that multi-step terraces parallel to the (0001) plane appear on the surface of each layer of an active layer with a multiple quantum well structure formed on an n-type cladding layer, and composition modulation occurs due to Ga segregation in each layer of the active layer, similar to the n-type cladding layer, and a region with a relatively low AlN mole fraction is generated in an inclined region inclined with respect to the (0001) plane connecting adjacent terraces, and a region with a relatively high AlN mole fraction is generated in the terrace region, and the light emission from the inclined region and the light emission from the terrace region, which have different peak emission wavelengths, are combined, which can result in a double peak in the EL (electroluminescence) spectrum of the entire active layer. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2014 / 178288 [Patent Document 2] International Publication No. 2016 / 157518 [Patent Document 3] International Publication No. 2019 / 159265 [Non-patent literature]

[0010] [Non-Patent Document 1] Y. Nagasawa, et al., "Comparison of AlxGa1-xN multiple quantum wells designed for 265 and 285nm deep-ultraviolet LEDs grown on AlN templates having macrosteps", Applied Physics Express 12, 064009 (2019) [Non-Patent Document 2] K. Kojima, et al., "Carrier localization structure combined with current micropaths in AlGaN quantum wells grown on an AlN template with macrosteps", Applied Physics letter 114, 011102 (2019) Summary of the Invention [Problem to be solved by the invention]

[0011] Ultraviolet light emitting elements made of AlGaN-based semiconductors are fabricated on substrates such as sapphire substrates by well-known epitaxial growth methods such as metal organic vapor phase epitaxy (MOVPE). However, when producing ultraviolet light emitting elements, the characteristics of the ultraviolet light emitting elements (emission wavelength, wall plug efficiency, forward bias, and other characteristics) vary due to drift in the crystal growth equipment, so it is not necessarily easy to produce them with a stable yield.

[0012] Drift in crystal growth equipment occurs due to changes in the effective temperature at the crystal growth site caused by deposits on the tray, chamber walls, etc. For this reason, in the past, in order to suppress drift, an experienced person would study the growth history and subtly change the set temperature or the composition of the source gas, or a growth schedule would be fixed for a certain period of time and maintenance such as cleaning would be performed in the same way at certain intervals, but it is difficult to completely eliminate drift.

[0013] The drift also affects the above-mentioned Ga segregation, and the manufacturing variation in the difference in the AlN mole fraction between the tilted region and the terrace region in the well layer can cause significant peak separation with a large wavelength difference in the EL spectrum. Depending on the application, such significant peak separation can result in defective products that cannot be shipped as products, and can be a factor in reducing yields.

[0014] The present invention has been made in consideration of the above-mentioned problems, and has an object to provide a stable nitride semiconductor ultraviolet light-emitting element in which characteristic fluctuations due to drift, etc. in a crystal growth apparatus are suppressed, thereby suppressing significant peak separation with large wavelength differences. [Means for solving the problem]

[0015] In order to achieve the above object, the present invention provides a method for producing a nitride semiconductor ultraviolet light emitting device having a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in a vertical direction, the method comprising: A first step of forming the n-type layer of an n-type AlGaN-based semiconductor by epitaxial growth on a base portion including a sapphire substrate; a second step of forming, on the n-type layer, the active layer having a quantum well structure including one or more well layers made of an AlGaN-based semiconductor by epitaxial growth; a third step of forming the p-type layer of a p-type AlGaN-based semiconductor on the active layer by epitaxial growth; The integer n is 3, 4, 5, or 6, In the second step, a target value Xwt of the AlN mole fraction during epitaxial growth of the well layer is set to (n-0.24) / 12≦Xwt≦(n+0.24) / 12 Set it within the range where forming a Ga-enriched well region having an AlN mole fraction lower than an average AlN mole fraction Xwa of the well layer and an Al-enriched well region having an AlN mole fraction higher than the average AlN mole fraction Xwa in the well layer; The Ga-rich well region or the Al-rich well region has an AlGaN composition ratio of an integer ratio. n Ga 12-n N 12 The present invention provides a method for producing a nitride semiconductor ultraviolet light emitting device, the first feature of which is to grow a metastable well region that is metastable AlGaN.

[0016] Furthermore, in addition to the first feature, the present invention is a vicinal substrate having a main surface inclined at a predetermined angle with respect to a (0001) plane, In the first and second steps, multi-step terraces parallel to a (0001) plane are exposed on the surfaces of the n-type layer and the active layer, In the second step, an inclined region inclined with respect to a (0001) plane connecting adjacent terraces of the multi-step terraces and a terrace region other than the inclined region are formed in each semiconductor layer in the active layer, The second feature of the present invention is to provide a method for manufacturing a nitride semiconductor ultraviolet light-emitting device, wherein the Ga-enriched well region is formed in the tilted region of the well layer, and the Al-enriched well region is formed in the terrace region of the well layer.

[0017] In addition, AlGaN-based semiconductors are those with the general formula Al 1-x Ga x N (0≦x≦1), but as long as the band gap energy is within a range in which the lower and upper limits are the band gap energies that GaN and AlN can have, respectively, it may contain a small amount of impurities such as group 3 elements such as B or In or group 5 elements such as P. Also, a GaN-based semiconductor is a nitride semiconductor basically composed of Ga and N, but may contain a small amount of impurities such as group 3 elements such as Al, B or In or group 5 elements such as P. Also, an AlN-based semiconductor is a nitride semiconductor basically composed of Al and N, but may contain a small amount of impurities such as group 3 elements such as Ga, B or In or group 5 elements such as P. Therefore, in the present application, GaN-based semiconductors and AlN-based semiconductors are each part of AlGaN-based semiconductors.

[0018] Furthermore, the n-type or p-type AlGaN-based semiconductor is an AlGaN-based semiconductor doped with Si or Mg as a donor or acceptor impurity. In the present application, an AlGaN-based semiconductor not specified as p-type or n-type means an undoped AlGaN-based semiconductor, but even if it is undoped, it may contain a small amount of donor or acceptor impurity that is inevitably mixed in. In addition, the first plane is not an exposed surface specifically formed in the manufacturing process of the n-type layer or a boundary surface with other semiconductor layers, but is a virtual plane extending in parallel in the vertical direction within the n-type layer. Furthermore, in this specification, the AlGaN-based semiconductor layer, the GaN-based semiconductor layer, and the AlN-based semiconductor layer are semiconductor layers composed of an AlGaN-based semiconductor, a GaN-based semiconductor, and an AlN-based semiconductor, respectively.

[0019] Next, we will explain the characteristics of “metastable AlGaN” in which the AlGaN composition ratio is expressed by a predetermined integer ratio.

[0020] If metastable AlGaN is not taken into consideration, ternary alloys such as AlGaN are in a crystalline state in which group 3 elements (Al and Ga) are mixed randomly, and can be approximately explained as a "random configuration." However, because the covalent bond radius of Al is different from that of Ga, the more symmetric the atomic arrangement of Al and Ga in the crystal structure, the more stable the structure will generally be.

[0021] In AlGaN-based semiconductors with the wurtzite structure, two types of arrangements can exist: a random arrangement with no symmetry, and a stable symmetric arrangement. Here, at a certain ratio, a state in which the symmetric arrangement becomes dominant appears. As will be described later, a symmetric arrangement structure of Al and Ga appears in "metastable AlGaN" where the AlGaN composition ratio (composition ratio of Al, Ga, and N) is expressed as a certain integer ratio.

[0022] In this symmetrical arrangement structure, even if the amount of Ga supplied to the crystal growth surface increases or decreases slightly, the high symmetry results in an energetically stable mixed crystal mole fraction, and it is possible to prevent the concentration of Ga, which is prone to mass transfer, from becoming uncontrollable.

[0023] Next, the "Ga-enriched well region" and the "Al-enriched well region" will be described.

[0024] The crystal growth surface of each semiconductor layer in the n-type layer, active layer, and p-type layer is not a completely flat surface. For example, in the manufacturing method of the nitride semiconductor ultraviolet light emitting device of the second feature, each semiconductor layer in the active layer is an epitaxial growth layer having a surface in which a multi-step terrace parallel to the (0001) plane is formed by step-flow growth. Therefore, in the well layer, Ga, which is easy to move in mass, is concentrated in the inclined region inclined with respect to the (0001) plane connecting the adjacent terraces of the well layer, forming a Ga-rich well region in which the AlN mole fraction is lower than the average AlN mole fraction in the well layer. On the other hand, unlike Ga, Al is easily adsorbed to the surface, and although it is thought that it moves somewhat after entering the site, it is strongly restricted. Therefore, the Al-rich well region is formed by relatively increasing the density of Al in some regions of the terrace region other than the inclined region due to the mass movement of Ga during the formation of the Ga-rich well region.

[0025] In the manufacturing method of the nitride semiconductor ultraviolet light emitting device according to the second aspect, multi-step terraces are formed in each semiconductor layer in the active layer by step flow growth, but when the inclination angle of the sapphire substrate used is extremely small, such as 0° to about 0.3°, hillocks or pits in the shape of hexagonal columns or hexagonal pyramids may be formed on the flat crystal growth surface. When the crystal growth surface of the well layer is flat, Ga, which is prone to mass transfer, may concentrate around the hillocks or the like, forming a Ga-rich well region. Therefore, in this case as well, the Al-rich well region is formed with a relative increase in the density of Al in a part of the terrace region other than the Ga-rich well region due to the mass transfer of Ga during the formation of the Ga-rich well region.

[0026] When a metastable well region is formed in a Ga-rich well region, the fluctuation of the Ga supply amount to the Ga-rich n-type region and the fluctuation of the average AlN mole fraction in the well layer are absorbed in the metastable well region. That is, when the Ga supply amount increases or the average AlN mole fraction decreases in the Ga-rich n-type region, the metastable well region increases, and when the Ga supply amount decreases or the average AlN mole fraction increases, the metastable well region decreases, and as a result, the fluctuation of the AlN mole fraction in the Ga-rich well region is suppressed. When metastable AlGaN is formed in an Al-rich well region, the fluctuation of the AlN mole fraction in the Al-rich well region is suppressed in the same way.

[0027] Therefore, according to the manufacturing method of the nitride semiconductor ultraviolet light emitting device having the first or second characteristic, in the second step, the target value Xwt of the AlN mole fraction is set within the range of (n-0.24) / 12≦Xwt≦(n+0.24) / 12, and a metastable well region made of metastable AlGaN having an AlGaN composition ratio of an integer ratio described later is grown in either the Ga-rich well region or the Al-rich well region formed in the well layer, so that the difference between the AlN mole fraction (n / 12) of the metastable well region and the target value Xwt of the AlN mole fraction of the entire well layer is suppressed to within 2% (=0.24 / 12), and the difference in AlN mole fraction between the Ga-rich well region and the Al-rich well region is also suppressed to within a certain range. The average AlN mole fraction Xwa of the well layer is between the AlN mole fractions of the Ga-rich well region and the Al-rich well region, and is the target value Xwt of the AlN mole fraction of the entire well layer or a value close to it. As a result, the occurrence of significant peak separation with large wavelength differences in the EL spectrum of the entire active layer, which combines the emission from the Ga-rich well region and the emission from the Al-rich well region and other regions in the well layer, which have different peak emission wavelengths, is suppressed.

[0028] Here, the target value Xwt of the AlN molar fraction is set within the range of ±2% of the AlN molar fraction (n / 12) in the metastable well region. However, when the target value Xwt is higher than the AlN molar fraction in the metastable well region (n / 12 < Xwt), the metastable well region is formed within the Ga-rich well region. When the target value Xwt is lower than the AlN molar fraction in the metastable well region (n / 12 > Xwt), the metastable well region is formed within the Al-rich well region. However, the average AlN molar fraction Xwa in the well layer becomes the target value Xwt or a value in its vicinity, but it can vary within the wafer due to influences such as drift in the crystal growth apparatus. Therefore, even when the target value Xwt is lower than the AlN molar fraction in the metastable well region, the metastable well region may be formed within the Ga-rich well region. Even when the target value Xwt is higher than the AlN molar fraction in the metastable well region, the metastable well region may be formed within the Al-rich well region. Also, when the target value Xwt is near the AlN molar fraction of the metastable AlGaN, the above tendency becomes prominent. However, regardless of whether the metastable well region is formed in either the Ga-rich well region or the Al-rich well region, the occurrence of a significant peak separation with a large wavelength difference in the EL spectrum is suppressed.

[0029] In the method for manufacturing a nitride semiconductor ultraviolet light-emitting device having the first or second feature, by forming the metastable well region within the Ga-rich well region or the Al-rich well region, in the EL spectrum of the nitride semiconductor ultraviolet light-emitting device, the wavelength difference between the extreme points of the first and second smallest minima of the second derivative of the EL intensity represented by a function with wavelength as a variable is preferably suppressed to 8 nm or less.

[0030] The wavelengths of the peak and shoulder peak of the EL spectrum are specified by the wavelengths of the first and second smallest minimum extreme points of the second derivative of the EL intensity. A shoulder peak is a bulge in luminescence intensity that occurs when two emission peaks in the EL spectrum are not clearly separated but are combined into a single peak, and one peak is superimposed on the shoulder part on the long-wavelength side or short-wavelength side of the other peak. Therefore, when the wavelength difference between the first and second smallest minimum extreme points of the second derivative of the EL intensity is 8 nm or less, it means that the wavelength difference between two peaks or between one peak and one shoulder peak in the EL spectrum is 8 nm or less, and the occurrence of significant peak separation with a large wavelength difference is suppressed in the EL spectrum.

[0031] Furthermore, in the method for producing a nitride semiconductor ultraviolet light-emitting device having the first or second characteristic, in the second step, it is preferable that a film thickness of the Ga-enriched well region or the Al-enriched well region in which the metastable well region is formed is controlled to be an integral multiple of a monoatomic layer and within a range of 4 to 12 monoatomic layers.

[0032] According to the above preferred embodiment, the peak or shoulder peak of the EL spectrum can be controlled within a predetermined wavelength range according to the AlN mole fraction of the metastable AlGaN and the film thickness of the Ga-enriched well region or the Al-enriched well region in which the metastable well region is formed.

[0033] Further, in the method for producing the nitride semiconductor ultraviolet light emitting device according to the first or second aspect, in the second step, the AlGaN composition ratio in the Ga-rich well region is an integer ratio of Al n-1 Ga 13-n N 12 The growth of the metastable well region, which is metastable AlGaN, is suppressed, and the Al-rich well region has an AlGaN composition ratio of an integer ratio of Al n+1 Ga 11-n N 12 It is preferable that the growth of the metastable well region, which is metastable AlGaN, be suppressed.

[0034] According to the above-mentioned preferred embodiment, in addition to at least two peaks or at least one peak and one shoulder peak, the appearance of peaks or shoulder peaks at wavelengths corresponding to the AlN molar fractions of (n-1) / 12 and (n+1) / 12 in the EL spectrum is suppressed, or even if such peaks or shoulder peaks appear, the emission intensity at such wavelengths becomes extremely low.

[0035] Furthermore, in one embodiment of the method for manufacturing a nitride semiconductor ultraviolet light-emitting device having the first or second feature described above, in a plurality of chips on the same wafer, in the second step, a first type chip in which the metastable well region grows in the Ga-enriched well region and a second type chip in which the metastable well region grows in the Al-enriched well region are mixed.

[0036] Furthermore, in the method for producing a nitride semiconductor ultraviolet light-emitting device having the first or second characteristic, in the second step, the well layers made of an AlGaN-based semiconductor and barrier layers made of an AlGaN-based semiconductor are alternately stacked by epitaxial growth to form the active layer having a multiple quantum well structure including two or more well layers.

[0037] According to the above preferred embodiment, the active layer has a multiple quantum well structure, and it is expected that the light emission efficiency will be improved compared to the case where only one well layer is used.

[0038] In order to achieve the above object, the present invention provides a nitride semiconductor ultraviolet light emitting device including a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in a vertical direction, the device comprising: the n-type layer is composed of an epitaxially grown layer of an n-type AlGaN-based semiconductor, the active layer disposed between the n-type layer and the p-type layer has a quantum well structure including one or more well layers formed of an epitaxially grown layer of an AlGaN-based semiconductor; the p-type layer is composed of an epitaxially grown layer of a p-type AlGaN-based semiconductor, a Ga-enriched well region having an AlN mole fraction lower than an average AlN mole fraction Xwa of the well layer, and an Al-enriched well region having an AlN mole fraction higher than the average AlN mole fraction Xwa, Integer n is 3, 4, 5, or 6, and the Ga-rich well region or the Al-rich well region has an AlGaN composition ratio of an integer ratio of Al n Ga 12-n N 12 A metastable well region is formed, which is metastable AlGaN. The first feature of the present invention is that in an EL spectrum, the wavelength difference between the first and second smallest minimum extreme points of the second derivative of the EL intensity, which is expressed as a function with wavelength as a variable, is 8 nm or less.

[0039] Furthermore, in addition to the first feature, the present invention is characterized in that each of the semiconductor layers in the n-type layer, the active layer, and the p-type layer is an epitaxially grown layer having a surface on which multiple terraces parallel to a (0001) plane are formed, each semiconductor layer in the active layer has an inclined region inclined with respect to a (0001) plane connecting adjacent terraces of the multi-stepped terraces, and a terrace region other than the inclined region; The second feature of the present invention is that the Ga-rich well region is present in the tilted region of the well layer, and the Al-rich well region is present in the terrace region of the well layer.

[0040] According to the nitride semiconductor ultraviolet light-emitting device having the first or second characteristic, a metastable well region made of metastable AlGaN having an AlGaN composition ratio of an integer ratio is formed in either the Ga-rich well region or the Al-rich well region formed in the well layer, so that in the EL spectrum of the entire active layer in which light emission from the Ga-rich well region and light emission from the Al-rich well region and other regions in the well layer, which have different peak emission wavelengths, are combined, the wavelength difference between two peaks or the wavelength difference between one peak and one shoulder peak is suppressed to 8 nm or less, and as a result, the occurrence of significant peak separation with a large wavelength difference is suppressed.

[0041] In the nitride semiconductor ultraviolet light emitting device according to the first or second aspect, the thickness of the Ga-enriched well region or the Al-enriched well region in which the metastable well region is formed is preferably an integral multiple of a monolayer and falls within the range of 4 to 12 monolayers.

[0042] According to the above preferred embodiment, the peak or shoulder peak of the EL spectrum is controlled within a predetermined wavelength range according to the AlN mole fraction of the metastable well region and the film thickness of the Ga-enriched well region or the Al-enriched well region in which the metastable well region is formed.

[0043] Furthermore, in the nitride semiconductor ultraviolet light emitting device according to the first or second aspect, the average AlN mole fraction Xwa is (n-0.24) / 12≦Xwa≦(n+0.24) / 12 It is preferable that the range is within the range.

[0044] According to the above preferred embodiment, the difference between the AlN mole fraction (n / 12) of the metastable well region and the average AlN mole fraction Xwa of the well layer is suppressed to within 2% (=0.24 / 12), and the difference in AlN mole fraction between the Ga-rich well region and the Al-rich well region is also suppressed to within a certain range. As a result, the occurrence of significant peak separation with a large wavelength difference is suppressed in the EL spectrum of the entire active layer, which combines the light emission from the Ga-rich well region and the light emission from the Al-rich well region and other regions in the well layer, which have different peak emission wavelengths.

[0045] Furthermore, in the nitride semiconductor ultraviolet light-emitting device according to the first or second aspect, it is preferable that the active layer has a multiple quantum well structure including two or more of the well layers, and a barrier layer made of an AlGaN-based semiconductor is present between two of the well layers.

[0046] According to the above preferred embodiment, since the active layer has a multiple quantum well structure, it is expected that the light emission efficiency will be improved compared to the case where only one well layer is used.

[0047] Furthermore, the nitride semiconductor ultraviolet light emitting device according to the first or second aspect further includes a base portion including a sapphire substrate, the sapphire substrate having a main surface inclined at a predetermined angle with respect to a (0001) plane, and the light emitting device structure is formed above the main surface, Each of the semiconductor layers from the principal surface of the sapphire substrate to the p-type layer is preferably an epitaxially grown layer having a surface on which multiple terraces parallel to the (0001) plane are formed.

[0048] According to the above-mentioned preferred embodiment, a sapphire substrate having an off-axis angle is used, and a sapphire substrate is formed on the main surface of the sapphire substrate. p-type layer The epitaxial growth can be carried out so that multi-step terraces appear on the surfaces of the layers up to this point, thereby achieving a nitride semiconductor ultraviolet light emitting device having the above-mentioned characteristics. Effect of the Invention

[0049] According to the manufacturing method of the nitride semiconductor ultraviolet light-emitting device or the nitride semiconductor ultraviolet light-emitting device having the above-mentioned characteristics, the occurrence of significant peak separation with a large wavelength difference is suppressed in the EL spectrum of the entire active layer, which is a combination of the light emission from the Ga-rich well region and the light emission from the Al-rich well region and other regions in the well layer, which have different peak emission wavelengths. [Brief description of the drawings]

[0050] [Figure 1] Schematic diagram of the wurtzite crystal structure of AlGaN. [Diagram 2] FIG. 2 is a plan view showing the positional relationship between each site on the A plane and each site on the B plane when viewed from the c-axis direction of the wurtzite crystal structure shown in FIG. 1. [Diagram 3] A schematic diagram showing the symmetric arrangement of Al and Ga on the site planes (A3 plane, B3 plane) of group 3 elements in metastable AlGaN, whose AlGaN composition ratios are expressed as integer ratios of Al1Ga2N3, Al1Ga1N2, and Al2Ga1N3. [Figure 4] 1 is a cross-sectional view of a main portion that illustrates a schematic example of the structure of a nitride semiconductor ultraviolet light emitting device according to a first embodiment. [Diagram 5] 5 is a cross-sectional view of a main portion, showing a schematic example of a laminated structure of an active layer of the nitride semiconductor ultraviolet light emitting device shown in FIG. 4. [Figure 6] FIG. 6 is a diagram showing a more detailed structure of the inclined region IA shown in FIG. 5. [Figure 7] FIG. 4 is a diagram showing a schematic diagram of a change in the AlN mole fraction in a Ga-enriched well region and an Al-enriched well region accompanying the mass transfer of Ga in a well layer. [Figure 8] FIG. 2 is a schematic diagram showing an EL spectrum having two separated peaks and an EL spectrum having one peak and one shoulder peak. [Figure 9] FIG. 2 is a graph for explaining the relationship between the peak and shoulder peak of an EL spectrum and the minimum value of the second derivative of the EL spectrum. [Figure 10] 13 is a graph showing the relationship between the emission wavelength of a quantum well structure consisting of an AlGaN well layer and an AlGaN barrier layer, and the film thickness of the well layer and the AlN mole fraction of the barrier layer when the AlN mole fraction of the Ga-enriched well region 220a is 50%. [Figure 11] 13 is a graph showing the relationship between the emission wavelength of a quantum well structure consisting of an AlGaN well layer and an AlGaN barrier layer, and the film thickness of the well layer and the AlN mole fraction of the barrier layer when the AlN mole fraction of the Ga-enriched well region 220a is 41.7%. [Figure 12] 13 is a graph showing the relationship between the emission wavelength of a quantum well structure consisting of an AlGaN well layer and an AlGaN barrier layer, and the film thickness of the well layer and the AlN mole fraction of the barrier layer when the AlN mole fraction of the Ga-enriched well region 220a is 33.3%. [Figure 13] 11 is a graph showing the relationship between the emission wavelength of a quantum well structure consisting of an AlGaN well layer and an AlGaN barrier layer and the film thickness of the well layer when the AlN mole fraction in a Ga-enriched well region 220a is 25%. [Figure 14] 5 is a plan view showing a schematic example of the structure of the nitride semiconductor ultraviolet light emitting device shown in FIG. 4 when viewed from above in FIG. 4. [Figure 15] 2 is a HAADF-STEM image showing a cross-sectional structure of a main portion including an active layer of a multiple quantum well structure of the nitride semiconductor ultraviolet light emitting device according to the first embodiment. [Figure 16] 4 is a graph showing the results of measuring the EL spectrum of the nitride semiconductor ultraviolet light emitting device according to the first embodiment in a wafer state. [Figure 17] FIG. 11 is a cross-sectional view of a main portion that illustrates an example of the structure of a nitride semiconductor ultraviolet light emitting device according to a second embodiment. [Figure 18] 18 is a cross-sectional view of a main part, showing a schematic example of a laminated structure of a main part including an active layer of the nitride semiconductor ultraviolet light emitting device shown in FIG. 17. [Figure 19] FIG. 1 is a cross-sectional view of a main portion that illustrates an example of an element structure of a general ultraviolet light emitting diode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] A nitride semiconductor ultraviolet light emitting device (hereinafter, simply referred to as "light emitting device") according to an embodiment of the present invention will be described with reference to the drawings. In the schematic diagrams used in the following description, the essential parts are emphasized to show the invention in a schematic manner for ease of understanding, and therefore the dimensional ratios of the various parts are not necessarily the same as those of an actual device. In the following description of this embodiment, the light emitting device will be described on the assumption that it is a light emitting diode.

[0052] [First embodiment] <Symmetric arrangement of Al and Ga in metastable AlGaN>

[0053] First, we will explain how a symmetric arrangement of Al and Ga appears in "metastable AlGaN" in which the AlGaN composition ratio is expressed as a predetermined integer ratio.

[0054] Figure 1 shows a schematic diagram of one unit cell (two monolayers) in the c-axis direction of AlGaN. In Figure 1, the white circles indicate sites where group 3 element atoms (Al, Ga) are located, and the black circles indicate sites where group 5 element atoms (N) are located. In the following explanation, a monolayer is abbreviated as ML. In Figure 1, one unit cell is abbreviated as 2ML.

[0055] In FIG. 1, the site planes of group 3 elements (A3 plane, B3 plane) and the site planes of group 5 elements (A5 plane, B5 plane) shown as hexagons are both parallel to the (0001) plane. In each site of the A3 plane and the A5 plane (collectively referred to as the A plane), there are six sites at each vertex of the hexagon and one site at the center of the hexagon. The same is true for the B3 plane and the B5 plane (collectively referred to as the B plane), but FIG. 1 shows only three sites present in the hexagon of the B plane. The sites of the A plane overlap in the c-axis direction, and the sites of the B plane overlap in the c-axis direction. However, an atom (N) at one site on the B5 surface forms four-coordinate bonds with atoms (Al, Ga) at three sites on the A3 surface located above the B5 surface and with atoms (Al, Ga) at one site on the B3 surface located below the B5 surface, and an atom (Al, Ga) at one site on the B3 surface forms four-coordinate bonds with an atom (N) at one site on the B5 surface located above the B3 surface and with atoms (N) at three sites on the A5 surface located below the B3 surface. Therefore, as shown in Figure 1, each site on the A surface does not overlap with each site on the B surface in the c-axis direction.

[0056] FIG. 2 shows the positional relationship between each site on the A plane and each site on the B plane as a plan view of the A3 plane and the B3 plane seen from the c-axis direction. The black and white circles in FIG. 2 indicate whether the site plane is the A3 plane or the B3 plane. In both the A3 plane and the B3 plane, each of the six vertices of the hexagon is shared by two other adjacent hexagons, and the central site is not shared with other hexagons, so that one hexagon essentially has sites for three atoms. Therefore, there are six sites for atoms of group 3 elements (Al, Ga) and six sites for atoms of group 5 elements (N) per unit cell (2ML). Therefore, there are the following five cases of AlGaN composition ratios expressed as integer ratios excluding GaN and AlN. 1) Al1Ga5N6, 2) Al2Ga4N6 (=Al1Ga2N3), 3) Al3Ga3N6 (=Al1Ga1N2), 4) Al4Ga2N6 (=Al2Ga1N3), 5) Al5Ga1N6.

[0057] Here, the Al1Ga2N3, Al1Ga1N2, and Al2Ga1N3 in 2) to 4) above can have the same symmetrical arrangement of Al and Ga on both the A3 and B3 planes as shown in Figure 3, so the metastable AlGaN with the AlGaN composition ratios in 2) to 4) above is formed in the c-axis direction in 1ML units. Figure 3 shows an example of an arrangement structure on only one of the A3 and B3 planes. In Figure 3, Ga is shown as a large black circle, and Al is shown as a small black circle.

[0058] On the other hand, the Al1Ga5N6 in 1) above can have a symmetrical arrangement structure of 2ML units, with one of the A3 and B3 planes having the arrangement structure of Al1Ga2N3 in 2) above and the other having an arrangement structure of GaN (all group 3 element sites are Ga).Furthermore, the Al1Ga5N6 in 5) above can have a symmetrical arrangement structure of 2ML units, with one of the A3 and B3 planes having the arrangement structure of Al2Ga1N3 in 4) above and the other having an arrangement structure of AlN (all group 3 element sites are Al).

[0059] Furthermore, the following four metastable AlGaNs with AlGaN composition ratios in integer ratios between the above 1) and 2), 2) and 3), 3) and 4), and 4) and 5) are assumed to be 6) to 9). 6) Al3Ga9N 12 (=Al1Ga3N4), 7) Al5Ga7N 12 , 8) Al7Ga5N 12 , 9) Al9Ga3N 12 (=Al3Ga1N4).

[0060] Here, the Al1Ga3N4 in 6) above can have a symmetrical arrangement structure of 2ML units, with one of the A3 and B3 planes being the arrangement structure of the Al1Ga1N2 in 3) above, and the other being the arrangement structure of GaN (all the sites of the group 3 elements are Ga). 12 The A3 plane and the B3 plane can have a symmetrical arrangement structure of 2ML units, with one of them being the arrangement structure of Al1Ga2N3 described above in 2) and the other being the arrangement structure of Al1Ga1N2 described above in 3). 12can have a symmetrical arrangement structure of 2ML units, with one of the A3 and B3 planes having the arrangement structure of Al1Ga1N2 described in 3) above and the other having the arrangement structure of Al2Ga1N3 described in 4). The Al3Ga1N4 described in 9) above can have a symmetrical arrangement structure of 2ML units, with one of the A3 and B3 planes having the arrangement structure of Al1Ga1N2 described in 3) above and the other having the arrangement structure of AlN (all Group 3 element sites are Al).

[0061] Therefore, the above 1), 5) to 9) Al1Ga5N6, Al5Ga1N6, Al3Ga9N 12 (=Al1Ga3N4), Al5Ga7N 12 , Al7Ga5N 12 , or Al9Ga3N 12 As described above, when (=Al3Ga1N4) has a 2ML unit arrangement structure in which the symmetric arrangement structure differs between the A3 plane and the B3 plane, metastable AlGaN with the AlGaN composition ratios of 1), 5) to 9) above is formed in the c-axis direction in 2ML units.

[0062] However, although not specifically exemplified, for each of the AlGaN composition ratios 1), 5) to 9), it is considered that, for example, by synthesizing different symmetric arrangement structures on the A3 plane and the B3 plane in the same plane, the A3 plane and the B3 plane can have the same symmetric arrangement structure of Al and Ga, similar to the Al1Ga2N3, Al1Ga1N2, and Al2Ga1N3 in 2) to 4) above. In that case, each metastable AlGaN with the AlGaN composition ratios 1), 5) to 9) above can be formed in the c-axis direction in 1ML units, similar to the metastable AlGaN with the AlGaN composition ratios 2) to 4) above.

[0063] From the above, the metastable AlGaN shown in 1) to 9) above has a symmetric atomic arrangement of Al and Ga, and is energetically stable AlGaN. However, in order to grow AlGaN while maintaining a certain crystal quality, it is necessary to perform crystal growth at a high temperature of 1000°C or higher. However, it is assumed that Ga atoms move around at 1000°C or higher even after they reach the site on the crystal surface. On the other hand, unlike Ga, Al is easily adsorbed on the surface, and it is thought that Al atoms move somewhat after entering the site, but the movement is strongly restricted. Therefore, even if it is metastable AlGaN, since the Al1Ga5N6 in 1) above has a high Ga composition ratio, at a growth temperature of around 1000°C, Ga moves violently, the symmetry of the atomic arrangement is disturbed, and the atomic arrangement of Al and Ga becomes close to a random state, and the stability described above is considered to be lower than other metastable AlGaN.

[0064] Here, the metastable AlGaN with the AlGaN composition ratios 1) to 9) above is expressed as Al n Ga 12-n N 12 and Al n Ga 12-n N 12 When expressed as a fraction, the AlN mole fraction is n / 12, but when expressed as a percentage, a fraction is generated after the decimal point. Therefore, for the sake of convenience, the six AlN mole fractions, which are expressed as fractions of 2 / 12 (= 1 / 6), 4 / 12 (= 1 / 3), 5 / 12, 7 / 12, 8 / 12 (= 2 / 3), and 10 / 12 (= 5 / 6), will be approximately expressed as 16.7%, 33.3%, 41.7%, 58.3%, 66.7%, and 83.3%.

[0065] <Element structure of light-emitting element> As shown in FIG. 4, the light emitting device 1 of the first embodiment includes a base 10 including a sapphire substrate 11, and a light emitting device structure 20 including a plurality of AlGaN-based semiconductor layers 21-24, a p-electrode 26, and an n-electrode 27. The light emitting device 1 is mounted (flip-chip mounted) with the light emitting device structure 20 side (upper side in FIG. 4) facing a mounting base (submount, etc.), and the light extraction direction is the base 10 side (lower side in FIG. 4). For convenience of explanation, in this specification, the direction perpendicular to the main surface 11a of the sapphire substrate 11 (or the upper surface of the base 10 and each of the AlGaN-based semiconductor layers 21-24) is referred to as the "up-down direction" (or the "vertical direction"), the direction from the base 10 toward the light emitting device structure 20 is referred to as the upward direction, and the opposite direction is referred to as the downward direction. A plane parallel to the upward direction is referred to as the "first plane". Furthermore, a plane parallel to the primary surface 11a of the sapphire substrate 11 (or the upper surfaces of the underlayer 10 and each of the AlGaN-based semiconductor layers 21-24) is referred to as a "second plane", and a direction parallel to the second plane is referred to as a "lateral direction".

[0066] The base portion 10 includes a sapphire substrate 11 and an AlN layer 12 formed directly on a primary surface 11a of the sapphire substrate 11. The sapphire substrate 11 is a slightly inclined substrate in which the primary surface 11a is inclined at an angle (off angle) within a certain range (for example, about 0.3° to 6°) with respect to the (0001) plane, and multi-step terraces are exposed on the primary surface 11a.

[0067] The AlN layer 12 is composed of AlN crystals epitaxially grown from the main surface of the sapphire substrate 11, and the AlN crystals have an epitaxial crystal orientation relationship with the main surface 11a of the sapphire substrate 11. Specifically, for example, <0001> The AlN crystal grows so that the C-axis direction of the AlN crystal is aligned with the C-axis direction of the AlN crystal. The AlN crystal constituting the AlN layer 12 may be an AlN-based semiconductor layer that may contain a small amount of Ga or other impurities. In this embodiment, the thickness of the AlN layer 12 is assumed to be about 2 μm to 3 μm. The structure of the base portion 10 and the substrate to be used are not limited to the above-mentioned configuration. For example, an AlGaN-based semiconductor layer having an AlN mole fraction equal to or greater than the AlN mole fraction of the AlGaN-based semiconductor layer 21 may be provided between the AlN layer 12 and the AlGaN-based semiconductor layer 21.

[0068] The AlGaN-based semiconductor layers 21-24 of the light-emitting element structure 20 have a structure in which, from the underlayer 10 side, an n-type cladding layer 21 (n-type layer), an active layer 22, an electron blocking layer 23 (p-type layer), and a p-type contact layer 24 (p-type layer) are epitaxially grown and stacked in that order.

[0069] In this embodiment, the AlN layer 12 of the underlayer 10, which is epitaxially grown in order from the main surface 11a of the sapphire substrate 11, and the semiconductor layers and electron block layer 23 in the n-type cladding layer 21 and active layer 22 of the light-emitting element structure 20 have surfaces on which multi-stepped terraces are formed parallel to the (0001) plane originating from the main surface 11a of the sapphire substrate 11. Note that the p-type contact layer 24 of the p-type layer is formed on the electron block layer 23 by epitaxial growth, and therefore may have a similar multi-stepped terrace, but does not necessarily have to have a surface on which a similar multi-stepped terrace is formed.

[0070] As shown in Fig. 4, in the light-emitting element structure 20, the active layer 22, the electron blocking layer 23, and the p-type contact layer 24 are formed on the first region R1 of the upper surface of the n-type cladding layer 21 by removing the portions of the active layer 22, the electron blocking layer 23, and the p-type contact layer 24 stacked on the second region R2 of the upper surface of the n-type cladding layer 21 by etching or the like. The upper surface of the n-type cladding layer 21 is exposed in the second region R2 excluding the first region R1. As shown in Fig. 4, the height of the upper surface of the n-type cladding layer 21 may differ between the first region R1 and the second region R2. In that case, the upper surface of the n-type cladding layer 21 is defined separately in the first region R1 and the second region R2.

[0071] The n-type cladding layer 21 is composed of an n-type AlGaN-based semiconductor, and layered regions 21a having a locally low AlN mole fraction are uniformly dispersed within the n-type cladding layer 21. As described above in the Background Art section, the layered regions 21a extend in an oblique direction relative to the surface of the n-type cladding layer 21, and the band gap energy becomes locally small, which makes it easier for carriers to localize and functions as a low-resistance current path.

[0072] In a preferred embodiment, the layer region 21a has an AlGaN composition ratio of an integer ratio. k Ga 12-k N 12 The n-type metastable AlGaN region, in which k=5, 6, 7, or 8, is dominant. Note that the integer k is an integer ratio of AlGaN in the well layer 220, which will be described later. n Ga 12-n N 12 It is set so that n+1≦k≦8, ​​more preferably n+2≦k≦8, ​​according to the AlN mole fraction Xws (=n / 12) of the metastable well region where (n=3, 4, 5, or 6). Therefore, in the above-mentioned preferred embodiment, the possible combinations of integers k and n are (k=5 to 8: n=3 to 4), (k=6 to 8: n=5), and (k=7 to 8: n=6).

[0073] Furthermore, in the above-mentioned preferred embodiment, the average AlN molar fraction Xna of the n-type cladding layer 21 is preferably set within a range of (Xns+2%) to (Xns+7%) with the AlN molar fraction Xns (=k / 12) of the metastable n-type region as a reference. As a result, the difference in AlN molar fraction between the n-type main body region 21b other than the layered region 21a of the n-type cladding layer 21 and the metastable n-type region in the layered region 21a is stably maintained at approximately 2% or more, so that the effect of carrier localization is obtained by the flow of electrons in the layered region 21a.

[0074] In this embodiment, the thickness of the n-type cladding layer 21 is assumed to be about 1 μm to 2 μm, similar to the thickness adopted in general nitride semiconductor ultraviolet light emitting devices, but the thickness may also be about 2 μm to 4 μm.

[0075] The active layer 22 has a multiple quantum well structure in which two or more well layers 220 made of AlGaN-based semiconductors (excluding AlN-based semiconductors and GaN-based semiconductors) and one or more barrier layers 221 made of AlGaN-based semiconductors or AlN-based semiconductors are alternately stacked. The barrier layer 221 does not necessarily have to be provided between the bottom well layer 220 and the n-type cladding layer 21. In this embodiment, the barrier layer 221 is not provided between the top well layer 220 and the electron blocking layer 23, but as a preferred embodiment, an AlGaN layer or an AlN layer that is thinner than the barrier layer 221 and has a higher AlN mole fraction may be provided.

[0076] The electron blocking layer 23 is made of a p-type AlGaN-based semiconductor. The p-type contact layer 24 is made of a p-type AlGaN-based semiconductor or a p-type GaN-based semiconductor. The p-type contact layer 24 is typically made of p-GaN.

[0077] Fig. 5 shows a schematic example of a stacked structure (multiple quantum well structure) of well layers 220 and barrier layers 221 in active layer 22. Fig. 5 shows an example in which well layers 220 and barrier layers 221 each have three layers. Three layers, namely, barrier layers 221 and well layers 220, are stacked in this order on n-type cladding layer 21, and electron block layer 23 is located on top of well layer 220.

[0078] The structure in which the terraces T in the well layer 220, the barrier layer 221, and the electron block layer 23 shown in FIG. 5 grow in a multi-step manner is a known structure, as disclosed in the above-mentioned Non-Patent Documents 1 and 2. Between the terraces T adjacent to each other in the lateral direction in each layer, an inclined region IA inclined with respect to the (0001) plane is formed, as described above. The region sandwiched between the terraces T on the top and bottom other than the inclined region IA is referred to as a terrace region TA. In this embodiment, the depth of one terrace T (the distance between the adjacent inclined regions IA) is assumed to be several tens of nm to several hundreds of nm. Therefore, the (0001) plane exposed in a step-like manner in the inclined region IA is distinguished from the terrace surface of the multi-step terrace T. FIG. 6 shows, for example, a step-like structure (macrostep structure) exposed on the surface of the inclined region IA of one well layer 220.

[0079] 5, in each layer of the well layer 220, due to mass transfer of Ga from the terrace region TA to the tilted region IA, a Ga-rich well region 220a is formed in the tilted region IA, in which the AlN mole fraction is lower than the average AlN mole fraction Xwa in the well layer 220. Furthermore, due to the formation of the Ga-rich well region 220a in the tilted region IA, that is, due to mass transfer of Ga from the terrace region TA to the tilted region IA, the density of Al relatively increases in a part of the terrace region TA, forming an Al-rich well region in which the AlN mole fraction is higher than the average AlN mole fraction Xwa.

[0080] In this embodiment, a Ga-rich barrier region 221a having an AlN mole fraction lower than the average AlN mole fraction Xba of the barrier layer 221 is formed in the gradient region IA also in the barrier layer 221. Furthermore, similar to the well layer 220, an Al-rich barrier region having an AlN mole fraction higher than the average AlN mole fraction Xba of the barrier layer 221 may be formed in a part of the terrace region TA.

[0081] In this embodiment, a Ga-enriched EB region 23a having an AlN molar fraction lower than the average AlN molar fraction Xea of ​​the electron block layer 23 is also formed in the inclined region IA in the electron block layer 23. Furthermore, similar to the well layer 220, an Al-enriched EB region having an AlN molar fraction higher than the average AlN molar fraction Xea of ​​the electron block layer 23 may be formed in a part of the terrace region TA.

[0082] That is, in the n-type cladding layer 21, carriers tend to localize in the layered region 21a having a locally low AlN mole fraction, and in the active layer 22, carriers tend to localize in the Ga-rich well region 220a having a locally low AlN mole fraction in the inclined region IA of the well layer 220 and in the Ga-rich barrier region 221a having a locally low AlN mole fraction in the inclined region IA of the barrier layer 221, and in the electron block layer 23, carriers tend to localize in the Ga-rich EB region 23a having a locally low AlN mole fraction in the inclined region IA. Therefore, carriers can be efficiently supplied to the Ga-rich well region 220a of the well layer 220 from the n-type cladding layer 21 side through the layered region 21a and from the electron block layer 23 side through the Ga-rich EB region 23a, respectively, resulting in an element structure that can improve the light emission efficiency by recombination of carriers (electrons and holes) in the well layer 220.

[0083] In this embodiment, the Ga-rich well region 220a or the Al-rich well region of the well layer 220 contains AlGaN having an integer ratio of Al n Ga 12-n N 12Here, the integer n is 3, 4, 5, or 6. The AlN mole fraction Xws (=n / 12) and the film thickness of the metastable well region are set according to the target value of the peak emission wavelength of the light-emitting element 1.

[0084] As will be described later, the target value Xwt of the AlN mole fraction during epitaxial growth of the well layer 220 is set so as to satisfy the condition expressed by the following inequality. (n-0.24) / 12≦Xwt≦(n+0.24) / 12

[0085] Therefore, as described above, the average AlN molar fraction Xwa of the well layer 220 is equal to or close to the target value Xwt of the AlN molar fraction of the well layer 220, and therefore generally falls within a range of ±2% of the AlN molar fraction (n / 12) of the metastable well region formed in the well layer 220.

[0086] When the average AlN mole fraction Xwa of the well layer 220 is higher than the AlN mole fraction Xws (=n / 12) of the metastable well region (the state on the left side of FIG. 7), the mass transfer of Ga from the terrace region TA to the inclined region IA reduces the AlN mole fraction Xw0 of the Ga-rich well region 220a in the inclined region IA to the AlN mole fraction Xws of the metastable well region. On the other hand, the AlN mole fraction Xw1 of the Al-rich well region formed in a part of the terrace region TA is suppressed from increasing locally in the Al-rich well region because the AlN mole fraction Xw0 of the Ga-rich well region 220a is not significantly lower than the AlN mole fraction Xws of the metastable well region. As a result, the AlN mole fraction difference ΔXw01 (=|Xw1-Xw0|) between the Ga-rich well region 220a and the Al-rich well region is suppressed within a certain range.

[0087] When the average AlN mole fraction Xwa of the well layer 220 is lower than the AlN mole fraction Xws (=n / 12) of the metastable well region (the state on the right side of FIG. 7), the AlN mole fraction Xw1 of the Al-rich well region formed in a part of the terrace region TA increases to the AlN mole fraction Xws of the metastable well region due to the mass transfer of Ga from the terrace region TA to the inclined region IA. On the other hand, the AlN mole fraction Xw0 of the Ga-rich well region 220a in the inclined region IA decreases below the AlN mole fraction Xws of the metastable well region, but the AlN mole fraction Xw1 of the Al-rich well region does not increase significantly above the AlN mole fraction Xws of the metastable well region, so that the local decrease in the AlN mole fraction Xw1 is suppressed even in the Ga-rich well region 220a. As a result, the difference in AlN mole fraction ΔXw01 (=|Xw1−Xw0|) between the Ga-enriched well region 220a and the Al-enriched well region is suppressed within a certain range.

[0088] In general, in a nitride semiconductor ultraviolet light-emitting device with a peak emission wavelength of about 285 nm or more, the AlN mole fraction of the AlGaN-based semiconductor constituting the well layer 220 is low compared to a device with a peak emission wavelength of less than about 285 nm, so that there are relatively fewer Al vacancies that become point defects, and holes that reach the terrace region TA in the well layer 220 undergo radiative recombination in the terrace region TA, which has a higher AlN mole fraction than the inclined region IA, resulting in emission at a shorter wavelength than the inclined region IA. In this case, double-peak emission may occur in which two emission peaks with different wavelengths appear separately in the EL spectrum rather than being combined into a single peak.

[0089] However, since the generation of Al vacancies in the well layer 220 can be suppressed by increasing the growth temperature of each semiconductor layer in the active layer 22 to, for example, 1200° C. or higher, the above-mentioned double peak emission can occur even if the peak emission wavelength is less than approximately 285 nm.

[0090] Furthermore, even if the two emission peaks are not clearly separated in the EL spectrum, the EL spectrum may have one peak and a bulge (shoulder peak) of emission intensity that appears when the other peak is superimposed on the shoulder part on the long wavelength side or short wavelength side of the peak. Figure 8 shows three cases, namely, a case where the EL spectrum has two separated peaks P1 and P2 (Case A), a case where the EL spectrum has one peak P1 and a shoulder peak SP2 that appears on the shoulder part on the long wavelength side of the peak (Case B), and a case where the EL spectrum has one peak P2 and a shoulder peak SP1 that appears on the shoulder part on the short wavelength side of the peak (Case C). Furthermore, although not shown, Case A is subdivided into three cases: a case where the emission intensities of the two separated peaks P1 and P2 are almost the same (Case A1), a case where the emission intensity of the peak P1 on the short wavelength side is greater than the peak P2 on the long wavelength side (Case A2), and a case where the emission intensity of the peak P2 on the long wavelength side is greater than the peak P1 on the short wavelength side (Case A3). In cases B and C, when the bulge in emission intensity at the shoulders of the shoulder peaks SP1 and SP2 is not significant, they can be regarded as a pseudo single peak.

[0091] In this embodiment, the peaks P1, P2 and the shoulder peaks SP1, SP2 shown in FIG. 8 are identified as extreme points with minimum values ​​in the second derivative of the EL intensity expressed as a function with the wavelength as a variable. The peaks P1, P2 are identified as the extreme points with the smallest minimum value, and the shoulder peaks SP1, SP2 are identified as the extreme points with the second smallest minimum value. Since noise components during measurement are superimposed on the EL spectrum, the second derivative may have extreme points with minimum values ​​with small absolute values ​​other than the minimum values ​​corresponding to the peaks P1, P2 and the shoulder peaks SP1, SP2. For this reason, in this embodiment, the peaks P1, P2 and the shoulder peaks SP1, SP2 are identified using the extreme points with the first and second smallest two minimum values. Note that the zero points of the first derivative of the EL intensity (the first derivative value of the EL intensity is 0) may be used to identify the peaks P1, P2.

[0092] FIG. 9 shows an example of the EL spectrum of case B, in which the EL spectrum is normalized by 1,000,000 in EL intensity, the first derivative of the EL spectrum, and the second derivative of the EL spectrum are vertically aligned with the same wavelength. From FIG. 9, it can be seen that the peak P1 and the shoulder peak SP2 are identified as the extreme points of the first and second smallest minimum values ​​of the second derivative of the EL intensity. Furthermore, it can be seen that the peak P1 is identified as a zero point of the first derivative of the EL intensity. Note that the EL spectrum shown in FIG. 9 is different from the EL spectrum of the light-emitting element assumed in this embodiment, in that the wavelength difference between the peak P1 and the shoulder peak SP2 is large at about 17 nm and the full width at half maximum is about 22 nm, and the peak P1 and the shoulder peak SP2 are clearly separated, making it an example that cannot be regarded as a pseudo single peak.

[0093] In this embodiment, since an Al-rich well region is formed in a part of the terrace region TA of the well layer 220, it is assumed that an intermediate region exists in the terrace region TA, in which the AlN mole fraction is the same or approximately the same as the average AlN mole fraction Xwa of the well layer 220. Therefore, the EL spectrum of the light-emitting element 1, regardless of the type of the above cases A to C, is a composite spectrum of the EL spectrum from the Ga-rich well region 220a with the AlN mole fraction Xw0, the EL spectrum from the Ga-rich well region 220a with the AlN mole fraction Xw0, the EL spectrum from the Al-rich well region with the AlN mole fraction Xw1, and the EL spectrum from the intermediate region in the terrace region TA with the AlN mole fraction Xwa.

[0094] The light-emitting element structure 20 of this embodiment is configured to efficiently supply carriers to the inclined region IA of the well layer 220 and its neighboring region through the layer region 21a from the n-type cladding layer 21 side, which functions as a low-resistance current path. Therefore, in the well layer 220, carriers are likely to be localized in the Ga-rich well region 220a in the inclined region IA, and the EL spectrum from the Ga-rich well region 220a may be the main EL spectrum among the EL spectra from the above three locations. However, when a metastable well region is formed in the Al-rich well region and the AlN mole fraction in the Al-rich well region is stably equal to the AlN mole fraction Xws (=n / 12) of the metastable well region, or when the occurrence of Al vacancies, which are point defects, is suppressed in the Al-rich well region, the EL spectrum from the Al-rich well region located near the inclined region IA may be the main EL spectrum among the EL spectra from the above three locations. On the other hand, in the intermediate region in the terrace region TA, which has a larger surface area than the inclined region IA, the supply of carriers is considered to be insufficient at a location away from the inclined region IA, so that the extent to which the EL spectrum from the intermediate region accounts for in the composite spectrum is considered to be limited. However, if the generation of Al vacancies, which are point defects, is sufficiently suppressed in the intermediate region, the extent to which the EL spectrum from the intermediate region accounts for in the composite spectrum is considered to be large. From the above, depending on the magnitude relationship of the EL spectra from the above three locations, the EL spectrum of the light-emitting element 1 can be any of the above cases A to C.

[0095] However, in this embodiment, the AlN molar fraction difference ΔXw01 (=|Xw1-Xw0|) between the Ga-enriched well region 220a and the Al-enriched well region is suppressed to a certain value or less (for example, about 4% or less), so that the wavelength difference Δλ between two peaks of the EL spectrum or the wavelength difference Δλ between one peak and one shoulder peak can be suppressed to 8 nm or less, and typically, the wavelength difference Δλ can be suppressed to 5 nm or less. The wavelength difference of 5 nm corresponds to an AlN molar fraction difference of about 3% and a film thickness difference of about 1 ML. As a result, even if the EL spectrum has two peaks or one peak and one shoulder peak as in the above cases A to C, the wavelength difference Δλ is small, so that an EL spectrum almost equivalent to an EL spectrum with a single peak can be obtained.

[0096] 10 to 13 are graphs showing simulation results (corresponding to peak emission wavelengths) of emission wavelengths obtained by changing the film thickness of the well layer within a range of 3ML to 14ML or 4ML to 14ML for a quantum well structure model in which the well layer 220 and the barrier layer 221 are made of AlGaN. As conditions for the above simulation, in FIG. 10, the AlN mole fraction Xw0 of the Ga enriched well region 220a of the well layer 220 is set to 50% (n=6) which is the AlN mole fraction Xws of the metastable well region, in FIG. 11, the AlN mole fraction Xw0 of the Ga enriched well region 220a of the well layer 220 is set to 41.7% (n=5) which is the AlN mole fraction Xws of the metastable well region, and in FIG. 12, the AlN mole fraction Xw0 of the Ga enriched well region 220a of the well layer 220 is set to 33.3% (n=6) which is the AlN mole fraction Xws of the metastable well region. % (n=4), and in FIG. 13, the AlN molar fraction Xw0 of the Ga-enriched well region 220a of the well layer 220 is set to 25% (n=3), which is the AlN molar fraction Xws of the metastable well region. In each of FIGS. 10 to 12, the AlN molar fraction of the Ga-enriched barrier region 221a of the barrier layer 221 is set to three values, 66.7% (two-thirds), 75% (three-quarters), and 83.3% (five-sixths). In FIG. 13, the AlN molar fraction of the Ga-enriched barrier region 221a of the barrier layer 221 is set to 66.7% (two-thirds). In the simulation results shown in FIGS. 10 to 13, it is assumed that ultraviolet light emission in the well layer 220 occurs in either the inclined region IA or the terrace region TA. For this reason, it is important that the film thickness condition of the well layer 220 is satisfied in the inclined region IA or the terrace region TA where the ultraviolet light emission occurs.

[0097] 10 to 13 show that within the range of 3ML to 14ML, the smaller the thickness of the well layer 220, the greater the quantum confinement effect in the well layer 220, and the shorter the emission wavelength becomes, and furthermore, from Figures 10 to 12, the greater the AlN molar fraction of the barrier layer 221, the greater the degree of change in the emission wavelength with respect to the change in the thickness of the well layer 220. Also, from Figure 10, it can be seen that when the AlN molar fraction Xw0 of the Ga-enriched well region 220a is 50%, within the above ranges of the thickness of the well layer 220 and the AlN molar fraction of the barrier layer 221, the emission wavelength changes in the range of approximately 246nm to 295nm, and when the thickness of the well layer 220 is within the range of 4ML to 12ML, the emission wavelength changes in the range of approximately 252nm to 287nm. 11 shows that when the AlN molar fraction Xw0 of the Ga-enriched well region 220a is 41.7%, the emission wavelength varies in the range of approximately 249 nm to 311 nm within the above ranges of the thickness of the well layer 220 and the AlN molar fraction of the barrier layer 221, and when the thickness of the well layer 220 is in the range of 4 ML to 12 ML, the emission wavelength varies in the range of approximately 257 nm to 301 nm. Fig. 12 shows that when the AlN molar fraction Xw0 of the Ga-enriched well region 220a is 33.3%, the emission wavelength varies in the range of approximately 261 nm to 328 nm within the above ranges of the thickness of the well layer 220 and the AlN molar fraction of the barrier layer 221, and when the thickness of the well layer 220 is in the range of 4 ML to 12 ML, the emission wavelength varies in the range of approximately 261 nm to 315 nm. 13, when the AlN molar fraction Xw0 of the Ga-enriched well region 220a is 25%, the emission wavelength varies in the range of approximately 275 nm to 320 nm when the film thickness of the well layer 220 is in the range of 4 ML to 12 ML and the AlN molar fraction of the barrier layer 221 is 66.7%. Furthermore, if the barrier layer 221 is made of AlN (AlN molar fraction=100%), the emission wavelength can be further expanded.

[0098] 10 to 13, the Ga-rich well region 220a or the Al-rich well region of the well layer 220 has an AlGaN composition ratio of Al. n Ga 12-n N 12A metastable well region where n=3, 4, 5, or 6 is formed. By adjusting the film thickness of the well layer 220 within the range of 4 ML to 10 ML and the AlN molar fraction of the Ga-enriched barrier region 221a of the barrier layer 221 within the range of 66.7% to 83.3% according to the AlN molar fraction Xws (=n / 12) of the metastable well region, it is found that the emission wavelength of the peak or shoulder peak corresponding to the AlN molar fraction Xws of the metastable well region can be set to approximately within the range of 252 nm to 310 nm.

[0099] 10 to 13, taking into consideration the lattice relaxation from the barrier layer and the possibility of the presence of a pseudo-metastable well region in which the AlN mole fraction has slightly changed from the AlN mole fraction Xws, other than the metastable well region, in the Ga-enriched well region 220a or the Al-enriched well region, the emission wavelength of the peak or shoulder peak corresponding to the AlN mole fraction Xws (n / 12, n=3 to 6) of the metastable well region and the film thickness of the metastable well region (4 ML to 10 ML) is expected to vary by about ±1.5 nm.

[0100] Therefore, for example, when the AlN mole fraction Xws of the metastable well region is 25% (n=3) and the AlN mole fraction of the barrier layer 221 is 66.7%, the emission wavelength of the peak or shoulder peak is within any of the ranges of 298 nm to 301 nm (8ML), 303 nm to 306 nm (9ML), and 309 nm to 312 nm (10ML), and when the AlN mole fraction Xws is 33.3% (n=4) and the AlN mole fraction of the barrier layer 221 is 83.3%, the emission wavelength of the peak or shoulder peak is within any of the ranges of 282 nm to 285 nm (7ML), 286 nm to 289 nm (8ML), 292 nm to 295 nm (9ML), and 297 nm to 300 nm (10ML). When the AlN mole fraction Xws is 41.7% (n=5) and the AlN mole fraction of the barrier layer 221 is 83.3%, it is within any of the ranges of 270 nm to 273 (6ML), 275 nm to 278 nm (7ML), 280 nm to 283 nm (8ML), and 285 nm to 287 nm (9ML), and when the AlN mole fraction Xws is 50% (n=6) and the AlN mole fraction of the barrier layer 221 is 83.3%, it is within any of the ranges of 251 nm to 254 nm (4ML), 256 nm to 259 nm (5ML), 261 nm to 264 nm (6ML), 265 nm to 268 nm (7ML), and 270 nm to 273 nm (8ML). The ML value in parentheses indicates the film thickness of the metastable well region. In the above example, it is assumed that metastable AlGaN can be formed in the c-axis direction in units of 1 ML even in the cases of n=3 and n=5.

[0101] In this embodiment, the emission wavelength range of the peak or shoulder peak determined by the AlN mole fraction Xws of the metastable well region, the AlN mole fraction of the barrier layer 221, and the film thickness of the metastable well region is referred to as the “intrinsic wavelength range” for convenience.

[0102] In the above-mentioned intrinsic wavelength range, the AlN mole fraction of the barrier layer 221 is assumed to be the AlN mole fraction in the gradient region IA of the barrier layer 221 when the metastable well region is formed in the Ga-enriched well region 220a, and the AlN mole fraction in the terrace region TA of the barrier layer 221 when the metastable well region is formed in the Al-enriched well region. Therefore, for example, when the AlN mole fraction in the gradient region IA of the barrier layer 221 is 83.3% and the metastable well region is formed in the Al-enriched well region of the terrace region TA, the AlN mole fraction in the terrace region TA of the barrier layer 221 is several percent higher than 83.3%, so that the range of variation of about ±1.5 nm in the above-mentioned intrinsic wavelength range may vary slightly depending on the film thickness of the metastable well region.

[0103] In a preferred embodiment, when the barrier layer 221 is made of an AlGaN-based semiconductor (excluding an AlN-based semiconductor), the AlN mole fraction of the terrace region TA of the barrier layer 221 is set to be 1% or more, preferably 2% or more, and more preferably 4% or more higher than the AlN mole fraction of the Ga-enriched barrier region 221a, within a range of approximately 51% to 90%. In order to fully ensure the effect of carrier localization in the Ga-enriched barrier region 221a, it is preferable that the difference in AlN mole fraction between the Ga-enriched barrier region 221a and the terrace region TA in the barrier layer 221 is 4 to 5% or more, but the effect of carrier localization can be expected even if it is about 1 to 2%.

[0104] In a further preferred embodiment, the Ga-rich barrier region 221a has an AlGaN composition ratio of an integer ratio. j Ga 12-j N 12 There exists a dominant metastable barrier region which is metastable AlGaN with (j=8, 9, or 10). The integer j is set within the range of 8 to 10 so that n+4≦j≦n+6 is satisfied based on the AlN mole fraction Xws (=n / 12) of the metastable well region, but is not limited to this condition.

[0105] The average AlN mole fraction Xba of the barrier layer 221 is preferably set within a range of (Xbs+2%) to (Xbs+8.3%) with the AlN mole fraction Xbs (=j / 12) of the metastable barrier region as a reference. As a result, the difference in AlN mole fraction between the Ga-enriched barrier region 221a and the terrace region TA of the barrier layer 221 is stably maintained at approximately 2% or more, and the above-mentioned effect of carrier localization in the Ga-enriched barrier region 221a is stably exerted.

[0106] The thickness of the barrier layer 221, including the terrace region TA and the inclined region IA, is set within the range of, for example, 6 nm to 8 nm.

[0107] The AlN molar fraction of the terrace regions TA of the electron block layer 23 is set to be within a range of approximately 69% to 90%, and is set to be 20% or more, preferably 25% or more, and more preferably 30% or more higher than the AlN molar fraction of the terrace regions of the well layer 220. Furthermore, the AlN molar fraction of the Ga-enriched EB regions 23a of the electron block layer 23 is set to be 20% or more, preferably 25% or more, and more preferably 30% or more higher than the AlN molar fraction of the Ga-enriched well regions 220a of the well layer 220.

[0108] In a preferred embodiment, the Ga-enriched EB region 23a has an AlGaN composition ratio of an integer ratio. m Ga 12-m N 12 The metastable EB region, which is p-type metastable AlGaN with m = 8, 9, or 10, dominates. Here, the AlGaN composition ratio (Al n Ga 12-n N 12 , n = 3-6) and the AlGaN composition ratio in the metastable EB region (Al m Ga 12-m N 12Among the combinations of n and m, (n=6, m=8, m=8-10), the difference in AlN mole fraction between the metastable EB region and the metastable well region is 20% or more in the combination where m>n+2. Therefore, in the above-mentioned preferred embodiment, among the combinations of integers n and m, (n=6, m=8), (n=7, m=9), and (n=7, m=8) do not satisfy the above condition and are excluded.

[0109] The average AlN molar fraction Xea of ​​the electron blocking layer 23 is preferably set within a range of (Xes+2%) to (Xes+7%) with respect to the AlN molar fraction Xes (=m / 12) of the metastable EB region. As a result, the AlN molar fraction difference between the Ga-enriched EB region 23a and the terrace region TA of the electron blocking layer 23 is stably maintained at approximately 2% or more, and the above-mentioned effect of carrier localization in the Ga-enriched EB region 23a is stably exerted.

[0110] The thickness of the electron blocking layer 23, including the terrace region TA and the inclined region IA, is set within a range of, for example, 15 nm to 30 nm (the optimum value is about 20 nm).

[0111] The average AlN molar fractions Xna, Xba, and Xea of ​​the n-type cladding layer 21, the barrier layer 221, and the electron blocking layer 23, like the average AlN molar fraction Xwa of the well layer 220, are set to or near the target values ​​of the AlN molar fractions of the n-type cladding layer 21, the barrier layer 221, and the electron blocking layer 23 at the time of deposition of each of them.

[0112] The p-electrode 26 is made of a multi-layer metal film such as Ni / Au, and is formed on the upper surface of the p-type contact layer 24. The n-electrode 27 is made of a multi-layer metal film such as Ti / Al / Ti / Au, and is formed in a part of the exposed surface in the second region R2 of the n-type cladding layer 21. The p-electrode 26 and the n-electrode 27 are not limited to the multi-layer metal film described above, and the electrode structure such as the metal constituting each electrode, the number of layers, and the order of layers may be changed as appropriate. FIG. 14 shows an example of the shape of the p-electrode 26 and the n-electrode 27 as viewed from the top of the light-emitting element 1. In FIG. 14, the line BL between the p-electrode 26 and the n-electrode 27 indicates the boundary between the first region R1 and the second region R2, and coincides with the outer peripheral side wall surfaces of the active layer 22, the electron blocking layer 23, and the p-type contact layer 24.

[0113] In this embodiment, as shown in FIG. 14, the planar shapes of the first region R1 and the p-electrode 26 are, as an example, comb-shaped, but the planar shapes and arrangements of the first region R1 and the p-electrode 26 are not limited to the example shown in FIG. 14.

[0114] When a forward bias is applied between the p-electrode 26 and the n-electrode 27, holes are supplied from the p-electrode 26 toward the active layer 22, and electrons are supplied from the n-electrode 27 toward the active layer 22. The supplied holes and electrons each reach the active layer 22 and recombine to emit light. This causes a forward current to flow between the p-electrode 26 and the n-electrode 27.

[0115] <Method of manufacturing light-emitting element> Next, the light emission shown in FIG. element An example of the manufacturing method of 1 will be described.

[0116] First, by metal-organic vapor phase epitaxy (MOVPE), the AlN layer 12 included in the base portion 10 and the nitride semiconductor layers 21 to 24 included in the light-emitting element structure 20 are epitaxially grown in order on the sapphire substrate 11. At this time, the n-type cladding layer 21 is doped with, for example, Si as a donor impurity, and the electron blocking layer 23 and the p-type contact layer 24 are doped with, for example, Mg as an acceptor impurity.

[0117] In this embodiment, in order to expose multi-step terraces parallel to the (0001) plane on at least the surfaces of the AlN layer 12, the n-type cladding layer 21, the active layer 22 (the well layer 220, the barrier layer 221), and the electron blocking layer 23, the sapphire substrate 11 is a slightly inclined substrate in which the main surface 11a is inclined at an angle (off angle) within a certain range (for example, up to about 0.3° to 6°) with respect to the (0001) plane and multi-step terraces are exposed on the main surface 11a.

[0118] The conditions for such epitaxial growth include, in addition to the use of the vicinal (0001) sapphire substrate 11 described above, a growth rate that makes it easy for multi-step terraces to appear (specifically, the growth rate is achieved by appropriately setting various conditions such as the growth temperature, and the supply amounts and flow rates of the source gas and carrier gas), etc. Note that these various conditions may differ depending on the type and structure of the film formation apparatus, so it is sufficient to actually fabricate several samples in the film formation apparatus and specify these conditions.

[0119] As growth conditions for the n-type cladding layer 21, the growth temperature, growth pressure, and donor impurity concentration are selected so that immediately after the start of growth, a growth starting point for the layer region 21a is formed by mass transfer of Ga in a step portion (inclined region) between multiple terraces formed on the upper surface of the AlN layer 12, and subsequently, as the n-type cladding layer 21 epitaxially grows, the layer region 21a grows obliquely upward by segregation associated with the mass transfer of Ga.

[0120] Specifically, the growth temperature is preferably 1050° C. or higher at which mass transfer of Ga occurs easily and 1150° C. or lower at which good n-type AlGaN can be prepared. k Ga 12-k N 12 When forming a metastable n-type region with (k=5, 6, 7, or 8), at a growth temperature exceeding 1170°C, the mass transfer of Ga becomes excessive, and even in metastable AlGaN, the AlN mole fraction tends to fluctuate randomly, so it may be difficult to stably form a metastable n-type region with an AlN mole fraction of 41.7% to 66.7%. As for the growth pressure, 75 Torr or less is preferable as a good growth condition for AlGaN, and 10 Torr or more is realistic and preferable as the control limit of the film formation apparatus. The donor impurity concentration is 1×10 18 ~5×10 18 cm -3 The above growth temperature, growth pressure, and the like are merely examples, and optimal conditions may be appropriately specified depending on the film formation apparatus used.

[0121] The supply amounts and flow rates of the source gases (trimethylaluminum (TMA) gas, trimethylgallium (TMG) gas, ammonia gas) and carrier gas used in the metal-organic compound vapor phase epitaxy are set with the average AlN mole fraction Xna of the n-type cladding layer 21 as a target value. The average AlN mole fraction Xna of the n-type cladding layer 21 is as described above, and a duplicated description will be omitted.

[0122] The donor impurity concentration does not necessarily need to be uniformly controlled in the vertical direction with respect to the thickness of the n-type cladding layer 21. For example, the impurity concentration of a predetermined thin portion in the n-type cladding layer 21 may be lower than the set concentration, for example, 1×10 18 cm -3 less than 1×10 17 cm -3The low impurity concentration layer may be a layer having a thickness controlled to 0 nm or less. The thickness of the low impurity concentration layer is preferably greater than 0 nm and less than 200 nm, more preferably greater than 10 nm and less than 100 nm, and even more preferably greater than 20 nm and less than 50 nm. The donor impurity concentration of the low impurity concentration layer may be lower than the above set concentration, and may be lower than the undoped layer (0 cm -3 ) may be included in part of the n-type cladding layer 21. Furthermore, a part or the whole of the low impurity concentration layer is preferably present in an upper layer region at a depth of 100 nm or less below the upper surface of the n-type cladding layer 21.

[0123] After the n-type cladding layer 21 having the layer region 21a and the n-type main body region 21b is formed in the above manner, the active layer 22 (well layer 220, barrier layer 221), the electron blocking layer 23, the p-type contact layer 24, etc. are subsequently formed on the entire upper surface of the n-type cladding layer 21 by a well-known epitaxial growth method such as metal-organic vapor phase epitaxy (MOVPE).

[0124] The acceptor impurity concentration of the electron blocking layer 23 is, for example, 1.0×10 16 ~1.0×10 18 cm -3 The acceptor impurity concentration of the p-type contact layer 24 is preferably about 1.0×10 18 ~1.0×10 20 cm -3 It is preferable that the acceptor impurity concentration is about 100 nm. It is not necessary to control the acceptor impurity concentration uniformly in the vertical direction with respect to the film thickness of each of the electron block layer 23 and the p-type contact layer 24.

[0125] In forming the active layer 22, in the same manner as for the n-type cladding layer 21, the well layer 220 is grown under growth conditions that make it easy to expose the above-mentioned multi-stepped terraces, with an average AlN mole fraction Xwa of the well layer 220 as a target value, and further, the barrier layer 221 is grown with an average AlN mole fraction Xba of the barrier layer 221 as a target value. The average AlN mole fractions Xwa and Xba of the well layer 220 and the barrier layer 221 are as described above, and therefore repeated explanations will be omitted.

[0126] In forming the electron blocking layer 23, the electron blocking layer 23 is grown under growth conditions that facilitate the appearance of the above-mentioned multi-step terraces in the same manner as the n-type cladding layer 21, with the average AlN mole fraction Xea of ​​the electron blocking layer 23 as a target value. The average AlN mole fraction Xea of ​​the electron blocking layer 23 is as described above, and a duplicated description will be omitted.

[0127] In this embodiment, the active layer 22 (the well layer 220 and the barrier layer 221), the electron blocking layer 23, and the p-type contact layer 24 The growth temperatures of the n-type cladding layer 21 are T1, the active layer 22 is T2, the electron blocking layer 23 is T3, and the p-type contact layer 24 When the growth temperature is T4, within the above-mentioned preferable temperature range (1050° C. to 1170° C.), it is preferable that the relationship shown in the following formulas (1) and (2) is satisfied. T3≧T2 (1) T3>T1>T4 (2)

[0128] Furthermore, the growth temperature T3 of the electron blocking layer 23 is preferably 1150° C. or higher when the AlN molar fraction of the metastable EB region is 83.3%, and is preferably 1100° C. or higher when the AlN molar fraction of the metastable EB region is 75% or 66.7%, and is more preferably higher than 1100° C. Note that the above temperatures are only examples, and for example, by increasing the flow rate of the nitrogen source gas and decreasing the growth rate, the above 1150° C. and 1100° C. can be reduced to 1100° C. and 1050° C., respectively.

[0129] When the growth temperature T3 of the electron blocking layer 23 is increased from the growth temperature T2 of the active layer 22, decomposition of GaN may occur in the well layer 220 located thereunder during the transition of the growth temperature, and the characteristics of the light-emitting element 1 may deteriorate due to the decomposition of GaN. , BIt is preferable to form an AlGaN layer or an AlN layer that is thinner than the rear layer 221 (for example, 3 nm or less, preferably 2 nm or less) and has a higher AlN mole fraction than the barrier layer 221 and the electron blocking layer 23 .

[0130] In the above manner, the active layer 22 (well layer 220, barrier layer 221), electron blocking layer 23, p-type contact layer 24, etc. are formed on the entire upper surface of the n-type cladding layer 21. Next, the second region R2 of the nitride semiconductor layers 21-24 is selectively etched by a well-known etching method such as reactive ion etching until the upper surface of the n-type cladding layer 21 is exposed, thereby exposing the second region R2 portion of the upper surface of the n-type cladding layer 21. Then, a p-electrode 26 is formed on the p-type contact layer 24 in the unetched first region R1 by a well-known film formation method such as electron beam evaporation, and the etched first region R2 is selectively etched by a well-known film formation method such as electron beam evaporation. 2 An n-electrode 27 is formed on the n-type cladding layer 21 in the region R2. After forming one or both of the p-electrode 26 and the n-electrode 27, a heat treatment may be performed by a well-known heat treatment method such as RTA (Rapid Thermal Annealing).

[0131] As an example, the light-emitting element 1 can be used in a state where it is flip-chip mounted on a base such as a submount, and then sealed with a predetermined resin such as silicone resin or amorphous fluororesin (e.g., a lens-shaped resin).

[0132] The cross-sectional structure of the AlGaN-based semiconductor layers 21 to 24 of the light-emitting element 1 fabricated in the above manner is , No. Etching of the second region R2 and p A sample is prepared before the electrode 26 and the n-electrode 27 are formed, a sample piece having a cross section perpendicular (or nearly perpendicular) to the upper surface of the sample is processed by focused ion beam (FIB), and the sample piece can be observed by HAADF-STEM imaging. HAADF-STEM images provide contrast proportional to the atomic weight, and heavy elements are displayed brightly. Therefore, areas with low AlN mole fraction are displayed relatively brightly. HAADF-STEM images are more suitable for observing differences in AlN mole fraction than normal STEM images (bright-field images).

[0133] As an example, Fig. 15 shows an HAADF-STEM image of a cross-sectional structure of a main part of the light-emitting device 1 produced as described above, including the active layer 22 having a multiple quantum well structure in which the well layer 220 and the barrier layer 221 are each three layers as shown in Fig. 5. Using the sample piece shown in Fig. 15, a composition analysis is performed in the active layer 22 by scanning an electron beam probe (diameter: about 2 nm) in a direction perpendicular to each surface of the inclined region IA and the terrace region TA of the well layer 220 by line analysis of energy dispersive X-ray spectroscopy (cross-sectional TEM-EDX), and the film thicknesses of the inclined region IA and the terrace region TA in the well layer 220 can be measured from the distribution of the obtained EDX counts of Ga in the scanning direction.

[0134] Furthermore, composition analysis within a specific semiconductor layer in the AlGaN-based semiconductor layers 21-24 can be performed by energy dispersive X-ray spectroscopy (cross-sectional TEM-EDX) or CL (cathodoluminescence) using the above-mentioned sample piece. Although a description of composition analysis by cross-sectional TEM-EDX and CL methods is omitted, detailed explanations are given in the specifications of the inventor's prior separate applications (PCT / JP2020 / 023050, PCT / JP2020 / 024828, PCT / JP2020 / 026558, PCT / JP2020 / 031620) and the like.

[0135] <EL spectrum of light-emitting element> Next, the measurement results of the EL spectrum of the light-emitting element 1 in a wafer state will be described with reference to Fig. 16. Five types of wafers (W1 to W5) were fabricated in the manner described in the manufacturing method section above, by controlling the growth temperature of the well layer within the range of 1080°C to 1170°C, and for each wafer, 1 to 5 chips were selected from those with an external quantum efficiency of 30% or more, and the EL spectra of a total of 12 chips were measured using an autoprobe equipped with a spectrometer.

[0136] All of the five types of wafers used in measuring the EL spectra have an AlN mole fraction Xws of 33.3% (n=4) in the metastable well region of the well layer 220 and an AlN mole fraction Xbs of 83.3% (j=10) in the metastable barrier region of the barrier layer 221. The target thickness of the well layer 220 is 8ML to 9ML, but there is a variation of about 1ML between and within the wafer, and the thickness of the inclined region IA is about 1ML larger than the thickness of the terrace region TA.

[0137] The EL spectrum measurement results for a total of 12 chips on five types of wafers (W1 to W5) were classified into three types depending on whether the emission wavelength of the peak or shoulder peak of the EL spectrum exists in one of three inherent wavelength ranges: 286 nm to 289 nm (8 ML), 292 nm to 295 nm (9 ML), or 297 nm to 300 nm (10 ML). The ML value in parentheses indicates the film thickness of the inclined region IA or terrace region TA where the metastable well region is formed. In addition, considering the growth temperature conditions of the well layer 220 of the five types of wafers (W1 to W5), the above three inherent wavelength ranges are all in wavelength regions where double peak emission of 285 nm or more can occur. Therefore, these ranges are suitable for verifying whether or not the occurrence of significant peak separation with a large wavelength difference is suppressed in a light-emitting element 1 fabricated so that the average AlN molar fraction Xwa of the well layer 220 falls within a range of ±2% of the AlN molar fraction (n / 12) of the metastable well region formed in either the Ga-enriched well region 220a or the Al-enriched well region.

[0138] The EL spectrum measurement results are called Type A, Type B, and Type C, in order from the shortest wavelength side of the three inherent wavelength ranges mentioned above, and are divided into three graphs according to type, which are shown in Figure 16. There are four chips in each type, and the EL spectrum of each chip is given a code (Wx-y) that combines the wafer number (Wx = W1 to W5) and the chip number (y = 1 to 5). In each graph, the horizontal axis is the wavelength, and the vertical axis is the normalized EL intensity, and the origin of the EL intensity is shifted by 0.3 for each chip.

[0139] In Figure 16, a solid line indicates a first type of EL spectrum (corresponding to the state on the left side of Figure 7) in which the emission wavelength of the longer wavelength peak or shoulder peak of the two peaks or one peak and one shoulder peak of the EL spectrum is within one of the above-mentioned intrinsic wavelength ranges, and a dashed line indicates a second type of EL spectrum (corresponding to the state on the right side of Figure 7) in which the emission wavelength of the shorter wavelength peak or shoulder peak is within one of the above-mentioned intrinsic wavelength ranges.

[0140] The types, characteristic wavelength ranges, peak and shoulder peak wavelengths, wavelength difference Δλ, and full width at half maximum (FWHM) of the 12 EL spectra shown in FIG. 16 are summarized in Table 1 below.

[0141] [Table 1]

[0142] From the measurement results shown in FIG. 16 and Table 1, the emission wavelengths of the peaks and shoulder peaks of the EL spectra are in the range of 280 nm to 310 nm in the 12 chips of the 5 types of wafers. Furthermore, it can be confirmed that the emission wavelengths of the peaks or shoulder peaks of the EL spectra are in the corresponding inherent wavelength ranges in the 12 chips of the 5 types of wafers, except for W2-2 and W3-1. In addition, although W2-2 and W3-1 are both second type EL spectra, the peaks on the short wavelength side are close to the corresponding inherent wavelength range with an error of 0.4 to 0.5 nm, and do not deviate significantly from the inherent wavelength range. As a result, it can be confirmed that a metastable well region is formed in the Ga-rich well region 220a or the Al-rich well region in the well layer 220.

[0143] Furthermore, since the EL spectrum measurement results are classified according to the above three inherent wavelength ranges, it can be seen that the film thickness of the inclined region IA or terrace region TA in which the metastable well regions of the above 12 chips are formed is in the range of 8 to 10 ML.

[0144] Furthermore, in the manufacture of the five types of wafers (W1 to W5), the target value Xwt of the AlN mole fraction during epitaxial growth of the well layer 220 is set to fall within a range of ±2% of the AlN mole fraction Xws (33.3%) in the metastable well region, so that in all of the 12 chips, the wavelength difference Δλ between two peaks in the EL spectrum or the wavelength difference Δλ between one peak and one shoulder peak is about 3.8 nm to about 7.6 nm, which can be confirmed to be suppressed to 8 nm or less, and the full width at half maximum (FWHM) of the EL spectrum is about 12.7 nm to about 15.6 nm, which can be confirmed to be within the target range of 12 nm to 20 nm for the EL spectrum in this embodiment. It is more preferable that the full width at half maximum of the EL spectrum is within a range of 12 nm to 16 nm, and the 12 chips above satisfy this more preferable condition.

[0145] In the EL spectra of chips W2-2, W4-1, and W5-2 with a wavelength difference Δλ of 7.6 nm, clear peak separation does not occur, but some of the spectra include those in which peak separation is about to occur, so it is considered that if the wavelength difference Δλ is about 8 nm or less, clear peak separation can be suppressed to a certain extent, and if the wavelength difference Δλ is 7.6 nm or less, clear peak separation can be sufficiently suppressed. Furthermore, if the wavelength difference Δλ is 7 nm or less, more preferably 6 nm or less, and even more preferably 5 nm or less, peak separation can be sufficiently suppressed.

[0146] Furthermore, since the target value Xwt of the AlN molar fraction during epitaxial growth of the well layer 220 is set to be within a range of ±2% of the AlN molar fraction Xws (33.3%) in the metastable well region, it can be seen that in wafers W1 to W3, within the same wafer, there is a mixture of chips in which the metastable well region is formed in the Ga-enriched well region 220a (first type of EL spectrum) and chips in which the metastable well region is formed in the Al-enriched well region (second type of EL spectrum).

[0147] The measurement result of the EL spectrum shown in FIG. 16 shows the case where the AlN mole fraction Xws of the metastable well region is 33.3% (n=4). However, from this measurement result, it is presumed that even when the AlN mole fraction Xws of the metastable well region is other than 33.3% (n=3, 5, or 6), particularly when the emission wavelength of the peak and shoulder peak is within the range of 280 nm to 310 nm, a metastable well region is formed in the Ga-rich well region 220a or Al-rich well region in the well layer 220, as in the case where the AlN mole fraction Xws is 33.3% (n=4), and as a result, the wavelength difference Δλ between two peaks in the EL spectrum or the wavelength difference Δλ between one peak and one shoulder peak is suppressed to 8 nm or less, and the full width at half maximum of the EL spectrum falls within the range of 12 nm to 20 nm.

[0148] Furthermore, in the EL spectrum measurement results shown in FIG. 16, the effect of suppressing double-peak emission was confirmed when the intrinsic wavelength range of the peak or shoulder peak was 285 nm or more. However, when the growth temperature of each semiconductor layer in active layer 22 was set higher than the growth temperatures of the above five types of wafers and the generation of Al vacancies in well layer 220 was suppressed, the effect of suppressing double-peak emission of light-emitting element 1 was also confirmed when the intrinsic wavelength range was less than 285 nm.

[0149] Furthermore, even if the AlN mole fraction Xws of the metastable well region is other than 33.3%, and even if the intrinsic wavelength range is less than 285 nm, chips in which the metastable well region is formed in a Ga-enriched well region 220a and chips in which the metastable well region is formed in an Al-enriched well region may be mixed within the same wafer.

[0150] [Second embodiment] In the light-emitting element 1 of the first embodiment, the p-type layer constituting the light-emitting element structure 20 is composed of two layers, the electron blocking layer 23 and the p-type contact layer 24. However, in the light-emitting element 2 of the second embodiment, the p-type layer has a p-type cladding layer 25 composed of one or more layers of p-type AlGaN-based semiconductor between the electron blocking layer 23 and the p-type contact layer 24.

[0151] Therefore, in the second embodiment, as shown in FIG. 17, the AlGaN-based semiconductor layers 21-25 of the light-emitting element structure 20 have a structure in which an n-type cladding layer 21 (n-type layer), an active layer 22, an electron blocking layer 23 (p-type layer), a p-type cladding layer 25 (p-type layer), and a p-type contact layer 24 (p-type layer) are epitaxially grown and stacked in this order from the base portion 10 side.

[0152] The base portion 10, and the AlGaN-based semiconductor layers 21-24, p-electrode 26, and n-electrode 27 of the light-emitting element structure 20 in the light-emitting element 2 of the second embodiment are the same as the base portion 10 and the AlGaN-based semiconductor layers 21-24, p-electrode 26, and n-electrode 27 of the light-emitting element structure 20 of the light-emitting element 1 of any of the first to third embodiments, and therefore redundant explanations will be omitted.

[0153] The p-type cladding layer 25 has a surface on which multi-step terraces are formed parallel to the (0001) plane originating from the primary surface 11a of the sapphire substrate 11, similar to the AlN layer 12 of the base portion 10, and the semiconductor layers and electron blocking layer 23 in the n-type cladding layer 21 and active layer 22 of the light-emitting element structure 20, which are epitaxially grown in order from the primary surface 11a of the sapphire substrate 11.

[0154] Fig. 18 shows a schematic diagram of an example of a stacked structure (multiple quantum well structure) of well layers 220 and barrier layers 221 in active layer 22. In Fig. 18, p-type cladding layer 25 is formed on electron block layer 23 having the stacked structure described in the first embodiment with reference to Fig. 5.

[0155] In the p-type cladding layer 25, as described above, an inclined region IA inclined with respect to the (0001) plane is formed between laterally adjacent terraces T. The regions sandwiched between the terraces T on the top and bottom other than the inclined region IA are referred to as terrace regions TA. The film thickness of the p-type cladding layer 25, including the terrace regions TA and the inclined region IA, is adjusted to within the range of 20 nm to 200 nm, for example.

[0156] As shown in FIG. 18, in the p-type cladding layer 25, a Ga-rich p-type region 25a having a lower AlN mole fraction than that of the terrace region TA is formed in the inclined region IA due to mass transfer of Ga from the terrace region TA to the inclined region IA.

[0157] The AlN mole fraction of the terrace region TA of the p-type cladding layer 25 is set to be within a range of 51% or more and less than the AlN mole fraction of the terrace region TA of the electron blocking layer 23. Furthermore, the AlN mole fraction of the Ga-enriched p-type region 25a of the p-type cladding layer 25 is set to be less than the AlN mole fraction of the Ga-enriched EB region 23a of the electron blocking layer 23.

[0158] Furthermore, the AlN molar fraction of the terrace region TA of the p-type cladding layer 25 is set to be at least 1%, preferably at least 2%, and more preferably at least 4% higher than the AlN molar fraction of the Ga-enriched p-type region 25a within the above range. In order to fully ensure the effect of carrier localization in the Ga-enriched p-type region 25a, it is preferable that the difference in AlN molar fraction between the Ga-enriched p-type region 25a of the p-type cladding layer 25 and the terrace region TA is 4 to 5% or more, but the effect of carrier localization can be expected even if it is about 1 to 2%.

[0159] In a preferred embodiment, the Ga-enriched EB region 23a of the electron block layer 23 includes an AlGaN composition ratio of an integer ratio in which the AlN mole fraction is 20% or more higher than the AlN mole fraction in the metastable well region. m Ga 12-m N 12 In the same manner as the metastable EB region composed of p-type metastable AlGaN with an AlN mole fraction of Xes (=m / 12) at m=8, 9, or 10 (m=8, 9, or 10) exists predominantly, the AlGaN composition ratio is an integer ratio of Al i Ga 12-i N 12In this case, a metastable p-type region composed of a p-type metastable AlGaN having an AlN molar fraction of Xps (= i / 12) and less than the AlN molar fraction Xes of the metastable EB region of the electron blocking layer 23 predominantly exists. The integer i is 6, 7, or 8, and satisfies i < m. Therefore, when the integer m is 8, the integer i is 6 or 7.

[0160] Furthermore, as a preferred embodiment, within the Ga-rich p-type region 25a of the p-type cladding layer 25, when a metastable p-type region having an AlN molar fraction of Xps (= i / 12) with the above AlGaN composition ratio (Al i Ga 12-i N 12 , i = 6 to 8) is formed, the average AlN molar fraction Xpa of the p-type cladding layer 25 is preferably set within the range of (Xps + 2%) to (Xps + 7%) based on the AlN molar fraction Xps (= i / 12) of the metastable p-type region. As a result, since the difference in the AlN molar fraction between the Ga-rich p-type region 25a and the terrace region TA of the p-type cladding layer 25 is stably maintained at approximately 2% or more, the effect of carrier localization in the above-described Ga-rich EB region 23a is stably exhibited.

[0161] Next, the method for growing the p-type cladding layer 25 will be briefly described. In forming the p-type cladding layer 25, in the same manner as the n-type cladding layer 21 and the electron blocking layer 23 described in the first embodiment, the p-type cladding layer 25 is grown with the average AlN molar fraction Xpa of the p-type cladding layer 25 as the target value under growth conditions where the above-described multi-step terraces are likely to appear.

[0162] [Alternative Embodiment] Hereinafter, modifications of the first and second embodiments will be described.

[0163] (1) In the above first and second embodiments, multi-step terraces T are formed in the well layers 220 and the barrier layers 221 in the active layer 22 by step-flow growth. However, when the inclination angle of the sapphire substrate used is extremely small, such as from 0° to about 0.3°, hillocks or pits in the shape of hexagonal columns or hexagonal pyramids may be formed on the flat crystal growth surface.

[0164] Therefore, in the above first and second embodiments, the Ga-enriched well region 220a is formed in the inclined region IA of the well layer 220, but in another embodiment in which the crystal growth surface of the well layer 220 is a flat surface, Ga, which is prone to mass transfer, may concentrate around the hillocks, etc., thereby forming the Ga-enriched well region 220a. In this case, the Al-enriched well region is formed by a relative increase in the density of Al in a portion of the terrace region TA other than the Ga-enriched well region 220a, due to the mass transfer of Ga during the formation of the Ga-enriched well region 220a.

[0165] In this other embodiment, as in the first and second embodiments, the Ga-rich well region 220a or the Al-rich well region of the well layer 220 has an AlGaN composition ratio of an integer ratio. n Ga 12-n N 12 Here, the integer n is 3, 4, 5, or 6. Furthermore, the target value Xwt of the AlN mole fraction during epitaxial growth of the well layer 220 is set so as to satisfy the condition represented by the following inequality, as in the first and second embodiments. (n-0.24) / 12≦Xwt≦(n+0.24) / 12

[0166] As for each semiconductor layer other than the well layer 220, like the well layer 220, the crystal growth surface is flat, or even if a multi-step terrace T is formed, its area is large. However, the AlN mole fraction, film thickness, and the like of each other part are the same as those explained in the first and second embodiments, and therefore a duplicated explanation will be omitted.

[0167] (2) In each of the above embodiments, it is assumed that the active layer 22 is configured with a multiple quantum well structure in which two or more well layers 220 made of AlGaN-based semiconductors and one or more barrier layers 221 made of AlGaN-based semiconductors or AlN-based semiconductors are alternately stacked, but the active layer 22 may be configured with a single quantum well structure having only one well layer 220 and without a barrier layer 221 (quantum barrier layer). It is clear that the effects of the well layer 220 employed in each of the above embodiments can be similarly achieved for such a single quantum well structure.

[0168] (3) In the above embodiments, as an example of the growth conditions for n-type cladding layer 21, the supply amounts and flow rates of the source gases and carrier gases used in metal-organic chemical vapor deposition have been described as being set according to the average AlN mole fraction of the entire n-type AlGaN layer constituting n-type cladding layer 21. In other words, it was assumed that when the average AlN mole fraction of the entire n-type cladding layer 21 is set to a constant value in the vertical direction, the supply amounts and flow rates of the source gases and the like are controlled to be constant. However, the supply amounts and flow rates of the source gases and the like do not necessarily have to be controlled to be constant.

[0169] (4) In each of the above embodiments, the first region R1 and the p-electrode 26 are, for example, comb-shaped in plan view. Ta However, the shape in plan view is not limited to a comb shape. Also, there may be a plurality of first regions R1, each of which is surrounded by one second region R2 in plan view.

[0170] (5) In each of the above embodiments, a sapphire substrate 11 having a main surface with an off-angle relative to the (0001) plane is used as the base portion 10 in which multi-stepped terraces are exposed on the surface of the AlN layer 12. However, the magnitude of the off-angle and the direction in which the off-angle is provided (specifically, the direction in which the (0001) plane is tilted, such as the m-axis direction or the a-axis direction) may be determined arbitrarily as long as multi-stepped terraces are exposed on the surface of the AlN layer 12 and a growth starting point for the layered region 21a is formed.

[0171] (5) In each of the above embodiments, the light-emitting element 1 is exemplified as a light-emitting element 1 having a base portion 10 including a sapphire substrate 11 as shown in Fig. 1, but the sapphire substrate 11 (and further, some or all of the layers included in the base portion 10) may be removed by lift-off or the like. Furthermore, the substrate constituting the base portion 10 is not limited to a sapphire substrate. [Industrial Applicability]

[0172] INDUSTRIAL APPLICABILITY The present invention can be used in a nitride semiconductor ultraviolet light emitting device having a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in the vertical direction. [Explanation of symbols]

[0173] 1 ,2 : Nitride semiconductor ultraviolet light emitting device 10: Base 11: Sapphire substrate 11a: Main surface of sapphire substrate 12: AlN layer 20: Light emitting device structure 21: n-type cladding layer (n-type layer) 21a: Layered region (n-type layer) 21b: n-type body region (n-type layer) 22: Active layer 220: Well layer 220a: Ga-rich well region 221: Barrier layer 221a: Ga-enriched barrier region 23: Electron blocking layer (p-type layer) 23a: Ga-enriched EB region 24: p-type contact layer (p-type layer) 25: p-type cladding layer (p-type layer) 25a: Ga-enriched p-type region 26:p electrode 27: n electrode 100: PCB 101: AlGaN-based semiconductor layer 102: Templates 103: n-type AlGaN semiconductor layer 104: Active layer 105: p-type AlGaN-based semiconductor layer 106: p-type contact layer 107: n electrode 108: p electrode BL: Boundary between the first and second areas IA: Slope area P1: EL spectrum peak P2: EL spectrum peak R1: 1st area R2: 2nd area SP1: Shoulder peak in the EL spectrum SP2: Shoulder peak in the EL spectrum T: Terrace TA: Terrace area

Claims

1. A method for manufacturing a nitride semiconductor ultraviolet light emitting device comprising a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in a vertical direction, the method comprising the steps of: A first step of forming the n-type layer of an n-type AlGaN-based semiconductor by epitaxial growth on a base portion including a sapphire substrate; a second step of forming, on the n-type layer, the active layer having a quantum well structure including one or more well layers made of an AlGaN-based semiconductor by epitaxial growth; a third step of forming the p-type layer made of a p-type AlGaN-based semiconductor on the active layer by epitaxial growth; Integer n is 3, 4, 5, or 6; In the second step, a target value Xwt of the AlN mole fraction during epitaxial growth of the well layer is defined as (n-0.24) / 12≦Xwt<n / 12 Set it within the range where forming a Ga-enriched well region having an AlN mole fraction lower than an average AlN mole fraction Xwa of the well layer and an Al-enriched well region having an AlN mole fraction higher than the average AlN mole fraction Xwa in the well layer; The Al-rich well region has an AlGaN composition ratio of an integer ratio of Al ｎ G １２－ｎ N １２ 4. A method for manufacturing a nitride semiconductor ultraviolet light emitting device, comprising: growing a metastable well region of metastable AlGaN having a structure similar to that described above.

2. the sapphire substrate is a vicinal substrate having a main surface inclined at a predetermined angle with respect to the (0001) plane, In the first and second steps, multi-step terraces parallel to a (0001) plane are exposed on the surfaces of the n-type layer and the active layer, In the second step, an inclined region inclined with respect to a (0001) plane connecting adjacent terraces of the multi-step terraces and a terrace region other than the inclined region are formed in each semiconductor layer in the active layer, 2. The method for manufacturing a nitride semiconductor ultraviolet light emitting device according to claim 1, wherein the Ga-rich well region is formed in the inclined region of the well layer, and the Al-rich well region is formed in the terrace region of the well layer.

3. 3. The method for producing a nitride semiconductor ultraviolet light-emitting device according to claim 1 or 2, characterized in that, by forming the metastable well region in the Ga-enriched well region or the Al-enriched well region, a wavelength difference between the first and second smallest minimum extreme points of a second derivative of EL intensity expressed as a function with wavelength as a variable in the EL spectrum of the nitride semiconductor ultraviolet light-emitting device is suppressed to 8 nm or less.

4. 4. The method for manufacturing a nitride semiconductor ultraviolet light-emitting device according to claim 1, wherein in the second step, a film thickness of the Ga-enriched well region or the Al-enriched well region in which the metastable well region is formed is controlled to be an integral multiple of a monoatomic layer and within a range of 4 to 12 monoatomic layers.

5. In the second step, the AlGaN composition ratio in the Ga-rich well region is an integer ratio of Al ｎ－１ G １３－ｎ N １２ The growth of a metastable well region made of metastable AlGaN having an Al composition ratio of an integer ratio is suppressed in the Al-rich well region. ｎ＋１ G １１－ｎ N １２ 5. The method for producing a nitride semiconductor ultraviolet light emitting device according to claim 1, wherein the growth of a metastable well region made of metastable AlGaN having a structure similar to that of the nitride semiconductor light emitting device is suppressed.

6. 6. The method for manufacturing a nitride semiconductor ultraviolet light-emitting device according to claim 1, wherein, in a plurality of chips on the same wafer, in the second step, a first type chip in which the metastable well region grows in the Ga-enriched well region and a second type chip in which the metastable well region grows in the Al-enriched well region are mixed.

7. 7. The method for producing a nitride semiconductor ultraviolet light-emitting device according to claim 1, wherein in the second step, the well layers made of an AlGaN-based semiconductor and barrier layers made of an AlGaN-based semiconductor are alternately stacked by epitaxial growth to form the active layer having a multiple quantum well structure including two or more well layers.

8. A nitride semiconductor ultraviolet light emitting device comprising a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in a vertical direction, the n-type layer is composed of an epitaxially grown layer of an n-type AlGaN-based semiconductor, the active layer disposed between the n-type layer and the p-type layer has a quantum well structure including one or more well layers formed of an epitaxially grown layer of an AlGaN-based semiconductor; the p-type layer is composed of an epitaxially grown layer of a p-type AlGaN-based semiconductor, a Ga-enriched well region having an AlN mole fraction lower than an average AlN mole fraction Xwa of the well layer, and an Al-enriched well region having an AlN mole fraction higher than the average AlN mole fraction Xwa, The integer n is 3, 4, 5, or 6, and the Al-rich well region has an AlGaN composition ratio of an integer ratio of Al ｎ G １２－ｎ N １２ A metastable well region is formed, which is metastable AlGaN having the structure: In the EL spectrum, the wavelength difference between the first and second minimum extreme points of the second derivative of the EL intensity expressed as a function of wavelength is 8 nm or less; The average AlN mole fraction X of the well layer is (n-0.24) / 12≦Xwt<n / 12 The nitride semiconductor ultraviolet light emitting device according to the present invention is characterized in that the above-mentioned range is satisfied.

9. each of the semiconductor layers in the n-type layer, the active layer, and the p-type layer is an epitaxially grown layer having a surface on which multiple terraces parallel to a (0001) plane are formed; each semiconductor layer in the active layer has an inclined region inclined with respect to a (0001) plane connecting adjacent terraces of the multi-stepped terraces, and a terrace region other than the inclined region; 9. The nitride semiconductor ultraviolet light emitting device according to claim 8, wherein the Ga-rich well region is present in the inclined region of the well layer, and the Al-rich well region is present in the terrace region of the well layer.

10. 10. The nitride semiconductor ultraviolet light-emitting device according to claim 8, wherein the thickness of the Ga-enriched well region or the Al-enriched well region in which the metastable well region is formed is an integral multiple of a monoatomic layer and is in the range of 4 to 12 monoatomic layers.

11. the active layer has a multiple quantum well structure including two or more of the well layers, 11. The nitride semiconductor ultraviolet light emitting device according to claim 8, further comprising a barrier layer made of an AlGaN-based semiconductor present between the two well layers.

12. Further comprising a base portion including a sapphire substrate; the sapphire substrate has a main surface inclined at a predetermined angle with respect to a (0001) plane, and the light emitting element structure is formed above the main surface; The nitride semiconductor ultraviolet light emitting device according to any one of claims 8 to 11, characterized in that each semiconductor layer from the main surface of the sapphire substrate to the p-type layer is an epitaxially grown layer having a surface on which multiple stepped terraces parallel to the (0001) plane are formed.

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