Nitride semiconductor ultraviolet light-emitting element

Abstract

The nitride semiconductor ultraviolet light-emitting device has a light-emitting device structure portion in which an n-type layer, an active layer, and a p-type layer including a wurtzite-structured AlGaN-based semiconductor are stacked in the vertical direction. The n-type layer is made of an n-type AlGaN-based semiconductor, the active layer includes one or more well layers made of an AlGaN-based semiconductor, the p-type layer is made of a p-type AlGaN-based semiconductor, and each semiconductor layer in the n-type layer and the active layer is an epitaxial growth layer having a surface formed with a multi-step terrace parallel to the (0001) plane. The n-type layer has a plurality of first Ga-rich regions, which are layered regions where the local AlN mole fraction is low and are evenly dispersed in the n-type layer and contain Al7Ga5N with an integer ratio of the AlGaN composition ratio. 12 The n-type AlGaN region has portions where the extending directions of the layered regions are inclined with respect to the upper surface of the n-type layer.

Description

[0001] Technical field

[0002] The present invention relates to a nitride semiconductor ultraviolet light-emitting device having a light-emitting element structure portion in which an n-type layer, an active layer, and a p-type layer of an AlGaN-based semiconductor having a wurtzite structure are stacked in the vertical direction, and a peak emission wavelength in the range of 265 nm to 300 nm. Background art

[0003] Generally, a nitride semiconductor light-emitting device is formed by epitaxial growth on a substrate such as sapphire to form a light-emitting element structure including a plurality of nitride semiconductor layers. The nitride semiconductor layer is represented by the general formula Al 1-x-y Ga x In y N (0≤x≤1, 0≤y≤1, 0≤x + y≤1).

[0004] The light-emitting element structure of a light-emitting diode has a double heterostructure in which an active layer composed of a nitride semiconductor layer is sandwiched between two cladding layers of an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. When the active layer is an AlGaN-based semiconductor, by adjusting the AlN mole fraction (also referred to as the Al composition ratio), the bandgap energy can be adjusted within the range where the bandgap energies that GaN and AlN can take (about 3.4 eV and about 6.2 eV) are set as the lower limit and the upper limit, respectively, to obtain an ultraviolet light-emitting device having an emission wavelength from about 200 nm to about 365 nm. Specifically, by flowing a forward current from the p-type nitride semiconductor layer toward the n-type nitride semiconductor layer, light emission corresponding to the above bandgap energy generated by the recombination of carriers (electrons and holes) is generated in the active layer. By supplying this 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.

[0005] 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 an AlGaN-based semiconductor having an AlN mole fraction higher than that of the active layer. However, it is difficult to form a good ohmic contact between the p-type nitride semiconductor layer having a high AlN mole fraction and the p electrode. Therefore, on the uppermost layer of the p-type nitride semiconductor layer, a p-type contact layer that can form a good ohmic contact with the p electrode including a p-type AlGaN-based semiconductor having a low AlN mole fraction (specifically, p-GaN) is generally formed. Since the AlN mole fraction of this p-type contact layer is smaller than that of the AlGaN-based semiconductor constituting the active layer, the ultraviolet light emitted from the active layer toward the p-type nitride semiconductor layer side is absorbed in this p-type contact layer and cannot be effectively extracted to the outside of the device. For this reason, a general ultraviolet light-emitting diode having an active layer of an AlGaN-based semiconductor adopts Figure 15The element structure schematically shown 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, refer to Patent Documents 1 and 2 below, etc.).

[0006] As Figure 15 shown, a general ultraviolet light-emitting diode is configured such that on a template 102 formed by stacking an AlGaN-based semiconductor layer 101 (for example, an AlN layer) on a substrate 100 such as a sapphire substrate, an n-type AlGaN-based semiconductor layer 103, an active layer 104, a p-type AlGaN-based semiconductor layer 105, and a p-type contact layer 106 are sequentially stacked, and a part of the active layer 104, the p-type AlGaN-based semiconductor layer 105, and the p-type contact layer 106 is etched away until the n-type AlGaN-based semiconductor layer 103 is exposed. An n electrode 107 is formed on the exposed surface of the n-type AlGaN-based semiconductor layer 103, and a p electrode 108 is formed on the surface of the p-type contact layer 106, respectively.

[0007] In addition, in order to improve the light emission efficiency (internal quantum efficiency) based on carrier recombination in the active layer, the active layer is made into a multi-quantum well structure, and an electron blocking layer is provided on the active layer, etc.

[0008] On the other hand, it has been reported that compositional modulation caused by Ga segregation occurs in the cladding layer composed of an n-type AlGaN-based semiconductor layer, and a layered region with a low AlN mole fraction is locally formed extending obliquely with respect to the surface of the cladding layer (for example, refer to Patent Document 3, Non-Patent Documents 1 and 2 below, etc.). Since the bandgap energy of the AlGaN-based semiconductor layer with a low local AlN mole fraction also locally becomes smaller, it has been reported in Patent Document 3 that carriers in this cladding layer tend to be locally distributed in the layered region, and a low-resistance current path can be provided with respect to the active layer, and an improvement in the light emission efficiency of the ultraviolet light-emitting diode can be achieved.

[0009] [Prior Art Documents]

[0010] [Patent Documents]

[0011] [Patent Document 1] International Publication No. 2014 / 178288

[0012] [Patent Document 2] International Publication No. 2016 / 157518

[0013] [Patent Document 3] International Publication No. 2019 / 159265

[0014] [Non-Patent Documents]

[0015] [Non-Patent Document 1] Y. Nagasawa, et al., "Comparison of Al xGa 1-x N multiple quantum wells designed for 265 and 285nm deep-ultraviolet LEDs grown on AlN templates having macrosteps″,Applied Physics Express 12,064009(2019)

[0016] [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

[0017] -Technical problems to be solved by the invention-

[0018] An ultraviolet light-emitting device composed of an AlGaN-based semiconductor is fabricated on a substrate such as a sapphire substrate by a known epitaxial growth method such as metalorganic chemical vapor deposition (MOVPE). However, when producing an ultraviolet light-emitting device, the characteristics of the ultraviolet light-emitting device (characteristics such as emission wavelength, wall plug efficiency, forward bias, etc.) vary due to the drift of the crystal growth apparatus, and thus it is not always easy to produce with a stable yield.

[0019] The drift of the crystal growth apparatus is caused by changes in the effective temperature of the crystal growth site due to attachments such as trays or chamber walls. Therefore, in order to suppress drift, the growth history has been studied in the past. Although experienced persons have made efforts such as subtly changing the set temperature or the composition of the source gas, or fixing the growth process for a certain period, and performing maintenance such as cleaning at the same time for a certain period, it is still difficult to completely eliminate drift.

[0020] The present invention has been completed in view of the above problems, and an object thereof is to provide a nitride semiconductor ultraviolet light-emitting device in which characteristic variations caused by drift of a crystal growth apparatus and the like are suppressed and stable production can be achieved.

[0021] -Means for solving the technical problems-

[0022] In order to achieve the above object, the present invention provides a nitride semiconductor ultraviolet light-emitting element, which is configured to have a light-emitting element structure portion in which an n-type layer, an active layer, and a p-type layer including a wurtzite-structured AlGaN-based semiconductor are stacked in the vertical direction, and the peak emission wavelength is in the range of 265 nm to 300 nm. Its first feature is that

[0023] the n-type layer is composed of an n-type AlGaN-based semiconductor,

[0024] 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 composed of an AlGaN-based semiconductor.

[0025] the p-type layer is composed of a p-type AlGaN-based semiconductor,

[0026] each semiconductor layer in the n-type layer and the active layer is an epitaxial growth layer having a surface formed with a multi-step platform parallel to the (0001) plane,

[0027] the n-type layer has a plurality of first Ga-rich regions, and the first Ga-rich regions are layered regions where the AlN mole fraction is locally low and evenly dispersed in the n-type layer and include an n-type AlGaN region of Al7Ga5N having an integer ratio of AlGaN composition, 12 of the n-type AlGaN region,

[0028] having a portion where each extending direction of the layered region on the first plane orthogonal to the upper surface of the n-type layer is inclined with respect to the intersection line of the upper surface of the n-type layer and the first plane.

[0029] Furthermore, although the AlGaN-based semiconductor is represented by the general formula Al 1-x Ga x N (0 ≤ x ≤ 1), impurities such as group 3 elements such as B or In or group 5 elements such as P may be contained in trace amounts as long as the bandgap energy is within the range where the bandgap energies of GaN and AlN can be taken as the lower limit and the upper limit, respectively. In addition, the GaN-based semiconductor is a nitride semiconductor mainly composed of Ga and N, and impurities such as group 3 elements such as Al, B or In or group 5 elements such as P may also be contained in trace amounts. In addition, the AlN-based semiconductor is a nitride semiconductor mainly composed of Al and N, and impurities such as group 3 elements such as Ga, B or In or group 5 elements such as P may also be contained in trace amounts. Therefore, in this application, the GaN-based semiconductor and the AlN-based semiconductor are each a part of the AlGaN-based semiconductor.

[0030] In addition, the n-type or p-type AlGaN-based semiconductor is an AlGaN-based semiconductor doped with Si, Mg, etc. as donor or acceptor impurities. The AlGaN-based semiconductor not specified as p-type and n-type in this application refers to an undoped AlGaN-based semiconductor. However, even if it is undoped, it may contain a trace amount of donor or acceptor impurities inevitably mixed in. In addition, the first plane is not an exposed surface specifically formed during the manufacturing process of the n-type layer or an interface with other semiconductor layers, but an imaginary plane extending parallel to the vertical direction within the n-type layer. In addition, 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.

[0031] The nitride semiconductor ultraviolet light-emitting element according to the first feature described above, as described below, suppresses characteristic variations such as drift caused by the crystal growth apparatus by using the quasi-stable AlGaN described later in the first Ga-rich region formed in the n-type layer, and it is expected that a nitride semiconductor ultraviolet light-emitting element having desired light-emitting characteristics can be stably produced.

[0032] First, the "quasi-stable AlGaN" represented by a given integer ratio of the AlGaN composition ratio will be described.

[0033] Generally, a ternary mixed crystal such as AlGaN is a crystal state in which group 3 elements (Al and Ga) are randomly mixed, and it is described as being approximately "random nonuniformity". However, since the covalent bond radius of Al is different from that of Ga, in the crystal structure, the one with higher symmetry in the atomic arrangement of Al and Ga generally becomes a stable structure.

[0034] There are two arrangements of the wurtzite structure AlGaN-based semiconductor: a random arrangement without symmetry and a stable symmetric arrangement. Here, a state in which the symmetric arrangement appears at a certain ratio and becomes dominant is present. In the "quasi-stable AlGaN" represented by a given integer ratio of the AlGaN composition ratio (the composition ratio of Al, Ga, and N) described later, a periodic symmetric arrangement structure of Al and Ga is found.

[0035] In this periodically symmetric arrangement structure, by slightly increasing the supply amount of Ga to the crystal growth surface, due to the high symmetry, a composition ratio of mixed crystals that is somewhat stable in terms of energy is formed, which can prevent the proliferation of regions where Ga, which is prone to mass transfer, extremely increases. That is, by utilizing the property of "quasi-stable AlGaN" formed in the first Ga-rich region within the n-type layer, as an AlGaN-based semiconductor, even if there are variations in the composition ratio of mixed crystals caused by drift or the like in the crystal growth apparatus, as described later, it is still possible to locally suppress the variation in the composition ratio of mixed crystals in the layered region that provides a low-resistance current path to the active layer. As a result, stable carrier supply from the n-type layer to the active layer can be achieved, suppressing variations in device characteristics, and thus it is expected that nitride semiconductor ultraviolet light-emitting elements with desired characteristics can be stably produced.

[0036] Next, the composition ratio of AlGaN in which Al and Ga are periodically symmetrically arranged within the (0001) plane will be described.

[0037] In Figure 1 is a schematic diagram showing one unit cell (two monolayers) in the c-axis direction of AlGaN. In Figure 1 the white circles indicate the positions where the atoms of group 3 elements (Al, Ga) are located, and the black circles indicate the positions where the atoms of group 5 elements (N) are located.

[0038] In Figure 1 the plane faces of group 3 elements (A3 plane, B3 plane) and the plane faces of group 5 elements (A5 plane, B5 plane) shown as hexagons are all parallel to the (0001) plane. For each part of the A3 plane and the A5 plane (collectively referred to as the A plane), there are six places at each vertex of the hexagon and one place at the center of the hexagon. The same applies to the B3 plane and the B5 plane (collectively referred to as the B plane). In Figure 1 only three parts within the hexagon existing in the B plane are shown. Each part of the A plane overlaps in the c-axis direction, and each part of the B plane overlaps in the c-axis direction. However, one atom (N) in one part of B5 forms four coordination bonds with three atoms (Al, Ga) in three parts of the A3 plane located above the B5 plane and one atom (Al, Ga) in one part of the B3 plane located below the B5 plane. One atom (Al, Ga) in one part of the B3 plane forms four coordination bonds with one atom (N) in one part of the B5 plane located above the B3 plane and three atoms (N) in three parts of the A5 plane located below the B3 plane. Therefore, as shown in Figure 1 each part of the A plane and each part of the B plane do not overlap in the c-axis direction.

[0039] Figure 2The positional relationship between the respective parts of surface A and the respective parts of surface B is illustrated as a plan view observed from the c-axis direction. For both surface A and surface B, the 6 vertices of the hexagon are shared by 2 adjacent other hexagons, and the central part is not shared with other hexagons. Therefore, within one hexagon, there are substantially 3 atomic parts. Thus, in each unit cell, there are 6 parts of atoms of group 3 elements (Al, Ga) and 6 parts of atoms of group 5 elements (N). Therefore, as the AlGaN composition ratios expressed as integer ratios other than GaN and AlN, there are the following 5 cases.

[0040] 1) Al1Ga5N6,

[0041] 2) Al2Ga4N6 (= Al1Ga2N3),

[0042] 3) Al3Ga3N6 (= Al1Ga1N2),

[0043] 4) Al4Ga2N6 (= Al2Ga1N3),

[0044] 5) Al5Ga1N6.

[0045] In Figure 3 , the A3 surface and the B3 surface of the above 5 combinations of group 3 elements are schematically shown. Ga is shown as a black circle, and Al is shown as a white circle.

[0046] For Figure 3 's (A) shown Al1Ga5N6, Ga is arranged at the 6 vertex parts of the A3 surface, the 6 vertex parts of the B3 surface, and 1 central part, and Al is arranged at 1 central part of the A3 surface.

[0047] For Figure 3 's (B) shown Al1Ga2N3, Ga is arranged at the 3 vertex parts and 1 central part of the A3 surface and the B3 surface, and Al is arranged at the 3 vertex parts of the A3 surface and the B3 surface.

[0048] For Figure 3 's (C) shown Al1Ga1N2, Ga is arranged at the 3 vertex parts and 1 central part of the A3 surface and the 3 vertex parts of the B3 surface, and Al is arranged at the 3 vertex parts of the A3 surface, the 3 vertex parts of the B3 surface, and 1 central part.

[0049] For Figure 3 's (D) shown Al2Ga1N3, Ga is arranged at the 3 vertex parts of the A3 surface and the B3 surface, and Al is arranged at the 3 vertex parts and 1 central part of the A3 surface and the B3 surface. It is equivalent to replacing the parts of Al and Ga when Figure 3 's (B) shown Al1Ga2N3.

[0050] When Figure 3 it is Al5Ga1N6 as shown in (E), Ga is arranged at one central part of the A3 plane, and Al is arranged at six vertex parts of the A3 plane, six vertex parts of the B3 plane, and one central part. This is equivalent to replacing the positions of Al and Ga in Al1Ga5N6 as shown in (A). Figure 3 of Al and Ga in Al1Ga5N6 as shown in (A).

[0051] In Figure 3 each of the figures (A) to (E), it can be seen that if it is assumed that the center moves to another hexagon at any one of the six vertices of the hexagon, it is equivalent to Al or Ga being at six vertex parts of the A3 plane, and Al or Ga being at three vertex parts and one central part of the A3 plane. Al or Ga being at one central part of the A3 plane is equivalent to Al or Ga being at three vertex parts of the A3 plane. The same applies to the B3 plane. In addition, in Figure 3 each of the figures (A), (C), and (E), the A3 plane and the B3 plane can also be replaced.

[0052] In Figure 3 each of the figures (A) to (E), regardless of whether it is the A3 plane or the B3 plane, the atomic arrangement of Al and Ga can maintain symmetry. In addition, even if the center of the hexagon is moved, the atomic arrangement of Al and Ga can still maintain symmetry.

[0053] In addition Figure 3 on the A3 plane and the B3 plane of each of the figures (A) to (E), if the hexagonal planes are repeatedly arranged in a honeycomb shape, when observing each part in the direction parallel to the (0001) plane, for example, in the [11-20] direction and the [10-10] direction, a state where Al and Ga are periodically repeated in position, or only one of Al and Ga is continuously in position appears. Therefore, it all becomes a periodically symmetric atomic arrangement.

[0054] Here, for the sake of convenience of explanation, Al x1 Ga 1-x1 N corresponding to the AlGaN composition ratios of the above 1) to 5), with the AlN mole fraction x1 (x1 = 1 / 6, 1 / 3, 1 / 2, 2 / 3, 5 / 6), is called "the first metastable AlGaN". The first metastable AlGaN has an atomic arrangement of Al and Ga that becomes a periodically symmetric arrangement and is an energetically stable AlGaN.

[0055] Next Figure 1When the plane shown by the hexagon extends over 2 unit cells (4 monolayers), there are 2 planes of group 3 elements (A3 plane, B3 plane) and 2 planes of group 5 elements (A5 plane, B5 plane). In every 2 unit cells, there are 12 sites of group 3 elements' atoms (Al, Ga) and 12 sites of group 5 elements' atoms (N). Therefore, as the AlGaN composition ratios expressed as integer ratios other than GaN and AlN, in addition to the AlGaN composition ratios of the above 1) - 5), there are the following 6 combinations.

[0056] 6) Al1Ga 11 N 12 (= GaN + Al1Ga5N6),

[0057] 7) Al3Ga9N 12 (= Al1Ga3N4 = Al1Ga5N6 + Al1Ga2N3),

[0058] 8) Al5Ga7N 12 (= Al1Ga2N3 + Al1Ga1N2),

[0059] 9) Al7Ga5N 12 (= Al1Ga1N2 + Al2Ga1N3),

[0060] 10) Al9Ga3N 12 (= Al3Ga1N4 = Al2Ga1N3 + Al5Ga1N6),

[0061] 11) Al 11 Ga1N 12 (= Al5Ga1N6 + AlN).

[0062] However, since these 6 AlGaN composition ratios of 6) - 11) are formed by combining 2 AlGaN composition ratios within the first metastable AlGaN, GaN, and AlN before and after them, the possibility of symmetry disorder in the c-axis direction is high. Therefore, although the stability is lower than that of the first metastable AlGaN, the symmetry of the atomic arrangement of Al and Ga within the A3 plane and B3 plane is the same as that of the first metastable AlGaN, and the stability is higher than that of the AlGaN in a randomly asymmetric arrangement state. Here, for the sake of convenient explanation, the Al corresponding to the AlN mole fraction x2 (x2 = 1 / 12, 1 / 4, 5 / 12, 7 / 12, 3 / 4, 11 / 12) of the AlGaN composition ratios of the above 6) - 11) x2 Ga1 -x2N is referred to as "the second metastable AlGaN". Accordingly, the first and second metastable AlGaNs are stable structures due to the symmetry of the atomic arrangements of Al and Ga in the crystal structure. Hereinafter, the first and second metastable AlGaNs are collectively referred to as "metastable AlGaN".

[0063] To grow AlGaN while maintaining a certain crystal quality, crystal growth needs to be carried out at a high temperature of 1000 °C or higher. However, it is assumed that Ga also moves around at a temperature of 1000 °C or higher after reaching the crystal surface. On the other hand, different from Ga, Al is easily adsorbed on the surface, and although its movement after entering the site is somewhat mobile, it is strongly restricted.

[0064] Therefore, even for metastable AlGaN, the AlN mole fractions of Al1Ga5N6 in the above (1), Al1Ga 11 N 12 in the above (6), and Al1Ga3N4 in the above (7) are all 25% or less. Due to the high Ga composition ratio, at a growth temperature near 1000 °C, the movement of Ga is intense, the symmetry of the atomic arrangement is disordered, the atomic arrangements of Al and Ga approach a random state, and the above-mentioned stability is lower than that of other metastable AlGaNs.

[0065] Next, the "first Ga-rich region" will be described. In the nitride semiconductor ultraviolet light-emitting device with the above characteristics, since each semiconductor layer in the n-type layer and the active layer is an epitaxial growth layer having a surface forming a multi-step platform parallel to the (0001) plane, in the n-type layer, Ga, which is easy to move in terms of mass, moves on the platform region and concentrates in the boundary region between adjacent platforms, forming a region with a lower AlN mole fraction than the platform region. This boundary region extends obliquely upward with respect to the (0001) plane along with the epitaxial growth of the n-type AlGaN layer in the n-type layer, and a layered region with a locally low AlN mole fraction is formed dispersedly on average in the n-type layer. Here, the metastable AlGaN with an AlGaN composition ratio of Al7Ga5N 12 belongs to the above-mentioned "second metastable AlGaN", so its stability is lower than that of the metastable AlGaN with an AlGaN composition ratio of Al1Ga1N2 or Al2Ga1N3, which is the "first metastable AlGaN" before and after it. However, in recent studies, it has been clearly judged that as long as the mass movement amount of Ga does not become excessively large, this layered region becomes the first Ga-rich region of the n-type AlGaN region containing the metastable AlGaN with an AlGaN composition ratio of Al7Ga5N 12 .

[0066] In the first Ga-rich region, due to the presence of the metastable AlGaN with an AlGaN composition ratio of Al7Ga5N 12For metastable AlGaN, fluctuations in the supply amount of Ga within the first Ga-rich region are absorbed by the metastable AlGaN. That is, within the first Ga-rich region, when the supply amount of Ga is increased, the metastable AlGaN increases, and when the supply amount of Ga is decreased, the metastable AlGaN decreases. As a result, fluctuations in the AlN mole fraction within the first Ga-rich region are suppressed. Therefore, within the first Ga-rich region, fluctuations in the supply amount of Ga due to drift of the crystal growth apparatus, etc. are absorbed, and an AlGaN with a composition ratio of Al7Ga5N (AlN mole fraction is 58.3% (seven twelfths)) is stably formed. 12 (AlN mole fraction is 58.3% (seven twelfths)) of metastable AlGaN. That is, with respect to fluctuations in the supply amount of Ga, fluctuations in the AlN mole fraction within the first Ga-rich region can be suppressed. In this specification, when the AlN mole fraction of seven twelfths is appropriately expressed as a percentage, it is approximately expressed as 58.3%.

[0067] Among them, as described above, in the crystal growth of AlGaN, since a state of randomly asymmetric arrangement and a state of regular symmetric arrangement can usually coexist, within the first Ga-rich region, a region of metastable AlGaN with an AlN mole fraction of 58.3% in the state of regular symmetric arrangement is stably formed, and at the same time, regions where the AlN mole fraction varies slightly (for example, by about 0 to 3%) from 58.3% coexist. Therefore, the AlN mole fraction within the first Ga-rich region is concentrated around the AlN mole fraction (58.3%) of metastable AlGaN with an AlGaN composition ratio of Al7Ga5N. 12 Near the AlN mole fraction (58.3%) of metastable AlGaN.

[0068] By stably forming the first Ga-rich region, which is a layered region with a locally low AlN mole fraction within the n-type layer, carriers within the n-type layer are locally distributed within the first Ga-rich region with a small bandgap energy within the n-type layer. Within the n-type layer, current preferentially and stably flows into the first Ga-rich region, suppressing fluctuations in the characteristics of the nitride semiconductor ultraviolet light-emitting element.

[0069] Furthermore, since fluctuations in the supply amount of Ga due to drift of the crystal growth apparatus, etc. are absorbed within the first Ga-rich region, the lower limit of the fluctuation range of the AlN mole fraction within the n-type layer is restricted near the AlN mole fraction (58.3%) of metastable AlGaN. That is, since the formation of a region where the AlN mole fraction within the n-type layer is smaller than near 58.3% is suppressed, a part of the light emitted from the well layer is absorbed in this region, preventing a decrease in luminous efficiency.

[0070] Furthermore, in the nitride semiconductor ultraviolet light-emitting element having the above first feature, it is preferable that the AlN mole fraction in the n-type main region other than the above layered region within the n-type layer is in the range of 60% to 66%.

[0071] According to the above preferred embodiment, since the AlN mole fraction in the n-type main region is in the range of 60% to 66%, the difference in the AlN mole fraction between the first Ga-rich region and the n-type main region is stably ensured to be about 1.7% or more. Therefore, carriers in the n-type layer are more stably locally distributed in the first Ga-rich region where the bandgap energy is smaller than that of the n-type main region. In the n-type layer, current preferentially and stably flows into the first Ga-rich region, achieving suppression of characteristic variations of the nitride semiconductor ultraviolet light-emitting element. From recent research, it is known that local distribution of carriers can be stably achieved when there is a difference in AlN mole fraction of 1.5% to 2% or more. Among them, due to the mass transfer of Ga from the n-type main region to the layered region including the first Ga-rich region, the AlN mole fraction in the n-type main region is not constant but varies, and after the mass transfer of Ga occurs, the AlN mole fraction may locally increase. Therefore, in the above preferred embodiment, in the n-type main region, a part of the region where the AlN mole fraction slightly exceeds 66% locally is allowed to form. That is, when the AlN mole fraction in the n-type main region is in the range of 60% to 66%, it means that in the n-type main region, there is a region where the AlN mole fraction is in the range of 60% to 66% that dominates. In addition, as will be described later, the AlN mole fraction in the range of 60% to 66% in the n-type main region can be captured as the average AlN mole fraction in the n-type main region or the target value of the AlN mole fraction during the growth of the n-type layer.

[0072] In addition, since the upper limit of the AlN mole fraction in the n-type main region of the n-type layer is specified at 66%, the AlN mole fraction will not locally decrease due to the mass transfer of Ga. In the n-type layer, a metastable AlGaN with an AlGaN composition ratio of Al2Ga 11 N3 is dominated to form. Assuming that the upper limit is 67% or more, due to the local decrease in the AlN mole fraction, a metastable AlGaN of Al2Ga1N3 can be stably formed in the n-type layer. As a result, it is difficult to sufficiently supply Ga for stably forming a metastable AlGaN of Al7Ga5N 12 from this metastable AlGaN of Al2Ga1N3 into the first Ga-rich region, and the AlN mole fraction of the n-type AlGaN-based semiconductor formed in the first Ga-rich region randomly varies, and the desired effect cannot be obtained.

[0073] Furthermore, the present invention provides a nitride semiconductor ultraviolet light-emitting element having, in addition to the above-described first feature, a second feature, wherein a boundary region portion between adjacent platforms of the multi-step platforms of the well layer has a second Ga-rich region where the AlN mole fraction is also locally low within the well layer, and within the second Ga-rich region, there exists an AlGaN region where the AlGaN composition ratio is an integer ratio, such as Al1Ga1N2 or Al5Ga7N. 12 of the AlGaN region.

[0074] According to the nitride semiconductor ultraviolet light-emitting element of the above-described second feature, as described below, by utilizing the metastable AlGaN formed in the second Ga-rich region within the well layer, characteristic variations caused by drift of the crystal growth apparatus and the like can be further suppressed, and it is expected that a nitride semiconductor ultraviolet light-emitting element having desired light-emitting characteristics can be produced more stably.

[0075] Hereinafter, the "well layer" and the "second Ga-rich region" will be described. Since the semiconductor layers in the n-type layer and the active layer have an epitaxial growth layer forming a multi-step platform surface parallel to the (0001) plane, the boundary region between adjacent platforms of the multi-step platform of the well layer becomes an inclined region inclined with respect to the (0001) plane connecting adjacent platforms (see Non-Patent Documents 1 and 2 above). However, this inclined region is a structure that aggregates multiple steps (step difference of one unit cell) and giant steps (step difference of multiple unit cells), and it is possible to distinguish the stepped (0001) plane and the platform surface of the multi-step platform exposed in the inclined region.

[0076] Due to the lateral growth of the step flow growth toward the side surface of the platform edge, the platform on the upper surface of the well layer moves laterally with respect to the platform on the lower surface of the well layer. Therefore, the film thickness of the inclined region of the well layer is thicker than the film thickness of the platform region other than the inclined region. In addition, similar to the formation of the first Ga-rich region where the AlN mole fraction is locally low in the n-type layer, a second Ga-rich region where the AlN mole fraction is locally low is formed in the inclined region of the well layer. Here, when the mass transfer amount of Ga is large enough, an AlGaN region of metastable AlGaN having an AlGaN composition ratio of Al1Ga1N2 or Al5Ga7N 12 or Al1Ga2N3 is formed.

[0077] That is, as a suitable combination of metastable AlGaN having an AlN mole fraction of 58.3% stably formed in the first Ga-rich region and metastable AlGaN formed in the second Ga-rich region having a peak emission wavelength in the range of 265 nm to 300 nm, the AlGaN composition ratio includes Al1Ga1N2, Al5Ga7N 12Or three metastable AlGaNs of Al1Ga1N2, Al5Ga7N, or Al1Ga2N3. That is, according to the target value of the peak emission wavelength, any one of the above three metastable AlGaNs can be selected. The ranges of the peak emission wavelengths that each of these three metastable AlGaNs can take will be described later.

[0078] In the second Ga-rich region, due to the presence of metastable AlGaN with an AlGaN composition ratio of Al1Ga1N2 or Al5Ga7N 12 or Al1Ga2N3, the variation in the Ga supply amount to the second Ga-rich region is absorbed by this metastable AlGaN. That is, in the second Ga-rich region, when the Ga supply amount is increased, the metastable AlGaN increases, and when the Ga supply amount is decreased, the metastable AlGaN decreases. As a result, the variation in the AlN mole fraction in the second Ga-rich region is suppressed. Therefore, in the second Ga-rich region, the variation in the Ga supply amount due to drift or the like of the crystal growth apparatus is absorbed, and metastable AlGaN with an AlGaN composition ratio of Al1Ga1N2 (AlN mole fraction of 50% (one-half)) or Al5Ga7N 12 (AlN mole fraction of 41.7% (five-twelfths)) or Al1Ga2N3 (AlN mole fraction of 33.3% (one-third)) is stably formed. That is, for the variation in the Ga supply amount, the variation in the AlN mole fraction in the second Ga-rich region is suppressed. In this specification, appropriately, when the AlN mole fraction of five-twelfths is expressed as a percentage, it is approximately expressed as 41.7%, and when the AlN mole fraction of one-third is expressed as a percentage, it is approximately expressed as 33.3%.

[0079] Among them, as described above, in the crystal growth of AlGaN, since a random asymmetric arrangement state and a regular symmetric arrangement state can usually coexist, in the second Ga-rich region, a region of metastable AlGaN with an AlN mole fraction of 50% or 41.7% or 33.3% in the regular symmetric arrangement state is stably formed, and at the same time, a region where the AlN mole fraction varies slightly (for example, by about 0 to 3%) from 50% or 41.7% or 33.3% coexists.

[0080] As described above, the bandgap energy of this inclined region is smaller than that of the plateau region, and like the first Ga-rich region of the n-type layer, local distribution of carriers is likely to occur. For this reason, the luminescence of the well layer becomes more significant in this inclined region than in the plateau region. In the above Non-Patent Documents 1 and 2, the same content for the well layer of the AlGaN-based semiconductor is reported. However, each plateau region of the well layer and the barrier layer is a region clamped between the upper surface plateau and the lower surface plateau of each layer in the c-axis direction. Therefore, regions other than each plateau region of the well layer and the barrier layer become the boundary regions (inclined regions) of each layer.

[0081] In addition, a multi-step platform formed by epitaxial growth of the active layer is continuously formed with a multi-step platform formed by epitaxial growth of the n-type layer. Therefore, carriers (electrons) supplied to the well layer along the current path in the first Ga-rich region are concentrated and supplied to the boundary region (tilted region) between adjacent platforms where light emission in the well layer is concentrated.

[0082] Therefore, in the first Ga-rich region predominantly present in the layered region of the n-type layer, a metastable AlGaN with an AlN mole fraction of 58.3%, i.e., an n-type AlGaN region, is stably formed. In addition, in the second Ga-rich region in the tilted region of the well layer, a metastable AlGaN with an AlN mole fraction of 50% or 41.7% or 33.3%, i.e., an AlGaN region, is stably formed, enabling stable carrier supply to the tilted region of the well layer and suppressing characteristic variations of the nitride semiconductor ultraviolet light-emitting element.

[0083] In addition, since the AlN mole fraction of the n-type layer is 58.3% or more and the ultraviolet light emitted from the well layer of the active layer passes through the n-type layer, a device structure for extracting ultraviolet light emission from the n-type layer side can be obtained.

[0084] Furthermore, the nitride semiconductor ultraviolet light-emitting element of the above second feature is preferably such that in the above second Ga-rich region, there is an AlGaN region of Al1Ga1N2 with an integer ratio of AlGaN composition ratio, and the AlN mole fraction outside the above boundary region portion of the above well layer is in the range of 50.1% to 54%.

[0085] By the above embodiment, the variation width of the AlN mole fraction in the well layer is suppressed within 4%. Moreover, even when the emission peaks caused by composition modulation from regions outside the AlN mole fraction of 50% overlap, a single-peak quantum well can be virtually formed, thus avoiding the separation of the emission peaks of the emission spectrum.

[0086] Even more, the nitride semiconductor ultraviolet light-emitting element of the above second feature is preferably such that in the above second Ga-rich region, there is an AlGaN region of Al5Ga7N 12 and the AlN mole fraction outside the above boundary region portion of the above well layer is in the range of 41.8% to 46%.

[0087] According to the above preferred embodiment, the variation range of the AlN mole fraction in the well layer is suppressed within 3.3%. Furthermore, even when the emission peaks caused by composition modulation from regions outside the AlN mole fraction of 41.7% overlap, a single-peak quantum well can be virtually formed, thus avoiding the separation of the emission peaks of the emission spectrum.

[0088] Furthermore, in the nitride semiconductor ultraviolet light-emitting device having the second feature described above, preferably, in the second Ga-rich region, there is an AlGaN region of Al1Ga2N3 having an integer ratio of AlGaN composition ratio, and the AlN mole fraction outside the boundary region portion of the well layer is in the range of 33.4% to 37%.

[0089] According to the above preferred embodiment, the variation range of the AlN mole fraction in the well layer is suppressed within 3.3%. Furthermore, even when the emission peaks caused by the composition modulation generated from regions outside the AlN mole fraction of 33.3% overlap, a single-peak quantum well can be virtually formed, so that the separation of the emission peaks of the emission spectrum can be avoided.

[0090] In addition, the present invention provides a nitride semiconductor ultraviolet light-emitting device having a third feature based on the first or second feature described above. The third feature is that the active layer has a multi-quantum well structure including two or more of the well layers, and a barrier layer made of an AlGaN-based semiconductor exists between the two well layers.

[0091] According to the nitride semiconductor ultraviolet light-emitting device having the third feature described above, the active layer becomes a multi-quantum well structure, and an improvement in the emission efficiency can be expected for the well layer compared to the case of only one layer.

[0092] In addition, in the nitride semiconductor ultraviolet light-emitting device having the third feature described above, more preferably, the barrier layer is made of an AlGaN-based semiconductor, and a third Ga-rich region where the AlN mole fraction is locally low also exists in a boundary region portion between adjacent platforms of the multi-step platform of at least the barrier layer closest to the p-type layer side among the barrier layers between the two well layers.

[0093] By the above preferred embodiment, in the barrier layer, similar to the first Ga-rich region of the n-type layer and the second Ga-rich region of the well layer, a local distribution of carriers can be generated in the third Ga-rich region. Therefore, when carriers (electrons) are supplied from the n-type layer to the second Ga-rich region in the boundary region (inclined region) between adjacent platforms where light emission is concentrated in the well layer, it can be effectively performed via the first Ga-rich region of the n-type layer and the third Ga-rich region of the barrier layer.

[0094] Here, in the multi-quantum well structure with two or more well layers, in the well layer closest to the p-type layer side, the emission efficiency is high. Therefore, in the barrier layer on the n-type layer side of this well layer, by forming the third Ga-rich region, the supply of the above carriers to the well layer can be performed more effectively.

[0095] Furthermore, in the nitride semiconductor ultraviolet light-emitting element having the above-described third feature, it is more preferable that in the third Ga-rich region of the barrier layer, there is an AlGaN region in which the AlGaN composition ratio is an integer ratio, such as Al2Ga1N3, Al3Ga1N4, or Al5Ga1N6.

[0096] By the above-described preferred embodiment, due to the presence of metastable AlGaN in the third Ga-rich region of the barrier layer, similar to the first Ga-rich region of the n-type layer and the second Ga-rich region of the well layer, the variation in the AlN mole fraction in the third Ga-rich region is suppressed, and a region of metastable AlGaN is stably formed in the third Ga-rich region. Therefore, the effect exerted by the third Ga-rich region of the barrier layer can be more stably achieved.

[0097] Furthermore, the nitride semiconductor ultraviolet light-emitting element having the above-described first, second, or third feature preferably further includes a base portion including a sapphire substrate, the sapphire substrate having a main surface that is inclined by only a given angle with respect to the (0001) plane, the light-emitting element structure portion being formed above the main surface, and at least each semiconductor layer from the main surface of the sapphire substrate to the surface of the active layer being an epitaxial growth layer having a surface formed with a multi-step platform parallel to the (0001) plane.

[0098] By the above-described preferred embodiment, a sapphire substrate having a tilt angle can be used for epitaxial growth such that multi-step platforms are exposed on the surfaces of the layers from the main surface of the sapphire substrate to the surface of the active layer, and the nitride semiconductor ultraviolet light-emitting element having the above-described features can be realized.

[0099] -Advantages of the Invention-

[0100] According to the nitride semiconductor ultraviolet light-emitting element having any of the above-described features, a nitride semiconductor ultraviolet light-emitting element having desired light-emitting characteristics and suppressing characteristic variations caused by drift or the like of the crystal growth apparatus can be stably provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0101] Figure 1 is a diagram schematically showing the wurtzite crystal structure of AlGaN.

[0102] Figure 2 is a top view showing the positional relationship between each part of the A plane and each part of the B plane as observed from the c-axis direction of the wurtzite crystal structure shown in Figure 1 is a top view showing the positional relationship between each part of the A plane and each part of the B plane as observed from the c-axis direction of the wurtzite crystal structure shown in

[0103] Figure 3 is a diagram schematically showing the arrangement of Al and Ga on the A3 plane and the B3 plane for each of five combinations of AlGaN composition ratios represented by integer ratios.

[0104] Figure 4 is a main part sectional view schematically showing an example of the structure of a nitride semiconductor ultraviolet light-emitting element to which an embodiment of the present invention relates.

[0105] Figure 5 is schematically showing Figure 4 a main part sectional view of an example of the stacked structure of the active layer of the nitride semiconductor ultraviolet light-emitting element shown.

[0106] Figure 6 is a graph showing the relationship between the emission wavelength of a quantum well structure including an AlGaN well layer and an AlGaN barrier layer, the film thickness of the well layer, and the AlN mole fraction of the barrier layer when the AlN mole fraction of the second Ga-rich region 220a is 50%.

[0107] Figure 7 is a graph showing the relationship between the emission wavelength of a quantum well structure including an AlGaN well layer and an AlGaN barrier layer, the film thickness of the well layer, and the AlN mole fraction of the barrier layer when the AlN mole fraction of the second Ga-rich region 220a is 41.7%.

[0108] Figure 8 is a graph showing the relationship between the emission wavelength of a quantum well structure including an AlGaN well layer and an AlGaN barrier layer, the film thickness of the well layer, and the AlN mole fraction of the barrier layer when the AlN mole fraction of the second Ga-rich region 220a is 33.3%.

[0109] Figure 9 is schematically showing from Figure 4 the upper side of Figure 4 a plan view of an example of the structure when observing the nitride semiconductor ultraviolet light-emitting element shown.

[0110] Figure 10 is a HAADF-STEM image showing the cross-sectional structure in the n-type cladding layer.

[0111] Figure 11 is a diagram showing five measurement regions A to E for performing line analysis of cross-sectional TEM-EDX in the n-type cladding layer in the HAADF-STEM image shown in Figure 10

[0112] Figure 12A is showing Figure 11 a diagram of the measurement results of the AlN mole fraction and the GaN mole fraction based on line analysis of cross-sectional TEM-EDX in the n-type cladding layer in the measurement region A shown.

[0113] Figure 12B is showing Figure 11 ​Graph of the measurement results of the AlN mole fraction and the GaN mole fraction based on line analysis of cross-sectional TEM-EDX in the n-type cladding layer in the measurement region B shown.

[0114] Figure 12C is a graph showing Figure 11 Graph of the measurement results of the AlN mole fraction and the GaN mole fraction based on line analysis of cross-sectional TEM-EDX in the n-type cladding layer in the measurement region C shown.

[0115] Figure 12D is a graph showing Figure 11 Graph of the measurement results of the AlN mole fraction and the GaN mole fraction based on line analysis of cross-sectional TEM-EDX in the n-type cladding layer in the measurement region D shown.

[0116] Figure 12E is a graph showing Figure 11 Graph of the measurement results of the AlN mole fraction and the GaN mole fraction based on line analysis of cross-sectional TEM-EDX in the n-type cladding layer in the measurement region E shown.

[0117] Figure 13 is a SEM image showing the measurement region of the AlN mole fraction based on the CL method in the n-type cladding layer.

[0118] Figure 14 is a graph showing Figure 13 Graphs of the first and second CL spectra calculated from the CL spectra of 10 points measured in each of the measurement regions shown.

[0119] Figure 15 is a main part cross-sectional view schematically showing an example of the element structure of a general ultraviolet light-emitting diode. Detailed Description of the Invention

[0120] Regarding the nitride semiconductor ultraviolet light-emitting element (hereinafter, simply referred to as "light-emitting element") of the embodiment of the present invention, it will be described with reference to the drawings. However, in the schematic diagrams of the drawings used in the following description, in order to facilitate understanding of the description, the content of the invention is schematically shown by emphasizing the main parts. Therefore, the dimensions of each part are not necessarily the same as the actual element in terms of size ratio. Hereinafter, in this embodiment, the case where the light-emitting element is a light-emitting diode will be described.

[0121] [First Embodiment]

[0122] [Element Structure of the Light-Emitting Element]

[0123] As Figure 4As shown, the light-emitting element 1 of the present embodiment includes: a base portion 10 including a sapphire substrate 11; and a light-emitting element structure portion 20 including a plurality of AlGaN-based semiconductor layers 21 to 25, a p electrode 26, and an n electrode 27. The light-emitting element 1 is mounted (flip-chip mounted) on a mounting base (submount) or the like toward the light-emitting element structure portion 20 side ( Figure 4 the upper side in the figure), and the light extraction direction is the base portion 10 side ( Figure 4 the lower side in the figure). However, in this specification, for convenience of explanation, the direction perpendicular to the main surface 11a of the sapphire substrate 11 (or the upper surfaces of the base portion 10 and each of the AlGaN-based semiconductor layers 21 to 25) is referred to as the "vertical direction" (or "longitudinal direction"), the direction from the base portion 10 toward the light-emitting element structure portion 20 is defined as the upper direction, and the opposite thereof is the lower direction. In addition, a plane parallel to the vertical direction is referred to as the "first plane". Further, a plane parallel to the main surface 11a of the sapphire substrate 11 (or the upper surfaces of the base portion 10 and each of the AlGaN-based semiconductor layers 21 to 25) is referred to as the "second plane", and the direction parallel to the second plane is referred to as the "lateral direction".

[0124] The base portion 10 is configured to include a sapphire substrate 11 and an AlN layer 12 formed directly on the main surface 11a of the sapphire substrate 11. The sapphire substrate 11 is a slightly inclined substrate in which the main surface 11a is inclined at an angle (tilt angle) within a certain range (for example, about 0 degrees to 6 degrees) with respect to the (0001) plane, and a stepped platform is exposed on the main surface 11a.

[0125] The AlN layer 12 is composed of AlN crystals epitaxially grown from the main surface of the sapphire substrate 11, and this AlN crystal has an epitaxial crystal orientation relationship with respect to the main surface 11a of the sapphire substrate 11. Specifically, for example, the AlN crystal grows such that the c-axis direction (<0001> direction) of the sapphire substrate 11 coincides with the c-axis direction of the AlN crystal. However, the AlN crystal constituting the AlN layer 12 may contain a small amount of Ga or other impurities, and may also be an AlN-based semiconductor layer. In the present embodiment, the film thickness of the AlN layer 12 is assumed to be about 2 μm to 3 μm. However, the structure of the base portion 10 and the substrate used or the like are not limited to the above configuration. For example, an AlGaN-based semiconductor layer having an AlN mole fraction equal to or higher 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.

[0126] The AlGaN-based semiconductor layers 21 to 25 of the light-emitting element structure portion 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 24 (p-type layer), and a p-type contact layer 25 (p-type layer) are epitaxially grown in sequence from the base portion 10 side and stacked in order.

[0127] In the present embodiment, each semiconductor layer in the AlN layer 12 of the base portion 10 and the n-type cladding layer 21 and the active layer 22 of the light-emitting element structure portion 20 that are epitaxially grown in sequence from the main surface 11a of the sapphire substrate 11 has a surface that forms a multi-step platform parallel to the (0001) plane derived from the main surface 11a of the sapphire substrate 11. However, for the electron blocking layer 23, the p-type cladding layer 24, and the p-type contact layer 25 of the p-type layer, since they are formed by epitaxial growth on the active layer 22, a multi-step platform of the same type can be formed, but the surface may not have a multi-step platform of the same type formed.

[0128] However, as Figure 4 shown, within the light-emitting element structure portion 20, the active layer 22, the electron blocking layer 23, the p-type cladding layer 24, and the p-type contact layer 25 are formed on the first region R1 on the upper surface of the n-type cladding layer 21 by removing the portion laminated on the second region R2 on the upper surface of the n-type cladding layer 21 by etching or the like. Then, the upper surface of the n-type cladding layer 21 is exposed in the second region R2 excluding the first region R1. As Figure 4 schematically shown, when there is a difference in height between the first region R1 and the second region R2 on the upper surface of the n-type cladding layer 21, the upper surface of the n-type cladding layer 21 is independently defined in the first region R1 and the second region R2.

[0129] The n-type cladding layer 21 is composed of an n-type AlGaN-based semiconductor. In the n-type cladding layer 21, there is a layered region where the AlN mole fraction is locally low and evenly dispersed. In the layered region, as described above, the first Ga-rich region 21a that predominantly contains Al7Ga5N 12 in which the AlGaN composition ratio becomes an integer ratio (that is, n-type metastable AlGaN with an AlN mole fraction of 58.3%) is dominant. Figure 4 In, as an example in which the first Ga-rich region 21a predominantly exists in the layered region, a case where the entire layered region becomes the first Ga-rich region 21a is schematically shown. Let the region other than the layered region in the n-type cladding layer 21 be called the n-type main region 21b.

[0130] In the present embodiment, the AlN mole fraction in the n-type main region 21b allows for a region where the AlN mole fraction locally slightly increases, and is adjusted within the range of 60% to 66%. As the film thickness of the n-type cladding layer 21, it is the same as the film thickness used in general nitride semiconductor ultraviolet light-emitting elements, assumed to be on the order of 1 μm to 2 μm, but this film thickness can also be on the order of 2 μm to 4 μm. Hereinafter, for simplicity of explanation, the AlGaN composition ratio existing in the first Ga-rich region 21a is made an integer ratio of Al7Ga5N 12 of the metastable AlGaN n-type AlGaN region, which is conveniently referred to as the "metastable n-type region". In addition, a region where the AlN mole fraction slightly varies with respect to 58.3% (seven-twelfths) of the AlN mole fraction outside the metastable n-type region existing in the first Ga-rich region 21a is referred to as the "n-type region near metastable". Here, in the multiple layered first Ga-rich regions 21a, the metastable n-type region does not necessarily have to exist continuously in a layered manner, and can also exist discontinuously by being segmented by the n-type region near metastable.

[0131] The active layer 22 has a multi-quantum well structure in which two or more well layers 220 formed of an AlGaN-based semiconductor and one or more barrier layers 221 formed of an AlGaN-based semiconductor or an AlN-based semiconductor are alternately stacked. It is not necessary to necessarily provide the barrier layer 221 between the lowermost well layer 220 and the n-type cladding layer 21. In addition, between the uppermost well layer 220 and the electron blocking layer 23, it is also possible to provide the barrier layer 221 or an AlGaN layer or an AlN layer having a higher AlN mole fraction with a film thickness thinner than that of the barrier layer 221.

[0132] In Figure 5 an example of the stacked structure (multi-quantum well structure) of the well layer 220 and the barrier layer 221 of the active layer 22 is schematically shown. Figure 5 In this case, the case where the well layer 220 has three layers is exemplified. Figure 5 The stepped structure in which the terraces T of the well layer 220 and the barrier layer 221 shown are grown in a multi-step shape is a well-known structure as disclosed in Non-Patent Documents 1 and 2 above. The boundary region BA between adjacent terraces T becomes an inclined region inclined with respect to the (0001) plane as described above. In the present embodiment, the depth of one terrace T (the distance between adjacent boundary regions BA) is assumed to be several tens of nanometers to several hundreds of nanometers.

[0133] As Figure 5 schematically shown, in the boundary region portion (inclined region) BA between adjacent terraces T of the multi-step terraces T in the well layer 220, a second Ga-rich region 220a where the AlN mole fraction is locally low is formed in the well layer 220. For convenience, the region outside the second Ga-rich region 220a in the well layer 220 is referred to as the well main region 220b.

[0134] In this embodiment, in the second Ga-rich region 220a, there is Al1Ga1N2 or Al5Ga7N in which the AlGaN composition ratio is an integer ratio. 12 or three quasi-stable AlGaN of Al1Ga2N3, i.e., any one of the quasi-stable AlGaN with an AlN mole fraction of 50% (one-half), 41.7% (five-twelfths), or 33.3% (one-third). Furthermore, the AlN mole fraction of the well body region 220b is adjusted to be within the range of 50.1% to 54% when the quasi-stable AlGaN with an AlN mole fraction of 50% exists in the second Ga-rich region 220a, is adjusted to be within the range of 41.8% to 46% when the quasi-stable AlGaN with an AlN mole fraction of 41.7% exists in the second Ga-rich region 220a, and is adjusted to be within the range of 33.4% to 37% when the quasi-stable AlGaN with an AlN mole fraction of 33.3% exists in the second Ga-rich region 220a. The film thickness of the well layer 220, including the terrace region TA and the inclined region BA, is adjusted to be within the range of 2 unit cells to 7 unit cells, for example.

[0135] In order to simplify the description below, the AlGaN composition ratio existing in the second Ga-rich region 220a is assumed to be an integer ratio of Al1Ga1N2 or Al5Ga7N 12 The quasi-stable AlGaN of Al1Ga2N3 or Al1Ga2N3 is conveniently referred to as a "quasi-stable well region". In addition, a region slightly changed from 50% (one-half) or 41.7% (five-twelfths) or 33.3% (one-third) of the AlN mole fraction outside the quasi-stable well region existing in the second Ga-rich region 220a is referred to as a "quasi-stable nearby well region". Here, the quasi-stable well region, in the second Ga-rich region 220a formed in the inclined region BA existing along the edge line of the terrace T when viewed from above, does not necessarily have to exist continuously along the edge line, and may also exist intermittently by being interrupted by the quasi-stable nearby n-type region.

[0136] As described above, the barrier layer 221 is made of AlGaN-based semiconductors, similarly to the n-type cladding layer 21 and the well layer 220, and has a surface having a multi-step terrace T formed parallel to the (0001) plane. Here, as an example, the AlN mole fraction of the barrier layer 221 as a whole is assumed to be in the range of 58.3% to 100%. Although the barrier layer 221 includes a case where the AlN mole fraction is 100% and a case where the AlN mole fraction is not 100%, there is also a case where the barrier layer 221 is made of AlGaN-based semiconductors. Therefore, Figure 5As schematically shown, when the barrier layer 221 is made of an AlGaN-based semiconductor with an AlN mole fraction of not 100%, similar to the n-type cladding layer 21 and the well layer 220, a third Ga-rich region 221a with a locally low AlN mole fraction can be formed in the boundary region (inclined region) BA between adjacent platforms T in the barrier layer 221. Here, for the sake of convenience, the region other than the third Ga-rich region 221a in the platform region within the barrier layer 221 is referred to as the barrier main region 221b. The barrier main region 221b mainly exists in the platform region TA within the barrier layer 221. As an example, when the overall AlN mole fraction of the third Ga-rich region 221a including the barrier layer 221 is within the range of 58.3% to 90% which is a part of the above-mentioned range of 58.3% to 100%, in order to fully ensure the local carrier distribution effect of the third Ga-rich region 221a, it is preferable that the difference in AlN mole fraction between the third Ga-rich region 221a and the barrier main region 221b is 4 to 5% or more. However, even if it is about 1%, a local carrier distribution effect can be expected. Therefore, in the present embodiment, the AlN mole fraction of the barrier main region 221b is within the range of 68% to 90%. In addition, the film thickness of the barrier layer 221 including the platform region TA and the inclined region BA is preferably adjusted within the range of 6 nm to 8 nm, for example.

[0137] Figure 6 , Figure 7 and Figure 8 are the simulation results (corresponding to the peak emission wavelength) of the emission wavelength obtained by graphically varying the film thickness of the well layer within the range of 3 ML (monoatomic layer) to 14 ML (15 unit cells to 7 unit cells) or 4 ML to 14 ML (2 unit cells to 7 unit cells) for a quantum well structure model in which the well layer 220 and the barrier layer 221 are made of AlGaN. As the conditions for the above simulation, Figure 6 in which the AlN mole fraction of the second Ga-rich region 220a in the well layer 220 is set to 50% (one-half) which is the AlN mole fraction of the metastable well region, Figure 7 in which the AlN mole fraction of the second Ga-rich region 220a in the well layer 220 is set to 41.7% (five-twelfths) which is the AlN mole fraction of the metastable well region, and in Figure 8 in which the AlN mole fraction of the second Ga-rich region 220a in the well layer 220 is set to 33.3% (one-third) which is the AlN mole fraction of the metastable well region, and in Figures 6 - 8 in each figure, the AlN mole fractions of the third Ga-rich region 221a in the barrier layer 221 are 66.7% (two-thirds), 75% (three-fourths), and 83.3% (five-sixths), respectively, in three cases. Figures 6 - 8In the simulation results shown, it is assumed that ultraviolet light emission from the well layer 220 occurs significantly in the boundary region (tilted region) BA. Therefore, it is particularly important that the film thickness condition of the well layer 220 is satisfied in this tilted region BA. In this specification, when marking two-thirds of the AlN mole fraction as a percentage, it is approximately expressed as 66.7%, and when marking five-sixths of the AlN mole fraction as a percentage, it is approximately expressed as 83.3%.

[0138] According to Figures 6 - 8 it can be seen that within the range where the film thickness of the well layer 220 is 3 ML to 14 ML, the smaller the film thickness of the well layer 220, the greater the quantum confinement effect on the well layer 220, and the shorter the emission wavelength. Furthermore, the greater the AlN mole fraction of the barrier layer 221, the greater the degree of change in the emission wavelength with respect to the change in the film thickness of the well layer 220. Also, as Figure 6 it can be seen that when the AlN mole fraction of the second Ga-rich region 220a is 50%, within the above ranges of the film thickness of the well layer 220 and the AlN mole fraction of the barrier layer 221, the emission wavelength varies in the range of approximately 246 nm to 295 nm. As Figure 7 it can be seen that when the AlN mole fraction of the second Ga-rich region 220a is 41.7%, within the above ranges of the film thickness of the well layer 220 and the AlN mole fraction of the barrier layer 221, the emission wavelength varies in the range of approximately 249 nm to 311 nm. As Figure 8 it can be seen that when the AlN mole fraction of the second Ga-rich region 220a is 33.3%, within the above ranges of the film thickness of the well layer 220 and the AlN mole fraction of the barrier layer 221, the emission wavelength varies in the range of approximately 261 nm to 328 nm. Furthermore, when the barrier layer 221 is composed of AlN, the emission wavelength can be further extended. In Figure 6 the emission wavelengths of 258 nm and 280 nm shown by the single-dot chain line are the control ranges (lower limit and upper limit) of the emission wavelengths envisaged in the light-emitting element 1 of the present embodiment when the AlN mole fraction of the second Ga-rich region 220a is 50%. In Figure 7 the emission wavelengths of 263 nm and 291 nm shown by the single-dot chain line are the control ranges (lower limit and upper limit) of the emission wavelengths envisaged in the light-emitting element 1 of the present embodiment when the AlN mole fraction of the second Ga-rich region 220a is 41.7%. In Figure 8 the emission wavelengths of 277 nm and 315 nm shown by the single-dot chain line are the control ranges (lower limit and upper limit) of the emission wavelengths envisaged in the light-emitting element 1 of the present embodiment when the AlN mole fraction of the second Ga-rich region 220a is 33.3%.

[0139] In the second Ga-rich region 220a of the well layer 220, the AlGaN composition ratio of the metastable AlGaN that predominantly exists becomes Al1Ga1N2 or Al5Ga7N 12 or Al1Ga2N3. As a result, when the AlN mole fraction in the second Ga-rich region 220a is 50% or 41.7%, for example, the film thickness of the well layer 220 is set in the range of 3 ML to 11 ML corresponding to the AlN mole fraction of the barrier layer 221, and the emission wavelength can be controlled within the range of 258 nm to 280 nm, or within the range of 263 nm to 291 nm. Furthermore, when the AlN mole fraction in the second Ga-rich region 220a is 33.3%, for example, the film thickness of the well layer 220 is set in the range of 5 ML to 11 ML corresponding to the AlN mole fraction of the barrier layer 221, and the emission wavelength can be controlled within the range of 277 nm to 315 nm. As a result, by appropriately selecting the AlGaN composition ratio of the metastable AlGaN existing in the second Ga-rich region 220a, the film thickness of the well layer 220, and the AlN mole fraction of the barrier layer 221 in the above-described manner, the peak emission wavelength can be controlled within the range of 265 nm to 300 nm.

[0140] The electron blocking layer 23 is made of a p-type AlGaN-based semiconductor. The p-type cladding layer 24 is made of a p-type AlGaN-based semiconductor. The p-type contact layer 25 is made of a p-type AlGaN-based semiconductor or a p-type GaN-based semiconductor. The p-type contact layer 25 is typically made of GaN. However, the film thicknesses of the respective layers such as the active layer 22, the electron blocking layer 23, the p-type cladding layer 24, and the p-type contact layer 25 are appropriately determined corresponding to the emission wavelength characteristics and electrical characteristics of the light-emitting element 1. In addition, since the p-type cladding layer 24 reduces the parasitic resistance of the p-type layer, it may be omitted.

[0141] The p-electrode 26 is formed, for example, of a multi-layer metal film such as Ni / Au on the upper surface of the p-type contact layer 25. The n-electrode 27 is formed, for example, of a multi-layer metal film such as Ti / Al / Ti / Au on a part of the exposed surface within the second region R2 of the n-type cladding layer 21. However, the p-electrode 26 and the n-electrode 27 are not limited to the above-described multi-layer metal films, and the electrode structure such as the metal constituting each electrode, the number of layers stacked, and the stacking order can be appropriately changed. In Figure 7 shows an example of the shapes of the p-electrode 26 and the n-electrode 27 as observed from the upper side of the light-emitting element 1. In Figure 7 the line BL existing between the p-electrode 26 and the n-electrode 27 shows the boundary line 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, the p-type cladding layer 24, and the p-type contact layer 25.

[0142] In this embodiment, as shown in Figure 9 , as an example, the top-view shapes of the first region R1 and the p-electrode 26 are comb-shaped, but the top-view shapes, arrangements, etc. of the first region R1 and the p-electrode 26 are not limited to the Figure 9 example.

[0143] 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. Each of the supplied holes and electrons reaches the active layer 22 and then bonds to emit light. In addition, a forward current flows between the p-electrode 26 and the n-electrode 27 thereby.

[0144] Figure 4 In , as schematically shown by a double line for one layer, there are multiple layers of the first Ga-rich region 21a of the n-type cladding layer 21 separated from each other in the vertical direction. In addition, in one first plane parallel to the vertical direction (for example, the cross-section shown in Figure 4 ), the extending direction of at least a part of the first Ga-rich region 21a is inclined with respect to the lateral direction (the extending direction of the intersection line of the first plane and the second plane). However, on the first plane shown in Figure 4 , although the layers of the first Ga-rich region 21a are schematically shown by parallel lines (double lines), the inclination angle formed by the extending direction and the lateral direction is not necessarily the same among the first Ga-rich regions 21a, and varies depending on the position within the same first Ga-rich region 21a. Therefore, the first Ga-rich region 21a on the first plane is not necessarily limited to extending in a straight line. In addition, this inclination angle varies according to the orientation of the first plane. Therefore, a part of the first Ga-rich region 21a can also cross other first Ga-rich regions 21a or branch from other first Ga-rich regions 21a on the first plane. The points that the inclination angle formed by the extending direction of the first Ga-rich region 21a and the lateral direction varies depending on the position and that the first Ga-rich region 21a is evenly dispersed in the n-type cladding layer 21 are clearly shown in the HAADF-STEM image shown in Figure 10 .

[0145] In addition, although the first Ga-rich region 21a is shown by one line (double line) respectively on the first plane in Figure 4 , in the direction perpendicular to this first plane, it also extends parallel or inclined to the second plane and has a two-dimensional expansion. Therefore, multiple first Ga-rich regions 21a exist in a striped pattern on multiple second planes within the n-type cladding layer 21.

[0146] As described above, the first Ga-rich region 21a is a layered region where the AlN mole fraction is locally low within the n-type cladding layer 21. That is, the AlN mole fraction of the first Ga-rich region 21a is lower than the AlN mole fraction of the n-type main region 21b. In addition, when the AlN mole fractions of the two regions gradually and continuously change near the boundary between the first Ga-rich region 21a and the n-type main region 21b, the boundary between the two regions cannot be clearly defined.

[0147] Therefore, in this case, taking the average AlN mole fraction of the entire n-type cladding layer 21, for example, the AlN mole fraction that is a prerequisite for the growth conditions of the n-type cladding layer 21 (the supply amount and flow rate of the source gas or carrier gas used in the metalorganic chemical vapor deposition method) as a reference, the portion where the AlN mole fraction is lower than this reference value can be relatively defined as the first Ga-rich region 21a. In addition, in addition to the above-described definition method, for example, according to the HAADF-STEM image described later, the portion with a large change in brightness can be defined as the boundary between the two layers. In the present invention, the definition of the boundary between the two layers itself is not important, and it is sufficient to fully grasp the existence of the first Ga-rich region 21a itself.

[0148] In fact, since the first Ga-rich region 21a is formed by the mass transfer of Ga from the n-type main region 21b, corresponding to the supply amount of Ga from the n-type main region 21b, the average AlN mole fraction within the first Ga-rich region 21a changes, and the AlN mole fraction within the first Ga-rich region 21a is not necessarily average. However, in the present embodiment, since a metastable n-type region is stably formed within the first Ga-rich region 21a, even if there is a slight change in the supply amount of the above Ga, the change is absorbed by the metastable n-type region, and the change in the AlN mole fraction within the first Ga-rich region 21a can be suppressed. For this reason, the minimum value of the AlN mole fraction within each first Ga-rich region 21a becomes 58.3% or a value near it of the AlN mole fraction of the metastable n-type region. As described above, within the first Ga-rich region 21a, along with the metastable n-type region, there also exists a near-metastable n-type region. Since the near-metastable n-type region is also formed by the mass transfer of Ga from the n-type main region 21b, generally, the AlN mole fraction of the near-metastable n-type region is higher than the AlN mole fraction of the metastable n-type region, and the average AlN mole fraction within the first Ga-rich region 21a is slightly higher than the AlN mole fraction of the metastable n-type region.

[0149] On the other hand, by supplying Ga to the first Ga-rich region 21a, the AlN molar fraction at the site where the mass of Ga in the n-type main region 21b has moved becomes relatively higher. In addition, due to the mass movement of Ga in the n-type main region 21b that does not reach the formation level of the first Ga-rich region 21a, the AlN molar fraction varies to some extent within the n-type main region 21b. However, as described above, the carriers in the n-type cladding layer 21 are locally distributed in the first Ga-rich region 21a where the bandgap energy is smaller than that of the n-type main region 21b, and since the current preferentially and stably flows into the first Ga-rich region 21a within the n-type cladding layer 21, even if the AlN molar fraction in the n-type main region 21b varies slightly, the characteristic variations of the light-emitting element 1 can be suppressed by the first Ga-rich region 21a.

[0150] Here, the above description of the first Ga-rich region 21a is directly applied to the second Ga-rich region 220a as well. That is, in the present embodiment, since a metastable well region is stably formed within the second Ga-rich region 220a, even if there is a slight variation in the supply amount of the above Ga, the variation is absorbed by the metastable well region, and the average AlN molar fraction of the second Ga-rich region 220a becomes a value of 50% or around it, or 41.7% or around it, or 33.3% or around it of the AlN molar fraction of the metastable well region. As described above, within the second Ga-rich region 220a, a near-metastable well region also exists along with the metastable well region. Since the near-metastable well region is also formed along with the mass movement of Ga from the well main region 220b, generally, the AlN molar fraction of the near-metastable well region is higher than that of the metastable well region, and the average AlN molar fraction within the second Ga-rich region 220a is slightly higher than that of the metastable well region.

[0151] On the other hand, by supplying Ga to the second Ga-rich region 220a, the AlN molar fraction at the site where the mass of Ga in the well main region 220b has moved becomes relatively higher. In addition, due to the mass movement of Ga in the well main region 220b that does not reach the formation level of the second Ga-rich region 220a, the AlN molar fraction varies to some extent within the well main region 220b. However, as described above, the carriers in the well layer 220 are locally distributed in the second Ga-rich region 220a where the bandgap energy is smaller than that of the well main region 220b, and since the current preferentially and stably flows into the second Ga-rich region 220a within the well layer 220, even if the AlN molar fraction in the well main region 220b varies slightly, the characteristic variations of the light-emitting element 1 can be suppressed by the second Ga-rich region 220a.

[0152] <Manufacturing method of light-emitting element>

[0153] Next, an example of a method for manufacturing the light-emitting element 1 illustrated Figure 4 will be described.

[0154] First, by metalorganic vapor phase epitaxy (MOVPE), the AlN layer 12 contained in the base portion 10 and the nitride semiconductor layers 21 to 25 contained in the light-emitting element structure portion 20 are sequentially epitaxially grown and stacked on the sapphire substrate 11. At this time, as a donor impurity, Si is doped, for example, in the n-type cladding layer 21, and as an acceptor impurity, Mg is doped, for example, in the electron blocking layer 23, the p-type cladding layer 24, and the p-type contact layer 25.

[0155] In the present embodiment, at least on each surface of the AlN layer 12, the n-type cladding layer 21, and the active layer 22 (the well layer 220 and the barrier layer 221), in order to expose a multi-step platform parallel to the (0001) plane, the sapphire substrate 11 uses a slightly inclined substrate in which the main surface 11a is inclined at an angle (tilt angle) within a certain range (for example, about 0 degrees to 6 degrees) with respect to the (0001) plane and a multi-step platform is exposed on the main surface 11a.

[0156] As conditions for the related epitaxial growth, in addition to the use of the (0001) sapphire substrate 11 of the slightly inclined substrate described above, for example, a growth rate that easily exposes a multi-step platform (specifically, for example, by appropriately setting various conditions such as the growth temperature, the supply amount or flow rate of the source gas or carrier gas, etc., this growth rate is achieved) can be cited. However, since these various conditions differ depending on the type or structure of the film-forming apparatus, several samples are actually fabricated in the film-forming apparatus, and these conditions can be determined.

[0157] As growth conditions for the n-type cladding layer 21, immediately after the start of growth, at the step difference portion (boundary region) between the multi-step platforms formed on the upper surface of the AlN layer 12, a growth start point of the first Ga-rich region 21a is formed by the mass transfer of Ga. Next, the growth temperature, the growth pressure, and the donor impurity concentration are selected so that, as the n-type cladding layer 21 is epitaxially grown, the first Ga-rich region 21a grows obliquely upward by segregation accompanying the mass transfer of Ga.

[0158] Specifically, as the growth temperature, it is preferably 1050°C or higher at which the mass movement of Ga is likely to occur and 1150°C or lower at which good n-type AlGaN can be modulated. Additionally, at a growth temperature exceeding 1170°C, the mass movement of Ga becomes excessive. Even for the first metastable AlGaN, since the AlN mole fraction is likely to vary randomly, it is difficult to stably form the metastable AlGaN with an AlN mole fraction of 58.3% as the second metastable AlGaN. Therefore, it is not preferred. As the growth pressure, 75 Torr or lower is preferably a good growth condition for AlGaN, and in reality, 10 Torr or higher is better as the control limit of the film-forming apparatus. The donor impurity concentration is 1×10 18 ~5×10 18 cm -3 is preferred. However, the above growth temperature, growth pressure, etc. are just examples, and the optimal conditions can be appropriately determined according to the film-forming apparatus used.

[0159] The supply amounts and flow rates of the source gases (trimethylaluminum (TMA) gas, trimethylgallium (TMG) gas, ammonia gas) or carrier gas used in the metalorganic chemical vapor deposition method are specified with the average AlN mole fraction Xa of the entire n-type cladding layer 21 as the target value. Here, let the average AlN mole fraction of the n-type main region 21b be Xb (= 60% - 66%), and let the average AlN mole fraction of the first Ga-rich region 21a where the metastable n-type region with an AlN mole fraction of 58.3% and the metastable near-n-type region with an AlN mole fraction slightly higher than 58.3% exist be Xc (> 58.3%). Considering the mass movement of Ga from the n-type main region 21b to the first Ga-rich region 21a, it becomes Xb > Xa > Xc. However, since the volume ratio of the first Ga-rich region 21a occupying the entire n-type cladding layer 21 is small, it can be approximately set as Xa = Xb.

[0160] In the first Ga-rich region 21a, a metastable n-type region with an AlN mole fraction of 58.3% stably exists. Since the target value Xa of the AlN mole fraction of the n-type cladding layer 21 is 69% to 74%, the difference (Xb - 58.3%) between the AlN mole fraction of 58.3% in the metastable n-type region and the average AlN mole fraction Xb of the n-type main region 21b can be stably ensured to be 1.7% or more. The carriers in the n-type layer are locally distributed in the first Ga-rich region 21a where the bandgap energy is smaller than that of the n-type main region 21b. In addition, if the lower limit values of the target value Xa and the average AlN mole fraction Xb of the n-type main region 21b are increased from 60% to 61% for example, the above difference becomes about 2.7% or more, and the local distribution of the carriers in the n-type layer into the first Ga-rich region 21a becomes more significant. Furthermore, since the upper limit of the target value Xa is 66%, in the n-type main region 21b, a metastable AlGaN with an AlGaN composition ratio of Al2Ga1N3 is not dominantly formed. Suppose when the upper limit of the target value Xa is 67% or more, a metastable AlGaN of Al2Ga1N3 is stably formed in the n-type main region 21b. In the first Ga-rich region, it is difficult to sufficiently supply Ga for stably forming a metastable AlGaN (metastable n-type region) of Al7Ga5N 12 from this metastable AlGaN of Al2Ga1N3. Therefore, by setting the upper limit of the target value Xa to 66%, a metastable n-type region with an AlN mole fraction of 58.3% can be stably formed in the first Ga-rich region 21a.

[0161] However, the donor impurity concentration does not necessarily need to be uniformly controlled in the up-down direction with respect to the film thickness of the n-type cladding layer 21. For example, the impurity concentration in a given thin film thickness part in the n-type cladding layer 21 may be lower than the above set concentration, for example, may be controlled to be less than 1×10 18 cm -3 and more preferably a low impurity concentration layer of 1×10 17 cm -3 or less. As the film thickness of this low impurity concentration layer, it is preferably greater than 0 nm and about 200 nm or less, more preferably 10 nm or more and 100 nm or less, and further preferably 20 nm or more and 50 nm or less. In addition, the donor impurity concentration of this low impurity concentration layer only needs to be lower than the above set concentration, and may also partly include an undoped layer (0 cm -3 ). Furthermore, it is preferable that part or all of this low impurity concentration layer exists in the upper layer region within a depth of 100 nm or less from the upper surface of the n-type cladding layer 21 toward the lower side.

[0162] When forming the n-type cladding layer 21 having the first Ga-rich region 21a and the n-type main region 21b according to the above-mentioned method, on the entire upper surface of the n-type cladding layer 21, then, by a known epitaxial growth method such as metalorganic vapor phase epitaxy (MOVPE), the active layer 22 (well layer 220, barrier layer 221), the electron blocking layer 23, the p-type cladding layer 24, the p-type contact layer 25, etc. are formed.

[0163] In the formation of the active layer 22, in the same manner as the n-type cladding layer 21, under the growth conditions that are likely to expose the above-mentioned multi-step platform, taking the AlN mole fraction (50.1% - 54%, 41.8% - 46% or 33.4% - 37%) set for the well main region 220b as the target value, the well layer 220 is grown. In addition, taking the AlN mole fraction (68% - 90% or 100%) set for the barrier main region 221b as the target value, the barrier layer 221 is grown.

[0164] Next, by a known etching method such as reactive ion etching, the second region R2 of the nitride semiconductor layers 21 - 25 laminated according to the above-mentioned method is selectively etched until the upper surface of the n-type cladding layer 21 is exposed, and the second region R2 portion of the upper surface of the n-type cladding layer 21 is exposed. Then, by a known film formation method such as electron beam evaporation, while forming the p electrode 26 on the p-type contact layer 25 in the unetched first region R1, the n electrode 27 is formed on the n-type cladding layer 21 in the etched second region R2. However, after the formation of one or both of the p electrode 26 and the n electrode 27, heat treatment can also be performed by a known heat treatment method such as RTA (rapid thermal annealing).

[0165] However, as an example, after the light-emitting element 1 is flip-chip mounted on a base such as a pedestal, it is used in a state of being sealed with a given resin such as silicone resin or amorphous fluororesin (for example, a lens-shaped resin).

[0166] <Cross-sectional Observation and Composition Analysis Results of the n-type Cladding Layer>

[0167] Next, a specimen for cross-sectional observation of the n-type cladding layer 21 is prepared, and a specimen piece having a cross-section perpendicular (or slightly perpendicular) to the upper surface of the n-type cladding layer 21 is processed from the specimen, and the results of observing the specimen piece with a scanning transmission electron microscope (STEM) are described with reference to the accompanying drawings.

[0168] The sample was fabricated by sequentially depositing an n-type cladding layer 21, an AlGaN layer with an AlN mole fraction higher than that of the n-type cladding layer 21, an AlGaN layer for protecting the sample surface, and a protective resin film on a base portion 10 including the above-mentioned sapphire substrate 11 and AlN layer 12 according to the above-mentioned manufacturing method for the n-type cladding layer 21 and the like. However, in the fabrication of this sample, the base portion 10 was used, and the main surface of the base portion used a sapphire substrate 11 having an inclination angle with respect to the (0001) plane and exposed a multi-step platform on the surface of the AlN layer 12. However, in the fabrication of this sample, the film thickness of the n-type cladding layer 21 was set to 3 μm, and the target value of the AlN mole fraction of the n-type cladding layer 21 was 63%. In addition, the donor impurity concentration was made about 3×10 18 cm -3 , and the implantation amount of the donor impurity (Si) was controlled.

[0169] In Figure 10 Figure shows a high-angle annular dark-field (HAADF)-STEM image of the cross-section of the above-mentioned sample piece. Figure 10 is a HAADF-STEM image observing the entire n-type cladding layer 21 including the upper portion of the AlN layer 12, the n-type cladding layer 21, and the active layer 22 of this sample piece.

[0170] The HAADF-STEM image can obtain a contrast proportional to the atomic weight, and heavy elements are shown brightly. Therefore, for the first Ga-rich region 21a and the n-type main region 21b in the n-type cladding layer 21, the first Ga-rich region 21a with a low AlN mole fraction is shown more brightly than the n-type main region 21b. The HAADF-STEM image is more suitable for observing the difference in AlN mole fraction than the normal STEM image (bright-field image).

[0171] By Figure 10 it can be seen that in the n-type cladding layer 21, a plurality of first Ga-rich regions 21a of a layered region with a locally low AlN mole fraction are dispersed in the vertical direction, and each first Ga-rich region 21a extends in a direction inclined with respect to the intersection line of the upper surface of the n-type cladding layer 21 and the first plane (the cross-section of the sample piece) on the screen of the HAADF-STEM image. Although each first Ga-rich region 21a extends linearly in the obliquely upward direction, it does not necessarily extend in a straight line, and it can be seen that the inclination angle with respect to the above-mentioned intersection line varies depending on the position within the same first Ga-rich region 21a. In addition, in Figure 10 the shown cross-section (corresponding to the first plane), it can be observed that a part of the first Ga-rich region 21a intersects with other first Ga-rich regions 21a or branches from other first Ga-rich regions 21a.

[0172] In the present embodiment, the composition analysis within the n-type cladding layer 21 of the above-described specimen wafer is performed by two analysis methods (line analysis of energy dispersive X-ray spectroscopy (cross-sectional TEM-EDX) and CL (cathodoluminescence) method).

[0173] In the composition analysis (EDX measurement) based on the EDX method, first, within almost the entire measurement region covering the Figure 10 shown HAADF-STEM image, an electron beam probe (diameter: approximately 2 nm) is scanned in the longitudinal direction (vertical direction) and the transverse direction (direction parallel to the second plane), and detection data (X-ray intensities corresponding to the respective compositions of Al and Ga) at probe positions distributed at intervals of approximately 7 nm in the longitudinal and transverse directions are obtained in a 512×512 matrix form.

[0174] Next, in order to perform line analysis based on EDX measurement for the first Ga-rich regions 21a dispersedly present throughout the measurement region, as Figure 11 shown, five measurement regions A to E having a substantially square shape (width: approximately 420 nm) are set within the entire measurement region. Figure 11 In Figure 10 the HAADF-STEM image, rectangular frames indicating the respective measurement regions A to E are superposed. Each of the five measurement regions is set to straddle at least one first Ga-rich region 21a confirmed on the HAADF-STEM image. In addition, the inclination of each measurement region is set for each measurement region so that the extending direction of at least one first Ga-rich region 21a within the measurement region is orthogonal to the scanning direction of the line analysis. The respective inclinations (angles formed by the longitudinal direction of the entire measurement region and the longitudinal direction of each measurement region) of the measurement regions A to E are approximately equal to 20°, and strictly speaking, they are not necessarily exactly the same. Here, different from the longitudinal and transverse directions of the entire measurement region, within Figure 11 each of the measurement regions A to E, for convenience of explanation, the scanning direction of the line analysis is defined as the longitudinal direction, and the direction orthogonal to the scanning direction is defined as the transverse direction. The vertical line shown at the center within each measurement region indicates the scanning direction, and the horizontal line at the center indicates the position where it is assumed that the above-described at least one first Ga-rich region 21a exists, and becomes the origin (0 nm) of the scanning position of the line analysis of the composition analysis result described later. Among them, an arrow is attached to the vertical line indicating the scanning direction to indicate the direction of the AlN layer 12. In addition, the scanning positions are set at intervals of approximately 8 nm in the vertical direction on both sides of the above-described origin on the vertical line at the center for each of the measurement regions A to E, within a range of a total of 36 to 49 points.

[0175] In the EDX measurement, since the diameter of the irradiated electron beam probe is as small as about 2 nm, although the spatial resolution is high, the X-rays emitted from each probe position are weak. Therefore, in the line analysis of the present embodiment, the detection data obtained from a plurality of probe positions arranged in the horizontal direction are accumulated at each scanning position to become the detection data at each scanning position. However, "arranged in the horizontal direction" means that the irradiation range of the electron beam probe overlaps a horizontal line that intersects the above-mentioned vertical line and extends in the horizontal direction at each scanning position.

[0176] Therefore, when all parts of a plurality of probe positions arranged in the horizontal direction are located within the metastable n-type region of the first Ga-rich region 21a at a certain scanning position, the accumulated detection data can accurately show the AlN molar fraction of the metastable n-type region with excellent accuracy. Similarly, when all parts of a plurality of probe positions arranged in the horizontal direction are located in the n-type main region 21b at a certain scanning position, the accumulated detection data can accurately show the AlN molar fraction of the n-type main region 21b with excellent accuracy.

[0177] However, when the extending direction of the metastable n-type region of the first Ga-rich region 21a is not correctly orthogonal to the scanning direction of the line analysis at a certain scanning position, or when the extending direction of the metastable n-type region of the first Ga-rich region 21a is curved or non-linear, etc., when a part of a plurality of probe positions arranged in the horizontal direction or a part of the probe range (diameter about 2 nm) of each probe position is located in the near-metastable n-type region outside the metastable n-type region or in the n-type main region 21b, the accumulated detection data shows the average AlN molar fraction of a plurality of pin positions, showing a value higher than the AlN molar fraction of the metastable n-type region.

[0178] Similarly, when most of a plurality of probe positions arranged in the horizontal direction are located in the n-type main region 21b at a certain scanning position, when a part of a plurality of probe positions or a part of the probe range (diameter about 2 nm) of each probe position is located in a region where the AlN molar fraction is locally low or high due to the mass movement of Ga passing through the n-type main region 21b or in a region where the AlN molar fraction is locally low outside the n-type main region 21b (a layered region other than the first Ga-rich region 21a, the metastable n-type region or the near-metastable n-type region within the first Ga-rich region 21a), the accumulated detection data shows the average AlN molar fraction of a plurality of probe positions, showing a value lower or higher than the average AlN molar fraction of the n-type main region 21b ( the target value of the AlN molar fraction of the n-type cladding layer 21).

[0179] In Figures 12A - 12E , the results of the composition analysis within the n-type cladding layer 21 of the six measurement regions A to E shown in Figure 11 by the line analysis of the EDX measurement are shown.Figures 12A - 12E A chart of the composition analysis results for each of the measurement regions A to E shown, with the horizontal axis showing the scanning position along the vertical line at the center of each measurement region and the vertical axis showing the measurement results of the AlN mole fraction and the GaN mole fraction. The 0 nm of the scanning position on the horizontal axis indicates the position of the horizontal line shown in the center of each measurement region (the position where at least one first Ga-rich region 21a is assumed to exist). The scanning positions are shown as positive values on the lower side (AlN layer 12 side) from the origin (0 nm) and negative values on the upper side (active layer 22 side).

[0180] In the EDX measurement, as described above, since the X-rays emitted from the probe position are weak, at each scanning position, even when the detection data (X-ray intensity of each component) of the probe position is accumulated in the horizontal direction, generally the measurement error is large. For example, when correcting based on the AlN mole fraction (100%) of the AlN layer 12 determined in advance for the AlN mole fraction, the measurement error of the detection data at each scanning position is about ±2 to 3% near the reference AlN layer 12, and the measurement accuracy decreases as the distance from the AlN layer 12 increases. Therefore, in the present embodiment, in the region far from the AlN layer 12, since the measurement error at each scanning position is suppressed to about ±2 to 3%, using the same specimen piece as the specimen piece used in the EDX measurement, the composition analysis of Al and Ga in the n-type cladding layer 21 based on the Rutherford backscattering (RBS) analysis method is performed, and the results obtained by the EDX measurement are corrected using the RBS analysis results. Figures 12A - 12E The AlN mole fraction and the GaN mole fraction of the measurement regions A to E shown show the results of this correction.

[0181] By Figure 12A It can be confirmed that in measurement region A, in region A1 with a scanning position of about -161 nm to about -121 nm, region A2 with a scanning position of about -105 nm to about -97 nm, and region A3 with a scanning position of about 0 nm to about 8 nm, there are first Ga-rich regions 21a. The AlN mole fraction at the 6 scanning positions in region A1 is 55.8% to 61.0% (2 points within 58.3% ± 2%, 2 points within 58.3% ± 1%). The AlN mole fraction at the 2 scanning positions in region A2 is 58.1% to 58.9% (2 points within 58.3% ± 1%). The AlN mole fraction at the 2 scanning positions in region A3 is 56.0% to 59.1% (1 point within 58.3% ± 1%).

[0182] By Figure 12BIt can be confirmed that in the measurement region B, in the region B1 from approximately 0 nm to approximately 8 nm at the scanning position, there is a first Ga-rich region 21a. The AlN mole fraction at two scanning positions within the region B1 is 58.3% to 60.0% (one point is within 58.3% ± 1%).

[0183] By Figure 12C It can be confirmed that in the measurement region C, in the region C1 from approximately 0 nm to approximately 8 nm at the scanning position and the region C2 from approximately 49 nm to approximately 57 nm at the scanning position, there is a first Ga-rich region 21a. The AlN mole fraction at two scanning positions within the region C1 is 59.3% to 61.1% (one point is within 58.3% ± 1%). The AlN mole fraction at two scanning positions within the region C2 is 59.0% to 60.7% (one point is within 58.3% ± 1%).

[0184] By Figure 12D It can be confirmed that in the measurement region D, in the region D1 at the scanning position of approximately -187 nm and the region D2 at the scanning position of approximately 0 nm, there is a first Ga-rich region 21a. The AlN mole fraction at one scanning position within the region D1 is 59.4%. The AlN mole fraction at one scanning position within the region D2 is 58.8%.

[0185] By Figure 12E It can be confirmed that in the measurement region E, in the region E1 from approximately -8 nm to approximately 0 nm at the scanning position and the region E2 from approximately 57 nm to approximately 65 nm at the scanning position, there is a first Ga-rich region 21a. The AlN mole fraction at two scanning positions within the region E1 is 58.1% to 59% (two points are within 58.3% ± 1%). The AlN mole fraction at two scanning positions within the region E2 is 58.9% to 59.5% (one point is within 58.3% ± 1%).

[0186] From the above, considering the measurement error of about ±2 to 3% at each of the above scanning positions and the possibility that the average AlN mole fraction at multiple probe positions arranged in the horizontal direction for the first Ga-rich region 21a shows a value higher than the AlN mole fraction of the quasi-stable n-type region, it can be confirmed that within the first Ga-rich region 21a of each of the regions A1 to A3, B1, C1, C2, D1, D2, E1, and E2 in the measurement regions A to E, there is a quasi-stable n-type region with an AlN mole fraction of 58.3%. In addition, it can be seen that the first Ga-rich region 21a exists respectively in the measurement regions A and B close to the upper part above the upper surface of the n-type cladding layer 21, the measurement regions C and D in the central part, and the measurement region E close to the lower part below the AlN layer 12, and is evenly dispersed within the n-type cladding layer 21.

[0187] In addition, by Figures 12A - 12E, it can be confirmed that the majority of the AlN mole fraction in the n-type host region 21b adjacent to each of the regions A1 to A3, B1, C1, C2, D1, D2, E1, and E2 in the measurement regions A to E is in the range of about 60% to about 66%. As described above, since the target value of the AlN mole fraction of the n-type cladding layer 21 of the sample used for EDX measurement is 63%, considering the measurement error of about ±2 to 3% at each scanning position and the possibility that the average AlN mole fraction at multiple probe positions arranged in the horizontal direction in the n-type host region 21b shows a value higher or lower than the average AlN mole fraction of the n-type host region 21b, it can be known that Figures 12A - 12E shows the AlN mole fraction of the n-type host region 21b with excellent accuracy.

[0188] Next, the results of measuring the AlN mole fractions of the first Ga-rich region 21a and the n-type host region 21b in the n-type cladding layer 21 by the CL (cathodoluminescence) method will be described. The sample wafer used for the measurement was made in the same manner as Figure 10 the sample wafer used for the observation of the HAADF-STEM image shown.

[0189] Figure 13 A scanning electron microscope (SEM) image showing the cross section of the n-type cladding layer 21 of the above sample wafer is shown. The measurement regions (a to d) surrounded by the dotted line in this cross section show the incident regions of the electron beam irradiated for each measurement. The measurement regions a and b are located at a distance of about 1700 nm from the upper surface of the AlN layer 12, and the measurement regions c and d are located at a distance of about 700 nm from the upper surface of the AlN layer 12. In each measurement region, an electron beam with a beam diameter of 50 nm (diameter) is moved in the horizontal direction and irradiated once at 50 nm intervals, for a total of 10 irradiations, and the CL spectrum of each irradiation is measured.

[0190] Figure 14 The first CL spectrum, which is the average of the two CL spectra with wavelength distributions closer to the short wavelength among the 10 CL spectra in each measurement region (a to d), and the second CL spectrum, which is the average of the two CL spectra with wavelength distributions closer to the long wavelength, are classified and shown for each measurement region (a to d).

[0191] Since the separation distance between the two ends of the centers of the 10 electron beams within each measurement region (a to d) is 450 nm, both the first Ga-rich region 21a and the n-type main region 21b exist within the 10 irradiation regions. Since the volume ratio of the first Ga-rich region 21a that occupies the entire n-type cladding layer 21 is small, the first CL spectrum mainly shows the CL spectrum of the n-type main region 21b. On the other hand, in the second CL spectrum, although it includes the CL spectrum of the first Ga-rich region 21a, because the average width of the cross-section perpendicular to the extending direction of the first Ga-rich region 21a is about 20 nm, the n-type main region 21b is partially included within the irradiation range of a beam diameter of 50 nm. Therefore, the second CL spectrum becomes a composite spectrum of the CL spectrum of the first Ga-rich region 21a and the CL spectrum of the n-type main region 21b. However, even when the centers of the electron beams of the two CL spectra with wavelength distributions closer to the long wavelength are located at the center in the width direction of the first Ga-rich region 21a, the electron beams in the central part within the irradiation range are concentrated in the first Ga-rich region 21a with a lower energy level, and the possibility of specifically exciting the first Ga-rich region 21a is high, and the second CL spectrum mainly shows the CL spectrum of the first Ga-rich region 21a.

[0192] Here, the reason for setting the first CL spectrum as the average of the two CL spectra with wavelength distributions closer to the short wavelength and the second CL spectrum as the average of the two CL spectra with wavelength distributions closer to the long wavelength is that the irradiation positions of the electron beams in each measurement region are randomly set, and the irradiation ranges of one CL spectrum closer to the shortest wavelength and one CL spectrum closer to the long wavelength are different for each measurement region. Considering that the measurement results vary greatly for each measurement region, or there may be cases where it is difficult to select one CL spectrum closer to the shortest wavelength and one CL spectrum closer to the long wavelength, etc., in order to suppress the non-uniformity for each measurement region, two CL spectra with wavelength distributions closer to the short wavelength and two CL spectra with wavelength distributions closer to the long wavelength are mechanically selected to obtain the average.

[0193] First, the first CL spectra of each measurement region (a to d) are studied. In the measurement regions a to d, the high-platform peak region of the emission wavelength extends to a range of about 2254 nm.

[0194] When the peak wavelength of about 224 nm in the measurement regions a to d is converted into the AlN mole fraction, it corresponds to about 65%. Considering the measurement error of about ±3% in terms of the AlN mole fraction conversion, the CL wavelength in the peak region of the above first CL spectrum is substantially consistent with the average AlN mole fraction Xb of the n-type main region 21b ( target value 63%).

[0195] Furthermore, in the first CL spectra of the measurement regions a to d, the band of the long-wavelength component on the long-wavelength side, which is longer than the above peak wavelength range, is wider than the band of the short-wavelength component on the short-wavelength side, which is shorter than the above peak wavelength range. It can be seen that a mass movement of Ga occurs in two irradiation ranges of the first CL spectra corresponding to the respective measurement regions (a to d). Furthermore, it can be seen that the above long-wavelength component repeats the CL wavelength (about 266 nm) of the metastable n-type region with an AlN mole fraction of 58.3% existing in the first Ga-rich region 21a, and a part of each irradiation range of the first CL spectra corresponding to the measurement regions a to d contains the first Ga-rich region 21a formed by the mass movement of Ga.

[0196] Next, the second CL spectra of the respective measurement regions (a to d) are studied. In the measurement regions a to d, the peak of the emission wavelength exists around about 266 nm. Furthermore, in the measurement regions a to d, it is considered that there is a gentle plateau-like undulation (measurement regions a and b), or a second peak (measurement region c), or a shoulder-like undulation (measurement region d) around about 254 nm on the short-wavelength side of the above peak wavelength.

[0197] When the peak wavelength of about 266 nm in the measurement regions a to d is converted into an AlN mole fraction, it corresponds to about 58.3%. By the CL method, although there is a measurement error of about ±3% in terms of the AlN mole fraction conversion, it is possible to distinguish the AlN mole fraction in the first Ga-rich region 21a. The wavelengths of the undulations around about 254 nm in the measurement regions a to d are consistent with the peak wavelength range of the first CL spectra of the respective measurement regions (a to d). Thus, from Figure 14 and the above description, it can be seen that each of the second CL spectra of the measurement regions a to d appears as a composite spectrum of the CL spectra of the metastable n-type region in the first Ga-rich region 21a and the metastable near n-type region with a slightly higher AlN mole fraction than that of the metastable n-type region, and the CL spectrum of the n-type main region 21b.

[0198] As described above, through Figure 14 the first CL spectra of the respective measurement regions a to d shown, it can be seen that the AlN mole fraction of the n-type main region 21b is almost the same as the target value of 63% of the AlN mole fraction of the n-type cladding layer 21. In addition, through the second CL spectra of the respective measurement regions a to d, it can be seen that in the first Ga-rich region 21a, while containing a metastable n-type region with an AlN mole fraction of 58.3%, there exists a metastable near n-type region with a higher AlN mole fraction than that of the metastable n-type region. In addition, although the analysis results shown in the first and second CL spectra of the respective measurement regions a to d shown in Figure 14 differ in terms of spatial resolution, etc. due to different analysis methods, compared with those shown in Figures 12A - 12EThe analysis results based on EDX measurement are quite consistent.

[0199] However, regarding the well layer 220, neither Figures 12A - 12E nor Figure 14 the compositional analysis based on the EDX method and the CL method shown was performed. Since the film thickness of the well layer 220 is extremely thin, ranging from 1.5 unit cells to 7 unit cells (3 - 14 ML), it is basically not suitable for compositional analysis based on the EDX method. On the other hand, the part that needs attention in the measurement of the CL spectrum of the well layer 220 is different from the compositional analysis of the n-type cladding layer 21 based on the CL method. The AlN mole fraction of the barrier layer 221, the n-type cladding layer 21, and the electron blocking layer adjacent to the well layer 220 is about 8.3% higher than the AlN mole fraction (50% or 41.7% or 33.3%) of the inclined region BA (the second Ga-rich region 220a) of the well layer 220. For this reason, the beam diameter of the electron beam irradiated toward the inclined region BA of the well layer 220 is as large as 50 nm. Since the high-energy electrons in the center of the electron beam are concentrated in the inclined region BA of the well layer 220 with a low energy state, the emission wavelength of the inclined region BA of the well layer 220 can be correctly measured using the CL method.

[0200] [Second Embodiment]

[0201] In the above first embodiment, when the barrier layer 221 is composed of an AlGaN-based semiconductor with an AlN mole fraction not equal to 100%, as an example, it is shown that the overall AlN mole fraction of the barrier layer 221 including the third Ga-rich region 221a is in the range of 58.3% to 90%, and the AlN mole fraction of the barrier main region 221b is in the range of 68% to 90%. In order to ensure the local carrier distribution effect of the third Ga-rich region 221a, the difference in the AlN mole fraction between the third Ga-rich region 221a and the barrier main region 221b is made 1% or more.

[0202] In the second embodiment, similar to the first Ga-rich region 21a of the n-type cladding layer 21 and the second Ga-rich region 220a of the well layer 220 in the first embodiment, the third Ga-rich region 221a of the barrier layer 221 is also preferably composed of the first or second metastable AlGaN. Here, since the overall AlN mole fraction of the barrier layer 221 is in the range of 58.3% to 90%, the first metastable AlGaN applicable to the third Ga-rich region 221a is Al2Ga1N3 or Al5Ga1N6 with an AlGaN composition ratio of an integer ratio. In addition, it is considered that Al3Ga1N4 of the second metastable AlGaN is also applicable to the third Ga-rich region 221a. However, for the second metastable AlGaN, Al 11 Ga1N 12Due to the excessively high composition ratio of Al, Ga, which is easily mobile, enters the symmetrically arranged sites, while the large amount of Al enters the sites randomly. The possibility that the atomic arrangements of Al and Ga cannot form a symmetric arrangement increases, and the atomic arrangements of Al and Ga approach a random state. Due to the above-mentioned decrease in stability, it is difficult to apply to the third Ga-rich region 221a.

[0203] However, Figures 6 - 8 In the simulation results of the emission wavelength of the well layer 220 shown, three cases where the AlN mole fraction of the third Ga-rich region 221a of the barrier layer 221 is assumed to be 58.3%, 75%, and 83.3% are considered. However, these cases correspond to the AlN mole fractions of metastable AlGaN with AlGaN composition ratios of Al2Ga1N3, Al3Ga l N4, and Al5Ga1N6.

[0204] When the third Ga-rich region 221a is composed of metastable AlGaN of Al2Ga1N3, Al3Ga1N4, and Al5Ga1N6, it is preferable that the AlN mole fraction of the barrier main region 221b corresponds to the three AlN mole fractions of the third Ga-rich region 221a and is within the ranges of 68% - 74%, 76% - 82%, or 85% - 90% respectively. Here, when the third Ga-rich region 221a is composed of Al5Ga1N6 of metastable AlGaN, in order to prevent the randomly mixed existence of Al 11 Ga1N 12 with low stability, it is preferable to set the AlN mole fraction of the barrier main region 221b not to exceed 90%.

[0205] The manufacturing method of the third Ga-rich region 221a and the barrier main region 221b of the barrier layer 221 is as described above. In the same manner as the n-type cladding layer 21, with the AlN mole fraction set for the barrier main region 221b as the target value, the barrier layer 221 is grown under growth conditions that are likely to expose the multi-step platforms.

[0206] When growing Al2Ga1N3, which is the first metastable AlGaN, in the third Ga-rich region 221a, the target value Xd of the AlN mole fraction of the barrier layer 221 is set within the range of 68% - 74%. In the same manner, when growing Al3Ga1N4, which is the second metastable AlGaN, in the third Ga-rich region 221a, the target value Xd of the AlN mole fraction of the barrier layer 221 is set within the range of 76% - 82%. When growing Al5Ga1N6, which is the first metastable AlGaN, in the third Ga-rich region 221a, the target value Xd of the AlN mole fraction of the barrier layer 221 is set within the range of 85% - 90%.

[0207] Therefore, the target value Xd of the AlN mole fraction of the barrier layer 221 is set to be more than 1% higher than the AlN mole fraction of the metastable AlGaN (target metastable AlGaN) formed in the third Ga-rich region 221a and less than the AlN mole fraction of the closest metastable AlGaN having a larger AlN mole fraction than that of the target metastable AlGaN. For this purpose, similar to the first Ga-rich region 21a of the n-type cladding layer 21, as the target metastable AlGaN is stably formed in the third Ga-rich region 221a, the difference in the AlN mole fraction between the third Ga-rich region 221a and the barrier main region 221b is ensured to be 1% or more, and the carriers in the barrier layer 221 are locally distributed in the third Ga-rich region 221a having a smaller bandgap energy than the barrier main region 221b.

[0208] By forming the third Ga-rich region 221a with a metastable AlGaN having a high stability, the variation in the mixed crystal mole fraction due to drift of the crystal growth apparatus or the like is suppressed. The third Ga-rich region 221a that generates a local distribution of carriers in the barrier layer 221 is stably formed corresponding to the AlN mole fraction of the metastable AlGaN used. As a result, similar to the n-type cladding layer 21, in the barrier layer 221, current preferentially and stably flows into the third Ga-rich region 221a, and the suppression of the characteristic variation of the light-emitting element 1 can be further achieved.

[0209] [Other Embodiments]

[0210] Hereinafter, modified examples of the above-described first and second embodiments will be described.

[0211] (1) In the above-described first and second embodiments, it is assumed that the active layer 22 is configured in a multi-quantum well structure in which two or more well layers 220 formed of an AlGaN-based semiconductor and one or more barrier layers 221 formed of an AlGaN-based semiconductor or an AlN-based semiconductor are alternately stacked. However, the active layer 22 may have a single quantum well structure in which the well layer 220 is only one layer and does not include the barrier layer 221 (quantum barrier layer). For the related single quantum well structure, it is also clear that the effects caused by the n-type cladding layer 21 employed in the above-described embodiments can be exhibited.

[0212] (2) In the above-described embodiment, as an example of the growth conditions of the n-type cladding layer 21, the supply amount and flow rate of the source gas or carrier gas used in the metalorganic chemical vapor deposition method are set corresponding to the average AlN mole fraction of the entire n-type AlGaN layer constituting the n-type cladding layer 21. That is, when the average AlN mole fraction of the entire n-type cladding layer 21 is set to a constant value in the vertical direction, it is assumed that the supply amount and flow rate of the above-described source gas, etc. are controlled to be constant. However, the supply amount and flow rate of the above-described source gas, etc. do not necessarily have to be controlled to be constant.

[0213] (3) In the above-described embodiment, as an example, the top view shapes of the first region R1 and the p-electrode 26 are comb-shaped, but the top view shape is not limited to the comb shape. In addition, the top view shape may be such that there are a plurality of first regions R1 and each is surrounded by one second region R2.

[0214] (4) In the above-described embodiment, although a sapphire substrate 11 having a tilt angle with respect to the (0001) plane is exemplified, and a case where the base portion 10 exposing a stepped platform is used on the surface of the AlN layer 12, the magnitude of the tilt angle or the direction of setting the tilt angle (specifically, the direction of tilting the (0001) plane, such as the m-axis direction or the a-axis direction, etc.) is such that a stepped platform is exposed on the surface of the AlN layer 12, and as long as the growth start point of the first Ga-rich region 21a is formed, it can be arbitrarily determined.

[0215] (5) In the above-described embodiment, as the light-emitting element 1, as Figure 1 exemplified, although a light-emitting element 1 including the base portion 10 of the sapphire substrate 11 is exemplified, the sapphire substrate 11 (and also a part or all of the layers included in the base portion 10) can be removed by lift-off or the like. In addition, the substrate constituting the base portion 10 is not limited to the sapphire substrate.

[0216] -Industrial Applicability-

[0217] The present invention can be applied to a nitride semiconductor ultraviolet light-emitting element configured to include a light-emitting element structure portion in which an n-type layer, an active layer, and a p-type layer of an AlGaN-based semiconductor having a wurtzite structure are stacked in the vertical direction.

[0218] -Symbol Explanation-

[0219] 1: Nitride semiconductor ultraviolet light-emitting element

[0220] 10: Base portion

[0221] 11: Sapphire substrate

[0222] 11a: Main surface of the sapphire substrate

[0223] 12: AlN layer

[0224] 20: Light-emitting element structure part

[0225] 21: n-type cladding layer (n-type layer)

[0226] 21a: First Ga-rich region (n-type layer)

[0227] 21b: n-type main region (n-type layer)

[0228] 22: Active layer

[0229] 220: Well layer

[0230] 220a: Second Ga-rich region

[0231] 220b: Well main region

[0232] 221: Barrier layer

[0233] 221a: Third Ga-rich region

[0234] 221b: Barrier main region

[0235] 23: Electron blocking layer (p-type layer)

[0236] 24: p-type cladding layer (p-type layer)

[0237] 25: p-type contact layer (p-type layer)

[0238] 26: p electrode

[0239] 27: n electrode

[0240] 100: Substrate

[0241] 101: AlGaN-based semiconductor layer

[0242] 102: Template

[0243] 103: n-type AlGaN-based semiconductor layer

[0244] 104: Active layer

[0245] 105: p-type AlGaN-based semiconductor layer

[0246] 106: p-type contact layer

[0247] 107: n electrode

[0248] 108: p electrode

[0249] BL: Boundary line between the first region and the second region

[0250] BA: Boundary region (tilted region)

[0251] R1: The first region

[0252] R2: The second region

[0253] T: Platform

[0254] TA: Platform area.

Claims

1. A nitride semiconductor ultraviolet light-emitting element includes a light-emitting element structure portion in which an n-type layer, an active layer, and a p-type layer including a wurtzite-structured AlGaN-based semiconductor are stacked in the vertical direction, and the peak emission wavelength is in the range of 265 nm to 300 nm. The nitride semiconductor ultraviolet light-emitting element is characterized in that the n-type layer is composed 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 composed of an AlGaN-based semiconductor, the p-type layer is composed of a p-type AlGaN-based semiconductor, each semiconductor layer in the n-type layer and the active layer is an epitaxial growth layer having a surface formed with a multi-step terrace parallel to the (0001) plane, The n-type layer has a plurality of first Ga-rich regions, which are layered regions with a locally low AlN mole fraction evenly dispersed within the n-type layer and contain Al7Ga5N with an integer ratio of the AlGaN composition ratio 12 in the n-type AlGaN region the nitride semiconductor ultraviolet light-emitting element has a portion where each extending direction of the layered region on a first plane orthogonal to the upper surface of the n-type layer is inclined with respect to the intersection line of the upper surface of the n-type layer and the first plane.

2. The nitride semiconductor ultraviolet light-emitting element according to claim 1, characterized in that the AlN mole fraction of the n-type main region other than the layered region in the n-type layer is in the range of 60% to 66%.

3. The nitride semiconductor ultraviolet light-emitting element according to claim 1 or 2, characterized in that a boundary region portion between adjacent terraces of the multi-step terrace of the well layer has a second Ga-rich region where the AlN mole fraction is also locally low within the well layer, Within the second Ga-rich region, there exists an AlGaN region where the composition ratio of AlGaN is an integer ratio, such as Al1Ga1N2, Al5Ga7N 12 or Al1Ga2N3.

4. The nitride semiconductor ultraviolet light-emitting element according to claim 3, characterized in that within the second Ga-rich region, there is an AlGaN region where the AlGaN composition ratio becomes an integer ratio of Al1Ga1N2, the AlN mole fraction outside the boundary region portion of the well layer is in the range of 50.1% to 54%.

5. The nitride semiconductor ultraviolet light-emitting element according to claim 3, characterized in that In the second Ga-rich region, there is an AlGaN region of Al5Ga7N in which the AlGaN composition ratio becomes an integer ratio 12 of AlGaN the AlN mole fraction outside the boundary region portion of the well layer is in the range of 41.8% to 46%.

6. The nitride semiconductor ultraviolet light-emitting element according to claim 3, characterized in that within the second Ga-rich region, there is an AlGaN region where the AlGaN composition ratio becomes an integer ratio of Al1Ga2N3, the AlN mole fraction outside the boundary region portion of the well layer is in the range of 33.4% to 37%.

7. The nitride semiconductor ultraviolet light-emitting element according to claim 1 or 2, characterized in that the active layer has a multi-quantum well structure including two or more of the well layers, a barrier layer composed of an AlGaN-based semiconductor exists between the two well layers.

8. The nitride semiconductor ultraviolet light-emitting element according to claim 7, characterized in that the barrier layer is composed of an AlGaN-based semiconductor, Among the barrier layers between the well layers located on the second layer, at least a boundary region portion between adjacent platforms of the multi-step platform of the barrier layer closest to the p-type layer side has a third Ga-rich region where the AlN mole fraction is locally low also within the barrier layer.

9. The nitride semiconductor ultraviolet light-emitting element according to claim 8, wherein an AlGaN region where the AlGaN composition ratio is an integer ratio, such as Al2Ga1N3, Al3Ga1N4, or Al5Ga1N6, exists within the third Ga-rich region of the barrier layer.

10. The nitride semiconductor ultraviolet light-emitting element according to claim 1 or 2, wherein it further includes a base portion including a sapphire substrate, the sapphire substrate has a main surface inclined by a given angle only with respect to the (0001) plane, and the light-emitting element structure portion is formed above this main surface, at least each semiconductor layer from the main surface of the sapphire substrate to the surface of the active layer is an epitaxial growth layer having a surface formed with a multi-step platform parallel to the (0001) plane.

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