Nitride semiconductor ultraviolet light emitting device and its manufacturing method

Abstract

This nitride semiconductor UV light-emitting element comprises an n-type layer, an active layer, and a p-type layer which are each formed from an AlGaN semiconductor having a wurtzite structure. Each of the semiconductor layers is an epitaxially grown layer having a surface in which multi-stage terraces parallel to the (0001) plane are formed. The n-type layer has a layer region where an extension direction in which the AlN molar fraction is locally low is inclined with respect to the upper surface. A first average AlN molar fraction in the entire range in the depth direction of the n-type layer is more than (n-0.25) / 12 but less than (n+0.25) / 12. On the upper side of at least one specific depth of one or more depths which correspond to a depth d from the upper end of the n-type layer and in which a second average AlN molar fraction becomes equal to n / 12, there is a region in which the AlN molar fraction is greater than n / 12. On the lower side of the specific depth, there is a region in which the AlN molar fraction is less than n / 12. In the layer region, an intermediate AlGaN region in which the AlN molar fraction is (n-0.5) / 12 is formed.

Description

[Technical field]

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

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

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

[0004] When the active layer is an AlGaN-based semiconductor, the n-type nitride semiconductor layer and the p-type nitride semiconductor layer sandwiching the active layer are composed of AlGaN-based semiconductors with a higher AlN molar fraction than the active layer. However, since it is difficult for a p-type nitride semiconductor layer with a high AlN molar fraction to form a good ohmic contact with a p-electrode, it is generally practiced to form a p-type contact layer that can make a good ohmic contact with a p-electrode made of a p-type AlGaN-based semiconductor (specifically, p-GaN) with a low AlN molar fraction on the top layer of the p-type nitride semiconductor layer. Since the AlN molar fraction of this p-type contact layer is smaller than that of the AlGaN-based semiconductor constituting the active layer, ultraviolet light emitted from the active layer toward the p-type nitride semiconductor layer side is absorbed by the p-type contact layer and cannot be effectively extracted outside the element. For this reason, a typical ultraviolet light-emitting diode with an active layer of an AlGaN-based semiconductor adopts an element structure as shown in FIG. 18, and effectively extracts ultraviolet light emitted from the active layer toward the n-type nitride semiconductor layer side to the outside of the element (for example, see Patent Documents 1 and 2 below).

[0005] As shown in FIG. 18, a typical ultraviolet light-emitting diode is configured by depositing an AlGaN semiconductor layer 101 (e.g., an AlN layer) on a substrate 100 such as a sapphire substrate, and then depositing an n-type AlGaN semiconductor layer 103, an active layer 104, a p-type AlGaN semiconductor layer 105, and a p-type contact layer 106 in that order on a template 102, and then etching away parts of the active layer 104, the p-type AlGaN semiconductor layer 105, and the p-type contact layer 106 until the n-type AlGaN semiconductor layer 103 is exposed, and then forming an n-electrode 107 on the exposed surface of the n-type AlGaN semiconductor layer 103 and a p-electrode 108 on the surface of the p-type contact layer 106.

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

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

[0008] [Patent Document 1] International Publication No. 2014 / 178288 [Patent Document 2] International Publication No. 2016 / 157518 [Patent Document 3] International Publication No. 2019 / 159265 [Patent Document 4] JP 2012-089754 A [Non-patent literature]

[0009] Non-patent document 1: Y. Nagasawa, et al., "Comparison of Al x Ga 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) 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) Non-patent document 3: Shigeaki Sumiya, et al., "AlGaN-Based Deep Ultraviolet Light-Emitting Diodes Grown on Epitaxial AlN / Sapphire Templates", Journal of Applied Physics, Vol. 47, NO. 1, 2008, pp.43-46 Summary of the Invention [Problem to be solved by the invention]

[0010] Ultraviolet light emitting elements made of AlGaN-based semiconductors are fabricated on substrates such as sapphire substrates by well-known epitaxial growth methods such as metal organic vapor phase epitaxy (MOVPE). However, when producing ultraviolet light emitting elements, the characteristics of the ultraviolet light emitting elements (emission wavelength, wall plug efficiency, forward bias, and other characteristics) vary due to drift in the crystal growth equipment, so it is not necessarily easy to produce them with a stable yield.

[0011] Drift in crystal growth equipment occurs due to changes in the effective temperature at the crystal growth site caused by deposits on the tray, chamber walls, etc. For this reason, in the past, in order to suppress drift, an experienced person would study the growth history and subtly change the set temperature or the composition of the source gas, or a growth schedule would be fixed for a certain period of time and maintenance such as cleaning would be performed in the same way at certain intervals, but it is difficult to completely eliminate drift.

[0012] Therefore, the inventors of the present application have discovered that by growing an n-type AlGaN-based semiconductor layer so that "metastable AlGaN" having an AlGaN composition ratio of an integer ratio, as described below, is predominantly present in a layered region having a locally low AlN mole fraction formed by Ga segregation in a cladding layer composed of an n-type AlGaN-based semiconductor layer, which provides a low-resistance current path to the active layer, characteristic fluctuations due to drift of the crystal growth apparatus, etc. can be suppressed, and nitride semiconductor ultraviolet light-emitting devices having the desired light-emitting characteristics can be stably produced, and the inventors have proposed using metastable AlGaN in the layered region of an n-type AlGaN-based semiconductor layer (see the specifications of international applications such as PCT / JP2020 / 024827, PCT / JP2020 / 024828, PCT / JP2020 / 026558, PCT / JP2020 / 031620, etc.).

[0013] <Characteristics of metastable AlGaN> For convenience of explanation, first, the characteristics of "metastable AlGaN" in which the AlGaN composition ratio is expressed as a predetermined integer ratio will be explained.

[0014] If metastable AlGaN is not taken into consideration, ternary alloys such as AlGaN are in a crystalline state in which group 3 elements (Al and Ga) are mixed randomly, and can be approximately explained as a "random configuration." However, because the covalent bond radius of Al is different from that of Ga, the more symmetric the atomic arrangement of Al and Ga in the crystal structure, the more stable the structure will generally be.

[0015] In AlGaN-based semiconductors with the wurtzite structure, two types of arrangements can exist: a random arrangement with no symmetry, and a stable symmetric arrangement. Here, at a certain ratio, a state in which the symmetric arrangement becomes dominant appears. As will be described later, a symmetric arrangement structure of Al and Ga appears in "metastable AlGaN" where the AlGaN composition ratio (composition ratio of Al, Ga, and N) is expressed as a certain integer ratio.

[0016] In this symmetrical arrangement structure, even if the amount of Ga supplied to the crystal growth surface increases or decreases slightly, the high symmetry results in an energetically stable mixed crystal mole fraction, and it is possible to prevent the concentration of Ga, which is prone to mass transfer, from becoming uncontrollable.

[0017] <Symmetric arrangement of Al and Ga in metastable AlGaN> Next, we will explain why a symmetric arrangement of Al and Ga appears in "metastable AlGaN".

[0018] Figure 1 shows a schematic diagram of one unit cell (two monolayers) in the c-axis direction of AlGaN. In Figure 1, the white circles indicate sites where group 3 element atoms (Al, Ga) are located, and the black circles indicate sites where group 5 element atoms (N) are located. In the following explanation, a monolayer is abbreviated as ML. In Figure 1, one unit cell is abbreviated as 2ML.

[0019] In FIG. 1, the site planes of group 3 elements (A3 plane, B3 plane) and the site planes of group 5 elements (A5 plane, B5 plane) shown as hexagons are both parallel to the (0001) plane. In each site of the A3 plane and the A5 plane (collectively referred to as the A plane), there are six sites at each vertex of the hexagon and one site at the center of the hexagon. The same is true for the B3 plane and the B5 plane (collectively referred to as the B plane), but FIG. 1 shows only three sites present in the hexagon of the B plane. The sites of the A plane overlap in the c-axis direction, and the sites of the B plane overlap in the c-axis direction. However, an atom (N) at one site on the B5 surface forms four-coordinate bonds with atoms (Al, Ga) at three sites on the A3 surface located above the B5 surface and with atoms (Al, Ga) at one site on the B3 surface located below the B5 surface, and an atom (Al, Ga) at one site on the B3 surface forms four-coordinate bonds with an atom (N) at one site on the B5 surface located above the B3 surface and with atoms (N) at three sites on the A5 surface located below the B3 surface. Therefore, as shown in Figure 1, each site on the A surface does not overlap with each site on the B surface in the c-axis direction.

[0020] FIG. 2 shows the positional relationship between each site on the A plane and each site on the B plane as a plan view of the A3 plane and the B3 plane seen from the c-axis direction. The black and white circles in FIG. 2 indicate whether the site plane is the A3 plane or the B3 plane. In both the A3 plane and the B3 plane, each of the six vertices of the hexagon is shared by two other adjacent hexagons, and the central site is not shared with other hexagons, so that one hexagon essentially has sites for three atoms. Therefore, there are six sites for atoms of group 3 elements (Al, Ga) and six sites for atoms of group 5 elements (N) per unit cell (2ML). Therefore, there are the following five cases of AlGaN composition ratios expressed as integer ratios excluding GaN and AlN. 1) Al 1 Ga 5 N 6 , 2) Al 2 Ga 4 N 6 (=Al 1 Ga 2 N 3 ), 3) Al 3 Ga 3 N 6 (=Al 1 Ga 1 N 2 ), 4) Al 4 Ga 2 N 6 (=Al 2 Ga 1 N 3 ), 5) Al 5 Ga 1 N 6 .

[0021] Here, the above 2) to 4) Al 1 Ga 2 N 3 and Al 1 Ga 1 N 2 and Al 2 Ga 1 N 3As shown in Fig. 3, both the A3 plane and the B3 plane can have the same symmetrical arrangement of Al and Ga, so metastable AlGaN with the AlGaN composition ratios 2) to 4) above is formed in the c-axis direction in 1ML units. Fig. 3 shows an example of an arrangement structure on only one of the A3 plane and the B3 plane. In Fig. 3, Ga is shown as a large black circle and Al is shown as a small black circle.

[0022] On the other hand, Al in 1) above 1 Ga 5 N 6 One of the A3 and B3 faces is Al described in 2) above. 1 Ga 2 N 3 The other is a GaN array structure (all Group 3 element sites are Ga). 1 Ga 5 N 6 One of the A3 and B3 faces is Al described in 4) above. 2 Ga 1 N 3 One side has an arrangement structure of AlN (where all the Group 3 element sites are Al) and the other side has an arrangement structure of AlN (where all the Group 3 element sites are Al).

[0023] Furthermore, the following four metastable AlGaNs with AlGaN composition ratios in integer ratios between the above 1) and 2), 2) and 3), 3) and 4), and 4) and 5) are assumed to be 6) to 9). 6) Al 3 Ga 9 N 12 (=Al 1 Ga 3 N 4 ), 7) Al 5 Ga 7 N 12 , 8) Al 7 Ga 5 N 12 , 9) Al 9 Ga 3 N 12 (=Al 3 Ga 1 N 4 ).

[0024] Here, Al in 6) above 1 Ga 3 N 4 The A3 and B3 surfaces are either Al or Al2O3. 1 Ga 1 N 2 The other is a GaN array structure (all Group 3 element sites are Ga). 5 Ga 7 N 12 The A3 and B3 surfaces are either Al or Al2O3. 1 Ga 2 N 3 The other is the Al structure of 3) above. 1 Ga 1 N 2 The above 8) Al can have a symmetrical arrangement structure of 2ML units. 7 Ga 5 N 12 The A3 and B3 surfaces are either Al or Al2O3. 1 Ga 1 N 2 The other is the Al structure of 4) above. 2 Ga 1 N 3 The above 9) Al can have a symmetrical arrangement structure of 2ML units. 3 Ga 1 N 4 The A3 and B3 surfaces are either Al or Al2O3. 1 Ga 1 N 2 One side has an arrangement structure of AlN (where all the Group 3 element sites are Al) and the other side has an arrangement structure of AlN (where all the Group 3 element sites are Al).

[0025] Therefore, the above 1), 5) to 9) Al 1 Ga 5 N 6 , Al 5 Ga 1 N 6 , Al 3 Ga 9 N 12 (=Al 1 Ga 3 N 4 ), Al 5Ga 7 N 12 , Al 7 Ga 5 N 12 , or Al 9 Ga 3 N 12 (=Al 3 Ga 1 N 4 As described above, when the symmetrical arrangement structure is different between the A3 plane and the B3 plane in a 2ML unit arrangement structure, the metastable AlGaN with the AlGaN composition ratios of 1), 5) to 9) above is formed in the c-axis direction in 2ML units.

[0026] However, although no specific examples are given for the AlGaN composition ratios of 1), 5) to 9) above, for example, by synthesizing different symmetrical arrangement structures on the A3 plane and the B3 plane on the same plane, the AlGaN composition ratios of 2) to 4) above can be obtained. 1 Ga 2 N 3 and Al 1 Ga 1 N 2 and Al 2 Ga 1 N 3 Similarly, it is considered that the A3 plane and the B3 plane can have the same symmetric arrangement structure of Al and Ga. In that case, each metastable AlGaN with the AlGaN composition ratios of 1) and 5) to 9) above can be formed in the c-axis direction in units of 1ML, similar to the metastable AlGaN with the AlGaN composition ratios of 2) to 4) above.

[0027] As a result, the metastable AlGaN shown in 1) to 9) above has a symmetrical atomic arrangement of Al and Ga, making it energetically stable. However, to grow AlGaN while maintaining a certain crystal quality, it is necessary to perform crystal growth at a high temperature of 1000°C or higher. However, it is expected that Ga atoms will continue to move around at temperatures of 1000°C or higher even after they reach a site on the crystal surface. On the other hand, unlike Ga, Al is easily adsorbed to the surface, and although it is thought that Al atoms will move somewhat after entering a site, the movement is strongly restricted. Therefore, even if it is metastable AlGaN, the above 1) Al 1 Ga 5 N 6Since the Ga composition ratio is high, at growth temperatures of around 1000°C, Ga moves violently, disrupting the symmetry of the atomic arrangement and making the atomic arrangement of Al and Ga closer to a random state, which is thought to result in a lower stability than other metastable AlGaN.

[0028] Here, the metastable AlGaN with the AlGaN composition ratios 1) to 9) above is expressed as Al n Ga 12-n N 12 and Al n Ga 12-n N 12 When expressed as a fraction, the AlN mole fraction is n / 12, but when expressed as a percentage, a fraction is generated after the decimal point. Therefore, for the sake of convenience, the six AlN mole fractions, which are expressed as fractions of 2 / 12 (= 1 / 6), 4 / 12 (= 1 / 3), 5 / 12, 7 / 12, 8 / 12 (= 2 / 3), and 10 / 12 (= 5 / 6), will be approximately expressed as 16.7%, 33.3%, 41.7%, 58.3%, 66.7%, and 83.3%.

[0029] <Issues when using metastable AlGaN in layered regions> As described above, the inventors of the present application have proposed using metastable AlGaN in the layer region of an n-type AlGaN-based semiconductor layer in order to suppress characteristic fluctuations caused by drift or the like in a crystal growth apparatus. However, problems that may arise when metastable AlGaN is predominantly formed in the layer region will be described below.

[0030] In a typical ultraviolet light-emitting diode that extracts ultraviolet light emitted from the above-mentioned active layer toward the n-type nitride semiconductor layer (n-type AlGaN-based semiconductor layer) side to the outside of the element, if the wavelength difference (λp-λae) between the peak emission wavelength (λp) of the light emitted from the active layer and the absorption edge wavelength (λae) determined by the average AlN mole fraction of the absorption spectrum of the n-type AlGaN-based semiconductor layer is 10 nm or more, absorption of the light emitted from the active layer in the n-type AlGaN-based semiconductor layer is suppressed.

[0031] The transmittance (I / I0, the ratio of the incident light intensity I0 to the transmitted light intensity I) when the light emitted from the active layer passes through the n-type AlGaN-based semiconductor layer in the depth direction attenuates exponentially with respect to the thickness of the n-type AlGaN-based semiconductor layer, which is the optical path length. While the thickness of the n-type AlGaN-based semiconductor layer is large, about 1 to 4 μm, the thickness of each layer in the layered region in the depth direction is short, about 20 nm on average. Therefore, since the AlN mole fraction of the layered region is about 4% lower than the average AlN mole fraction of the n-type AlGaN-based semiconductor layer, the absorption edge wavelength in the layered region shifts slightly to the longer wavelength side than the absorption edge wavelength λae of the entire n-type AlGaN-based semiconductor layer, but since the optical density (common logarithm of the reciprocal of the transmittance) of the layered region is smaller than that of the entire n-type AlGaN-based semiconductor layer, the absorption on the longer wavelength side than the absorption edge wavelength λae is extremely limited.

[0032] In order for metastable AlGaN to be predominantly formed in the layered region, the average AlN mole fraction of the n-type AlGaN-based semiconductor layer is set to be about 4% higher than the AlN mole fraction of the metastable AlGaN. On the other hand, since the AlGaN composition ratio of metastable AlGaN is an integer ratio, the AlN mole fraction can take discrete values ​​in increments of about 8.33%. Therefore, the setting range of the average AlN mole fraction of the n-type AlGaN-based semiconductor layer is also a discrete range about 4% higher than that of metastable AlGaN.

[0033] In an n-type AlGaN-based semiconductor layer in which metastable AlGaN is predominantly formed in the layered region, if the absorption edge wavelength λae determined by the average AlN mole fraction of the n-type AlGaN-based semiconductor layer cannot ensure a wavelength difference (λp-λae) of 10 nm or more with respect to the peak emission wavelength λp, a part of the emission spectrum (especially a part distributed on the short wavelength side) is absorbed in the n-type AlGaN-based semiconductor layer, resulting in a decrease in external quantum efficiency. Therefore, in order to avoid this decrease in external quantum efficiency, it is necessary to increase the discrete AlN mole fraction that metastable AlGaN can take by one step (about 8.33%) at a time, and similarly set the average AlN mole fraction of the n-type AlGaN-based semiconductor layer to be higher in a stepwise manner so that the wavelength difference (λp-λae) is 10 nm or more.

[0034] On the other hand, when the average AlN mole fraction of the n-type AlGaN-based semiconductor layer becomes high, the contact resistance between the n-electrode and the n-type AlGaN-based semiconductor layer and the bulk resistivity for the current flowing in the n-type AlGaN-based semiconductor layer become high, so that the parasitic resistance of the current path between the n-electrode and the active layer becomes high, and the wall plug efficiency decreases. In particular, the contact resistance between the n-type AlGaN-based semiconductor layer and the n-electrode (for example, a laminated structure of Ti / Al / Ti / Au: the bottom layer is Ti and the top layer is Au) tends to increase as the AlN mole fraction of the n-type AlGaN-based semiconductor layer becomes large, and this becomes particularly noticeable when it exceeds 60%, so it is preferable to set the average AlN mole fraction not to exceed 60% (see Non-Patent Document 3). If the AlN mole fraction of the n-type AlGaN-based semiconductor layer is 60% or less, the contact resistance can be reduced to 0.01 Ωcm by appropriately selecting the heat treatment temperature. 2 or less, the forward voltage Vf will be at a level that does not pose a practical problem (see Patent Document 4).

[0035] Therefore, if the wavelength difference (λp-λae) is made too large beyond 10 nm in order to avoid a decrease in the external quantum efficiency, the contact resistance and bulk resistivity may increase, resulting in a decrease in the wall plug efficiency. For this reason, if the average AlN mole fraction is set one step higher (about 8.33%) in order to avoid a decrease in the external quantum efficiency with respect to the target peak emission wavelength λp, the wavelength difference (λp-λae) changes from less than 10 nm to more than 10 nm, and the wall plug efficiency decreases. In this case, it is preferable to set the average AlN mole fraction without forming metastable AlGaN predominantly in the layered region so that the wavelength difference does not greatly exceed 10 nm. This makes it possible to avoid a decrease in the external quantum efficiency while simultaneously suppressing a decrease in the wall plug efficiency according to the target peak emission wavelength λp.

[0036] The present invention has been made in consideration of the above-mentioned problems, and has an object to prevent a decrease in light emission efficiency caused by the dominant formation of metastable AlGaN in a layered region having a locally low AlN mole fraction in an n-type AlGaN-based semiconductor layer by deliberately suppressing the formation of metastable AlGaN in the layered region.

Means for Solving the Problem

[0037] In order to achieve the above object, the present invention provides a nitride semiconductor ultraviolet light-emitting device including a light-emitting element structure portion in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in the vertical direction, wherein 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 made 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, the active layer, and the p-type layer is an epitaxial growth layer having a surface on which stepped terraces parallel to the (0001) plane are formed, the n-type layer has a layered region where the local AlN molar fraction is low and is dispersed in the n-type layer, each extending direction of the layered region on a first plane orthogonal to the upper surface of the n-type layer has a portion inclined with respect to the intersection line of the upper surface of the n-type layer and the first plane, an integer n is 6 or 7, a first average AlN molar fraction Xna1 over the entire depth direction of the n-type layer is, (n - 0.25) / 12 < Xna1 < (n + 0.25) / 12 within a range where, a second average AlN molar fraction Xna2(d) at a depth d from the upper end of the n-type layer changes according to the depth d, and at at least one specific depth of one or more depths where Xna2(d) = n / 12 in the n-type layer, there is a region where Xna2(d) < n / 12 above the specific depth and a region where Xna2(d) > n / 12 below the specific depth, and an intermediate AlGaN region having an AlN molar fraction of (n - 0.5) / 12 is formed in the layered region. A nitride semiconductor ultraviolet light-emitting device is provided having a first feature.

[0038] Furthermore, in the nitride semiconductor ultraviolet light emitting device according to the first aspect, a third average AlN mole fraction Xna3 in a region from an upper end of the n-type layer to the specific depth is (n-0.25) / 12 <Xna3<(n+0.25) / 12 It is preferable that the range is within the range.

[0039] Furthermore, in the nitride semiconductor ultraviolet light emitting device according to the first aspect, the second average AlN mole fraction Xna2(d) is, in the entire region in the depth direction of the n-type layer, (n-0.25) / 12 <Xna2(d)<(n+0.25) / 12 It is preferable that the range is within the range.

[0040] In order to achieve the above object, the present invention provides a method for producing a nitride semiconductor ultraviolet light emitting device having a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in a vertical direction, the method comprising: a first step of epitaxially growing the n-type layer of an n-type AlGaN-based semiconductor on a base including a sapphire substrate having a primary surface inclined at a predetermined angle with respect to a (0001) plane, and exposing multi-step terraces parallel to the (0001) plane on a surface of the n-type layer; a second step of epitaxially growing the active layer having a quantum well structure including one or more well layers made of an AlGaN-based semiconductor on the n-type layer, and exposing multi-step terraces parallel to a (0001) plane on a surface of the well layer; a third step of forming the p-type layer of a p-type AlGaN-based semiconductor on the active layer by epitaxial growth; In the first step, The integer n is 6 or 7, A first average AlN mole fraction Xna1 throughout the depth direction of the n-type layer is (n-0.25) / 12 <Xna1<(n+0.25) / 12 is in the range where The second average AlN molar fraction Xna2(d) at a depth d from the upper end of the n-type layer changes according to the depth d, and at at least one specific depth where Xna2(d) = n / 12 within the n-type layer, there is a region where Xna2(d) < n / 12 above the specific depth and a region where Xna2(d) > n / 12 below the specific depth, and a locally AlN molar fraction-low layer region uniformly dispersed within the n-type layer is formed by extending obliquely upward, so that an intermediate AlGaN region with an AlN molar fraction of (n - 0.5) / 12 is formed within the layer region, A method for manufacturing a nitride semiconductor ultraviolet light-emitting device is provided, which is characterized by forming the n-type layer as the first feature.

[0041] Furthermore, in the method for manufacturing a nitride semiconductor ultraviolet light-emitting device according to the first feature, in the first step, the third average AlN molar fraction Xna3 over the region from the upper end of the n-type layer to the specific depth is (n - 0.25) / 12 < Xna3 < (n + 0.25) / 12 It is preferable to form the n-type layer so as to be within the range.

[0042] Furthermore, in the method for manufacturing a nitride semiconductor ultraviolet light-emitting device according to the first feature, in the first step, the second average AlN molar fraction Xna2(d) is (n - 0.25) / 12 < Xna2(d) < (n + 0.25) / 12 It is preferable to form the n-type layer so as to be within the range.

[0043] Incidentally, the AlGaN-based semiconductor has the general formula Al 1-x Ga xN (0≦x≦1), but as long as the band gap energy is within a range in which the lower and upper limits are the band gap energies that GaN and AlN can have, respectively, it may contain a small amount of impurities such as group 3 elements such as B or In or group 5 elements such as P. Also, a GaN-based semiconductor is a nitride semiconductor basically composed of Ga and N, but may contain a small amount of impurities such as group 3 elements such as Al, B or In or group 5 elements such as P. Also, an AlN-based semiconductor is a nitride semiconductor basically composed of Al and N, but may contain a small amount of impurities such as group 3 elements such as Ga, B or In or group 5 elements such as P. Therefore, in the present application, GaN-based semiconductors and AlN-based semiconductors are each part of AlGaN-based semiconductors.

[0044] Furthermore, the n-type or p-type AlGaN-based semiconductor is an AlGaN-based semiconductor doped with Si or Mg as a donor or acceptor impurity. In the present application, an AlGaN-based semiconductor not specified as p-type or n-type means an undoped AlGaN-based semiconductor, but even if it is undoped, it may contain a small amount of donor or acceptor impurity that is inevitably mixed in. In addition, the first plane is not an exposed surface specifically formed in the manufacturing process of the n-type layer or a boundary surface with other semiconductor layers, but is a virtual plane extending in parallel in the vertical direction within the n-type layer. Furthermore, in this specification, the AlGaN-based semiconductor layer, the GaN-based semiconductor layer, and the AlN-based semiconductor layer are semiconductor layers composed of an AlGaN-based semiconductor, a GaN-based semiconductor, and an AlN-based semiconductor, respectively.

[0045] In the nitride semiconductor ultraviolet light emitting device according to the first aspect, the first average AlN mole fraction Xna1 is controlled within a range of ±0.25 / 12 (±approximately ±2.08%) around n / 12, which is the discrete AlN mole fraction of metastable AlGaN when the integer n is 6 or 7, and the region other than the layered region in the n-type layer (hereinafter referred to as the "n-type main body region") contains a first metastable AlGaN region (Al n Ga 12-n N 12) is formed stably. Furthermore, the second average AlN mole fraction Xna2(d) is also distributed in the vicinity of n / 12. As a result, in the layered region, a second metastable AlGaN region (Al n-1 Ga 13-n N 12 ) is not formed predominantly, but instead, an intermediate AlGaN region (Al n-0.5 Ga 12.5-n N 12 ) is formed.

[0046] Therefore, in a nitride semiconductor ultraviolet light-emitting device that utilizes metastable AlGaN in an n-type layer, when metastable AlGaN is formed predominantly in the layer region, if the target peak emission wavelength λp is within a specific wavelength range in which it is difficult to simultaneously avoid a decrease in external quantum efficiency and a decrease in wall-plug efficiency due to the selection of the AlN mole fraction being discrete values ​​in increments of approximately 8.33%, an intermediate AlGaN region is formed in the layer region and a first metastable AlGaN region is formed in the n-type body region, thereby making it possible to avoid a decrease in external quantum efficiency while simultaneously suppressing a decrease in wall-plug efficiency.

[0047] Furthermore, according to the method for manufacturing the nitride semiconductor ultraviolet light-emitting device having the first characteristic described above, the nitride semiconductor ultraviolet light-emitting device having the first characteristic described above is produced, so that it is possible to prevent a decrease in light-emitting efficiency caused by the dominant formation of metastable AlGaN in the layer region.

[0048] Furthermore, in addition to the first feature, the present invention is characterized in that the second average AlN mole fraction Xna2(d) is, in a region from an upper end of the n-type layer to the specific depth, Xna2(d)≦n / 12 The second feature of the present invention is that the nitride semiconductor ultraviolet light emitting device satisfies the above range.

[0049] Furthermore, in addition to the first feature, the present invention is characterized in that in the first step, the second average AlN mole fraction Xna2(d) is, in a region from an upper end of the n-type layer to the specific depth, Xna2(d)≦n / 12 The second feature of the present invention is to provide a method for producing a nitride semiconductor ultraviolet light emitting device, in which the n-type layer is formed so that the n-type layer is within the range given by

[0050] According to the nitride semiconductor ultraviolet light-emitting device having the second characteristic or the nitride semiconductor ultraviolet light-emitting device having the second characteristic, the contact resistance between the n-electrode and the n-type layer, and the bulk resistivity for a current flowing through an upper portion of the n-type layer can be further reduced, and a decrease in the wall-plug efficiency can be further suppressed while avoiding a decrease in the external quantum efficiency.

[0051] Furthermore, in the nitride semiconductor ultraviolet light emitting device according to the second aspect, the second average AlN mole fraction Xna2(d) is, in a region of the n-type layer deeper than the specific depth, Xna2(d)≧n / 12 It is preferable that the range is within the range.

[0052] Furthermore, in the manufacturing method of the nitride semiconductor ultraviolet light emitting device according to the second aspect, in the first step, the second average AlN mole fraction Xna2(d) is, in a region of the n-type layer deeper than the specific depth, Xna2(d)≧n / 12 It is preferable to form the n-type layer so that the n-type layer is in the range.

[0053] According to any of the preferred embodiments described above, in a region deeper than the specific depth of the n-type layer, absorption of light emitted from the active layer can be further suppressed, and a decrease in external quantum efficiency can be further suppressed.

[0054] Furthermore, in the nitride semiconductor ultraviolet light emitting device according to the first or second aspect, the peak emission wavelength is preferably set to a predetermined value within a range of 280 nm to 315 nm when the integer n is 7, and is preferably set to a predetermined value within a range of 300 nm to 330 nm when the integer n is 6.

[0055] Further, in the method for producing the nitride semiconductor ultraviolet light emitting device having the above-mentioned characteristics, in the second step, It is preferable that the active layer is formed so that the peak emission wavelength of the nitride semiconductor ultraviolet light emitting device is a predetermined value within the range of 280 nm to 315 nm when the integer n is 7, and is a predetermined value within the range of 300 nm to 330 nm when the integer n is 6.

[0056] According to any of the above preferred embodiments, it is possible to prevent a decrease in luminous efficiency caused by the dominant formation of metastable AlGaN in the layer region for a specific peak emission wavelength set to a predetermined value within the range of 280 nm to 315 nm when the integer n is 7, and for a specific peak emission wavelength set to a predetermined value within the range of 300 nm to 330 nm when the integer n is 6. The specific peak emission wavelength is a peak emission wavelength within the above specific wavelength range at which a decrease in luminous efficiency (external quantum efficiency or wall plug efficiency) may occur due to the dominant formation of metastable AlGaN in the layer region.

[0057] Furthermore, in the nitride semiconductor ultraviolet light-emitting device according to the first or second aspect, it is preferable that the active layer has a multiple quantum well structure including two or more of the well layers, and a barrier layer made of an AlGaN-based semiconductor is present between two of the well layers.

[0058] Furthermore, in the method for producing a nitride semiconductor ultraviolet light-emitting device having the first or second characteristic, in the second step, the well layers made of an AlGaN-based semiconductor and barrier layers made of an AlGaN-based semiconductor are alternately stacked by epitaxial growth to form the active layer having a multiple quantum well structure including two or more well layers.

[0059] According to any of the preferred embodiments described above, the active layer has a multiple quantum well structure, and it is expected that the light emission efficiency will be improved compared to the case where only one well layer is used.

[0060] Furthermore, it is preferable that the nitride semiconductor ultraviolet light-emitting element according to the first or second aspect further comprises a base portion including a sapphire substrate, the sapphire substrate having a main surface inclined at a predetermined angle with respect to a (0001) plane, the light-emitting element structure being formed above the main surface, and each semiconductor layer from the main surface of the sapphire substrate to the p-type layer is an epitaxially grown layer having a surface on which multi-step terraces parallel to the (0001) plane are formed.

[0061] According to the preferred embodiment, a sapphire substrate having an off-axis angle is used, and a laser beam is formed from the main surface of the sapphire substrate. p-type layer The epitaxial growth can be carried out so that multi-step terraces appear on the surfaces of the layers up to this point, thereby achieving a nitride semiconductor ultraviolet light emitting device having the above-mentioned characteristics. Effect of the Invention

[0062] According to the nitride semiconductor ultraviolet light emitting device or the manufacturing method of the nitride semiconductor ultraviolet light emitting device of the first or second feature, the formation of metastable AlGaN in the layered region having a locally low AlN mole fraction in the n-type AlGaN-based semiconductor layer is intentionally suppressed at a specific peak emission wavelength, thereby preventing a decrease in luminous efficiency caused by the dominant formation of metastable AlGaN in the layered region. Furthermore, when setting a target peak emission wavelength within a predetermined wavelength range, an embodiment in which metastable AlGaN is used in the layered region of the n-type layer and an embodiment in which metastable AlGaN is not used are selectively used according to the target peak emission wavelength, thereby preventing a decrease in luminous efficiency and increasing the degree of freedom in setting the peak emission wavelength. [Brief description of the drawings]

[0063] [Figure 1] Schematic diagram of the wurtzite crystal structure of AlGaN. [Diagram 2] FIG. 2 is a plan view showing the positional relationship between each site on the A plane and each site on the B plane when viewed from the c-axis direction of the wurtzite crystal structure shown in FIG. 1. [Diagram 3] A schematic diagram showing the symmetric arrangement of Al and Ga on the site planes (A3 plane, B3 plane) of group 3 elements in metastable AlGaN, whose AlGaN composition ratios are expressed as integer ratios of Al1Ga2N3, Al1Ga1N2, and Al2Ga1N3. [Figure 4] 1 is a cross-sectional view of a main portion that illustrates a schematic example of the structure of a nitride semiconductor ultraviolet light emitting device according to a first embodiment. [Diagram 5] 5 is a cross-sectional view of a main portion, showing a schematic example of a laminated structure of an active layer of the nitride semiconductor ultraviolet light emitting device shown in FIG. 4. [Figure 6] FIG. 6 is a diagram showing a more detailed structure of the inclined region IA shown in FIG. 5. [Figure 7] 13 is a graph showing the relationship between the emission wavelength of a quantum well structure consisting of an AlGaN well layer and an AlGaN barrier layer, and the film thickness of the well layer and the AlN mole fraction of the barrier layer when the AlN mole fraction of the Ga-enriched well region 220a is 50%. [Figure 8] 13 is a graph showing the relationship between the emission wavelength of a quantum well structure consisting of an AlGaN well layer and an AlGaN barrier layer, and the film thickness of the well layer and the AlN mole fraction of the barrier layer when the AlN mole fraction of the Ga-enriched well region 220a is 41.7%. [Figure 9] 13 is a graph showing the relationship between the emission wavelength of a quantum well structure consisting of an AlGaN well layer and an AlGaN barrier layer, and the film thickness of the well layer and the AlN mole fraction of the barrier layer when the AlN mole fraction of the Ga-enriched well region 220a is 33.3%. [Figure 10] 1 is a graph showing the relationship between the emission wavelength of a quantum well structure consisting of a GaN well layer and an AlGaN barrier layer, and the film thickness of the well layer and the AlN mole fraction of the barrier layer. [Figure 11] 5 is a plan view showing a schematic example of the structure of the nitride semiconductor ultraviolet light emitting device shown in FIG. 4 when viewed from above in FIG. 4. [Figure 12]11 is a graph showing an example of a change in a second average AlN mole fraction Xna2(d) at a depth d from the upper end of an n-type layer depending on the depth d. [Figure 13] SEM image showing the main part of the cross section of the sample piece where the AlN mole fraction was measured by the CL method. [Figure 14] 14 is a diagram showing first and second CL spectra on six Y coordinates on the measurement cross section of the sample piece shown in FIG. 13. [Figure 15] HAADF-STEM image showing four measurement areas A to D where the AlN mole fraction in the layered region of the specimen was measured by cross-sectional TEM-EDX line analysis. [Figure 16] FIG. 11 is a cross-sectional view of a main portion that illustrates an example of the structure of a nitride semiconductor ultraviolet light emitting device according to a second embodiment. [Figure 17] 17 is a cross-sectional view of a main part, showing a schematic example of a laminated structure of a main part including an active layer of the nitride semiconductor ultraviolet light emitting device shown in FIG. 16. [Figure 18] FIG. 1 is a cross-sectional view of a main portion that illustrates an example of an element structure of a general ultraviolet light emitting diode. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] A nitride semiconductor ultraviolet light emitting device (hereinafter, simply referred to as "light emitting device") according to an embodiment of the present invention will be described with reference to the drawings. In the schematic diagrams used in the following description, the essential parts are emphasized to show the invention in a schematic manner for ease of understanding, and therefore the dimensional ratios of the various parts are not necessarily the same as those of an actual device. In the following description of this embodiment, the light emitting device will be described on the assumption that it is a light emitting diode.

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

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

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

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

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

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

[0071] In the n-type cladding layer 21 made of an n-type AlGaN-based semiconductor, multi-step terraces parallel to the (0001) plane are formed on the growth surface during the epitaxial growth process, and Ga, which is easily transferred by mass, is concentrated in an inclined region inclined with respect to the (0001) plane connecting adjacent terraces, forming a Ga-rich n-type region in which the AlN mole fraction is lower than that of the terrace region. As the epitaxial growth progresses, the inclined region extends obliquely upward, and as a result, layered regions 21a with locally low AlN mole fraction are formed uniformly distributed in the n-type cladding layer 21. The extension direction of each layered region 21a on the first plane has a portion inclined with respect to the intersection line between the upper surface of the n-type cladding layer 21 and the first plane.

[0072] In one embodiment, during the epitaxial growth process of the n-type cladding layer 21, as a result of the formation of a Ga-rich n-type region in the inclined region, i.e., due to mass transfer of Ga from the terrace region to the inclined region, the density of Al may relatively increase in a part of the terrace region, forming an Al-rich n-type region in which the AlN mole fraction is higher than the average AlN mole fraction.

[0073] As described above in the Background Art section, layer region 21a extends in a diagonal direction relative to the surface of n-type cladding layer 21, and since the band gap energy is locally small, carriers tend to localize and function as a low-resistance current path. The region other than layer region 21a in n-type cladding layer 21 is called n-type body region 21b.

[0074] In this embodiment, the first average AlN molar fraction Xna1 over the entire depth direction of the n-type clad layer 21 is within the range represented by the inequality of the following formula (1). Here, n in the following formula (1) is the integer 6 or 7. (n - 0.25) / 12 < Xna1 < (n + 0.25) / 12 (1)

[0075] Furthermore, in this embodiment, the second average AlN molar fraction Xna2(d) at the depth d (nm) from the upper surface of the n-type clad layer 21 changes according to the depth d, and Xna2(d) = n / 12 There exists at least one depth d0 of 1 or more such that. And at at least one specific depth dx among those depths d0, there exists a region where Xna2(d) < n / 12 above the specific depth dx, and there exists a region where Xna2(d) > n / 12 below it.

[0076] As a typical example of the depth d0 and the specific depth dx, it is assumed that there is one depth d0 and d0 = dx. Furthermore, in this typical example, as an example, as in the embodiment shown in FIG. 12 to be described later, at the start of growth of the n-type clad layer 21, Xna2(d) > n / 12, and it is assumed that Xna2(d) decreases until Xna2(d) < n / 12 as growth progresses. Here, at least one depth d0 is not such that Xna2(d0) = n / 12 Rather, it also includes the case where Xna2(d0) = n / 12 in a continuous range of depths d. The same applies to the specific depth dx.

[0077] Furthermore, as one embodiment, it is preferable that the third average AlN molar fraction Xna3 over the region from the upper surface of the n-type clad layer 21 to the specific depth dx is within the range represented by the inequality of the following formula (2). (n - 0.25) / 12 < Xna3 < (n + 0.25) / 12 (2) Here, when there are a plurality of specific depths dx, it is preferable that the above relationship is satisfied for each of the specific depths dx.

[0078] Furthermore, as one embodiment, it is more preferable that the second average AlN mole fraction Xna2(d) is within the range represented by the inequality of the following formula (3) over the entire region of the n-type cladding layer 21 in the depth direction. (n-0.25) / 12 <Xna2(d)<(n+0.25) / 12 (3)

[0079] Furthermore, as one embodiment, it is preferable that Xna2(d) is within the range represented by the inequality in formula (4) below in the region from the upper surface of the n-type cladding layer 21 to the specific depth dx. Furthermore, in the above preferred embodiment in which Xna2(d) is within the range represented by formula (3), it is more preferable that Xna2(d) is within the range represented by the inequality in formula (5) below in the region from the upper surface of the n-type cladding layer 21 to the specific depth dx. Xna2(d)≦n / 12 (4) (n-0.25) / 12 <Xna2(d)≦n / 12 (5)

[0080] Furthermore, in the above preferred embodiment in which Xna2(d) is within the range represented by formula (4) or formula (5), it is preferable that Xna2(d) is within the range represented by the inequality of the following formula (6), more preferably, formula (7), in a region deeper than the specific depth dx. n / 12≦Xna2(d) (6) n / 12≦Xna2(d)<(n+0.25) / 12 (7)

[0081] In addition, the first average AlN mole fraction Xna1 is an integer ratio of AlGaN composition ratio. n Ga 12-n N 12Since the AlN mole fraction (n / 12) of the first metastable AlGaN region is controlled within a range of ±0.25 / 12 (±approximately ±2.08%) around the reference, the first metastable AlGaN region is uniformly formed in n-type body region 21b. In the region where the second average AlN mole fraction Xna2(d) is lower than the AlN mole fraction (n / 12) of the first metastable AlGaN region, the first metastable AlGaN region is formed in the Al-rich n-type region of n-type body region 21b.

[0082] In the layered region 21a, the AlN mole fraction is one step smaller (about 8.33%) than that of the first metastable AlGaN region, and the AlGaN composition ratio is an integer ratio of Al n-1 Ga 13-n N 12 A second metastable AlGaN region having an AlN mole fraction of 100% may be formed due to mass transfer of Ga during the formation of layer region 21a. However, the second metastable AlGaN region is not dominantly formed within layer region 21a, but instead, an intermediate AlGaN region (AlN mole fraction 100% to 100% of the first and second metastable AlGaN regions) is formed. n-0.5 Ga 12.5-n N 12 ) is formed.

[0083] In this embodiment, the thickness of the n-type cladding layer 21 is assumed to be about 1 μm to 2 μm, similar to the thickness adopted in general nitride semiconductor ultraviolet light emitting devices, but the thickness may also be about 2 μm to 4 μm.

[0084] The active layer 22 has a multiple quantum well structure in which two or more well layers 220 made of AlGaN-based semiconductors (excluding AlN-based semiconductors) and one or more barrier layers 221 made of AlGaN-based semiconductors (excluding GaN-based semiconductors) or AlN-based semiconductors are alternately stacked. The barrier layer 221 does not necessarily have to be provided between the bottom well layer 220 and the n-type cladding layer 21. In this embodiment, the barrier layer 221 is not provided between the top well layer 220 and the electron blocking layer 23, but as a preferred embodiment, an AlGaN layer or an AlN layer that is thinner than the barrier layer 221 and has a higher AlN mole fraction may be provided.

[0085] The electron blocking layer 23 is made of a p-type AlGaN-based semiconductor. The p-type contact layer 24 is made of a p-type AlGaN-based semiconductor or a p-type GaN-based semiconductor. The p-type contact layer 24 is typically made of p-GaN.

[0086] Fig. 5 shows a schematic example of a stacked structure (multiple quantum well structure) of well layers 220 and barrier layers 221 in active layer 22. Fig. 5 shows an example in which well layers 220 and barrier layers 221 each have three layers. Three layers, namely, barrier layers 221 and well layers 220, are stacked in this order on n-type cladding layer 21, and electron block layer 23 is located on top of well layer 220.

[0087] The structure in which the terraces T in the well layer 220, the barrier layer 221, and the electron block layer 23 shown in FIG. 5 grow in a multi-step manner is a known structure, as disclosed in the above-mentioned Non-Patent Documents 1 and 2. Between the terraces T adjacent to each other in the lateral direction in each layer, an inclined region IA inclined with respect to the (0001) plane is formed, as described above. The region sandwiched between the terraces T on the top and bottom other than the inclined region IA is referred to as a terrace region TA. In this embodiment, the depth of one terrace T (the distance between the adjacent inclined regions IA) is assumed to be several tens of nm to several hundreds of nm. Therefore, the (0001) plane exposed in a step-like manner in the inclined region IA is distinguished from the terrace surface of the multi-step terrace T. FIG. 6 shows, for example, a step-like structure (macrostep structure) exposed on the surface of the inclined region IA of one well layer 220.

[0088] 5, when the well layer 220 is made of an AlGaN-based semiconductor and the AlN mole fraction is not 0%, Ga-rich well regions 220a having an AlN mole fraction lower than the average AlN mole fraction Xwa in the well layer 220 are formed in the tilted regions IA due to mass transfer of Ga from the terrace regions TA to the tilted regions IA in each sublayer of the well layer 220. Furthermore, in one embodiment, with the formation of the Ga-rich well regions 220a in the tilted regions IA, that is, due to mass transfer of Ga from the terrace regions TA to the tilted regions IA, the density of Al may relatively increase in a part of the terrace regions TA, forming an Al-rich well region having an AlN mole fraction higher than the average AlN mole fraction Xwa.

[0089] In addition, as a preferred embodiment, when the well layer 220 is composed of an AlGaN-based semiconductor and the AlN mole fraction is not 0%, the average AlN mole fraction Xwa of the well layer 220 is preferably adjusted to be within the range of Xw0+2% to Xw0+3% when metastable AlGaN with an AlN mole fraction Xw0 is formed in the Ga-enriched well region 220a. According to this preferred embodiment, the difference in AlN mole fraction between the inclined region IA and the terrace region TA in the well layer 220 is 4% or less, which can suppress the occurrence of double emission peaks due to the difference in AlN mole fraction. Furthermore, even if the average AlN molar fraction Xwa of the well layer 220 is outside the range of Xw0+2% to Xw0+3%, as long as a Ga-rich well region 220a with a locally low AlN molar fraction is formed in the inclined region IA of the well layer 220, the AlN molar fraction in the Ga-rich well region 220a corresponding to the target value of the peak emission wavelength is set to Xw1%, and the average AlN molar fraction Xwa of the well layer 220 may be set to any value within the range of Xw1+2% to Xw1+3%, where Xw1% is the AlN molar fraction in the Ga-rich well region 220a corresponding to the target value of the peak emission wavelength.

[0090] In this embodiment, when the barrier layer 221 is composed of an AlGaN-based semiconductor (excluding an AlN-based semiconductor), a Ga-rich barrier region 221a in which the AlN mole fraction is lower than the average AlN mole fraction Xba of the barrier layer 221 is formed in the gradient region IA in the barrier layer 221. Furthermore, in one embodiment, similar to the well layer 220, an Al-rich barrier region in which the AlN mole fraction is higher than the average AlN mole fraction Xba of the barrier layer 221 may be formed in a part of the terrace region TA.

[0091] In a preferred embodiment, when a metastable AlGaN region with an AlN mole fraction of Xb0 is formed in the Ga-enriched barrier region 221a of the barrier layer 221, the average AlN mole fraction Xba of the barrier layer 221 is preferably adjusted to fall within a range of Xb0+2% to Xb0+8%, thereby ensuring a difference in AlN mole fraction of about 2% or more between the Ga-enriched barrier region 221a and the terrace region TA of the barrier layer 221.

[0092] Furthermore, as a preferred embodiment, the AlN molar fraction of the terrace region TA of the barrier layer 221 is set to be 1% or more, preferably 2% or more, and more preferably 4% or more higher than the AlN molar fraction of the Ga-enriched barrier region 221a, within a range of approximately 51% to 90%. In order to fully ensure the effect of carrier localization in the Ga-enriched barrier region 221a, it is preferable that the difference in AlN molar fraction between the Ga-enriched barrier region 221a and the terrace region TA in the barrier layer 221 is 4 to 5% or more, but the effect of carrier localization can be expected even if it is about 1 to 2%.

[0093] In this embodiment, a Ga-enriched EB region 23a having an AlN molar fraction lower than the average AlN molar fraction Xea of ​​the electron block layer 23 is also formed in the inclined region IA in the electron block layer 23. Furthermore, in one embodiment, similar to the well layer 220, an Al-enriched EB region having an AlN molar fraction higher than the average AlN molar fraction Xea of ​​the electron block layer 23 may be formed in a part of the terrace region TA.

[0094] The AlN molar fraction of the terrace regions TA of the electron block layer 23 is set to be within a range of approximately 69% to 90%, and is set to be 20% or more, preferably 25% or more, and more preferably 30% or more higher than the AlN molar fraction of the terrace regions of the well layer 220. Furthermore, the AlN molar fraction of the Ga-enriched EB regions 23a of the electron block layer 23 is set to be 20% or more, preferably 25% or more, and more preferably 30% or more higher than the AlN molar fraction of the Ga-enriched well regions 220a of the well layer 220.

[0095] As a preferred embodiment, when a metastable AlGaN region with an AlN mole fraction of Xe0 is formed in the Ga-enriched EB region 23a of the electron block layer 23, the average AlN mole fraction Xea of ​​the electron block layer 23 is preferably adjusted to be within a range of approximately Xe0+2% to Xe0+8%. This ensures that the difference in AlN mole fraction between the Ga-enriched EB region 23a and the terrace region TA of the electron block layer 23 is approximately 2% or more.

[0096] In the layered structure (macrostep structure) of the light-emitting element structure 20 shown in Figure 5, in the n-type cladding layer 21, carriers are likely to localize in the layered region 21a having a locally low AlN mole fraction. In the active layer 22, carriers are likely to localize in the Ga-rich well region 220a having a locally low AlN mole fraction in the gradient region IA of the well layer 220 and in the Ga-rich barrier region 221a having a locally low AlN mole fraction in the gradient region IA of the barrier layer 221. In the electron blocking layer 23, carriers are likely to localize in the Ga-rich EB region 23a having a locally low AlN mole fraction in the gradient region IA. Therefore, carriers can be efficiently supplied to the Ga-enriched well region 220a of the well layer 220 from the n-type cladding layer 21 side via the layered region 21a, and from the electron blocking layer 23 side via the Ga-enriched EB region 23a, resulting in an element structure that can improve the light emitting efficiency by recombination of carriers (electrons and holes) in the well layer 220.

[0097] In this embodiment, the thickness of the well layer 220, including the terrace region TA and the inclined region IA, is set, for example, within a range of 3 ML to 14 ML in accordance with the target value of the peak emission wavelength λp of the light-emitting element 1. The thickness of the barrier layer 221, including the terrace region TA and the inclined region IA, is set, for example, within a range of 6 nm to 8 nm. Furthermore, the thickness of the electron blocking layer 23, including the terrace region TA and the inclined region IA, is set, for example, within a range of 15 nm to 30 nm (the optimum value is about 20 nm).

[0098] When the well layer 220 is composed of an AlGaN-based semiconductor and the AlN mole fraction is not 0%, the AlN mole fraction and film thickness of the well layer 220 (particularly, the Ga-enriched well region 220a in the gradient region IA) and the AlN mole fraction of the barrier layer and electron block layer 23 adjacent to the well layer 220 (particularly, the Ga-enriched barrier region 221a and the Ga-enriched EB region 23a in the gradient region IA) are set according to the target value of the peak emission wavelength λp of the light-emitting element 1.

[0099] When the well layer 220 is composed of a GaN-based semiconductor and has an AlN mole fraction of 0%, the AlN mole fraction of the barrier layers and electron block layer 23 adjacent to the well layer 220 (particularly, the Ga-enriched barrier region 221a and the Ga-enriched EB region 23a in the gradient region IA) and the film thickness of the well layer 220 are set according to the target value of the peak emission wavelength λp of the light-emitting element 1.

[0100] 7, 8 and 9 are graphs showing the simulation results of the emission wavelength (corresponding to the peak emission wavelength) obtained by changing the film thickness of the well layer within the range of 3ML to 14ML or 4ML to 14ML for a quantum well structure model in which the well layer 220 and the barrier layer 221 are made of AlGaN. As a condition of the above simulation, it is assumed that metastable AlGaN having an AlGaN composition ratio of an integer ratio is predominantly present in the Ga-enriched well region 220a of the well layer 220, and the AlN molar fraction of the Ga-enriched well region 220a of the well layer 220 is set to 50% (one half), which is the AlN molar fraction of metastable AlGaN, in Fig. 7, 41.7% (five twelfths), which is the AlN molar fraction of metastable AlGaN, in Fig. 8, and 33.3% (one third), which is the AlN molar fraction of metastable AlGaN, in Fig. 9, and the AlN molar fraction of the Ga-enriched barrier region 221a of the barrier layer 221 is set to three values, 66.7% (two thirds), 75% (three quarters), and 83.3% (five sixths), respectively, in Fig. 7 to Fig. 9. In the simulation results shown in Fig. 7 to Fig. 9, it is assumed that ultraviolet light emission in the well layer 220 occurs significantly in the gradient region IA. For this reason, it is important that the film thickness condition of the well layer 220 is satisfied in the gradient region IA.

[0101] 7 to 9, it can be seen that within the range of 3ML to 14ML, the smaller the thickness of the well layer 220, the greater the quantum confinement effect in the well layer 220, and the shorter the emission wavelength becomes, and further, the greater the AlN molar fraction of the barrier layer 221, the greater the degree of change in the emission wavelength with respect to the change in the thickness of the well layer 220. Also, from Fig. 7, it can be seen that when the AlN molar fraction of the Ga-enriched well region 220a is 50%, the emission wavelength changes in the range of approximately 246nm to 295nm within the above ranges of the thickness of the well layer 220 and the AlN molar fraction of the barrier layer 221. From Fig. 8, it can be seen that when the AlN molar fraction of the Ga-enriched well region 220a is 41.7%, the emission wavelength changes in the range of approximately 249nm to 311nm within the above ranges of the thickness of the well layer 220 and the AlN molar fraction of the barrier layer 221. 9 shows that when the AlN molar fraction of the Ga-enriched well region 220a is 33.3%, the emission wavelength varies in the range of approximately 261 nm to 328 nm within the above ranges of the film thickness of the well layer 220 and the AlN molar fraction of the barrier layer 221. Furthermore, if the barrier layer 221 is made of AlN (AlN molar fraction=100%), the emission wavelength can be further expanded.

[0102] 7 to 9, the Ga-enriched well region 220a of the well layer 220 has an AlGaN composition ratio of Al 1 Ga 1 N 2 Or Al 5 Ga 7 N 12 Or Al 1 Ga 2 N 3 It can be seen that by adjusting the film thickness of the well layer 220 within the range of 3 ML to 14 ML and the AlN molar fraction of the Ga-enriched barrier region 221a of the barrier layer 221 within the range of 66.7% to 100% according to the AlN molar fraction of the metastable AlGaN, the peak emission wavelength can be set within the range of 246 nm to 328 nm.

[0103] 10 is a graph showing the simulation results (corresponding to the peak emission wavelength) of the emission wavelength obtained by changing the thickness of the well layer within a range of 4ML to 10ML for two types of AlN mole fraction of the barrier layer, 66.7% (AlGaN) and 100% (AlN), for a quantum well structure model in which the well layer is made of GaN and the barrier layer is made of AlGaN or AlN. From FIG. 10, it can be seen that the emission wavelength changes approximately within a range of 270nm to 325nm within the range. Therefore, even if the well layer is made of GaN (AlN mole fraction=0%), it can be seen that the peak emission wavelength can be set within a range of 270nm to 325nm by adjusting the thickness of the well layer 220 within a range of 4ML to 10ML and the AlN mole fraction of the Ga-enriched barrier region 221a of the barrier layer 221 within a range of 66.7% to 100%, respectively.

[0104] As a preferred embodiment of this invention, it is assumed that the target value of the peak emission wavelength λp of the light-emitting element 1 is set, for example, within the range of 280 nm to 315 nm when n=7, and within the range of 300 nm to 330 nm when n=6.

[0105] When n=7, the first average AlN mole fraction Xna1 is within the range shown in the above formula (1), that is, within the range of about 56.3% to about 60.4%, and the absorption edge wavelength (λae) of the n-type cladding layer 21 determined by the first average AlN mole fraction Xna1 is located within the range of about 263 nm to about 269 nm. Therefore, if the target value of the peak emission wavelength λp is within the above range of 280 nm to 315 nm, the wavelength difference (λp-λae) between the peak emission wavelength (λp) and the absorption edge wavelength (λae) is ensured to be 10 nm or more, and the emission absorption in the n-type cladding layer 21 is sufficiently suppressed. As a comparative example for the case where n=7, if we assume that a second metastable AlGaN region having an AlN mole fraction of 50% is predominantly formed in the layer region 21a, the first average AlN mole fraction Xna1 will be approximately 54.2%, and the absorption edge wavelength (λae) will be approximately 273 nm. If the target value for the peak emission wavelength λp is less than 283 nm, the wavelength difference will be less than 10 nm, which may cause emission absorption in the n-type cladding layer 21 and result in a decrease in external quantum efficiency.

[0106] When n=6, the first average AlN molar fraction Xna1 is within the range shown in the above formula (1), that is, within the range of about 47.9% to about 52.1%, and the absorption edge wavelength (λae) of the n-type cladding layer 21 determined by the first average AlN molar fraction Xna1 is located within the range of about 276 nm to about 283 nm. Therefore, if the target value of the peak emission wavelength λp is within the above range of 300 nm to 330 nm, the wavelength difference (λp-λae) between the peak emission wavelength (λp) and the absorption edge wavelength (λae) is sufficiently ensured to be 10 nm or more, and the emission absorption in the n-type cladding layer 21 is sufficiently suppressed. In addition, it is possible to lower the lower limit of the target value from 300 nm to about 293 nm. As a comparative example for the case where n=6, if we assume that a second metastable AlGaN region having an AlN mole fraction of approximately 41.7% is dominantly formed in the layered region 21a, the first average AlN mole fraction Xna1 will be approximately 45.8%, and the absorption edge wavelength (λae) will be approximately 286 nm. If the target value for the peak emission wavelength λp is set to less than 296 nm, the wavelength difference will be less than 10 nm, which may cause emission absorption in the n-type cladding layer 21 and result in a decrease in external quantum efficiency.

[0107] The AlN mole fraction (n / 12) of the first metastable AlGaN region formed in the n-type body region 21b is approximately 58.3% when n=7 and 50% when n=6. The contact resistance between the n-type cladding layer 21 and the n-electrode 27 formed on its exposed surface is higher than when the AlN mole fraction is less than 50%, but lower than when the AlN mole fraction is 60%, and a significant increase in the contact resistance is suppressed.

[0108] Since the first average AlN mole fraction Xna1 is controlled within the range shown by the above formula (1), when n=7, there may be a region in the n-type body region 21b where the AlN mole fraction exceeds 60%. However, since the n-electrode 27 is formed on the exposed surface in the second region R2 of the n-type cladding layer 21 and the contact area with the upper surface of the n-type cladding layer 21 is sufficiently large, the contact area includes a first metastable AlGaN region with an AlN mole fraction of about 58.3% and a layered region with an even smaller AlN mole fraction in addition to the region where the AlN mole fraction exceeds 60%, so that the average contact resistance between the n-electrode 27 and the n-type cladding layer 21 is kept low. In the case of n=7, in a preferred embodiment in which Xna2(d) is within the range expressed by the inequality of the above formula (4) in the region from the upper surface of the n-type cladding layer 21 to a specific depth dx, the average contact resistance between the n-electrode 27 and the n-type cladding layer 21 is kept even lower. Furthermore, when n=6, the average contact resistance between the n-electrode 27 and the n-type cladding layer 21 is suppressed to be even lower than when n=7.

[0109] Furthermore, in this embodiment, in order to suppress the absorption of light emission in the n-type cladding layer 21, the first average AlN mole fraction Xna1 and the second average AlN mole fraction Xna2(d) are not set unnecessarily high, and therefore an unnecessary increase in bulk resistivity is also suppressed.

[0110] The p-electrode 26 is made of a multi-layer metal film such as Ni / Au, and is formed on the upper surface of the p-type contact layer 24. The n-electrode 27 is made of a multi-layer metal film such as Ti / Al / Ti / Au, and is formed in a part of the exposed surface in the second region R2 of the n-type cladding layer 21. The p-electrode 26 and the n-electrode 27 are not limited to the multi-layer metal film described above, and the electrode structure such as the metal constituting each electrode, the number of layers, and the order of layers may be changed as appropriate. FIG. 11 shows an example of the shape of the p-electrode 26 and the n-electrode 27 as viewed from the top of the light-emitting element 1. In FIG. 11, the line BL between the p-electrode 26 and the n-electrode 27 indicates the boundary between the first region R1 and the second region R2, and coincides with the outer peripheral side wall surfaces of the active layer 22, the electron blocking layer 23, and the p-type contact layer 24.

[0111] In this embodiment, as shown in FIG. 11, the planar shape of the first region R1 and the p-electrode 26 is, as an example, a comb shape, but the planar shape and arrangement of the first region R1 and the p-electrode 26 are not limited to the example shown in FIG. 11.

[0112] When a forward bias is applied between the p-electrode 26 and the n-electrode 27, holes are supplied from the p-electrode 26 toward the active layer 22, and electrons are supplied from the n-electrode 27 toward the active layer 22. The supplied holes and electrons each reach the active layer 22 and recombine to emit light. This causes a forward current to flow between the p-electrode 26 and the n-electrode 27.

[0113] <Method of manufacturing light-emitting element> Next, the light emission shown in FIG. element An example of the manufacturing method of 1 will be described.

[0114] First, by metal-organic vapor phase epitaxy (MOVPE), the AlN layer 12 included in the base portion 10 and the nitride semiconductor layers 21 to 24 included in the light-emitting element structure 20 are epitaxially grown in order on the sapphire substrate 11. At this time, the n-type cladding layer 21 is doped with, for example, Si as a donor impurity, and the electron blocking layer 23 and the p-type contact layer 24 are doped with, for example, Mg as an acceptor impurity.

[0115] In this embodiment, in order to expose multi-step terraces parallel to the (0001) plane on at least the surfaces of the AlN layer 12, the n-type cladding layer 21, the active layer 22 (the well layer 220, the barrier layer 221), and the electron blocking layer 23, the sapphire substrate 11 is a slightly inclined substrate in which the main surface 11a is inclined at an angle (off angle) within a certain range (for example, up to about 0.3° to 6°) with respect to the (0001) plane and multi-step terraces are exposed on the main surface 11a.

[0116] The conditions for such epitaxial growth include, in addition to the use of the vicinal (0001) sapphire substrate 11 described above, a growth rate that makes it easy for multi-step terraces to appear (specifically, the growth rate is achieved by appropriately setting various conditions such as the growth temperature, and the supply amounts and flow rates of the source gas and carrier gas), etc. Note that these various conditions may differ depending on the type and structure of the film formation apparatus, so it is sufficient to actually fabricate several samples in the film formation apparatus and specify these conditions.

[0117] As growth conditions for the n-type cladding layer 21, the growth temperature, growth pressure, and donor impurity concentration are selected so that immediately after the start of growth, a growth starting point for the layer region 21a is formed by mass transfer of Ga in a step portion (inclined region) between multiple terraces formed on the upper surface of the AlN layer 12, and subsequently, as the n-type cladding layer 21 epitaxially grows, the layer region 21a grows obliquely upward by segregation associated with the mass transfer of Ga.

[0118] Specifically, the growth temperature is preferably 1050°C or higher at which mass transfer of Ga easily occurs, and 1150°C or lower at which good n-type AlGaN can be prepared. In addition, as a preferred embodiment, in the case of forming a first metastable AlGaN region with an AlN mole fraction of n / 12 in the n-type main body region 21b, at a growth temperature exceeding 1170°C, the mass transfer of Ga becomes excessive, and even in the case of metastable AlGaN, the AlN mole fraction tends to vary randomly, so that it may be difficult to stably form a metastable AlGaN region with an AlN mole fraction of 50% to 58.3%. As for the growth pressure, 75 Torr or lower is preferred as a good growth condition for AlGaN, and 10 Torr or higher is realistic and preferred as a control limit of the film formation apparatus. The donor impurity concentration is 1×10 18 ~5×10 18 cm -3 The above growth temperature, growth pressure, and the like are merely examples, and optimal conditions may be appropriately specified depending on the film formation apparatus used.

[0119] The supply amounts and flow rates of the source gases (trimethylaluminum (TMA) gas, trimethylgallium (TMG) gas, ammonia gas) and carrier gas used in the metal organic compound vapor phase epitaxy are set with the first average AlN mole fraction Xna1 of the n-type cladding layer 21 described above as a target value.

[0120] In this embodiment, the second average AlN mole fraction Xna2(d) changes on either side of the AlN mole fraction (n / 12) of the first metastable AlGaN region in response to a change in depth d. The change in the AlN mole fraction Xna2(d) can be achieved by actively modulating the supply amounts and flow rates of the source gas and carrier gas, or modulating the growth temperature. Furthermore, a natural change in the substrate surface temperature that occurs as the n-type cladding layer 21 grows thicker with its growth may be utilized. In this case, the supply amounts and flow rates of the source gas and carrier gas are set according to the tendency of change in the substrate surface temperature.

[0121] The donor impurity concentration does not necessarily need to be uniformly controlled in the vertical direction with respect to the thickness of the n-type cladding layer 21. For example, the impurity concentration of a predetermined thin portion in the n-type cladding layer 21 may be lower than the set concentration, for example, 1×10 18 cm -3 less than 1×10 17 cm -3 The low impurity concentration layer may be a layer having a thickness controlled to 0 nm or less. The thickness of the low impurity concentration layer is preferably greater than 0 nm and less than 200 nm, more preferably greater than 10 nm and less than 100 nm, and even more preferably greater than 20 nm and less than 50 nm. The donor impurity concentration of the low impurity concentration layer may be lower than the above set concentration, and may be lower than the undoped layer (0 cm -3 ) may be included in part of the n-type cladding layer 21. Furthermore, a part or the whole of the low impurity concentration layer is preferably present in an upper layer region at a depth of 100 nm or less below the upper surface of the n-type cladding layer 21.

[0122] After the n-type cladding layer 21 having the layer region 21a and the n-type main body region 21b is formed in the above manner, the active layer 22 (well layer 220, barrier layer 221), the electron blocking layer 23, the p-type contact layer 24, etc. are subsequently formed on the entire upper surface of the n-type cladding layer 21 by a well-known epitaxial growth method such as metal-organic vapor phase epitaxy (MOVPE).

[0123] The acceptor impurity concentration of the electron blocking layer 23 is, for example, 1.0×10 16 ~1.0×10 18 cm -3 The acceptor impurity concentration of the p-type contact layer 24 is preferably about 1.0×10 18 ~1.0×10 20 cm -3 It is preferable that the acceptor impurity concentration is about 100 nm. It is not necessary to control the acceptor impurity concentration uniformly in the vertical direction with respect to each film thickness of the electron block layer 23 and the p-type contact layer 24.

[0124] In forming the active layer 22, in the same manner as for the n-type cladding layer 21, the well layer 220 is grown under growth conditions that make it easy to expose the above-mentioned multi-stepped terraces, with an average AlN mole fraction Xwa of the well layer 220 as a target value, and further, the barrier layer 221 is grown with an average AlN mole fraction Xba of the barrier layer 221 as a target value. The average AlN mole fractions Xwa and Xba of the well layer 220 and the barrier layer 221 are as described above, and therefore repeated explanations will be omitted.

[0125] In forming the electron blocking layer 23, the electron blocking layer 23 is grown under growth conditions that facilitate the appearance of the above-mentioned multi-step terraces in the same manner as the n-type cladding layer 21, with the average AlN mole fraction Xea of ​​the electron blocking layer 23 as a target value. The average AlN mole fraction Xea of ​​the electron blocking layer 23 is as described above, and a duplicated description will be omitted.

[0126] In this embodiment, the active layer 22 (the well layer 220 and the barrier layer 221), the electron blocking layer 23, and the p-type contact layer 24The growth temperatures of the n-type cladding layer 21 are T1, the active layer 22 is T2, the electron blocking layer 23 is T3, and the p-type contact layer 24 When the growth temperature is T4, within the above-mentioned preferable temperature range (1050° C. to 1170° C.), it is preferable that the relationship shown in the following formulas (8) and (9) is satisfied. T3≧T2 (8) T3>T1>T4 (9)

[0127] The growth temperature T3 of the electron blocking layer 23 can be reduced by, for example, increasing the flow rate of the nitrogen source gas and decreasing the growth rate.

[0128] When the growth temperature T3 of the electron blocking layer 23 is increased from the growth temperature T2 of the active layer 22, decomposition of GaN may occur in the well layer 220 located thereunder during the transition of the growth temperature, and the characteristics of the light-emitting element 1 may deteriorate due to the decomposition of GaN. , B It is preferable to form an AlGaN layer or an AlN layer that is thinner than the rear layer 221 (for example, 3 nm or less, preferably 2 nm or less) and has a higher AlN mole fraction than the barrier layer 221 and the electron blocking layer 23 .

[0129] In the above manner, the active layer 22 (well layer 220, barrier layer 221), electron blocking layer 23, p-type contact layer 24, etc. are formed on the entire upper surface of the n-type cladding layer 21. Next, the second region R2 of the nitride semiconductor layers 21-24 is selectively etched by a well-known etching method such as reactive ion etching until the upper surface of the n-type cladding layer 21 is exposed, thereby exposing the second region R2 portion of the upper surface of the n-type cladding layer 21. Then, a p-electrode 26 is formed on the p-type contact layer 24 in the unetched first region R1 by a well-known film formation method such as electron beam evaporation, and the etched first region R2 is selectively etched by a well-known film formation method such as electron beam evaporation. 2An n - electrode 27 is formed on the n - type clad layer 21 within the region R2. Incidentally, after forming one or both of the p - electrode 26 and the n - electrode 27, heat treatment may be performed by a well - known heat treatment method such as RTA (rapid thermal annealing).

[0130] Incidentally, as an example, the light - emitting element 1 can be used in a state where it is flip - chip mounted on a base such as a sub - mount and then sealed with a predetermined resin such as a silicone resin or an amorphous fluororesin (for example, a resin in a lens shape).

[0131] The cross - sectional structure of the AlGaN - based semiconductor layers 21 to 24 of the light - emitting element 1 manufactured in the above - mentioned manner is , No. Etching of the two regions R2 and p A sample before forming the electrode 26 and the n - electrode 27 is prepared, and a sample piece having a cross - section perpendicular (or substantially perpendicular) to the upper surface of the sample is processed by a focused ion beam (FIB) and can be observed by a HAADF - STEM image of the sample piece. In a HAADF - STEM image, a contrast proportional to the atomic weight is obtained, and heavy elements are displayed brightly. Therefore, a region with a low AlN molar fraction is displayed relatively brightly. The HAADF - STEM image is more suitable for observing the difference in AlN molar fraction than a normal STEM image (bright - field image).

[0132] Furthermore, the composition analysis within a specific semiconductor layer in the AlGaN - based semiconductor layers 21 to 24 can be performed by energy - dispersive X - ray spectroscopy (cross - sectional TEM - EDX) or CL (cathodoluminescence) method using the above - mentioned sample piece. Regarding the composition analysis by cross - sectional TEM - EDX and CL method, detailed explanations are given in the specifications of the applicant's previous separate applications (PCT / JP2020 / 024827, PCT / JP2020 / 024828, PCT / JP2020 / 026558, PCT / JP2020 / 031620, etc.).

[0133] <Composition analysis results of the n - type clad layer> Next, the results of measuring the second average AlN mole fraction Xna2(d) of n-type cladding layer 21 by Rutherford backscattering (RBS) analysis, the results of measuring the AlN mole fractions of layer region 21a and n-type body region 21b in n-type cladding layer 21 by CL (cathodoluminescence) analysis, and the results of measuring the AlN mole fraction of layer region 21a in n-type cladding layer 21 by cross-sectional TEM-EDX analysis will be described.

[0134] A sample was prepared for composition analysis of the n-type cladding layer 21, and a specimen piece having a cross section perpendicular (or nearly perpendicular) to the upper surface of the n-type cladding layer 21 was processed from the sample using a focused ion beam (FIB) to prepare a specimen piece for measurement.

[0135] The above sample was prepared by sequentially depositing the n-type cladding layer 21, an AlGaN layer having a higher AlN mole fraction than the n-type cladding layer 21, an AlGaN layer for protecting the sample surface, and a protective resin film on the underlayer 10 consisting of the sapphire substrate 11 and AlN layer 12, in accordance with the above-mentioned method for preparing the n-type cladding layer 21, etc. In preparing the sample, a sapphire substrate 11 was used whose main surface was off-angled with respect to the (0001) plane, and the underlayer 10 in which multi-step terraces were exposed on the surface of the AlN layer 12 was used. Furthermore, a donor impurity concentration of about 3×10 18 cm -3 The amount of donor impurity (Si) implanted was controlled so as to satisfy the following condition.

[0136] The first average AlN mole fraction Xna1, the second average AlN mole fraction Xna2(d) at a depth d (nm), and the third average AlN mole fraction Xna3 of the n-type cladding layer 21 of the above sample satisfy the above formulas (1), (3), and (2) when n = 7. Therefore, a first metastable AlGaN region with an AlN mole fraction of 7 / 12 (about 58.3%) is formed in the n-type body region 21b, and an intermediate AlGaN region with an AlN mole fraction of 6.5 / 12 (about 54.2%) is formed in the layered region 21a.

[0137] FIG. 12 shows the measured value of the second average AlN mole fraction Xna2(d) by RBS analysis. The AlN mole fraction Xna2(d) varies within a range of about 57.8% to about 59.9% according to the change in the depth d, and satisfies the above formula (3). The first average AlN mole fraction Xna1 is calculated to be about 58.4% from Xna2(d) shown in FIG. 12, which is slightly higher than the AlN mole fraction (about 58.3%) in the first metastable AlGaN region, but is generally the same. As will be described later, there is one specific depth dx, which is about 1061 nm. The third average AlN mole fraction Xna3 is calculated to be about 57.9% from Xna2(d) shown in FIG. 12, which is smaller than the first average AlN mole fraction Xna1, and satisfies formula (2). When the second average AlN molar fraction Xna2(d) satisfies formula (3), the first average AlN molar fraction Xna1 naturally satisfies the above formula (1), and the third average AlN molar fraction Xna3 also naturally satisfies formula (2).

[0138] As shown in Fig. 12, the second average AlN mole fraction Xna2(d) is constant at about 57.8% in the upper layer region from 0 nm to about 750 nm in depth (nm), and gradually increases from about 57.8% to about 59.9% as the depth (nm) increases from about 750 nm to about 1900 nm, and becomes equal to the AlN mole fraction (about 58.3%) of the first metastable AlGaN region at a depth (nm) of about 1061 nm. Therefore, in the example shown in Fig. 12, the specific depth dx is about 1061 nm. The second average AlN mole fraction Xna2(d) is smaller than the AlN mole fraction (about 58.3%) of the first metastable AlGaN region in the region where the depth d (nm) is 0 nm to about 1061 nm, and satisfies the formula (5). In the region where the depth d (nm) is about 1061 nm to about 1900 nm, the second average AlN mole fraction Xna2(d) is larger than the AlN mole fraction (about 58.3%) of the first metastable AlGaN region, and satisfies the formula (7). Therefore, the second average AlN mole fraction Xna2(d) shown in FIG. 12 is one of the typical examples that satisfies the formulas (5) and (7). In other words, d=about 1061 nm is the specific depth dx.

[0139] In addition, in the RBS analysis, for example,2+ An ion beam (beam diameter: 2.2 mm) is applied vertically to the top surface of the n-type cladding layer 21 of the sample at an acceleration voltage of 2.3 MeV. Since the vertical measurement range is large at about 300 nm, the film thickness to be analyzed must be greater than 300 nm.

[0140] 13 is a scanning electron microscope (SEM) image showing a main portion including the n-type cladding layer 21 on the measurement cross section of the above-mentioned sample piece. The measurement range of the sample piece (the range of the incident points of the electron beam irradiated for measurement) is 6.25 μm and 2.2 μm in the X direction (horizontal direction parallel to the second plane) and Y direction (vertical direction perpendicular to the second plane), respectively, and the incident points of the electron beam are set in a 121 mesh × 41 mesh grid pattern. The mesh spacing is approximately 52 nm in the X direction and approximately 55 nm in the Y direction.

[0141] The Y values ​​(Y coordinates) written in the measurement range of the sample piece shown in Fig. 13 represent the mesh numbers counted from the top end of each measurement range, with the top end being Y = 0. In Fig. 13, Y = 4 and Y = 38 are located near the top and bottom ends of the n-type cladding layer 21, respectively. Therefore, the film thickness of the n-type cladding layer 21 is approximately 1.9 µm.

[0142] An electron beam with a beam diameter of 50 nm was irradiated once at each of the grid-like electron beam incidence points within the measurement range of the sample piece, and the CL spectrum was measured at each incidence point.

[0143] FIG. 14 shows the first CL spectrum (solid line) and the second CL spectrum (dashed line) derived in the following manner for 121 CL spectra obtained by scanning in the X direction at each of six Y coordinates of Y=10, Y=16, Y=21, Y=26, Y=31, and Y=35 of the sample piece A. The six Y coordinates (Y=10 to 35) correspond to about 330 nm to about 1700 nm in terms of depth d from the upper end of the n-type cladding layer 21. The first and second CL spectra of the six Y coordinates are displayed on the same graph with their origins shifted in the vertical axis direction so that they can be distinguished from each other. The vertical axis of FIG. 14 indicates the emission intensity (arbitrary unit), and the emission intensities of the two Y coordinates (Y=10, 35) are multiplied by 0.5 (Y=10) and 1.5 (Y=35) to make them easier to see. The horizontal axis of FIG. 14 indicates the wavelength (nm).

[0144] In addition, for reference, three CL wavelengths (approximately 253 nm, approximately 266 nm, and approximately 279 nm) corresponding to three metastable AlGaN (AlN mole fractions of 50%, 58.3%, and 66.7%) are shown by dashed vertical lines in FIG. 14.

[0145] 14 was calculated by extracting 6 to 7 or more CL spectra whose emission intensity peaks are shifted to the same wavelength longer than the AlN mole fraction (about 58.3%) of the first metastable AlGaN region and averaging the extracted CL spectra. Therefore, the measurement region for the first CL spectrum contains more layer region 21a than other measurement regions for the same Y coordinate.

[0146] 14, 6 to 7 or more CL spectra whose emission intensity peaks are shifted to the same wavelength on the shorter wavelength side than the AlN mole fraction (about 58.3%) of the first metastable AlGaN region are extracted from the CL spectra of the same Y coordinate, and the extracted CL spectra are averaged to calculate the second CL spectrum. Therefore, the measurement region for the second CL spectrum contains more n-type body region 21b (especially Al-enriched n-type region) than other measurement regions of the same Y coordinate.

[0147] In the first CL spectrum of each Y coordinate shown in FIG. 14, the wavelength λ0(Y) showing the maximum signal intensity In0(Y) is within the range of about 268 nm to about 271 nm, and the intermediate AlGaN region (Al n-0.5 Ga 12.5-n N 12 The CL wavelength (approximately 273 nm) corresponds to the AlN mole fraction ((n-0.5) / 12) of n = 7, and the first metastable AlGaN region (Al n Ga 12-n N 12 ) and the CL wavelength (about 266 nm) corresponding to the AlN mole fraction (n / 12) of n-type body region 21b. This is because the first CL spectrum includes the CL spectrum from layer region 21a, the CL spectrum from the first metastable AlGaN region in n-type body region 21b, and the CL spectrum from the Al-rich n-type region in n-type body region 21b, and in particular, the first two CL spectra are mainly included, resulting in a composite spectrum of these.

[0148] Here, as described above, the Al-rich n-type region in the n-type main body region 21b is formed in conjunction with the formation of the layer region 21a (Ga-rich n-type region) due to the mass transfer of Ga from the terrace region to the inclined region during the epitaxial growth process of the n-type cladding layer 21. Therefore, the intermediate AlGaN region (Al n-0.5 Ga 12.5-n N 12 ) is formed, as well as a second intermediate AlGaN region (Al n+0.5 Ga 11.5-n N 12 The second intermediate AlGaN region is a first metastable AlGaN region and a third metastable AlGaN region (Al n+1 Ga 11-n N 12 ) The CL wavelength corresponding to the AlN mole fraction ((n+0.5) / 12) of the second intermediate AlGaN region when n=7 is approximately 259 nm.

[0149] Furthermore, in the first CL spectrum of each Y coordinate, the signal intensity In11(Y) at wavelength λs1 (approximately 266 nm) corresponding to the AlN mole fraction (approximately 58.3%) of the first metastable AlGaN region is approximately 67% to approximately 96% of the maximum signal intensity In0(Y), and it can be seen that the measurement region for the first CL spectrum of each Y coordinate includes an n-type body region 21b in which the first metastable AlGaN region is dominantly formed.

[0150] In the first CL spectrum of each Y coordinate, the signal intensity In1m(Y) at the wavelength λsm (about 273 nm) corresponding to the AlN mole fraction (about 54.2%) of the intermediate AlGaN region is about 73% to about 93% of the maximum signal intensity In0(Y). n-0.5 Ga 12.5-n N 12 ) is predominantly formed.

[0151] In the above measurement results, if the signal intensity In1m(Y) is 70% or more of the maximum signal intensity In0(Y), it is determined that an intermediate AlGaN region (Al n-0.5 Ga 12.5-n N 12 ) is predominantly formed. The same can be said for the case of n=6. However, in order to ensure the measurement accuracy by the CL method, it is preferable to limit the Y coordinate of the first CL spectrum to the Y coordinate in the intermediate region excluding the upper end region from the upper end of the n-type cladding layer 21 down to 150 nm and the lower end region from the lower end up to 150 nm, and to eliminate the influence of the active layer 22 present above the n-type cladding layer 21 and the influence of the underlayer 10 (in one embodiment, the uppermost layer is AlN 12) present below the n-type cladding layer 21.

[0152] Furthermore, in the first CL spectrum for each Y coordinate, the signal intensity In12(Y) at wavelength λs2 (approximately 279 nm) corresponding to the AlN mole fraction (50%) of the second metastable AlGaN region is approximately 13% to approximately 40% of the maximum signal intensity In0(Y). This indicates that, although the second metastable AlGaN region exists in the measurement region for the first CL spectrum for each Y coordinate, it is not formed predominantly.

[0153] If the signal intensity In12(Y) of the above measurement results is less than 50% of the maximum signal intensity In0(Y), it can be determined that the second metastable AlGaN region is not dominantly formed in the layer region 21a. The same can be said for the case where n=6. However, in order to ensure the measurement accuracy by the CL method, it is preferable to limit the Y coordinate of the first CL spectrum to the Y coordinate within the intermediate region.

[0154] In the second CL spectrum of each Y coordinate shown in FIG. 14, the wavelength λ1(Y) showing the maximum signal intensity In1(Y) is in the range of about 258 nm to about 261 nm, and the AlN mole fraction Xn1(d) at the depth d corresponding to each wavelength λ1(Y) is in the range of about 61.5% to about 63.5%, and all of them show a value higher than the second average AlN mole fraction Xna2(d). Therefore, it can be seen that an Al-rich n-type region is formed in the n-type main body region 21b due to the mass transfer of Ga during the formation of the layered region 21a. Incidentally, near the upper end of the n-type cladding layer 21, the AlN mole fraction Xn1(d) is about 61.5%.

[0155] Furthermore, in the second CL spectrum of each Y coordinate, the signal intensity In21(Y) at wavelength λs1 (approximately 266 nm) corresponding to the AlN mole fraction (approximately 58.3%) of the first metastable AlGaN region is approximately 55% to approximately 84% of the maximum signal intensity In1(Y), and it can be seen that the measurement region for the second CL spectrum of each Y coordinate includes an n-type body region 21b in which the first metastable AlGaN region is dominantly formed.

[0156] Furthermore, in the first and second CL spectra for each Y coordinate, the signal intensities In21(Y) at the wavelength λs1 (approximately 266 nm) corresponding to the AlN mole fraction (approximately 58.3%) of the first metastable AlGaN region are approximately the same, which indicates that the first metastable AlGaN region is uniformly formed in the n-type body region 21b.

[0157] In addition, in the second CL spectrum of each Y coordinate, the signal intensity In2m(Y) at the wavelength λsm (about 259 nm) corresponding to the AlN mole fraction (62.5%) of the second intermediate AlGaN region is about 90% to about 100% of the maximum signal intensity In2(Y), and the second intermediate AlGaN region (Al n+0.5 Ga 11.5-n N 12 ) is dominantly formed in the n-type body region 21b.

[0158] Therefore, on the surface of the n-type cladding layer 21 (particularly the exposed surface in the second region R2), the first metastable AlGaN region having an AlN molar fraction of approximately 58.3% that is uniformly and predominantly formed in the n-type main body region 21b, and the layered region 21a having an even lower AlN molar fraction are exposed, and it can be seen that even if there are some Al-rich n-type regions with an AlN molar fraction slightly exceeding 60%, the contact resistance between the surface of the n-type cladding layer 21 and the n-electrode 27 is kept low.

[0159] FIG. 15 is a HAADF-STEM image showing four measurement regions A to D where the AlN mole fraction in the layer region 21a of the above-mentioned sample piece is measured by line analysis of cross-sectional TEM-EDX.

[0160] In the composition analysis (EDX measurement) using the cross-sectional TEM-EDX method, first, an electron beam probe (diameter: approximately 2 nm) was scanned vertically (up and down) and horizontally (direction parallel to the second plane) in the entire measurement area covering the four measurement areas A to D shown in FIG. 15 to obtain detection data (X-ray intensity corresponding to each composition of Al and Ga) at each probe location distributed in a 512 × 512 matrix at intervals of approximately 4 nm in the vertical and horizontal directions.

[0161] Next, in order to perform line analysis by EDX measurement on the layered regions 21a distributed in the entire measurement region, the above-mentioned four measurement regions A to D (shown by dashed lines in FIG. 15) were set in the entire measurement region. Each of the measurement regions A to D is rectangular, and the inclination and size are set for each measurement region so that the extension direction of at least one layered region 21a in the measurement region is perpendicular to the scanning direction of the line analysis. In addition, the inclinations of the measurement regions A to D (the angle between the vertical direction of the entire measurement region and the vertical direction of each measurement region) are approximately equal at about 20°, but are not necessarily the same strictly. Here, for convenience of explanation, in each of the measurement regions A to D in FIG. 15, apart from the vertical and horizontal directions of the entire measurement region, the scanning direction of the line analysis is set as the vertical direction, and the direction perpendicular to the scanning direction is set as the horizontal direction. The central vertical line shown in each measurement region indicates the scanning direction, and the x mark on the vertical line indicates the vertical position of the layered region 21a to be measured for the AlN mole fraction. The positions of the x marks in the measurement regions A to D roughly correspond to 10, 21, 31, and 36 on the Y coordinate in the measurement range of the AlN mole fraction by the CL method shown in FIG.

[0162] In EDX measurement, the diameter of the irradiating electron beam probe is small at about 2 nm, so the spatial resolution is high, but since the X-rays emitted from each probe point are weak, in the line analysis of this embodiment, detection data obtained from multiple probe points aligned in the horizontal direction at each scanning position are accumulated to obtain the detection data at each scanning position. Note that "aligned in the horizontal direction" means that the irradiation range of the electron beam probe overlaps with a horizontal line that intersects the vertical line and extends in the horizontal direction at each scanning position.

[0163] The AlN mole fractions in layer region 21a indicated by crosses in measurement regions A to D derived based on the cumulative detection data obtained in the above manner are as follows: In addition, the AlN mole fraction difference Δ obtained by subtracting the AlN mole fraction (about 54.17%) of the intermediate AlGaN region from the measurement results of the AlN mole fraction in layer region 21a in each of measurement regions A to D is shown in parentheses to the right of the measurement results of the AlN mole fraction. Measurement area A (Y=approx. 10): 52.62% (Δ=-1.55%) Measurement area B (Y=approx. 21): 54.52% (Δ=0.35%) Measurement area C (Y=approx. 31): 54.63% (Δ=0.46%) Measurement area D (Y=approx. 36): 54.05% (Δ=-0.12%)

[0164] From the measurement results of the AlN mole fraction in layer region 21a in each of measurement regions A to D, it is found that an intermediate AlGaN region with an AlN mole fraction of 6.5 / 12 (approximately 54.2%) is predominantly formed in layer region 21a, which coincides with the results derived from the first CL spectrum in FIG.

[0165] [Second embodiment] In the light-emitting element 1 of the first embodiment, the p-type layer constituting the light-emitting element structure 20 is composed of two layers, the electron blocking layer 23 and the p-type contact layer 24. However, in the light-emitting element 2 of the second embodiment, the p-type layer has a p-type cladding layer 25 composed of one or more layers of p-type AlGaN-based semiconductor between the electron blocking layer 23 and the p-type contact layer 24.

[0166] Therefore, in the second embodiment, as shown in FIG. 16, the AlGaN-based semiconductor layers 21-25 of the light-emitting element structure 20 have a structure in which an n-type cladding layer 21 (n-type layer), an active layer 22, an electron blocking layer 23 (p-type layer), a p-type cladding layer 25 (p-type layer), and a p-type contact layer 24 (p-type layer) are epitaxially grown and stacked in this order from the underlayer 10 side.

[0167] The base portion 10, and the AlGaN-based semiconductor layers 21-24, p-electrode 26, and n-electrode 27 of the light-emitting element structure 20 in the light-emitting element 2 of the second embodiment are the same as the base portion 10 and the AlGaN-based semiconductor layers 21-24, p-electrode 26, and n-electrode 27 of the light-emitting element structure 20 of the light-emitting element 1 of any of the first to third embodiments, and therefore redundant explanations will be omitted.

[0168] The p-type cladding layer 25 has a surface on which multi-step terraces are formed parallel to the (0001) plane originating from the primary surface 11a of the sapphire substrate 11, similar to the AlN layer 12 of the base portion 10, and the semiconductor layers and electron blocking layer 23 in the n-type cladding layer 21 and active layer 22 of the light-emitting element structure 20, which are epitaxially grown in order from the primary surface 11a of the sapphire substrate 11.

[0169] Fig. 17 shows a schematic diagram of an example of a stacked structure (multiple quantum well structure) of well layers 220 and barrier layers 221 in active layer 22. In Fig. 17, p-type cladding layer 25 is formed on electron block layer 23 having the stacked structure described in the first embodiment with reference to Fig. 5.

[0170] In the p-type cladding layer 25, as described above, an inclined region IA inclined with respect to the (0001) plane is formed between laterally adjacent terraces T. The regions sandwiched between the terraces T on the top and bottom other than the inclined region IA are referred to as terrace regions TA. The film thickness of the p-type cladding layer 25, including the terrace regions TA and the inclined region IA, is adjusted to within the range of 20 nm to 200 nm, for example.

[0171] As shown in FIG. 17, in the p-type cladding layer 25, a Ga-rich p-type region 25a having a lower AlN mole fraction than the terrace region TA is formed in the inclined region IA due to mass transfer of Ga from the terrace region TA to the inclined region IA.

[0172] The AlN mole fraction of the terrace region TA of the p-type cladding layer 25 is set to be within a range of 51% or more and less than the AlN mole fraction of the terrace region TA of the electron blocking layer 23. Furthermore, the AlN mole fraction of the Ga-enriched p-type region 25a of the p-type cladding layer 25 is set to be less than the AlN mole fraction of the Ga-enriched EB region 23a of the electron blocking layer 23.

[0173] Furthermore, the AlN molar fraction of the terrace region TA of the p-type cladding layer 25 is set to be at least 1%, preferably at least 2%, and more preferably at least 4% higher than the AlN molar fraction of the Ga-enriched p-type region 25a within the above range. In order to fully ensure the effect of carrier localization in the Ga-enriched p-type region 25a, it is preferable that the difference in AlN molar fraction between the Ga-enriched p-type region 25a of the p-type cladding layer 25 and the terrace region TA is 4 to 5% or more, but the effect of carrier localization can be expected even if it is about 1 to 2%.

[0174] Next, a brief description will be given of a method for growing the p-type cladding layer 25. In forming the p-type cladding layer 25, in the same manner as the n-type cladding layer 21 and the electron blocking layer 23 described in the first embodiment, the p-type cladding layer 25 is grown under growth conditions that make it easy to expose the above-mentioned multi-step terraces, with the average AlN mole fraction Xpa of the p-type cladding layer 25 as a target value.

[0175] [Another embodiment] Modifications of the first and second embodiments will be described below.

[0176] (1) In each of the above embodiments, it is assumed that the active layer 22 is configured with a multiple quantum well structure in which two or more well layers 220 made of AlGaN-based semiconductors and one or more barrier layers 221 made of AlGaN-based semiconductors or AlN-based semiconductors are alternately stacked, but the active layer 22 may be configured with a single quantum well structure having only one well layer 220 and without a barrier layer 221 (quantum barrier layer). It is clear that the effects of the well layer 220 employed in each of the above embodiments can be similarly achieved for such a single quantum well structure.

[0177] (2) In each of the above embodiments, an n-type AlGaN-based semiconductor layer (hereinafter referred to as an "n-type underlayer") having a higher AlN mole fraction than the n-type cladding layer 21 may be provided between the n-type cladding layer 21 and the underlayer 10. As a result, the average AlN mole fraction over the entire depth direction of the combination of the n-type cladding layer 21 and the n-type underlayer may be higher than the range of the above formula (1).

[0178] The n-type underlayer provided below the n-type cladding layer 21 has a higher AlN mole fraction than the n-type cladding layer 21, and therefore does not absorb light emitted from the active layer. Even if the n-type underlayer has a higher AlN mole fraction than the n-type cladding layer 21, it does not come into contact with the n-electrode 27 and does not form a current path between the n-electrode 27 and the active layer, and therefore does not increase the parasitic resistance between the n-electrode 27 and the active layer, and does not become a factor in reducing the wall plug efficiency. In other words, since the n-type underlayer does not substantially function as a part of the light-emitting element structure 20, it can be said that there is no particular advantage or significant disadvantage in providing the n-type underlayer.

[0179] (3) In the above embodiments, the first region R1 and the p-electrode 26 are, for example, comb-shaped in plan view. Ta However, the shape in plan view is not limited to a comb shape. Also, there may be a plurality of first regions R1, each of which is surrounded by one second region R2 in plan view.

[0180] (4) In each of the above embodiments, a sapphire substrate 11 having a main surface with an off-angle relative to the (0001) plane is used as the base portion 10 in which multi-stepped terraces are exposed on the surface of the AlN layer 12. However, the magnitude of the off-angle and the direction in which the off-angle is provided (specifically, the direction in which the (0001) plane is tilted, such as the m-axis direction or the a-axis direction) may be determined arbitrarily as long as multi-stepped terraces are exposed on the surface of the AlN layer 12 and a growth starting point for the layered region 21a is formed.

[0181] (5) In each of the above embodiments, the light-emitting element 1 is exemplified as a light-emitting element 1 having a base portion 10 including a sapphire substrate 11 as shown in Fig. 1, but the sapphire substrate 11 (and further, some or all of the layers included in the base portion 10) may be removed by lift-off or the like. Furthermore, the substrate constituting the base portion 10 is not limited to a sapphire substrate. [Industrial Applicability]

[0182] INDUSTRIAL APPLICABILITY The present invention can be used in a nitride semiconductor ultraviolet light emitting device having a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in the vertical direction. [Explanation of symbols]

[0183] 1 , 2 : Nitride semiconductor ultraviolet light emitting device 10: Base 11: Sapphire substrate 11a: Main surface of sapphire substrate 12: AlN layer 20: Light emitting device structure 21: n-type cladding layer (n-type layer) 21a: Layered region (n-type layer) 21b: n-type body region (n-type layer) 22: Active layer 220: Well layer 220a: Ga-rich well region 221: Barrier layer 221a: Ga-enriched barrier region 23: Electron blocking layer (p-type layer) 23a: Ga-enriched EB region 24: p-type contact layer (p-type layer) 25: p-type cladding layer (p-type layer) 25a: Ga-enriched p-type region 26:p electrode 27: n electrode 100: PCB 101: AlGaN-based semiconductor layer 102: Templates 103: n-type AlGaN semiconductor layer 104: Active layer 105: p-type AlGaN semiconductor layer 106: p-type contact layer 107: n electrode 108: p electrode BL: Boundary between the first and second areas IA: Slope area R1: 1st area R2: 2nd area T: Terrace TA: Terrace area

Claims

1. A nitride semiconductor ultraviolet light emitting device comprising a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in a vertical direction, the n-type layer is made 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 made of an AlGaN-based semiconductor; the p-type layer is made of a p-type AlGaN-based semiconductor, each of the semiconductor layers in the n-type layer, the active layer, and the p-type layer is an epitaxially grown layer having a surface on which multiple terraces parallel to a (0001) plane are formed; the n-type layer has a layered region having a locally low AlN mole fraction that is dispersed in the n-type layer, an extension direction of each of the layered regions on a first plane perpendicular to an upper surface of the n-type layer has a portion inclined with respect to an intersection line between the upper surface of the n-type layer and the first plane, The integer n is 6 or 7, A first average AlN mole fraction Xna1 throughout the entire region in the depth direction of the n-type layer is (n-0.25) / 12<Xna1<(n+0.25) / 12 is in the range where a second average AlN mole fraction Xna2(d) at a depth d from an upper end of the n-type layer varies depending on the depth d, and at least one specific depth among one or more depths at which Xna2(d)=n / 12 in the n-type layer, a region where Xna2(d)<n / 12 exists above the specific depth and a region where Xna2(d)>n / 12 exists below the specific depth; The second average AlN mole fraction Xna2(d) is, in the entire area in the depth direction of the n-type layer, (n-0.25) / 12<Xna2(d)<(n+0.25) / 12 is in the range where A nitride semiconductor ultraviolet light emitting device, comprising: an intermediate AlGaN region having an AlN mole fraction of (n-0.5) / 12 formed within said layer region.

2. a third average AlN mole fraction Xna3 in a region from the upper end of the n-type layer to the specific depth, (n-0.25) / 12<Xna3<(n+0.25) / 12 2. The nitride semiconductor ultraviolet light emitting device according to claim 1, wherein the range is:

3. The second average AlN mole fraction Xna2(d) is, in a region from an upper end of the n-type layer to the specific depth, Xna2(d)≦n / 12 3. The nitride semiconductor ultraviolet light emitting device according to claim 1, wherein the nitride semiconductor ultraviolet light emitting device has a wavelength of 100 nm or more and a wavelength of 100 nm or more.

4. The second average AlN mole fraction Xna2(d) in a region of the n-type layer deeper than the specific depth, Xna2(d)≧n / 12 4. The nitride semiconductor ultraviolet light emitting device according to claim 3, wherein the range is:

5. 5. The nitride semiconductor ultraviolet light emitting device according to claim 1, wherein the integer n is 7, and the peak emission wavelength is set to a predetermined value within a range of 280 nm to 315 nm.

6. 5. The nitride semiconductor ultraviolet light emitting device according to claim 1, wherein the integer n is 6, and the peak emission wavelength is set to a predetermined value within a range of 300 nm to 330 nm.

7. the active layer has a multiple quantum well structure including two or more of the well layers, 7. The nitride semiconductor ultraviolet light emitting device according to claim 1, further comprising a barrier layer made of an AlGaN-based semiconductor present between the two well layers.

8. Further comprising a base portion including a sapphire substrate; the sapphire substrate has a main surface inclined at a predetermined angle with respect to a (0001) plane, and the light emitting element structure is formed above the main surface; The nitride semiconductor ultraviolet light emitting device according to any one of claims 1 to 7, characterized in that each semiconductor layer from the main surface of the sapphire substrate to the p-type layer is an epitaxially grown layer having a surface on which multiple stepped terraces parallel to the (0001) plane are formed.

9. A method for manufacturing a nitride semiconductor ultraviolet light emitting device comprising a light emitting device structure in which an n-type layer, an active layer, and a p-type layer made of an AlGaN-based semiconductor having a wurtzite structure are stacked in a vertical direction, the method comprising the steps of: a first step of epitaxially growing an n-type layer of an n-type AlGaN-based semiconductor on a base including a sapphire substrate having a primary surface inclined at a predetermined angle with respect to a (0001) plane, and exposing multi-step terraces parallel to the (0001) plane on a surface of the n-type layer; a second step of epitaxially growing the active layer having a quantum well structure including one or more well layers made of an AlGaN-based semiconductor on the n-type layer, and exposing multi-step terraces parallel to a (0001) plane on a surface of the well layer; a third step of forming the p-type layer made of a p-type AlGaN-based semiconductor on the active layer by epitaxial growth; In the first step, The integer n is 6 or 7, A first average AlN mole fraction Xna1 throughout the entire region in the depth direction of the n-type layer is (n-0.25) / 12<Xna1<(n+0.25) / 12 is in the range where a second average AlN mole fraction Xna2(d) at a depth d from an upper end of the n-type layer varies depending on the depth d, and at least one specific depth among one or more depths at which Xna2(d)=n / 12 in the n-type layer, there is a region above the specific depth where Xna2(d)<n / 12 exists and a region below the specific depth where Xna2(d)>n / 12 exists; The second average AlN mole fraction Xna2(d) is, in the entire area in the depth direction of the n-type layer, (n-0.25) / 12<Xna2(d)<(n+0.25) / 12 and a layered region having a locally low AlN mole fraction that is uniformly dispersed in the n-type layer and extends obliquely upward; so as to form an intermediate AlGaN region within the layered region, the intermediate AlGaN region having an AlN mole fraction of (n-0.5) / 12; A method for producing a nitride semiconductor ultraviolet light emitting device, comprising forming the n-type layer.

10. In the first step, a third average AlN mole fraction Xna3 in a region from an upper end of the n-type layer to the specific depth is (n-0.25) / 12<Xna3<(n+0.25) / 12 10. The method for manufacturing a nitride semiconductor ultraviolet light emitting device according to claim 9, wherein the n-type layer is formed so that the n-type layer is within the range where

11. In the first step, the second average AlN mole fraction Xna2(d) is, in a region from an upper end of the n-type layer to the specific depth, Xna2(d)≦n / 12 11. The method for manufacturing a nitride semiconductor ultraviolet light emitting device according to claim 9, wherein the n-type layer is formed so that the n-type layer is within the range where

12. In the first step, the second average AlN mole fraction Xna2(d) is, in a region of the n-type layer deeper than the specific depth, Xna2(d)≧n / 12 12. The method for manufacturing a nitride semiconductor ultraviolet light emitting device according to claim 11, wherein the n-type layer is formed so that the n-type layer is within the range where

13. 13. The method for manufacturing a nitride semiconductor ultraviolet light-emitting device according to claim 9, wherein the integer n is 7, and in the second step, the active layer is formed so that a peak emission wavelength of the nitride semiconductor ultraviolet light-emitting device is a predetermined value within a range of 280 nm to 315 nm.

14. 13. The method for manufacturing a nitride semiconductor ultraviolet light-emitting device according to claim 9, wherein the integer n is 6, and in the second step, the active layer is formed so that a peak emission wavelength of the nitride semiconductor ultraviolet light-emitting device is a predetermined value within a range of 300 nm to 330 nm.

15. 15. The method for producing a nitride semiconductor ultraviolet light-emitting device according to claim 9, wherein in the second step, the well layers made of an AlGaN-based semiconductor and barrier layers made of an AlGaN-based semiconductor are alternately stacked by epitaxial growth to form the active layer having a multiple quantum well structure including two or more well layers.

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