Semiconductor laser element

Abstract

Provided is a semiconductor laser element in which carrier loss can be reduced. A semiconductor laser element according to the present invention has, in the following order in the upward direction, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each of the layers being composed of a nitride semiconductor, wherein the p-side semiconductor layer has a first portion that is undoped, an electron barrier layer containing p-type impurities, and a second portion that has one or more p-type semiconductor layers containing p-type impurities, the first portion having: a first p-side composition-graded layer that is composed of InxGa1−xN, the In composition ratio x decreasing within the range of 0 to less than 1 with progression in the upward direction; a second p-side composition-graded layer that is disposed between the first p-side composition-graded layer and the electron barrier layer and composed of AlyGa1−yN, the Al composition ratio y increasing within the range of greater than 0 to less than 1 with progression in the upward direction; and an intermediate layer that is disposed between the first p-side composition-graded layer and the second p-side composition-graded layer and composed of AlzGa1−zN, the Al composition ratio z being within the range of greater than 0 to less than y.

Description

[Technical field]

[0001] The present disclosure relates to a semiconductor laser device. [Background technology]

[0002] Patent Document 1 describes a semiconductor laser device having an undoped p-side compositionally graded layer and an undoped p-side intermediate layer between an active layer and an electron barrier layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-115539 A Summary of the Invention [Problem to be solved by the invention]

[0004] An object of one aspect of a semiconductor laser device according to the present disclosure is to provide a semiconductor laser device capable of reducing carrier loss. [Means for solving the problem]

[0005] One aspect of the semiconductor laser element of the present disclosure is a semiconductor laser element having, in this order from the top, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each made of a nitride semiconductor, in which the p-side semiconductor layer has one or more semiconductor layers and has, in this order from the top, a first portion that is undoped, a second portion that has a band gap energy larger than that of the first portion and contains a p-type impurity, and one or more p-type semiconductor layers that contain a p-type impurity, and the first portion is In x Ga 1-x a first p-side composition gradient layer having an In composition ratio x that decreases upward within a range of 0 to less than 1; and an Al y Ga 1-ya second p-side compositionally graded layer having an Al composition ratio y that increases upward in a range exceeding 0 and less than 1; and a second p-side compositionally graded layer disposed between the first p-side compositionally graded layer and the second p-side compositionally graded layer, the second p-side compositionally graded layer being made of Al. z Ga 1-z and an intermediate layer made of N and having an Al composition ratio z exceeding 0 and less than y. Effect of the Invention

[0006] A semiconductor laser device capable of reducing carrier loss can be obtained. [Brief description of the drawings]

[0007] [Figure 1] 1 is a schematic cross-sectional view of a semiconductor laser device according to one embodiment of the present invention. [Diagram 2] 2 is a diagram illustrating an example of a layer structure of a p-side semiconductor layer of the semiconductor laser element of FIG. 1. [Diagram 3] 2 is a diagram illustrating an example of a layer structure of an n-side semiconductor layer of the semiconductor laser element of FIG. 1. [Figure 4] FIG. 2 is a diagram illustrating another example of the layer structure of the active layer. [Diagram 5] 2 is a partial enlarged view of the first p-side compositionally graded layer and its vicinity of the semiconductor laser device of FIG. 1. [Figure 6] 2 is a partial enlarged view of the second p-side compositionally graded layer and its vicinity of the semiconductor laser device of FIG. 1. [Figure 7] 1 shows the results of SIMS analysis of a wafer on which a p-side semiconductor layer similar to that of Example 1 was grown. [Figure 8] 1 is a graph showing IL characteristics of the semiconductor laser devices of Example 1 and Comparative Example 1. [Figure 9] 13 is a graph showing IL characteristics of the semiconductor laser devices of Example 2, Example 3, and Comparative Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the embodiment shown below is merely an example of a method for realizing the technical idea of ​​the present invention, and the present invention is not limited to the following embodiment. Furthermore, in the following description, the same names and symbols indicate the same or similar components, and detailed description will be omitted as appropriate.

[0009] FIG. 1 is a schematic cross-sectional view of a semiconductor laser device 100 according to this embodiment, showing a cross section in a direction perpendicular to the cavity direction of the semiconductor laser device 100. FIG. 2 is a diagram showing a schematic example of a layer structure of the p-side semiconductor layer 4 of the semiconductor laser device 100. In FIG. 2, the magnitude relationship of the band gap energy of each layer is shown. The dashed dotted line in FIG. 2 is a line indicating the position of the bottom surface of the ridge 4a. The bottom surface of the ridge 4a refers to a surface connecting the lowest sides of both sides of the ridge 4a.

[0010] 1, the semiconductor laser element 100 has an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4, each made of a nitride semiconductor, arranged in this order from the top. In this specification, the direction from the n-side semiconductor layer 2 to the p-side semiconductor layer 4 is referred to as "up" or "upward," and the opposite direction is referred to as "down" or "downward." Such up-down directions do not need to coincide with the direction of gravity when the semiconductor laser element 100 is used.

[0011] The p-side semiconductor layer 4 has a first portion 41, an electron barrier layer 42, and a second portion 43, in this order from the top. The first portion 41 has one or more semiconductor layers. The first portion 41 is undoped. The electron barrier layer 42 has a band gap energy larger than that of the first portion 41 and contains a p-type impurity. The second portion 43 has one or more p-type semiconductor layers containing a p-type impurity. In this specification, "undoped" refers to not being intentionally doped. A concentration that does not exceed the detection limit in the analysis results of secondary ion mass spectrometry (SIMS) or the like may be called undoped. Alternatively, an impurity concentration of 1×10 17 / cm 3The state where the concentration of p-type impurities is less than 10 may be regarded as undoped. For example, the first portion 41 may be said to be undoped when the concentrations of p-type impurities and n-type impurities are below the detection limit. However, if the first portion 41 is in contact with the electron barrier layer 42 having a high concentration of p-type impurities, p-type impurities may be detected in the analysis results even if the first portion 41 is formed without being intentionally doped with p-type impurities. In this case, the concentration of p-type impurities detected is 1×10 18 / cm 3 It is preferable that the thickness is less than 100 nm. In addition, when the first portion 41 and the like are formed undoped, unintended impurities such as H and C may be contained, but in this case, they can also be called undoped. In addition, in this specification, the thickness of a certain layer or part refers to the shortest distance from the lower surface to the upper surface of the layer or part. When the lower surface and / or the upper surface have partial concave and / or convex portions such as V pits, the shortest distance between flat portions of the lower surface and / or the upper surface that do not have such concave and / or convex portions may be the thickness of the layer or part. In addition, the distance from a certain layer to another layer refers to the shortest distance from a certain layer to another layer. When the lower surface and / or the upper surface of those layers have partial concave and / or convex portions such as V pits, the shortest distance between flat portions of the lower surface and / or the upper surface that do not have such concave and / or convex portions may be the distance between those layers. The magnitude relationship of the band gap energy can be determined from the composition of the semiconductor to be compared. For example, when two layers to be compared are both made of AlGaN, the layer with a relatively higher Al composition ratio is determined to be the layer with a relatively higher band gap energy.

[0012] The first portion 41 has a first p-side compositionally graded layer 411, a second p-side compositionally graded layer 412 disposed between the first p-side compositionally graded layer 411 and the electron barrier layer 42, and an intermediate layer 413 disposed between the first p-side compositionally graded layer 411 and the second p-side compositionally graded layer 412. The first p-side compositionally graded layer 411 is composed of In x Ga 1-x The second p-side composition gradient layer 412 is made of Al. y Ga 1-yThe intermediate layer 413 (first intermediate layer) is made of Al. z Ga 1-z N, and the Al composition ratio z is greater than 0 and less than y.

[0013] By providing the second p-side compositionally graded layer 412, the band gap energy can be gradually changed toward the electron barrier layer 42, and the influence of a band spike occurring at a heterointerface with a band gap energy difference can be reduced. The influence of the band spike is that electrons are generated at the band spike, and holes (positive holes) are relatively decreased, resulting in an increase in carrier loss. The first p-side compositionally graded layer 411 is In x Ga 1-x N, and the second p-side compositionally graded layer 412 is Al y Ga 1-y N. Therefore, when these layers are grown, the source gas is switched midway. By growing the intermediate layer 413 between these layers, the switched source gas can be stabilized, and the second p-side compositionally graded layer 412 can be formed satisfactorily. These layers can be formed by, for example, metal organic chemical vapor deposition (MOCVD). By forming the second p-side compositionally graded layer 412 satisfactorily, the influence of band spikes in the p-side semiconductor layer 4 can be reduced, and carrier loss can be reduced.

[0014] (Semiconductor laser element 100) 1, the semiconductor laser element 100 has a substrate 1 and, provided thereon, an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4. The semiconductor laser element 100 is an edge-emitting laser element having a light-emitting end face and a light-reflecting end face intersecting with the main surfaces of the semiconductor layers such as the active layer 3.

[0015] For example, the p-side semiconductor layer 4 is provided with a ridge 4a that protrudes upward. The ridge 4a has a mesa structure. The top view shape of the ridge 4a is long in the direction connecting the light emitting end face and the light reflecting end face, and is, for example, a rectangular shape with a short side parallel to the light reflecting end face and a long side perpendicular to the light reflecting end face. The part of the active layer 3 directly below the ridge 4a and its vicinity is the optical waveguide region. An insulating film 5 can be provided on the side surface of the ridge 4a and on the surface of the p-side semiconductor layer 4 that is continuous with the side surface of the ridge 4a. The substrate 1 is made of, for example, an n-type semiconductor, and an n-electrode 8 is provided on its lower surface. A p-electrode 6 is provided in contact with the upper surface of the ridge 4a, and a p-side pad electrode 7 is further provided thereon.

[0016] The semiconductor laser element 100 can oscillate, for example, a visible laser beam. The semiconductor laser element 100 can oscillate, for example, a blue or green laser beam. The peak wavelength of the laser beam oscillated by the semiconductor laser element 100 is, for example, 400 nm or more and 600 nm or less, and may be 420 nm or more and 580 nm or less, or may be 500 nm or more and 580 nm or less. As the wavelength of the laser beam oscillated by the semiconductor laser element 100 becomes longer, the amount of light leaking outward from the optical guide layer increases due to the influence of the wavelength dispersion of the refractive index. As a result, the threshold current increases, and the current density during laser oscillation increases. And, as the current density increases, the effective transition interval expands due to the shielding of the localized level and band filling, and the oscillation wavelength shifts to a shorter wavelength. By providing the first p-side compositionally graded layer 411, the laser oscillation threshold current density can be reduced, and the effect of suppressing the short wavelength shift can be expected. For this reason, the peak wavelength of the laser light oscillated by the semiconductor laser device 100 is preferably 500 nm or more, and may be 500 nm or more and 580 nm or less.

[0017] (Substrate 1) The substrate 1 may be, for example, a nitride semiconductor substrate made of GaN or the like. The n-side semiconductor layer 2, active layer 3, and p-side semiconductor layer 4 grown on the substrate 1 may be, for example, semiconductors grown substantially in the c-axis direction. For example, a GaN substrate having a +c plane ((0001) plane) as the main surface may be used, and each semiconductor layer may be grown on the +c plane. Here, the +c plane as the main surface may include a surface having an off-angle of about ±1 degree or less. By using a substrate having a +c plane as the main surface, it is possible to obtain the advantage of excellent mass productivity.

[0018] (n-side semiconductor layer 2) The n-side semiconductor layer 2 can be a multi-layer structure made of a nitride semiconductor such as GaN, InGaN, or AlGaN. The n-side semiconductor layer 2 includes one or more n-type semiconductor layers. An example of the n-type semiconductor layer is a layer made of a nitride semiconductor containing n-type impurities such as Si or Ge. The n-side semiconductor layer 2 can have an n-side cladding layer and an n-side optical guide layer, and may include other layers. The n-side cladding layer has a larger band gap energy than the n-side optical guide layer. Although not as much as the p-type impurities, the n-type impurities also cause light absorption, so the n-side optical guide layer is preferably undoped, or if it contains n-type impurities, the n-type impurity concentration is lower than the n-type impurity concentration of the n-side cladding layer.

[0019] An example of the layer structure of the n-side semiconductor layer 2 is shown in FIG. 3. FIG. 3 is a diagram showing a schematic example of the layer structure of the n-side semiconductor layer 2 of the semiconductor laser element 100. The n-side semiconductor layer 2 can have an n-side compositionally graded layer 21 arranged in contact with the lower surface of the active layer 3, an n-type semiconductor layer 22 arranged below the n-side compositionally graded layer 21, and an n-side intermediate portion 23 arranged between the n-side compositionally graded layer 21 and the n-type semiconductor layer 22. The n-side compositionally graded layer 21 has a composition that changes so that the band gap energy increases toward the bottom. The n-type semiconductor layer 22 contains n-type impurities and has a band gap energy larger than the band gap energy of any layer constituting the first portion 41 of the p-side semiconductor layer 4. The distance from the n-side compositionally graded layer 21 to the n-type semiconductor layer 22 is preferably larger than the distance from the first p-side compositionally graded layer 411 to the electron barrier layer 42. This allows the electric field intensity distribution in the semiconductor laser device 100 to be biased toward the n-side semiconductor layer 2, thereby reducing loss due to light absorption in the p-type impurity-containing electron barrier layer 42 and the layers thereover. The n-side semiconductor layer 2 may include layers other than these.

[0020] The band gap energy of the n-side compositional graded layer 21 becomes smaller as it approaches the active layer 3. The refractive index of the n-side compositional graded layer 21 becomes higher as it approaches the active layer 3. This makes it possible to strengthen the light confinement in the active layer 3. The n-side compositional graded layer 21 is, for example, an n-side optical guide layer. The n-side compositional graded layer 21 is, for example, In a Ga 1-aThe layer may be made of N, and the In composition ratio a increases in the range of 0 to less than 1 as it goes upward. The average value of the composition gradient layer can be used as a criterion for determining the magnitude relationship between the band gap energy and the impurity concentration of the composition gradient layer and other layers. The average value of the composition gradient layer refers to the sum of the multiplication of the band gap energy, etc., and the thickness of each sublayer constituting the composition gradient layer, divided by the total thickness. When the n-side composition gradient layer 21 is a composition gradient layer whose lattice constant increases as it approaches the active layer 3, it is preferable to add an n-type impurity to the composition gradient layer. In other words, the composition gradient layer can be said to be made up of multiple sublayers whose compositions are slightly different. For this reason, it is difficult to avoid the generation of fixed charges in the composition gradient layer even if the composition change rate is reduced. By adding an n-type impurity, the fixed charges can be shielded, and the degree of voltage rise caused by the generation of fixed charges can be reduced.

[0021] The n-type semiconductor layer 22 is, for example, an n-type AlGaN layer. The n-type semiconductor layer 22 is, for example, an n-side cladding layer. The n-type semiconductor layer 22 and the n-side intermediate portion 23 may be in contact with each other, or another layer may be disposed between the n-type semiconductor layer 22 and the n-side intermediate portion 23. The distance from the n-side compositionally graded layer 21 to the n-type semiconductor layer 22 may be less than 600 nm, and may be 400 nm or less.

[0022] The n-side intermediate portion 23 is a layer having a band gap energy smaller than the band gap energy of the n-type semiconductor layer 22. The n-type impurity concentration of the n-side intermediate portion 23 may be smaller than the n-type impurity concentration of the n-type semiconductor layer 22. The n-side intermediate portion 23 is, for example, an n-type GaN layer containing n-type impurities.

[0023] The n-side intermediate portion 23 may have a compositionally graded layer 232 whose band gap energy decreases toward the active layer 3. In this compositionally graded layer, the band gap energy at its lower end is smaller than the band gap energy of the n-type semiconductor layer 22, and the band gap energy at its upper end is equal to or larger than the band gap energy at the lower end of the n-side compositionally graded layer 21. The n-side intermediate portion 23 may have, in order from the n-side compositionally graded layer 21 side, a compositionally graded layer 232 and an intermediate layer 233. The n-side intermediate portion 23 may have, in order from the n-side compositionally graded layer 21 side, an intermediate layer 231, a compositionally graded layer 232, and an intermediate layer 233. The intermediate layer 231 and the intermediate layer 233 are not compositionally graded layers, or are compositionally graded layers having a compositional change rate smaller than the compositional change rate of the compositionally graded layer 232. The compositionally graded layer 232 is preferably doped with an n-type impurity, like the n-side compositionally graded layer 21. This can reduce the degree of voltage rise caused by the generation of fixed charges. The intermediate layer 231 and the intermediate layer 233 may be undoped, but contain, for example, an n-type impurity. The thickness of the n-side intermediate portion 23 may be 100 nm or more. This can reduce light leakage to the n-type semiconductor layer 22. The thickness of the n-side intermediate portion 23 may be 400 nm or less. This can improve light confinement in the active layer 3.

[0024] (Active layer 3) The active layer 3 may have a multi-layer structure made of nitride semiconductor layers such as GaN, InGaN, etc. The active layer 3 has a single quantum well structure or a multiple quantum well structure. For example, the active layer 3 has, in order from the n-side semiconductor layer 2 side, an n-side barrier layer 31, a well layer 32, and a p-side barrier layer 33. When the active layer 3 has a multiple quantum well structure, it has a plurality of well layers 32 and intermediate barrier layers sandwiched between the well layers 32.

[0025] The distance from the well layer 32 closest to the n-side semiconductor layer 2 to the lower surface of the active layer 3 can be, for example, 10 nm or less. The distance from the well layer 32 closest to the n-side semiconductor layer 2 to the lower surface of the active layer 3 can be 0 nm, that is, the lower surface of the well layer 32 can be the lower surface of the active layer 3. When the n-side semiconductor layer 2 has the n-side compositionally graded layer 21, it is preferable that a layer having a band gap energy larger than the band gap energy of the upper end of the n-side compositionally graded layer 21 is disposed as at least a part of the n-side barrier layer 31. This can improve the probability of radiative recombination in the well layer 32. The n-side barrier layer 31 is disposed between the well layer 32 closest to the n-side semiconductor layer 2 and the n-side compositionally graded layer 21. The distance from the well layer 32 closest to the n-side semiconductor layer 2 to the lower surface of the active layer 3 can be 1 nm or more, and may be 1 nm or more and 10 nm or less.

[0026] The distance from the well layer 32 closest to the p-side semiconductor layer 4 to the upper surface of the active layer 3 can be, for example, 5 nm or less. It is preferable that a layer having a band gap energy larger than the band gap energy of the lower end of the first p-side compositionally graded layer 411 is disposed as at least a part of the p-side barrier layer 33. This can improve the probability of radiative recombination in the well layer 32. The p-side barrier layer 33 is disposed between the well layer 32 closest to the p-side semiconductor layer 4 and the p-side semiconductor layer 4. The distance from the well layer 32 closest to the p-side semiconductor layer 4 to the upper surface of the active layer 3 may be 1 nm or more, and may be 1 nm or more and 5 nm or less. It is preferable that the active layer 3 is formed without adding p-type impurities. This can suppress light absorption loss caused by the addition of p-type impurities. Each layer of the active layer 3 is, for example, an undoped layer.

[0027] The well layer 32 is, for example, In x Ga 1-x In the case of a semiconductor laser device with an oscillation wavelength of 500 nm or more, x Ga 1-xThe In composition ratio x of the N well layer is, for example, 0.23 or more, although it varies somewhat depending on the layer structure other than the active layer 3. The upper limit of the In composition ratio x of the well layer 32 is, for example, 0.50 or less. In this case, the oscillation wavelength of the semiconductor laser element is considered to be about 600 nm or less.

[0028] Another example of the active layer is shown in FIG. 4. FIG. 4 is a diagram showing a schematic diagram of another example of the layer structure of the active layer. The active layer 3A shown in FIG. 4 has an n-side barrier layer 31, a plurality of well layers 32, an intermediate barrier layer 34, and a p-side barrier layer 33. The plurality of well layers 32 are located between the n-side barrier layer 31 and the p-side barrier layer 33. The plurality of well layers 32 include a first well layer and a second well layer. The first well layer and the second well layer are located between the n-side barrier layer 31 and the p-side barrier layer 33. The intermediate barrier layer 34 is a layer sandwiched between the two well layers 32. The intermediate barrier layer 34 is located between the first well layer and the second well layer. The number of well layers 32 is a plurality, for example, two. The band gap energy of the n-side barrier layer 31, the intermediate barrier layer 34, and the p-side barrier layer 33 is larger than the band gap energy of the well layer 32. The thickness of the multiple well layers 32 is, for example, 1 nm or more and 4 nm or less. The multiple well layers 32 may have the same thickness.

[0029] The driving voltage of the semiconductor laser element 100 tends to decrease as the thickness of the intermediate barrier layer 34 is reduced. On the other hand, the p-side barrier layer 33 preferably has a certain thickness as a base layer before stacking the p-side semiconductor layer 4, for the purpose of recovering crystallinity. For this reason, the thickness of the intermediate barrier layer 34 is preferably smaller than the thickness of the p-side barrier layer 33. This makes it possible to reduce the driving voltage of the semiconductor laser element 100 and improve the crystallinity of the p-side semiconductor layer 4 at the same time. The crystallinity can be evaluated, for example, by dislocation density. The thickness of each layer refers to the thickness in the stacking direction. The intermediate barrier layer 34 is, for example, an undoped GaN layer. The thickness of the intermediate barrier layer 34 is preferably 3 nm or less, and more preferably 2.5 nm or less. This can improve the efficiency of injection of electrons into the well layer 32. The thickness of the intermediate barrier layer 34 can be 1 nm or more, preferably 1.5 nm or more, and more preferably 2 nm or more. By making the thickness of the intermediate barrier layer 34 1.5 nm or more, it is possible to suppress a decrease in carrier confinement efficiency and suppress a decrease in the output of the semiconductor laser device 100. The thickness of the intermediate barrier layer 34 can be set to 1 nm or more and 3 nm or less, preferably 1.5 nm or more and 3 nm or less, and more preferably 1.5 nm or more and 2.5 nm or less. The thickness of the intermediate barrier layer 34 may be set to 2 nm or more and 2.5 nm or less.

[0030] The thickness of the p-side barrier layer 33 may be greater than the thickness of one well layer 32. The thickness of the p-side barrier layer 33 is preferably 2.5 nm or greater. This allows the crystallinity of the p-side semiconductor layer 4 to be improved more effectively. When the thickness of the p-side barrier layer 33 becomes large to a certain extent, an increase in the driving voltage of the semiconductor laser element 100 is observed. The thickness of the p-side barrier layer 33 is preferably 7 nm or less. This allows the increase in the driving voltage of the semiconductor laser element 100 to be suppressed. The thickness of the p-side barrier layer 33 is preferably 2.5 nm or greater and 7 nm or less. The thickness of the p-side barrier layer 33 may be greater than 2.5 nm. The thickness of the p-side barrier layer 33 may be greater than 2.5 nm and 7 nm or less. The p-side barrier layer 33 is preferably a GaN layer, and more preferably an undoped GaN layer. This allows the crystallinity of the p-side semiconductor layer 4 to be improved more effectively.

[0031] The semiconductor laser element 100 having the active layer 3A may be a semiconductor laser element 100 having an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4, each of which is made of a nitride semiconductor, in this order from the top, and the active layer 3A has an n-side barrier layer 31, a p-side barrier layer 33, a plurality of well layers 32 including a first well layer and a second well layer located between the n-side barrier layer 31 and the p-side barrier layer 33, and an intermediate barrier layer 34 located between the first well layer and the second well layer, and the thickness of the intermediate barrier layer 34 is smaller than the thickness of the p-side barrier layer 33. In this case, the p-side semiconductor layer 4 may have one or more semiconductor layers, and may have, from the top, a first portion 41 that is undoped, an electron barrier layer 42 that has a band gap energy larger than that of the first portion 41 and contains a p-type impurity, and a second portion 43 that has one or more p-type semiconductor layers containing a p-type impurity, in this order from the top. In this case, the p-side semiconductor layer 4 may have the first p-side compositionally graded layer 411. The p-side semiconductor layer 4 in this case does not have to have the second p-side compositionally graded layer 412 and the intermediate layer 413.

[0032] (p-side semiconductor layer 4) The p-side semiconductor layer 4 can be a multi-layer structure made of nitride semiconductors such as GaN, InGaN, and AlGaN. The p-side semiconductor layer 4 can have a p-side cladding layer and a p-side light guide layer, and may include other layers. When a transparent conductive film is provided as the p-electrode 6, it can function as a cladding layer, so that a cladding layer does not need to be provided in the p-side semiconductor layer 4. The p-side semiconductor layer 4 includes one or more p-type semiconductor layers. An example of the p-type semiconductor layer is a layer made of a nitride semiconductor containing p-type impurities such as Mg. Since the activation rate of p-type impurities is lower than that of n-type impurities such as Si, in order to obtain sufficient hole injection in the p-type semiconductor layer, it is necessary to dope the p-type impurities at a higher concentration, and the free carrier absorption loss due to the p-type impurities increases.

[0033] (Part 1 41) The first portion 41 is a portion of the p-side semiconductor layer 4 that connects the active layer 3 to the p-type impurity-containing layer. The first portion 41 may be disposed in contact with the upper surface of the active layer 3. The first portion 41 is a portion that does not contain a p-type semiconductor layer. As long as the p-type impurity concentration and thickness are such that they do not affect the free carrier absorption loss, a layer containing a p-type impurity may be included in part of the first portion 41. However, in order to dope Mg required for p-type doping, a layer containing 1×10 18 / cm 3A p-type impurity of about 100 nm or more is required, and in this case, there is a high possibility that free carrier absorption loss will increase. Therefore, it is preferable that the first portion 41 is a portion that does not contain a p-type semiconductor layer. It is preferable that the first portion 41 has a low p-type impurity concentration throughout the first portion 41, to the extent that the p-type impurity concentration by analysis such as SIMS is below the detection limit. For example, the first portion 41 is formed without intentionally adding p-type impurities throughout the first portion 41 during manufacturing. The first portion 41 can be formed so that it is undoped throughout the first portion 41. Since the thicker the first portion 41 is, the more light leakage to the second portion 43 can be reduced, the thickness of the first portion 41 is preferably 400 nm or more. The upper limit of the thickness of the first portion 41 can be set to a level at which the supply of holes from the second portion 43 is not hindered. In addition, since the thicker the first portion 41 is, the more electrons tend to overflow, from this point of view, it can be said that the thinner the first portion 41 is, the more the probability of electron overflow can be reduced. From this point of view, the thickness of the first portion 41 can be set to, for example, 660 nm or less. The first portion 41 can reduce the probability of electron overflow due to the band gap difference between it and the electron barrier layer 42. For this reason, the layer in the first portion 41 that is in contact with the electron barrier layer 42 is preferably a layer having a band gap energy smaller than the band gap energy of the electron barrier layer 42.

[0034] When the first portion 41 is a lightly doped portion rather than an undoped portion, it is preferable that the p-type impurity concentration throughout the first portion 41 is lower than the p-type impurity concentration of the electron barrier layer 42, and more preferably lower than the p-type impurity concentrations of both the electron barrier layer 42 and the second portion 43. The n-type impurity concentration of the first portion 41 is preferably 2×10 18 / cm 3 Preferably, the first portion 41 has an n-type impurity concentration that is low enough that n-type impurities are not detected by SIMS analysis (i.e., background level). In other words, it is preferable that the first portion 41 does not substantially contain n-type impurities.

[0035] The lower end of the ridge 4a is preferably located in the first portion 41. This allows the lower end of the ridge 4a to be close to the active layer 3. For example, the ridge 4a is composed of a part of the first portion 41, an electron barrier layer 42, and a second portion 43.

[0036] (1st p-side compositionally graded layer 411) The first p-side compositional graded layer 411 is a layer in which the band gap energy increases toward the top. With such a configuration, the light confinement to the active layer 3 can be strengthened. The first p-side compositional graded layer 411 can be in contact with the active layer 3. By disposing the first p-side compositional graded layer 411 close to the active layer 3 in this manner, the light confinement effect of the first p-side compositional graded layer 411 can be improved. The first p-side compositional graded layer 411 can be in contact with the second p-side compositional graded layer 412. The distance from the first p-side compositional graded layer 411 to the electron barrier layer may be 150 nm or more. The distance from the first p-side compositional graded layer 411 to the electron barrier layer may be 600 nm or less. By strengthening the light confinement to the active layer 3 by the first p-side compositional graded layer 411, the laser oscillation threshold current density can be reduced. This can suppress the shielding of the localized level, and can suppress the short-wavelength shift of the oscillation wavelength due to the increase in current injection, which is advantageous for increasing the oscillation wavelength.

[0037] The first p-side compositionally graded layer 411 has an upper surface and a lower surface, and its band gap energy increases from the lower surface to the upper surface. The band gap energy on the lower surface side of the first p-side compositionally graded layer 411 is smaller than the band gap energy on the upper surface side. In FIG. 2, the first p-side compositionally graded layer 411 is shown in a slope shape, but as described later, the compositionally graded layer can be said to be a collection of multiple sublayers having different compositions. Therefore, in the first p-side compositionally graded layer 411, the band gap energy increases in a step-like manner from the lower surface to the upper surface. An n-side compositionally graded layer 21 that is paired with the first p-side compositionally graded layer 411 may be provided in the n-side semiconductor layer 2. For example, as a band gap energy structure, the first p-side compositionally graded layer 411 and the n-side compositionally graded layer can be formed symmetrically with the active layer 3 in between.

[0038] The first p-side compositionally graded layer 411 functions as, for example, a p-side light guide layer. The thickness of the first p-side compositionally graded layer 411 is thicker than the thickness of the well layer of the active layer 3, and is thicker than the thickness of the p-side barrier layer 33 if the p-side barrier layer 33 is present. In order to improve the light confinement effect, the thickness of the first p-side compositionally graded layer 411 is preferably 100 nm or more. The thickness of the first p-side compositionally graded layer 411 can be 500 nm or less. The thickness of the first p-side compositionally graded layer 411 may be 200 nm or less. When the p-side barrier layer 33 is provided, the band gap energy of the lower end of the first p-side compositionally graded layer 411 is preferably smaller than the band gap energy of the p-side barrier layer 33. The band gap energy of the upper end of the first p-side compositionally graded layer 411 may have a band gap energy equal to or greater than that of the p-side barrier layer 33.

[0039] The first p-side compositional gradient layer 411 has a structure in which the refractive index decreases from the active layer 3 side toward the electron barrier layer 42 side, and the band gap energy increases from the active layer 3 side toward the electron barrier layer 42 side. This makes it possible to suppress the overflow of electrons while directing light toward the active layer 3. The composition of the first p-side compositional gradient layer 411 changes at a first rate of change so that the band gap energy increases upward. The first rate of change is the rate of change of the band gap energy, and may be calculated from the rate of change of the In composition ratio x. For example, the In content of the first p-side compositional gradient layer 411 obtained by analysis such as SIMS may be plotted in a table, and the slope of the table may be taken as the rate of change of the In composition ratio x. The In content obtained by the analysis may be plotted in a table on a linear scale, a linear approximation curve may be created from the table, and the slope of the approximation curve may be taken as the rate of change of the In composition ratio x.

[0040] The first p-side compositionally graded layer 411 can be said to be composed of a plurality of sub-layers 411a, 411b, 411c, 411y, and 411z having different compositions, as shown in Fig. 5. Fig. 5 is a partial enlarged view of the first p-side compositionally graded layer 411 and its vicinity, and a plurality of other sub-layers are present between the sub-layer 411c and the sub-layer 411y. The first p-side compositionally graded layer 411 is made of In x Ga 1-x In the case where the first p-side compositionally graded layer 411 is composed of N, the In composition ratio x a and the In composition ratio x of the uppermost sublayer 411z z The relationship is 0≦x z <x a The In composition ratio x of the sub-layer 411a constituting the lower end of the first p-side compositionally graded layer 411 is a The upper limit of the In composition ratio x is, for example, 0.25. a is preferably 0.1 or less.

[0041] It is preferable that the difference in lattice constant between adjacent sublayers is small. This can reduce distortion. For this reason, it is preferable that the first p-side compositionally graded layer 411 has a small thickness and the composition is changed little by little. Specifically, it is preferable that the In composition ratio x of the first p-side compositionally graded layer 411 decreases every thickness of 25 nm or less from the lower surface to the upper surface. That is, it is preferable that the thickness of each of the sublayers 411a, 411b, 411c, 411y, and 411z is 25 nm or less. It is more preferable that the thickness of each of the sublayers 411a, 411b, 411c, 411y, and 411z is 20 nm or less, and may be 10 nm or less, or may be 5 nm or less. The lower limit of the thickness of each of the sublayers 411a, 411b, 411c, 411y, and 411z is, for example, about one atomic layer (about 0.25 nm). Furthermore, the difference in the In composition ratio x between adjacent sublayers (e.g., sublayer 411a and sublayer 411b) is preferably 0.005 or less, and more preferably 0.001 or less. The lower limit of the difference in the In composition ratio x between adjacent sublayers is, for example, about 0.00007. It is preferable that such a range is satisfied throughout the first p-side composition gradient layer 411. In other words, it is preferable that all sublayers are within such a range. The number of times the composition changes in the first p-side composition gradient layer 411, i.e., the number of sublayers in the first p-side composition gradient layer 411, is preferably 90 or more.

[0042] When the n-side compositionally graded layer 21 is provided, the preferred ranges of the composition, composition change rate, and thickness thereof may be similar to those of the first p-side compositionally graded layer 411 .

[0043] (2nd p-side compositionally graded layer 412) The second p-side compositionally graded layer 412 is disposed between the first p-side compositionally graded layer 411 and the electron barrier layer 42. The second p-side compositionally graded layer 412 has an upper surface and a lower surface, and its band gap energy increases from the lower surface to the upper surface. The band gap energy on the lower surface side of the second p-side compositionally graded layer 412 is smaller than the band gap energy on the upper surface side. As with the first p-side compositionally graded layer 411, the compositionally graded layer can be said to be an aggregate of multiple sublayers having different compositions, so in the second p-side compositionally graded layer 412, the band gap energy increases stepwise from the lower surface to the upper surface.

[0044] The thickness of the second p-side compositionally graded layer 412 can be 2 nm or more. This can effectively reduce band spikes. The thickness of the second p-side compositionally graded layer 412 may be 10 nm or more, and may be 50 nm or more. This can reduce light leakage to the electron barrier layer 42 and the second portion 43, and can reduce light absorption loss. The thickness of the second p-side compositionally graded layer 412 may be thicker than the thickness of the well layer of the active layer 3. If the p-side barrier layer 33 is present, the thickness of the second p-side compositionally graded layer 412 may be thicker than the thickness of the p-side barrier layer 33. The thickness of the second p-side compositionally graded layer 412 is preferably 250 nm or less, and may be 200 nm or less, or may be 100 nm or less. The thickness of the second p-side compositionally graded layer 412 may be 2 nm or more and 250 nm or less, and may be 10 nm or more and 250 nm or less.

[0045] The band gap energy of the lower end of the second p-side compositionally graded layer 412 is larger than the band gap energy of the upper end of the first p-side compositionally graded layer 411. The band gap energy of the upper end of the second p-side compositionally graded layer 412 is smaller than the band gap energy of the electron barrier layer 42. The band gap energy of the upper end of the second p-side compositionally graded layer 412 may be larger than the band gap energy of the layer constituting the lower end of the second portion 43. The Al composition ratio y of the lower end of the second p-side compositionally graded layer 412 is preferably 0.05 or less, exceeding the Al composition ratio z of the intermediate layer 413. This makes it easy to stably form the second p-side compositionally graded layer 412 made of AlGaN. The Al composition ratio y of the upper end of the second p-side compositionally graded layer 412 is preferably 0.1 or less. This makes it easy to reduce the influence of band spikes.

[0046] The second p-side compositionally graded layer 412 has a structure in which the refractive index decreases from the active layer 3 side toward the electron barrier layer 42 side, and the band gap energy increases from the active layer 3 side toward the electron barrier layer 42 side. The second p-side compositionally graded layer 412 changes in composition such that the band gap energy increases upward at a second rate of change smaller than the first rate of change. This makes it easier to reduce the effect of band spikes. The second rate of change can be obtained in the same manner as the first rate of change. That is, the second rate of change is the rate of change of the band gap energy, and may be calculated from the rate of change of the Al composition ratio y. The Al content of the second p-side compositionally graded layer 412 obtained by analysis such as SIMS can be plotted in a table, and the slope of the table can be used as the rate of change of the Al composition ratio y. The Al content obtained by the analysis can be plotted in a table on a linear scale, a linear approximation curve can be created from the table, and the slope of the approximation curve can be used as the rate of change of the Al composition ratio y. Even if at least one of the In content of the first p-side composition gradient layer 411 and the Al content of the second p-side composition gradient layer 412 varies nonlinearly within the layer, the magnitude relationship between the first change rate and the second change rate can be compared by creating a linear approximation curve for these.

[0047] The second p-side compositionally graded layer 412 can be said to be composed of a plurality of sub-layers 412a, 412b, 412c, 412y, and 412z having different compositions, as shown in Fig. 6. Fig. 6 is a partially enlarged view of the second p-side compositionally graded layer 412 and its vicinity, and a plurality of other sub-layers are present between the sub-layer 412c and the sub-layer 412y. The second p-side compositionally graded layer 412 is made of Al y Ga 1-y When the second p-side compositionally graded layer 412 is composed of N, the Al composition ratio y a and the Al composition ratio y of the uppermost sublayer 412z z The relationship between a <y z The Al composition ratio y of the sub-layer 412a constituting the lower end of the second p-side compositionally graded layer 412 is <1. a is z <y a The Al composition ratio y of the sub-layer 412z constituting the upper end of the second p-side compositionally graded layer 412 can be set to ≦0.05. z is y a <y z It can be ≦0.1.

[0048] The second p-side composition gradient layer 412 preferably has an Al composition ratio y increasing at intervals of 25 nm or less from the bottom surface to the top surface. That is, the thickness of each of the sublayers 412a, 412b, 412c, 412y, and 412z is preferably 25 nm or less. This can reduce distortion between adjacent sublayers. The thickness of each of the sublayers 412a, 412b, 412c, 412y, and 412z is more preferably 20 nm or less, may be 10 nm or less, or may be 5 nm or less. The lower limit of the thickness of each of the sublayers 412a, 412b, 412c, 412y, and 412z is, for example, about one atomic layer (about 0.25 nm). The difference in the Al composition ratio y between adjacent sublayers (for example, the sublayer 412a and the sublayer 412b) is preferably 0.005 or less, more preferably 0.001 or less. The lower limit of the difference in the Al composition ratio y between adjacent sublayers is, for example, about 0.00007. It is preferable that such a range is satisfied throughout the entire second p-side compositionally graded layer 412. In other words, it is preferable that all sublayers are within such a range. The number of times the composition changes in the second p-side compositionally graded layer 412, i.e., the number of sublayers of the second p-side compositionally graded layer 412, is preferably 10 or more, and more preferably 30 or more.

[0049] When the compositionally graded layer 232 is provided in the n-side intermediate portion 23, the preferred ranges of the composition, composition change rate, and thickness thereof may be similar to those of the second p-side compositionally graded layer 412.

[0050] (Middle class 413) The intermediate layer 413 is disposed between the first p-side compositionally graded layer 411 and the second p-side compositionally graded layer 412. The intermediate layer 413 is in contact with the second p-side compositionally graded layer 412. The intermediate layer 413 may be in contact with the first p-side compositionally graded layer 411.

[0051] The intermediate layer 413 is Al z Ga 1-zN. The Al composition ratio z of the intermediate layer 413 is less than the Al composition ratio y of the second p-side composition gradient layer 412. Although the Al composition ratio y of the second p-side composition gradient layer 412 is not constant, the Al composition ratio z of the intermediate layer 413 is smaller than any of the Al composition ratios. Since the minimum Al composition ratio y in the second p-side composition gradient layer 412 is the Al composition ratio y at the bottom end, the Al composition ratio y at the bottom end of the second p-side composition gradient layer 412 may be compared with the Al composition ratio z of the intermediate layer 413. The Al composition ratio z of the intermediate layer 413 may be greater than 0 and less than 0.01. In order to stably form the second p-side composition gradient layer 412, such a small Al composition ratio z is suitable. The Al composition ratio z of the intermediate layer 413 may be greater than 0 and less than 0.01, or greater than 0 and less than 0.005. The intermediate layer 413 is not a compositionally graded layer, or is a compositionally graded layer having a compositional change rate smaller than that of the second p-side compositionally graded layer 412. The intermediate layer 413 may have an Al composition change of, for example, 0.0001 or less per nm. When the composition of the intermediate layer 413 is not constant, its median value is taken as the Al composition ratio z.

[0052] The thickness of the intermediate layer 413 can be 2 nm or more, may be 10 nm or more, or may be 50 nm or more. This makes it easy to stably form the second p-side compositionally graded layer 412. The thickness of the intermediate layer 413 can be 250 nm or less, may be 200 nm or less, or may be 100 nm or less. This makes it possible to efficiently form the first portion 41 including the intermediate layer 413. The thickness of the intermediate layer 413 can be 0.5 times or more and 1.5 times or less the thickness of the second p-side compositionally graded layer 412.

[0053] When intermediate layer 231 is provided in n-side intermediate portion 23, the preferred ranges of composition, composition change rate, and thickness thereof can be similar to those of intermediate layer 413.

[0054] (Middle class 414) The second p-side compositionally graded layer 412 may be in contact with the electron barrier layer 42, or the intermediate layer 414 may be disposed between the second p-side compositionally graded layer 412 and the electron barrier layer 42. The intermediate layer 413 and the intermediate layer 414 may be referred to as a first intermediate layer and a second intermediate layer, respectively. The intermediate layer 414 may be in contact with the second p-side compositionally graded layer 412 and the electron barrier layer 42.

[0055] The band gap energy of the intermediate layer 414 is the same as the band gap energy of the upper end of the second p-side composition graded layer 412, or is a value between the band gap energy of the upper end of the second p-side composition graded layer 412 and the band gap energy of the electron barrier layer 42. By providing such an intermediate layer 414, it is possible to improve the light confinement in the active layer 3. In addition, by providing the intermediate layer 414, it is possible to further separate the electron barrier layer 42 and the second portion 43 containing p-type impurities from the active layer 3, thereby reducing the light absorption loss. By reducing the light absorption loss, it is possible to improve the efficiency of the semiconductor laser element 100. The efficiency of the semiconductor laser element 100 can be exemplified by the slope efficiency, which is the slope in a characteristic graph of the current and the light output at a current value equal to or higher than the threshold current.

[0056] The intermediate layer 414 is not a compositionally graded layer, or is a compositionally graded layer having a compositional change rate smaller than that of the second p-side compositionally graded layer 412. The intermediate layer 414 may have an Al compositional change of 0.001 or less per nm, for example. When the composition of the intermediate layer 414 is not constant, the intermediate layer 414 may have a median value as the composition of the intermediate layer 414 and the band gap energy of the intermediate layer 414 may be compared with that of other layers. The lower end of the ridge 4a is preferably located in the intermediate layer 414. This makes it possible to reduce the variation in the effective refractive index difference inside and outside the ridge 4a due to the variation in the depth of the ridge 4a, as compared with the case where the lower end of the ridge 4a is located in the compositionally graded layer.

[0057] For example, the intermediate layer 414 is made of AlGaN. When the lower surface of the intermediate layer 414 is in contact with the upper surface of the second p-side compositionally graded layer 412, the lattice constant of the lower end of the intermediate layer 414 is preferably the same as the lattice constant of the upper end of the second p-side compositionally graded layer 412. The smaller the difference between the lattice constant of the intermediate layer 414 and the lattice constant of the upper end of the second p-side compositionally graded layer 412, the more the influence of the band spike can be reduced. When the intermediate layer 414 is made of AlGaN, the difference between the maximum Al composition ratio in the intermediate layer 414 and the Al composition ratio y of the upper end of the second p-side compositionally graded layer 412 is preferably 0.02 or less, and more preferably 0.01 or less. This makes it possible to reduce the influence of the band spike.

[0058] The thickness of the intermediate layer 414 is thicker than the sub-layers constituting the second p-side compositionally graded layer 412. The thickness of the intermediate layer 414 may be thicker than 25 nm and may be 50 nm or more. The thickness of the intermediate layer 414 may be 600 nm or less, or may be 250 nm or less.

[0059] The sum of the thickness of the second p-side compositional graded layer 412 and the thickness of the intermediate layer 414 is preferably 100 nm or more. This can reduce light leakage to the electron barrier layer 42 and the second portion 43, and can reduce light absorption loss. The sum of the thickness of the second p-side compositional graded layer 412 and the thickness of the intermediate layer 414 is preferably 500 nm or less. This can place the electron barrier layer 42 at a position not too far from the active layer 3, and can suppress the total amount of electrons overflowing to the second portion 43. The ratio of the thickness of the second p-side compositional graded layer 412 to the sum of the thickness of the second p-side compositional graded layer 412 and the thickness of the intermediate layer 414 is preferably 0.5 or less. Since the average value of the band gap energy of the second p-side compositional graded layer 412 is smaller than the band gap energy of the intermediate layer 414, the light confinement to the active layer 3 decreases as the thickness of the second p-side compositional graded layer 412 is increased. Therefore, by making the thickness of the second p-side compositional graded layer 412 relatively thin, the light confinement to the active layer 3 can be improved. This is expected to reduce the oscillation threshold current of the semiconductor laser device 100. In addition, since the amount of light reaching the second portion 43 can be reduced, the absorption loss of light can be reduced, and the optical output can be increased.

[0060] When the intermediate layer 233 is provided in the n-side intermediate portion 23, the preferable ranges of the composition, composition change rate, and thickness thereof can be similar to those of the intermediate layer 414.

[0061] (Electron barrier layer 42) The electron barrier layer 42 contains a p-type impurity such as Mg. The electron barrier layer 42 may be disposed in contact with the upper surface of the first portion 41. The band gap energy of the electron barrier layer 42 is larger than that of the first portion 41. When the first portion 41 has a multilayer structure as described above, the electron barrier layer 42 is a layer having a band gap energy larger than any of the layers constituting the first portion 41. Since the electron barrier layer 42 is a layer having such a large band gap energy, the electron barrier layer 42 can function as a barrier against electrons overflowing from the active layer 3. The electron barrier layer 42 preferably has a band gap energy difference of 0.1 eV or more with respect to the uppermost layer of the first portion 41. The band gap energy difference between the electron barrier layer 42 and the uppermost layer of the first portion 41 can be, for example, 1 eV or less. The electron barrier layer 42 is, for example, a layer having the highest band gap energy in the p-side semiconductor layer 4. The electron barrier layer 42 may be a layer having a thickness smaller than that of the first p-side compositionally graded layer 411.

[0062] The electron barrier layer 42 may have a multi-layer structure. In this case, one or more layers constituting the electron barrier layer 42 have a band gap energy larger than the band gap energy of any layer constituting the first portion 41. The electron barrier layer 42 may be a compositionally graded layer. The electron barrier layer 42 may be, for example, a compositionally graded layer in which the composition changes so that the band gap energy decreases toward the top. When the first portion 41 or the electron barrier layer 42 has a superlattice layer, the magnitude relationship is compared using the average band gap energy of the superlattice layer, not the band gap energy of each layer constituting the superlattice layer. The electron barrier layer 42 is made of, for example, AlGaN. When the electron barrier layer 42 is made of AlGaN, its Al composition ratio may be 0.08 or more and 0.5 or less. The thickness of the electron barrier layer 42 may be, for example, 4 nm or more and 100 nm or less.

[0063] (Second part 43) The second portion 43 has one or more p-type semiconductor layers containing p-type impurities. The second portion 43 may be disposed in contact with the upper surface of the electron barrier layer. The concentration of p-type impurities in the p-type semiconductor layer of the second portion 43 is, for example, 1×10 18 / cm 3 Can be more than 1×10 22 / cm 3 As described above, the driving voltage can be reduced by reducing the thickness of the second portion 43, so the thickness of the second portion 43 is preferably 260 nm or less. The thickness of the second portion 43 can be 10 nm or more. The second portion 43 may include an undoped layer. The second portion 43 preferably contains p-type impurities throughout. This makes it possible to reduce the resistance of the second portion 43 more than when the second portion 43 includes an undoped layer. In the case of a superlattice layer, the average p-type impurity concentration can be regarded as the p-type impurity concentration of the superlattice layer, so when the second portion 43 includes a superlattice layer, the superlattice layer may have a laminated structure of an undoped layer and a p-type impurity-containing layer.

[0064] The thickness of the second portion 43 may be smaller than the thickness of the first portion 41. Since the first portion 41 is relatively thick, the peak of the light intensity can be moved away from the p-type impurity-containing layer, and the loss due to free carrier absorption in the p-type impurity-containing layer can be reduced. Therefore, the efficiency of the semiconductor laser element 100, such as the slope efficiency, can be improved. In addition, since the second portion 43 is relatively thin, the driving voltage of the semiconductor laser element 100 can be reduced, and the efficiency can be improved. The reason why the voltage is reduced by reducing the thickness of the portion containing p-type impurities is that in nitride semiconductors, p-type impurities such as Mg have a lower activation rate than n-type impurities such as Si, and the p-type impurity-containing layer has a relatively high resistance. Although the first portion 41 is undoped, it is located between the electron barrier layer 42 and the active layer 3, and therefore tends to exhibit n-type conductivity rather than complete insulation due to factors such as electron overflow. For these reasons, it is believed that by reducing the thickness of the second portion 43 containing p-type impurities and having a relatively high resistance, it is possible to obtain the effect of lowering the driving voltage and suppressing the increase in driving voltage caused by increasing the thickness of the undoped first portion 41. Since the first portion 41 is relatively thick, it is preferable to provide the lower end of the ridge 4a in the first portion 41. This can strengthen the optical confinement in the lateral direction.

[0065] The second portion 43 may have a lower p-type semiconductor layer and an upper p-type semiconductor layer. The upper p-type semiconductor layer forms the upper surface of the ridge 4a. In other words, the upper p-type semiconductor layer is the uppermost layer of the second portion 43. The upper p-type semiconductor layer functions as a p-side contact layer. The lower p-type semiconductor layer is disposed between the upper p-type semiconductor layer and the electron barrier layer 42, and has a band gap energy larger than the band gap energy of the upper p-type semiconductor layer.

[0066] The lower p-type semiconductor layer is made of, for example, AlGaN. The upper p-type semiconductor layer is made of, for example, GaN. The lower p-type semiconductor layer may function as a p-side cladding layer. The lower p-type semiconductor layer may be a p-type GaN layer, which can further reduce the resistance of the second portion 43. In this case, it is preferable to form the p-electrode from a material that functions as a cladding layer, such as ITO. The upper p-type semiconductor layer may have a thickness of, for example, 5 nm or more and 30 nm or less. The lower p-type semiconductor layer may have a thickness of, for example, 1 nm or more and 260 nm or less. The lower p-type semiconductor layer is thicker than, for example, the electron barrier layer 42. In this case, in order to reduce free carrier absorption loss, it is preferable that the p-type impurity concentration of the lower p-type semiconductor layer is lower than the p-type impurity concentration of the electron barrier layer 42.

[0067] (insulating film 5, n-electrode 8, p-electrode 6, p-side pad electrode 7) The insulating film 5 can be formed of a single layer or multilayer of oxides or nitrides of, for example, Si, Al, Zr, Ti, Nb, Ta, etc. The n-electrode 8 is provided, for example, on almost the entire lower surface of the n-type substrate 1. The p-electrode 6 is provided on the upper surface of the ridge 4a. When the width of the p-electrode 6 is narrow, a p-side pad electrode 7 wider than the p-electrode 6 is provided on the p-electrode 6, and a wire or the like may be connected to the p-side pad electrode 7. Examples of materials for each electrode include a single layer or multilayer of conductive oxides containing at least one selected from metals or alloys such as Ni, Rh, Cr, Au, W, Pt, Ti, and Al, Zn, In, and Sn. Examples of conductive oxides include ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), and GZO (Gallium-doped Zinc Oxide). The thickness of the electrodes may be such that they can usually function as electrodes of semiconductor elements. For example, the thickness may be about 0.05 μm to 2 μm.

[0068] The p-electrode 6 is preferably a transparent conductive film having a refractive index smaller than that of the active layer 3. This allows it to function as a cladding layer. Furthermore, the p-electrode 6 is preferably a transparent conductive film having a refractive index smaller than that of the second portion 43. This allows a better light confinement effect to be obtained. In addition, when a p-side cladding layer is provided in the second portion 43, for example, an AlGaN layer having a relatively high Al composition ratio and doped with p-type impurities is provided as the p-side cladding layer, but the higher the Al composition ratio, the higher the resistance tends to be. If the p-electrode 6 functions as a cladding layer, it is not necessary to provide a p-side cladding layer in the second portion 43, or even if a p-side cladding layer is provided, its Al composition ratio can be lowered. Therefore, the resistance can be reduced, and the driving voltage of the semiconductor laser element 100 can be reduced. As the p-electrode 6 functioning as a cladding layer, for example, a p-electrode 6 made of ITO can be given.

[0069] (Manufacturing method) The manufacturing method of the semiconductor laser element 100 according to the embodiment includes, for example, the first to fifth steps as shown below. The first step is a step of forming an n-side semiconductor layer 2 on a substrate 1. The second step is a step of forming an active layer 3 on the n-side semiconductor layer 2. The third step is a step of forming a first portion 41 having one or more semiconductor layers undoped on the upper surface of the active layer 3. The fourth step is a step of forming an electron barrier layer 42 on the upper surface of the first portion 41 by doping with a p-type impurity. The fifth step is a step of forming a second portion 43 having one or more p-type semiconductor layers formed by doping with a p-type impurity on the upper surface of the electron barrier layer 42. In the third step, the layers constituting the first portion 41 are formed in order from bottom to top. The manufacturing method of the semiconductor laser element 100 may further include a sixth step of forming a ridge 4a protruding upward by removing a part of the p-side semiconductor layer 4 including the first portion 41, the electron barrier layer 42, and the second portion 43. The effects and preferred configurations of the layers obtained in each step are as described above.

[0070] Example 1 As Example 1, a semiconductor laser element 100 was fabricated. An MOCVD apparatus was used to fabricate an epitaxial wafer that would become the semiconductor laser element 100. As raw materials, trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH3), silane gas, and bis(cyclopentadienyl)magnesium (Cp2Mg) were appropriately used.

[0071] An n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4 were grown in this order on an n-type GaN substrate (substrate 1) having a +c-plane as the upper surface.

[0072] The n-side semiconductor layer 2 is made of a 1.5 μm-thick AlN layer doped with Si, in that order from the n-type GaN substrate side. 0.016 Ga 0.984 N layer and 150 nm thick In doped with Si 0.05 Ga 0.95 N layer and 900 nm thick Al doped with Si 0.072 Ga 0.928 An N layer (n-type semiconductor layer 22), a GaN layer (n-side intermediate portion 23) with a thickness of 3500 nm and doped with Si, and a GaN layer with a thickness of 200 nm and doped with Si, the starting end of which is GaN and the In 0.05 Ga 0.95 and an n-side compositionally graded layer 21 grown by substantially monotonically increasing the In composition with N as a termination.

[0073] The active layer 3 is made up of, in order from the n-type GaN substrate side, a GaN layer doped with Si, an undoped In composition gradient layer, and an undoped In 0.2 Ga 0.8 It was formed to have an N layer and an undoped GaN layer.

[0074] The p-side semiconductor layer 4 is made up of, in order from the n-type GaN substrate side, an undoped first p-side compositionally graded layer 411 having a thickness of 200 nm, an undoped intermediate layer 413 having a thickness of 50 nm, an undoped second p-side compositionally graded layer 412 having a thickness of 70 nm, and an undoped Al 0.045 Ga 0.955The first p-side compositionally graded layer 411 is formed to have an intermediate layer 414 made of N, an electron barrier layer 42 made of AlGaN with a thickness of 10 nm and doped with Mg, a lower p-type semiconductor layer made of AlGaN with a thickness of 100 nm and doped with Mg, and an upper p-type semiconductor layer made of GaN with a thickness of 18 nm and doped with Mg. 0.05 Ga 0.95 The second p-side compositionally graded layer 412 has an AlGaN layer with an Al composition ratio of 1% or less at the start of growth and an AlGaN layer with an Al composition ratio of 1% or less at the end of growth. 0.045 Ga 0.955 The result was N.

[0075] Then, the epitaxial wafer on which the above layers have been formed was taken out of the MOCVD apparatus, and the ridge 4a, the insulating film 5, the p-electrode 6, the p-side pad electrode 7, and the n-electrode 8 were formed, and reflective films were formed on the light-emitting end face and the light-reflecting end face, respectively, and the semiconductor laser device 100 was obtained by singulating. The depth of the ridge 4a was about 270 nm. That is, the ridge 4a was formed so that its lower end was located in the second p-side compositionally graded layer 412. In addition, an ITO film having a thickness of 200 nm was formed as the p-electrode 6. The peak wavelength of the laser light emitted by the semiconductor laser device 100 in accordance with Example 1 was about 455 nm.

[0076] (SIMS analysis) FIG. 7 shows the results of SIMS analysis of a wafer in which the same p-side semiconductor layer 4 as in Example 1 was grown and stopped before the formation of the ridge 4a and the like. In FIG. 7, the dashed line indicates the amount of In detected, and the solid line indicates the amount of Al detected. The vertical and horizontal axes of FIG. 7 are linear scales. From FIG. 7, it was confirmed that the intermediate layer 414 was formed between the first p-side compositionally graded layer 411 and the second p-side compositionally graded layer 412. In FIG. 7, the line indicating the amount of In detected and the line indicating the amount of Al detected intersect at the lower end (right end in the figure) of the intermediate layer 414, and Al is detected in the majority of the intermediate layer 414.

[0077] Example 2 An n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4 were grown in this order on an n-type GaN substrate (substrate 1) having a +c-plane as the upper surface.

[0078] The n-side semiconductor layer 2 is made of a 1.25 μm-thick AlN layer doped with Si, in that order from the n-type GaN substrate side. 0.018 Ga 0.982 N layer and 250 nm thick Al doped with Si 0.08 Ga 0.92 N layer and 150 nm thick In doped with Si 0.04 Ga 0.96 N layer, a 10 nm thick GaN layer doped with Si, and a 650 nm thick Al layer doped with Si. 0.08 Ga 0.92 An N layer (n-type semiconductor layer 22), a 200 nm thick GaN layer (n-side intermediate portion 23) doped with Si, and an undoped In layer (n-side intermediate portion 24) doped with Si and having a thickness of 230 nm. 0.03 Ga 0.97 N layer.

[0079] The active layer 3 is made up of a GaN layer doped with Si and an In layer doped with Si, in that order from the n-type GaN substrate side. 0.05 Ga 0.95 N layer, Si-doped GaN layer, and undoped In 0.25 Ga 0.75 N layer, undoped GaN layer, and undoped In 0.25 Ga 0.75 It was formed to have an N layer and an undoped GaN layer.

[0080] The p-side semiconductor layer 4 is made up of, in order from the n-type GaN substrate side, an undoped first p-side compositionally graded layer 411 having a thickness of 180 nm, an undoped intermediate layer 413 having a thickness of 50 nm, an undoped second p-side compositionally graded layer 412 having a thickness of 100 nm, and an undoped Al 0.04 Ga 0.96 The first p-side compositionally graded layer 411 is formed to have an intermediate layer 414 made of N, an electron barrier layer 42 made of AlGaN with a thickness of 10.9 nm and doped with Mg, a lower p-type semiconductor layer made of AlGaN with a thickness of 100 nm and doped with Mg, and an upper p-type semiconductor layer made of GaN with a thickness of 18 nm and doped with Mg. 0.05 Ga 0.95The second p-side compositionally graded layer 412 has an AlGaN layer with an Al composition ratio of 1% or less at the start of growth and an AlGaN layer with an Al composition ratio of 1% or less at the end of growth. 0.04 Ga 0.96 The result was N.

[0081] Then, the epitaxial wafer on which the above layers have been formed was taken out of the MOCVD apparatus, and a ridge 4a, an insulating film 5, a p-electrode 6, a p-side pad electrode 7, and an n-electrode 8 were formed, and a reflective film was formed on each of the light-emitting end face and the light-reflecting end face, and the semiconductor laser device 100 was obtained by singulating. The depth of the ridge 4a was about 270 nm. That is, the ridge 4a was formed so that its lower end was located in the intermediate layer 414. In addition, an ITO film having a thickness of 200 nm was formed as the p-electrode 6. The peak wavelength of the laser light emitted by the semiconductor laser device 100 in Example 2 was about 527 nm.

[0082] Example 3 The semiconductor laser device 100 according to Example 3 was fabricated in the same manner as in Example 2, except for the following points. The last two layers in the n-side semiconductor layer 2 were a 250 nm thick GaN layer (n-side intermediate portion 23) doped with Si and a 180 nm thick undoped In 0.03 Ga 0.97 An N layer was used. In the p-side semiconductor layer 4, the thickness of the second p-side compositionally graded layer 412 was set to 300 nm, and the intermediate layer 414 was eliminated. The undoped intermediate layer 413 having a thickness of 50 nm was left as it was. The depth of the ridge 4a was left at approximately 270 nm. That is, the ridge 4a was formed so that its lower end was located in the second p-side compositionally graded layer 412. The peak wavelength of the laser light emitted by the semiconductor laser device 100 in accordance with Example 3 was 527 nm.

[0083] Comparative Example 1 The semiconductor laser device according to Comparative Example 1 was fabricated in the same manner as the semiconductor laser device 100 according to Example 1, except that the intermediate layer 413 and the second p-side compositionally graded layer 412 were not formed, and an undoped GaN layer having a thickness of 120 nm was formed in their place. The depth of the ridge 4a was left at approximately 270 nm. That is, the ridge 4a was formed so that its lower end was located in the undoped GaN layer having a thickness of 120 nm. The peak wavelength of the laser light emitted by the semiconductor laser device according to Comparative Example 1 was 455 nm.

[0084] Comparative Example 2 The semiconductor laser device according to Comparative Example 2 was fabricated in the same manner as in Example 2, except for the following points. The last two layers in the n-side semiconductor layer 2 were a 250 nm thick GaN layer doped with Si and a 180 nm thick undoped In 0.03 Ga 0.97 An N layer was used. In the p-side semiconductor layer 4, the second p-side compositionally graded layer 412 was not formed, and an undoped GaN layer having a thickness of 150 nm was formed in its place. The peak wavelength of the laser light emitted by the semiconductor laser device in Comparative Example 2 was 527 nm.

[0085] (IL characteristics) FIG. 8 shows the IL characteristics of the semiconductor laser elements of Example 1 and Comparative Example 1. In the graph of FIG. 8, the horizontal axis indicates the current, and the vertical axis indicates the optical output. In FIG. 8, the solid line indicates Example 1, and the dashed line indicates Comparative Example 1. FIG. 9 shows the IL characteristics of the semiconductor laser elements of Example 2, Example 3, and Comparative Example 2. In the graph of FIG. 9, the horizontal axis indicates the current, and the vertical axis indicates the optical output. In FIG. 9, the solid line indicates Example 2, the dashed line indicates Example 3, and the dashed line indicates Comparative Example 2. As shown in FIG. 8 and FIG. 9, it was confirmed that the slope efficiency was improved and the optical output was improved by providing the first p-side compositional graded layer 411, the intermediate layer 413, and the second p-side compositional graded layer 412. This is considered to be an effect of reducing the carrier loss in the p-side semiconductor layer 4.

[0086] Through the contents described so far in this specification, the following technical matters are disclosed. (Item 1) A semiconductor laser element having, in this order from top to bottom, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each of which is made of a nitride semiconductor, The p-side semiconductor layer is a first portion having one or more semiconductor layers and being undoped; an electron barrier layer having a band gap energy larger than that of the first portion and containing a p-type impurity; a second portion having one or more p-type semiconductor layers containing p-type impurities, The first portion is In x Ga 1-x a first p-side compositionally graded layer made of N and having an In composition ratio x that decreases upward within a range of 0 to less than 1; an Al layer disposed between the first p-side compositionally graded layer and the electron barrier layer; y Ga 1-y a second p-side compositionally graded layer made of N and having an Al composition ratio y that increases upward within a range exceeding 0 and less than 1; An Al z Ga 1-z and an intermediate layer made of N, wherein the Al composition ratio z is greater than 0 and less than y. (Item 2) The first p-side compositionally graded layer has a composition that changes at a first rate so that the band gap energy increases upward, 2. The semiconductor laser element according to item 1, wherein the second p-side compositionally graded layer changes in composition at a second rate of change smaller than the first rate of change so that the band gap energy increases upward. (Item 3) The semiconductor laser element according to item 1 or 2, wherein the Al composition ratio y at an upper end of the second p-side compositionally graded layer is 0.1 or less. (Item 4) The semiconductor laser device according to any one of Items 1 to 3, wherein the second p-side compositionally graded layer has a thickness of 2 nm or more. (Item 5) The semiconductor laser device according to any one of Items 1 to 4, wherein the Al composition ratio z of the intermediate layer is greater than 0 and is not greater than 0.01. (Item 6) The semiconductor laser element according to any one of items 1 to 5, wherein the Al composition ratio y at the bottom end of the second p-side compositionally graded layer exceeds the Al composition ratio z of the intermediate layer and is 0.05 or less. (Item 7) The intermediate layer is a first intermediate layer, The semiconductor laser element according to any one of items 1 to 6, wherein the first portion has a second intermediate layer disposed between the second p-side compositional gradient layer and the electron barrier layer, and having a band gap energy equal to that of an upper end of the second p-side compositional gradient layer or between the upper end of the second p-side compositional gradient layer and the electron barrier layer. (Item 8) The semiconductor laser element according to item 7, wherein a ratio of a thickness of the second p-side compositionally graded layer to a sum of a thickness of the second p-side compositionally graded layer and a thickness of the second intermediate layer is 0.5 or less. (Item 9) The n-side semiconductor layer is an n-side composition gradient layer disposed in contact with a lower surface of the active layer, the composition of which changes downward so that the band gap energy becomes larger; an n-type semiconductor layer that is disposed below the n-side compositionally graded layer, has a band gap energy larger than the band gap energy of any layer that constitutes the first portion of the p-side semiconductor layer, and contains an n-type impurity; an n-side intermediate portion disposed between the n-side composition graded layer and the n-type semiconductor layer, 9. The semiconductor laser element according to any one of items 1 to 8, wherein a distance from the n-side compositional gradient layer to the n-type semiconductor layer is greater than a distance from the first p-side compositional gradient layer to the electron barrier layer. (Item 10) The active layer is An n-side barrier layer; A p-side barrier layer; a plurality of well layers including a first well layer and a second well layer located between the n-side barrier layer and the p-side barrier layer; an intermediate barrier layer located between the first well layer and the second well layer; 10. The semiconductor laser device according to any one of items 1 to 9, wherein a thickness of the intermediate barrier layer is smaller than a thickness of the p-side barrier layer. [Explanation of symbols]

[0087] 100 Semiconductor laser element 1 Board 2 n-side semiconductor layer 21 N-side composition gradient layer 22 n-type semiconductor layer 23 n-side middle part 231 Middle Class 232 Compositionally graded layer 233 Middle Class 3, 3A active layer 31 n-side barrier layer 32 Well Layer 33 p-side barrier layer 34 Intermediate barrier layer 4 p-side semiconductor layer 41 Part 1 411 1st p-side compositionally graded layer 411a, 411b, 411c, 411y, 411z sub-layers 412 2nd p-side compositionally graded layer 412a, 412b, 412c, 412y, 412z sub-layers 413 Middle Class 414 Middle Class 42 Electron Barrier Layer 43 Part 2 4a Ridge 5. Insulating film 6p electrode 7 P-side pad electrode 8n electrode

Claims

1. A semiconductor laser element having, in this order from above, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each of which is made of a nitride semiconductor, The p-side semiconductor layer is a first portion having one or more semiconductor layers and being undoped; an electron barrier layer having a band gap energy larger than that of the first portion and containing a p-type impurity; a second portion having one or more p-type semiconductor layers containing p-type impurities, The first portion is In x G 1-x a first p-side composition gradient layer made of N, in which an In composition ratio x decreases upward within a range of 0 or more and less than 1; An Al layer is disposed between the first p-side compositionally graded layer and the electron barrier layer. y G 1-y a second p-side compositionally graded layer made of N, in which the Al composition ratio y increases upward within a range exceeding 0 and less than 1; An Al layer is disposed between the first p-side compositionally graded layer and the second p-side compositionally graded layer. z G 1-z and an intermediate layer made of N and having an Al composition ratio z exceeding 0 and less than y.

2. the first p-side compositionally graded layer has a composition that changes at a first rate of change such that the band gap energy increases upward; 2. The semiconductor laser device according to claim 1, wherein the composition of said second p-side compositionally graded layer changes at a second rate of change smaller than the first rate of change so that the band gap energy increases upward.

3. 2. The semiconductor laser device according to claim 1, wherein the Al composition ratio y at an upper end of the second p-side compositionally graded layer is 0.1 or less.

4. The semiconductor laser device according to claim 1 , wherein the second p-side compositionally graded layer has a thickness of 2 nm or more.

5. 2. The semiconductor laser device according to claim 1, wherein the Al composition ratio z of said intermediate layer is greater than 0 and is equal to or smaller than 0.

01.

6. 2. The semiconductor laser device according to claim 1, wherein an Al composition ratio y at a lower end of said second p-side compositionally graded layer exceeds an Al composition ratio z of said intermediate layer and is equal to or smaller than 0.

05.

7. the intermediate layer is a first intermediate layer, 7. The semiconductor laser element according to claim 1, wherein the first portion includes a second intermediate layer disposed between the second p-side composition graded layer and the electron barrier layer, and having a band gap energy equal to that of an upper end of the second p-side composition graded layer or between an upper end of the second p-side composition graded layer and the electron barrier layer.

8. 8. The semiconductor laser device according to claim 7, wherein a ratio of a thickness of said second p-side compositionally graded layer to a sum of a thickness of said second p-side compositionally graded layer and a thickness of said second intermediate layer is 0.5 or less.

9. The n-side semiconductor layer is an n-side composition gradient layer disposed in contact with a lower surface of the active layer, the composition of which changes downward so that the band gap energy becomes larger; an n-type semiconductor layer that is disposed below the n-side composition gradient layer, has a band gap energy larger than the band gap energy of any layer that constitutes the first portion of the p-side semiconductor layer, and contains an n-type impurity; an n-side intermediate portion disposed between the n-side composition graded layer and the n-type semiconductor layer, 2. The semiconductor laser device according to claim 1, wherein a distance from said n-side compositionally graded layer to said n-type semiconductor layer is greater than a distance from said first p-side compositionally graded layer to said electron barrier layer.

10. The active layer is An n-side barrier layer; A p-side barrier layer; a plurality of well layers including a first well layer and a second well layer located between the n-side barrier layer and the p-side barrier layer; an intermediate barrier layer located between the first well layer and the second well layer; The semiconductor laser device according to claim 1 , wherein a thickness of said intermediate barrier layer is smaller than a thickness of said p-side barrier layer.

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