Semiconductor laser element

Abstract

To provide a semiconductor laser element in which the loss of carriers can be reduced.SOLUTION: A semiconductor laser element includes an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each of which is formed of a nitride semiconductor, in this order to the upper side. The p-side semiconductor layer includes a first part that is undoped, an electron barrier layer containing p-type impurities, and a second part including one or more p-type semiconductor layers containing the p-type impurities. The first part includes a first p-side composition inclination layer containing InxGa1-xN in which the In composition ratio x decreases in the range of 0 or more and less than 1 toward the upper side, a second p-side composition inclination layer disposed between the first p-side composition inclination layer and the electron barrier layer and containing AlyGa1-yN in which the Al composition ratio y increases in the range of more than 0 and less than 1 toward the upper side, and an intermediate layer disposed between the first p-side composition inclination layer and the second p-side composition inclination layer and containing AlzGa1-zN in which the Al composition ratio z is more than 0 and less than y.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a semiconductor laser element.

Background Art

[0002] Patent Document 1 describes a semiconductor laser element having an undoped p-side composition gradient 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

Summary of the Invention

Problems to be Solved by the Invention

[0004] One aspect of the semiconductor laser element in the present disclosure aims to obtain a semiconductor laser element capable of reducing carrier loss.

Means for Solving the Problems

[0005] One aspect of the semiconductor laser element in the present disclosure is a semiconductor laser element having an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each made of a nitride semiconductor, in this order upward, wherein the p-side semiconductor layer has one or more semiconductor layers, and has, in this order upward, an undoped first portion, an electron barrier layer having a larger bandgap energy than the first portion and containing a p-type impurity, and a second portion having one or more p-type semiconductor layers containing a p-type impurity, and the first portion is composed of In x Ga 1-x N, and is a first p-side composition gradient layer in which the In composition ratio x decreases in the range of 0 or more and less than 1 upward, and is disposed between the first p-side composition gradient layer and the electron barrier layer, and is Al y Ga 1-yIt consists of N, and has a second p-side composition gradient layer in which the Al composition ratio y increases in the range exceeding 0 and less than 1 as it goes upward, and is disposed between the first p-side composition gradient layer and the second p-side composition gradient layer, and Al z Ga 1-z It consists of N, and has an intermediate layer in which the Al composition ratio z exceeds 0 and is less than y.

Advantages of the Invention

[0006] A semiconductor laser element capable of reducing carrier loss can be obtained.

Brief Description of the Drawings

[0007]

Figure 1

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Modes for Carrying Out the Invention

[0008] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the embodiments shown below are examples of methods for embodying the technical idea of the present invention, and do not specify the present invention to the following embodiments. Further, in the following description, the same names and reference numerals indicate the same or equivalent members, and detailed descriptions will be omitted as appropriate.

[0009] FIG. 1 is a schematic cross-sectional view of a semiconductor laser element 100 according to this embodiment, showing a cross-section in a direction perpendicular to the resonator direction of the semiconductor laser element 100. FIG. 2 is a diagram schematically showing an example of the layer structure of the p-side semiconductor layer 4 of the semiconductor laser element 100. In FIG. 2, the magnitude relationship of the bandgap energies of each layer is schematically 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 lowermost sides of both side surfaces of the ridge 4a.

[0010] As shown in FIG. 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, in this order upward. 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 an up-down direction does not have to coincide with the direction of gravity when using the semiconductor laser element 100.

[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 upward. The first portion 41 has one or more semiconductor layers. The first portion 41 is undoped. The electron barrier layer 42 has a larger bandgap energy than the first portion 41 and contains p-type impurities. The second portion 43 has one or more p-type semiconductor layers containing p-type impurities. Note that, in this specification, undoped means not intentionally doping. A concentration not exceeding the detection limit in the analysis results such as secondary ion mass spectrometry (SIMS) may be referred to as undoped. Alternatively, the impurity concentration is 1×10 17 / cm 3A state of being less than may be regarded as undoped. For example, it may be said that the first portion 41 is 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 an electron barrier layer 42 having a high p-type impurity concentration, p-type impurities may be detected in the analysis result even if it is formed without intentionally doping p-type impurities. The concentration of p-type impurities detected in this case is preferably less than 1×10 18 / cm 3 When the first portion 41 or the like is formed undoped, unintentional impurities such as H and C may be contained, but this case can also be called undoped. In this specification, the thickness of a certain layer or portion refers to the shortest distance from the lower surface to the upper surface of that layer or portion. When the lower surface and / or the upper surface has partial recesses and / or protrusions such as V-pits, the shortest distance between the flat portions without such recesses and / or protrusions on the lower surface and / or the upper surface may be taken as the thickness of that layer or portion. Also, the distance from one layer to another layer refers to the shortest distance from one layer to another layer. When the lower surface and / or the upper surface of those layers have partial recesses and / or protrusions such as V-pits, the shortest distance between the flat portions without such recesses and / or protrusions on the lower surface and / or the upper surface may be taken as the distance between those layers. The magnitude relationship of the bandgap energy can be determined from the composition of the semiconductor to be compared. For example, when both of the two layers to be compared are composed of AlGaN, the layer with a relatively larger Al composition ratio is determined to be the layer with a relatively larger bandgap energy.

[0012] The first portion 41 has a first p-side composition gradient layer 411, a second p-side composition gradient layer 412 disposed between the first p-side composition gradient layer 411 and the electron barrier layer 42, and an intermediate layer 413 disposed between the first p-side composition gradient layer 411 and the second p-side composition gradient layer 412. The first p-side composition gradient layer 411 is made of In x Ga 1-x N, and the In composition ratio x decreases in the range of 0 or more and less than 1 as it goes upward. The second p-side composition gradient layer 412 is made of Al y Ga 1-yIt consists of N, and as it goes upward, the Al composition ratio y increases in the range exceeding 0 and less than 1. The intermediate layer 413 (the first intermediate layer) is Al z Ga 1-z It consists of N, and the Al composition ratio z exceeds 0 and is less than y.

[0013] By providing the second p-side composition gradient layer 412, the bandgap energy can be gently changed toward the electron barrier layer 42, and the influence of the band spike generated at the heterointerface with a bandgap energy difference can be reduced. The influence of the band spike means that electrons are generated by the band spike and holes (positive holes) relatively decrease, and the loss of carriers increases. The first p-side composition gradient layer 411 is In x Ga 1-x It consists of N, and the second p-side composition gradient layer 412 is Al y Ga 1-y It consists of N. Therefore, when growing these layers, the source gas needs to be switched halfway. By growing the intermediate layer 413 between these layers, the switched source gas can be stabilized, and the second p-side composition gradient layer 412 can be formed well. These layers can be formed, for example, by the metal organic chemical vapor deposition (MOCVD) method. By forming the second p-side composition gradient layer 412 well, the influence of the band spike in the p-side semiconductor layer 4 can be reduced, and the loss of carriers can be reduced.

[0014] (Semiconductor laser element 100) As shown in FIG. 1, the semiconductor laser element 100 includes a substrate 1, an n-side semiconductor layer 2 provided above the substrate 1, 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 that intersect the main surface of the semiconductor layer such as the active layer 3.

[0015] For example, a ridge 4a protruding upward is provided on the p-side semiconductor layer 4. The ridge 4a has a mesa structure. The top view shape of the ridge 4a is a shape that is long in the direction connecting the light emitting end face and the light reflecting end face. For example, it is a rectangular shape with the direction parallel to the light reflecting end face as the short side and the direction perpendicular to the light reflecting end face as the long side. The portion directly under the ridge 4a and its vicinity in the active layer 3 are the optical waveguide regions. An insulating film 5 can be provided on the side surface of the ridge 4a and the surface of the p-side semiconductor layer 4 continuous from 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. Further, 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, visible laser light. The semiconductor laser element 100 can oscillate, for example, blue or green laser light. The peak wavelength of the laser light oscillated by the semiconductor laser element 100 is, for example, 400 nm or more and 600 nm or less, 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 light oscillated by the semiconductor laser element 100 becomes longer, the leakage light to the outside of 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 at the time of laser oscillation becomes large. And the larger the current density is, the more the effective transition interval is expanded by the shielding of the localized level and the band filling, and the oscillation wavelength shifts to the short wavelength. By providing the first p-side composition gradient layer 411, the laser oscillation threshold current density can be reduced, and an 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 element 100 is preferably 500 nm or more, and may be 500 nm or more and 580 nm or less.

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

[0018] (n-side semiconductor layer 2) The n-side semiconductor layer 2 can have a multilayer 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. Examples of the n-type semiconductor layer include layers made of a nitride semiconductor containing an n-type impurity 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 bandgap energy than the n-side optical guide layer. Although not as much as p-type impurities, n-type impurities are also a factor in light absorption. Therefore, the n-side optical guide layer is preferably undoped or, when containing an n-type impurity, has an n-type impurity concentration lower than that 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 schematically showing an 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 include an n-side composition gradient layer 21 disposed in contact with the lower surface of the active layer 3, an n-type semiconductor layer 22 disposed below the n-side composition gradient layer 21, and an n-side intermediate portion 23 disposed between the n-side composition gradient layer 21 and the n-type semiconductor layer 22. The composition of the n-side composition gradient layer 21 changes such that the bandgap energy increases as it goes downward. The n-type semiconductor layer 22 contains n-type impurities and has a bandgap energy larger than that of any layer constituting the first portion 41 of the p-side semiconductor layer 4. The distance from the n-side composition gradient layer 21 to the n-type semiconductor layer 22 is preferably larger than the distance from the first p-side composition gradient layer 411 to the electron barrier layer 42. Thereby, since the electric field strength distribution in the semiconductor laser element 100 can be biased toward the n-side semiconductor layer 2, the loss due to light absorption in the electron barrier layer 42 containing p-type impurities and the layers above it can be reduced. The n-side semiconductor layer 2 may include other layers in addition to these.

[0020] The bandgap energy of the n-side composition gradient layer 21 becomes smaller as it approaches the active layer 3. The refractive index of the n-side composition gradient layer 21 becomes higher as it approaches the active layer 3. Thereby, the light confinement in the active layer 3 can be enhanced. The n-side composition gradient layer 21 is, for example, an n-side optical guide layer. The n-side composition gradient layer 21 is, for example, In a Ga 1-aIt may be a layer composed of N and having an In composition ratio a increasing in the range of 0 or more and less than 1 as going upward. When determining the magnitude relationship with other layers such as the bandgap energy and impurity concentration of the composition gradient layer, the average value of the composition gradient layer can be used. The average value of the composition gradient layer refers to the value obtained by dividing the total value of the multiplication of the bandgap energy etc. and the thickness of each sublayer constituting the composition gradient layer by the total thickness. When the n-side composition gradient layer 21 is a composition gradient layer in which the 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 also be said to be composed of a plurality of sublayers with slightly different compositions. For this reason, in the composition gradient layer, it is difficult to avoid the generation of fixed charges even if the composition change rate is reduced. Since the addition of an n-type impurity can shield the fixed charges, the degree of voltage increase due to 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 composition gradient layer 21 to the n-type semiconductor layer 22 can be less than 600 nm, and may be 400 nm or less.

[0022] The n-side intermediate portion 23 is a layer having a bandgap energy smaller than the bandgap 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 an n-type impurity.

[0023] The n-side intermediate portion 23 may have a composition gradient layer 232 in which the bandgap energy decreases as it approaches the active layer 3. In this composition gradient layer, the bandgap energy at the lower end thereof is smaller than the bandgap energy of the n-type semiconductor layer 22, and the bandgap energy at the upper end thereof is the same as or larger than the bandgap energy at the lower end of the n-side composition gradient layer 21. The n-side intermediate portion 23 may have, in order from the n-side composition gradient layer 21 side, the composition gradient layer 232 and the intermediate layer 233. The n-side intermediate portion 23 may have, in order from the n-side composition gradient layer 21 side, the intermediate layer 231, the composition gradient layer 232, and the intermediate layer 233. The intermediate layer 231 and the intermediate layer 233 are either not composition gradient layers or composition gradient layers having a composition change rate smaller than the composition change rate of the composition gradient layer 232. Similar to the n-side composition gradient layer 21, it is preferable to add an n-type impurity to the composition gradient layer 232. Thereby, the degree of voltage increase due to the generation of fixed charges can be reduced. 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. Thereby, light leakage to the n-type semiconductor layer 22 can be reduced. The thickness of the n-side intermediate portion 23 may be 400 nm or less. Thereby, light confinement to the active layer 3 can be improved.

[0024] (Active layer 3) The active layer 3 can have a multilayer structure composed of a nitride semiconductor layer such as GaN or InGaN. 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 an intermediate barrier layer 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 may be 0 nm, that is, the lower surface of the well layer 32 may be the lower surface of the active layer 3. When the n-side semiconductor layer 2 has an n-side composition gradient layer 21, as at least a part of the n-side barrier layer 31, a layer having a bandgap energy larger than the bandgap energy at the upper end of the n-side composition gradient layer 21 is preferably disposed. Thereby, the probability of luminescence recombination in the well layer 32 can be improved. 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 composition gradient 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 may 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. As at least a part of the p-side barrier layer 33, a layer having a bandgap energy larger than the bandgap energy at the lower end of the first p-side composition gradient layer 411 is preferably disposed. Thereby, the probability of luminescence recombination in the well layer 32 can be improved. 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. The active layer 3 is preferably formed without adding p-type impurities. Thereby, the light absorption loss caused by the addition of p-type impurities can be suppressed. Each layer of the active layer 3 is, for example, an undoped layer.

[0027] The well layer 32 is, for example, an In x Ga 1-x N well layer. When the semiconductor laser element has an oscillation wavelength of 500 nm or more, In x Ga 1-xThe In composition ratio x of the well layer is somewhat increased or decreased depending on the layer structure other than the active layer 3, but is, for example, 0.23 or more. As the upper limit of the In composition ratio x of the well layer 32, for example, 0.50 or less can be mentioned. At this time, 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 schematically showing another example of the layer structure of the active layer. The active layer 3A shown in FIG. 4 includes 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 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 plural, for example, two. The bandgap energies of the n-side barrier layer 31, the intermediate barrier layer 34, and the p-side barrier layer 33 are larger than the bandgap energy of the well layer 32. The thickness of the plurality of well layers 32 is, for example, 1 nm or more and 4 nm or less. The thicknesses of the plurality of well layers 32 may be the same.

[0029] The smaller the thickness of the intermediate barrier layer 34, the more the driving voltage of the semiconductor laser element 100 tends to decrease. On the other hand, for the p-side barrier layer 33, it is preferable to have a certain thickness as a base layer before laminating 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. Thereby, it is possible to achieve both a reduction in the driving voltage of the semiconductor laser element 100 and an improvement in the crystallinity of the p-side semiconductor layer 4. Crystallinity can be evaluated, for example, by dislocation density. Note that the thickness of each layer refers to the thickness in the lamination 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. Thereby, the injection efficiency of electrons into the well layer 32 can be improved. 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. When the thickness of the intermediate barrier layer 34 is 1.5 nm or more, a decrease in the carrier confinement efficiency can be suppressed, and a decrease in the output of the semiconductor laser element 100 can be suppressed. The thickness of the intermediate barrier layer 34 can be 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 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 more. Thereby, the crystallinity of the p-side semiconductor layer 4 can be improved more effectively. When the thickness of the p-side barrier layer 33 becomes larger 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. Thereby, an increase in the driving voltage of the semiconductor laser element 100 can be suppressed. The thickness of the p-side barrier layer 33 is preferably 2.5 nm or more 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 more 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. Thereby, the crystallinity of the p-side semiconductor layer 4 can be improved more effectively.

[0031] The semiconductor laser element 100 having the active layer 3A has an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4, each made of a nitride semiconductor, in this order upward. 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. The thickness of the intermediate barrier layer 34 is smaller than the thickness of the p-side barrier layer 33. The semiconductor laser element 100 may be used. In this case, the p-side semiconductor layer 4 may have one or more semiconductor layers, an undoped first portion 41, an electron barrier layer 42 having a larger bandgap energy than the first portion 41 and containing a p-type impurity, and a second portion 43 having one or more p-type semiconductor layers containing a p-type impurity, in this order upward. In this case, the p-side semiconductor layer 4 may have a first p-side composition gradient layer 411. In this case, the p-side semiconductor layer 4 may not have a second p-side composition gradient layer 412 and an intermediate layer 413.

[0032] (p-side semiconductor layer 4) The p-side semiconductor layer 4 can have a multilayer structure composed 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 optical guide layer, and may include other layers. When a transparent conductive film is provided as the p-electrode 6, since it can function as a cladding layer, it is not necessary to provide a cladding layer in the p-side semiconductor layer 4. The p-side semiconductor layer 4 includes one or more p-type semiconductor layers. Examples of the p-type semiconductor layer include a layer made of a nitride semiconductor containing a p-type impurity such as Mg. Since the activation rate of the p-type impurity is lower than that of the n-type impurity such as Si, in the p-type semiconductor layer, in order to obtain sufficient hole injection, it is necessary to dope the p-type impurity at a higher concentration, and the free carrier absorption loss due to the p-type impurity increases.

[0033] (First part 41) The first part 41 is a part connecting from the active layer 3 to the p-type impurity-containing layer in the p-side semiconductor layer 4. The first part 41 may be disposed in contact with the upper surface of the active layer 3. The first part 41 is a part that does not contain a p-type semiconductor layer. If 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 a part of the first part 41. However, if Mg necessary for p-type doping is to be doped, 1×10 18 / cm 3The p-type impurities of the above level are necessary, and in this case, the free carrier absorption loss is likely to increase. Therefore, it is preferable that the first portion 41 does not contain a p-type semiconductor layer. It is preferable that the p-type impurity concentration of the first portion 41 is low to such an extent that the p-type impurity concentration is below the detection limit by analysis such as SIMS throughout the first portion 41. For example, the first portion 41 is formed without intentionally adding p-type impurities during manufacturing throughout the first portion 41. The first portion 41 can be formed to be undoped throughout. Since the thicker the first portion 41 is, the more the 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 value of the thickness of the first portion 41 can be set to such an extent that the supply of holes from the second portion 43 is not hindered. Also, since the thicker the first portion 41 is, the more electrons tend to overflow, from this perspective, it can be said that the thinner the first portion 41 is, the lower the probability of electron overflow. From this perspective, the thickness of the first portion 41 can be, for example, 660 nm or less. The first portion 41 can reduce the probability of electron overflow due to the bandgap difference between the first portion 41 and the electron barrier layer 42. For this reason, it is preferable that the layer of the first portion 41 in contact with the electron barrier layer 42 has a bandgap energy smaller than the bandgap energy of the electron barrier layer 42.

[0034] In addition, when the first portion 41 is a portion with low-concentration doping instead of undoping, it is preferable that the p-type impurity concentration is lower than the p-type impurity concentration of the electron barrier layer 42 throughout the first portion 41. Furthermore, it is preferable that the p-type impurity concentration is lower than the p-type impurity concentration of any of the electron barrier layer 42 and the second portion 43. Regarding the n-type impurity concentration of the first portion 41, examples include less than 2×10 18 / cm 3 . Preferably, the first portion 41 has an n-type impurity concentration that is low to such an extent that n-type impurities are not detected by SIMS analysis (that is, at the background level). In other words, it is preferable that the first portion 41 does not substantially contain n-type impurities.

[0035] Preferably, the lower end of the ridge 4a is located in the first portion 41. Thereby, the lower end of the ridge 4a can be brought closer 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] (First p-side composition gradient layer 411) The first p-side composition gradient layer 411 is a layer in which the bandgap energy increases as it goes upward. By having such a configuration, the optical confinement in the active layer 3 can be enhanced. The first p-side composition gradient layer 411 can be in contact with the active layer 3. By arranging the first p-side composition gradient layer 411 near the active layer 3 in this way, the optical confinement effect by the first p-side composition gradient layer 411 can be improved. The first p-side composition gradient layer 411 can be in contact with the second p-side composition gradient layer 412. The distance from the first p-side composition gradient layer 411 to the electron barrier layer may be 150 nm or more. The distance from the first p-side composition gradient layer 411 to the electron barrier layer may be 600 nm or less. By strengthening the optical confinement in the active layer 3 by the first p-side composition gradient layer 411, the laser oscillation threshold current density can be reduced. Thereby, the shielding of the localized level can be suppressed, and the short-wavelength shift of the oscillation wavelength accompanying the increase in current injection can be suppressed, which is advantageous for the lengthening of the oscillation wavelength.

[0037] The first p-side compositionally graded layer 411 has an upper surface and a lower surface, and its bandgap energy increases from the lower surface towards the upper surface. The bandgap energy on the lower surface side of the first p-side compositionally graded layer 411 is smaller than the bandgap 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 will be described later, it can be said that the compositionally graded layer is an aggregate of a plurality of sub-layers having different compositions from each other. Therefore, in the first p-side compositionally graded layer 411, it can be said that the bandgap energy increases stepwise from the lower surface towards the upper surface. An n-side compositionally graded layer 21 paired with the first p-side compositionally graded layer 411 may be provided in the n-side semiconductor layer 2. For example, as a bandgap energy structure, the first p-side compositionally graded layer 411 and the n-side compositionally graded layer can be formed to be symmetric with the active layer 3 interposed therebetween.

[0038] The first p-side compositionally graded layer 411 functions as, for example, a p-side optical guide layer. The thickness of the first p-side compositionally graded layer 411 is greater than the thickness of the well layer of the active layer 3, and also greater than the thickness of the p-side barrier layer 33 when the p-side barrier layer 33 is present. In order to improve the optical 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. The bandgap energy at the lower end of the first p-side compositionally graded layer 411 is preferably smaller than the bandgap energy of the p-side barrier layer 33 when the p-side barrier layer 33 is provided. The bandgap energy at the upper end of the first p-side compositionally graded layer 411 may have a bandgap energy equal to or higher than that of the p-side barrier layer 33.

[0039] The first p-side compositionally graded 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 bandgap energy increases from the active layer 3 side toward the electron barrier layer 42 side. Thereby, while attracting light to the active layer 3, it is possible to suppress the overflow of electrons. The first p-side compositionally graded layer 411 changes in composition at a first rate such that the bandgap energy increases upward. The first rate is the rate of change of the bandgap energy, and may be obtained by calculation from the rate of change of the In composition ratio x. For example, the In content of the first p-side compositionally graded layer 411 obtained by analysis such as SIMS is plotted in a table, and the slope thereof may be set 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 approximate curve may be created from the table, and the slope of the approximate curve may be set as the rate of change of the In composition ratio x.

[0040] As shown in FIG. 5, the first p-side compositionally graded layer 411 can also be said to be composed of a plurality of sub-layers 411a, 411b, 411c, 411y, 411z having different compositions from each other. FIG. 5 is a partially enlarged view of the first p-side compositionally graded layer 411 and its vicinity, and a plurality of other sub-layers exist between the sub-layer 411c and the sub-layer 411y. When the first p-side compositionally graded layer 411 is composed of In x Ga 1-x N, the relationship between the In composition ratio x a of the lowermost sub-layer 411a of the first p-side compositionally graded layer 411 and the In composition ratio x z of the uppermost sub-layer 411z is 0 ≦ x z < x a . The upper limit value of the In composition ratio x a of the sub-layer 411a constituting the lower end of the first p-side compositionally graded layer 411 is, for example, 0.25. Considering the suppression of deterioration of crystallinity, the In composition ratio x a is preferably 0.1 or less.

[0041] It is preferable that the lattice constant difference between adjacent sub-layers is small. Thereby, the strain can be reduced. For this purpose, it is preferable that the first p-side composition gradient layer 411 changes its composition little by little with a small thickness. Specifically, it is preferable that the In composition ratio x decreases for each thickness of 25 nm or less from the lower surface to the upper surface in the first p-side composition gradient layer 411. That is, it is preferable that the thicknesses of the respective sub-layers 411a, 411b, 411c, 411y, and 411z are 25 nm or less. The thicknesses of the respective sub-layers 411a, 411b, 411c, 411y, and 411z are more preferably 20 nm or less, may be 10 nm or less, and may be 5 nm or less. The lower limit value of the thickness of each of the sub-layers 411a, 411b, 411c, 411y, and 411z is about 1 atomic layer (about 0.25 nm), for example. Further, the difference in the In composition ratio x between adjacent sub-layers (for example, sub-layer 411a and sub-layer 411b) is preferably 0.005 or less, and more preferably 0.001 or less. The lower limit value of the difference in the In composition ratio x between adjacent sub-layers is about 0.00007, for example. Such a range is preferably satisfied over the entire first p-side composition gradient layer 411. That is, it is preferable that all the sub-layers are within such a range. The number of times the composition changes in the first p-side composition gradient layer 411, that is, the number of sub-layers of the first p-side composition gradient layer 411 is preferably 90 or more.

[0042] When providing the n-side composition gradient layer 21, the preferable ranges of its composition, composition change rate, and thickness can be the same as those of the first p-side composition gradient layer 411.

[0043] (Second p-side composition gradient 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 bandgap energy increases from the lower surface toward the upper surface. The bandgap energy on the lower surface side of the second p-side compositionally graded layer 412 is smaller than the bandgap energy on the upper surface side. Similar to the first p-side compositionally graded layer 411, since the compositionally graded layer can be said to be an aggregate of a plurality of sub-layers having different compositions from each other, in the second p-side compositionally graded layer 412, it can be said that the bandgap energy increases stepwise from the lower surface toward the upper surface.

[0044] The thickness of the second p-side compositionally graded layer 412 can be 2 nm or more. Thereby, the band spike can be effectively reduced. The thickness of the second p-side compositionally graded layer 412 may be 10 nm or more, and may be 50 nm or more. Thereby, light leakage to the electron barrier layer 42 and the second portion 43 can be reduced, and light absorption loss can be reduced. The thickness of the second p-side compositionally graded layer 412 may be greater than the thickness of the well layer of the active layer 3. When there is a p-side barrier layer 33, the thickness of the second p-side compositionally graded layer 412 may be greater 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, may be 200 nm or less, and 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 bandgap energy at the lower end of the second p-side compositionally graded layer 412 is larger than the bandgap energy at the upper end of the first p-side compositionally graded layer 411. The bandgap energy at the upper end of the second p-side compositionally graded layer 412 is smaller than the bandgap energy of the electron barrier layer 42. The bandgap energy at the upper end of the second p-side compositionally graded layer 412 may be larger than the bandgap energy of the layer constituting the lower end of the second portion 43. The Al composition ratio y at the lower end of the second p-side compositionally graded layer 412 preferably exceeds the Al composition ratio z of the intermediate layer 413 and is 0.05 or less. Thereby, it is easy to stably form the second p-side compositionally graded layer 412 made of AlGaN. The Al composition ratio y at the upper end of the second p-side compositionally graded layer 412 is preferably 0.1 or less. Thereby, it is easy to reduce the influence of the band spike.

[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 bandgap 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 bandgap energy increases upward at a second change rate smaller than the first change rate. Thereby, it is easy to reduce the influence of the band spike. The second change rate can be obtained by the same method as the first change rate. That is, the second change rate is the change rate of the bandgap energy and may be obtained by calculation from the change rate 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 thereof can be used as the change rate of the Al composition ratio y. The Al content obtained by the analysis may be plotted in a table on a linear scale, a linear approximate curve may be created from the table, and the slope of the approximate curve may be used as the change rate of the Al composition ratio y. Even when at least one of the In content of the first p-side compositionally graded layer 411 and the Al content of the second p-side compositionally graded layer 412 changes non-linearly within the layer, the magnitude relationship between the first change rate and the second change rate can be compared by creating these linear approximate curves.

[0047] As shown in FIG. 6, the second p-side composition inclined layer 412 can be said to be composed of a plurality of sub-layers 412a, 412b, 412c, 412y, 412z having different compositions from each other. FIG. 6 is a partially enlarged view of the second p-side composition inclined layer 412 and its vicinity, and there are also a plurality of sub-layers other than these between the sub-layer 412c and the sub-layer 412y. When the second p-side composition inclined layer 412 is composed of Al y Ga 1-y N, the Al composition ratio y a of the lowermost sub-layer 412a of the second p-side composition inclined layer 412 and the Al composition ratio y z of the uppermost sub-layer 412z satisfy the relationship y a <y z <1. The Al composition ratio y a of the sub-layer 412a constituting the lower end of the second p-side composition inclined layer 412 can be set to z < y a ≦ 0.05. z is the Al composition ratio of the intermediate layer 413. The Al composition ratio y z of the sub-layer 412z constituting the upper end of the second p-side composition inclined layer 412 can be set to y a <y z ≦ 0.1.

[0048] It is preferable that in the second p-side composition gradient layer 412, the Al composition ratio y increases for each thickness of 25 nm or less from the bottom surface to the top surface. That is, it is preferable that the thicknesses of the respective sub-layers 412a, 412b, 412c, 412y, and 412z are 25 nm or less. Thereby, the strain generated between adjacent sub-layers can be reduced. More preferably, the thicknesses of the respective sub-layers 412a, 412b, 412c, 412y, and 412z are 20 nm or less, may be 10 nm or less, and may be 5 nm or less. The lower limit of the thickness of each sub-layer 412a, 412b, 412c, 412y, and 412z is, for example, about 1 atomic layer (about 0.25 nm). Further, the difference in the Al composition ratio y between adjacent sub-layers (for example, sub-layer 412a and sub-layer 412b) is preferably 0.005 or less, and more preferably 0.001 or less. The lower limit of the difference in the Al composition ratio y between adjacent sub-layers is, for example, about 0.00007. It is preferable that such a range is satisfied over the entire second p-side composition gradient layer 412. That is, it is preferable that all sub-layers are within such a range. The number of times the composition changes in the second p-side composition gradient layer 412, that is, the number of sub-layers of the second p-side composition gradient layer 412, is preferably 10 or more, and more preferably 30 or more.

[0049] When providing the composition gradient layer 232 in the n-side intermediate portion 23, the preferable ranges of its composition, composition change rate, and thickness can be the same as those of the second p-side composition gradient layer 412.

[0050] (Intermediate layer 413) The intermediate layer 413 is disposed between the first p-side composition gradient layer 411 and the second p-side composition gradient layer 412. The intermediate layer 413 is in contact with the second p-side composition gradient layer 412. The intermediate layer 413 may be in contact with the first p-side composition gradient layer 411.

[0051] The intermediate layer 413 is Al z Ga 1-zIt consists of N. The Al composition ratio z of the intermediate layer 413 is less than the Al composition ratio y of the 2p-side composition gradient layer 412. The Al composition ratio y of the 2p-side composition gradient layer 412 is not constant, but the Al composition ratio z of the intermediate layer 413 is smaller than any of its values. Since the minimum Al composition ratio y in the 2p-side composition gradient layer 412 is the Al composition ratio y at the lower end, the Al composition ratio y at the lower end of the 2p-side composition gradient layer 412 and the Al composition ratio z of the intermediate layer 413 may be compared. The Al composition ratio z of the intermediate layer 413 can be more than 0 and 0.01 or less. In order to stably form the 2p-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 more than 0 and less than 0.01, or may be more than 0 and 0.005 or less. The intermediate layer 413 is either not a composition gradient layer or a composition gradient layer having a composition change rate smaller than the composition change rate of the 2p-side composition gradient layer 412. For example, the change in the Al composition per 1 nm of the intermediate layer 413 may be 0.0001 or less. 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, and may be 50 nm or more. Thereby, it is easy to stably form the 2p-side composition gradient layer 412. The thickness of the intermediate layer 413 can be 250 nm or less, may be 200 nm or less, and may be 100 nm or less. Thereby, the first portion 41 including the intermediate layer 413 can be efficiently formed. The thickness of the intermediate layer 413 can be 0.5 times or more and 1.5 times or less the thickness of the 2p-side composition gradient layer 412.

[0053] When the intermediate layer 231 is provided in the n-side intermediate portion 23, the preferable ranges of its composition, composition change rate, and thickness can be the same as those of the intermediate layer 413.

[0054] (Intermediate layer 414) The second p-side compositional gradient layer 412 may be in contact with the electron barrier layer 42, or an intermediate layer 414 may be disposed between the second p-side compositional gradient 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 compositional gradient layer 412 and the electron barrier layer 42.

[0055] The bandgap energy of the intermediate layer 414 is the same as the bandgap energy at the upper end of the second p-side compositional gradient layer 412, or is a value between the bandgap energy at the upper end of the second p-side compositional gradient layer 412 and the bandgap energy of the electron barrier layer 42. By providing such an intermediate layer 414, light confinement in the active layer 3 can be improved. Also, by providing the intermediate layer 414, the electron barrier layer 42 and the second portion 43 containing p-type impurities can be further away from the active layer 3, so that light absorption loss can be reduced. By reducing the light absorption loss, the efficiency of the semiconductor laser element 100 can be improved. The efficiency of the semiconductor laser element 100 includes the slope efficiency which is the slope in the characteristic graph of current and optical output at a current value equal to or higher than the threshold current.

[0056] The intermediate layer 414 is not a compositional gradient layer or is a compositional gradient layer having a composition change rate smaller than the composition change rate of the second p-side compositional gradient layer 412. The intermediate layer 414 may have, for example, a change in Al composition of 0.001 or less per 1 nm. When the composition of the intermediate layer 414 is not constant, the median value may be used as the composition of the intermediate layer 414 to compare the magnitudes of the bandgap energies with other layers. The lower end of the ridge 4a is preferably located in the intermediate layer 414. Thereby, compared with the case where the lower end of the ridge 4a is located in a compositional gradient layer, 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 can be reduced.

[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 composition gradient layer 412, it is preferable that the lattice constant at the lower end of the intermediate layer 414 is the same as the lattice constant at the upper end of the second p-side composition gradient layer 412. The smaller the difference between the lattice constant of the intermediate layer 414 and the lattice constant at the upper end of the second p-side composition gradient 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 at the upper end of the second p-side composition gradient layer 412 is preferably 0.02 or less, and more preferably 0.01 or less. Thereby, the influence of the band spike can be reduced.

[0058] The thickness of the intermediate layer 414 is thicker than the sub-layers that make up the second p-side composition gradient 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, and may also be 250 nm or less.

[0059] It is preferable that the sum of the thickness of the second p-side compositionally graded layer 412 and the thickness of the intermediate layer 414 is 100 nm or more. Thereby, light leakage to the electron barrier layer 42 and the second portion 43 can be reduced, and absorption loss of light can be reduced. It is preferable that the sum of the thickness of the second p-side compositionally graded layer 412 and the thickness of the intermediate layer 414 is 500 nm or less. Thereby, since the electron barrier layer 42 can be disposed at a position not too far from the active layer 3, the total amount of electron overflow to the second portion 43 can be suppressed. The ratio of the thickness of the second p-side compositionally graded layer 412 to the sum of the thickness of the second p-side compositionally graded layer 412 and the thickness of the intermediate layer 414 is preferably 0.5 or less. Since the average value of the bandgap energy of the second p-side compositionally graded layer 412 is smaller than the bandgap energy of the intermediate layer 414, the light confinement in the active layer 3 decreases as the thickness of the second p-side compositionally graded layer 412 increases. Therefore, by relatively thinning the thickness of the second p-side compositionally graded layer 412, the light confinement in the active layer 3 can be improved. Thereby, a decrease in the oscillation threshold current of the semiconductor laser element 100 can be expected. Also, since the light reaching the second portion 43 can be reduced, the absorption loss of light can be reduced, and the light output can be increased.

[0060] When the intermediate layer 233 is provided in the n-side intermediate portion 23, the preferable ranges of its composition, composition change rate, and thickness can be the same as 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 greater than the band gap energy 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 greater than that of any layer 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 to electrons overflowing from the active layer 3. Preferably, the difference in band gap energy between the electron barrier layer 42 and the uppermost layer of the first portion 41 is 0.1 eV or more. These differences in band gap energy 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 compositional gradient layer 411.

[0062] The electron barrier layer 42 may have a multilayer structure. In this case, one or more layers constituting the electron barrier layer 42 have a band gap energy greater than that of any layer constituting the first portion 41. The electron barrier layer 42 may be a compositional gradient layer. The electron barrier layer 42 may be, for example, a compositional gradient layer whose composition changes such that the band gap energy decreases upward. When the first portion 41 and the electron barrier layer 42 have superlattice layers, the magnitude relationship is compared using the average band gap energy of the superlattice layer instead of 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 AlGaN, its Al composition ratio may be 0.08 or more and 0.5 or less. The thickness of the electron barrier layer 42 can be, for example, 4 nm or more and 100 nm or less.

[0063] (Second portion 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 the p-type impurities in the p-type semiconductor layer included in the second portion 43 can be, for example, 1×10 18 / cm 3 or more, and can be 1×10 22 / cm 3 or less. As described above, since the driving voltage can be reduced by making the thickness of the second portion 43 thin, 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. Preferably, the entire second portion 43 contains p-type impurities. Thereby, the resistance of the second portion 43 can be reduced as compared with the case of including an undoped layer. In the case of a superlattice layer, since the average p-type impurity concentration can be regarded as the p-type impurity concentration of the superlattice layer, when the second portion 43 has 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 such as the slope efficiency of the semiconductor laser element 100 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 decreases by reducing the thickness of the portion containing the p-type impurity is that in the nitride semiconductor, 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, since it is located between the electron barrier layer 42 and the active layer 3, it tends to exhibit n-type conductivity rather than perfect insulation due to factors such as electron overflow. From these facts, it is considered that by reducing the thickness of the second portion 43 containing the p-type impurity, which is relatively high in resistance, the driving voltage can be lowered, and the effect of suppressing the increase in the driving voltage due to increasing the thickness of the undoped first portion 41 can be obtained. Since the first portion 41 is relatively thick, it is preferable to provide the lower end of the ridge 4a on the first portion 41. Thereby, the lateral light confinement can be strengthened.

[0065] The second portion 43 may include a lower p-type semiconductor layer and an upper p-type semiconductor layer. The upper p-type semiconductor layer constitutes the upper surface of the ridge 4a. That is, 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 bandgap energy larger than the bandgap 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, whereby the resistance of the second portion 43 can be made lower. In this case, it is preferable to form the p electrode from a material that functions as a cladding layer such as ITO. The thickness of the upper p-type semiconductor layer can be, for example, 5 nm or more and 30 nm or less. The thickness of the lower p-type semiconductor layer can be, for example, 1 nm or more and 260 nm or less. The lower p-type semiconductor layer is, for example, thicker than 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, for example, from a single-layer film or a multilayer film of an oxide or nitride such as Si, Al, Zr, Ti, Nb, Ta, etc. The n electrode 8 is provided, for example, over substantially 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 having a width wider than that of the p electrode 6 may be provided on the p electrode 6, and a wire or the like may be connected to the p-side pad electrode 7. The material of each electrode includes, for example, a single-layer film or a multilayer film of a metal or alloy such as Ni, Rh, Cr, Au, W, Pt, Ti, Al, etc., or a conductive oxide containing at least one selected from Zn, In, Sn. Examples of the conductive oxide include ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), GZO (Gallium-doped Zinc Oxide), etc. The thickness of the electrode is usually a thickness that can function as an electrode of a semiconductor device. For example, about 0.05 μm to 2 μm can be mentioned.

[0068] The p - electrode 6 is preferably a transparent conductive film having a refractive index smaller than that of the active layer 3. Thereby, it can 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. Thereby, the light confinement effect can be obtained more effectively. Also, when providing a p - side cladding layer on the second portion 43, for example, an AlGaN layer with a relatively high Al composition ratio and p - type impurities added is provided as the p - side cladding layer. However, the higher the Al composition ratio, the higher the resistance tends to be. If the p - electrode 6 functions as a cladding layer, it may not be necessary to provide a p - side cladding layer on the second portion 43, or even when providing a p - side cladding layer, its Al composition ratio can be lowered. For this reason, the resistance can be reduced, and the driving voltage of the semiconductor laser element 100 can be reduced. As the p - electrode 6 that functions as a cladding layer, for example, a p - electrode 6 made of ITO can be mentioned.

[0069] (Manufacturing method) The manufacturing method of the semiconductor laser element 100 according to the embodiment has, for example, the following first to fifth steps. The first step is a step of forming an n - side semiconductor layer 2 on the 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 undopedly forming a first portion 41 having one or more semiconductor layers on the upper surface of the active layer 3. The fourth step is a step of forming an electron barrier layer 42 by doping p - type impurities on the upper surface of the first portion 41. The fifth step is a step of forming a second portion 43 having one or more p - type semiconductor layers formed by doping p - type impurities 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 have 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 operational effects, preferable configurations, etc. of the layers obtained by 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 the epitaxial wafer for the semiconductor laser element 100. Further, trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH3), silane gas, and bis(cyclopentadienyl)magnesium (Cp2Mg) were appropriately used as raw materials.

[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) with the +c plane as the upper surface.

[0072] The n-side semiconductor layer 2 includes, in order from the side of the n-type GaN substrate, an AlGaN layer with a thickness of 1.5 μm doped with Si, an InGaN layer with a thickness of 150 nm doped with Si, an AlGaN layer with a thickness of 900 nm doped with Si (n-type semiconductor layer 22), a GaN layer with a thickness of 3500 nm doped with Si (n-side intermediate portion 23), and an n-side composition gradient layer 21 grown with the starting end being GaN and the ending end being InGaN and the In composition being substantially monotonically increased. 0.016 Ga 0.984 N layer and an InGaN layer with a thickness of 150 nm doped with Si, an AlGaN layer with a thickness of 900 nm doped with Si (n-type semiconductor layer 22), a GaN layer with a thickness of 3500 nm doped with Si (n-side intermediate portion 23), and an n-side composition gradient layer 21 grown with the starting end being GaN and the ending end being InGaN and the In composition being substantially monotonically increased. 0.05 Ga 0.95 N layer and an AlGaN layer with a thickness of 900 nm doped with Si (n-type semiconductor layer 22), a GaN layer with a thickness of 3500 nm doped with Si (n-side intermediate portion 23), and an n-side composition gradient layer 21 grown with the starting end being GaN and the ending end being InGaN and the In composition being substantially monotonically increased. 0.072 Ga 0.928 N layer (n-type semiconductor layer 22), a GaN layer with a thickness of 3500 nm doped with Si (n-side intermediate portion 23), and an n-side composition gradient layer 21 grown with the starting end being GaN and the ending end being InGaN and the In composition being substantially monotonically increased. 0.05 Ga 0.95 N and the In composition being substantially monotonically increased.

[0073] The active layer 3 includes, in order from the side of the n-type GaN substrate, a GaN layer doped with Si, an undoped In composition gradient layer, an undoped InGaN layer, and an undoped GaN layer. 0.2 Ga 0.8 N layer and an undoped GaN layer.

[0074] The p-side semiconductor layer 4 includes, in order from the side of the n-type GaN substrate, an undoped first p-side composition gradient layer 411 with a thickness of 200 nm, an undoped intermediate layer 413 with a thickness of 50 nm, an undoped second p-side composition gradient layer 412 with a thickness of 70 nm, and an undoped AlGaN layer with a thickness of 130 nm. 0.045 Ga 0.955It was formed to have an intermediate layer 414 made of N, an electron barrier layer 42 made of AlGaN with a thickness of 10 nm to which Mg was added, a lower p-type semiconductor layer made of AlGaN with a thickness of 100 nm to which Mg was added, and an upper p-type semiconductor layer made of GaN with a thickness of 18 nm to which Mg was added. The first p-side composition gradient layer 411 had In 0.05 Ga 0.95 N as the starting point of growth and GaN as the ending point of growth. The second p-side composition gradient layer 412 had AlGaN with an Al composition ratio of 1% or less as the starting point of growth and Al 0.045 Ga 0.955 N as the ending point of growth.

[0075] Then, the epitaxial wafer on which the above layers were formed was taken out from the MOCVD apparatus, a ridge 4a, an insulating film 5, a p electrode 6, a p-side pad electrode 7, and an n electrode 8 were formed, reflection films were formed on the light-emitting end face and the light-reflecting end face, respectively, and the semiconductor laser element 100 was obtained by singulation. The depth of the ridge 4a was about 270 nm. That is, the ridge 4a was formed such that its lower end was located in the second p-side composition gradient layer 412. Also, as the p electrode 6, an ITO film with a thickness of 200 nm was formed. The peak wavelength of the laser light emitted from the semiconductor laser element 100 according to Example 1 was about 455 nm.

[0076] (SIMS analysis) For the wafer on which the p-side semiconductor layer 4 similar to that in Example 1 was grown and stopped before forming the ridge 4a and the like, the results of SIMS analysis are shown in FIG. 7. In FIG. 7, the broken line is the line showing the detected amount of In, and the solid line is the line showing the detected amount of Al. The vertical axis and the horizontal axis in FIG. 7 are linear scales. It was confirmed from FIG. 7 that the intermediate layer 414 was formed between the first p-side composition gradient layer 411 and the second p-side composition gradient layer 412. In FIG. 7, the line showing the detected amount of In and the line showing the detected amount of Al intersect at the lower end (the 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 the +c plane as the upper surface.

[0078] The n-side semiconductor layer 2 is, in order from the side of the n-type GaN substrate, an AlGaN layer with a thickness of 1.25 μm doped with Si, 0.018 Ga 0.982 a GaN layer with a thickness of 250 nm doped with Si, 0.08 Ga 0.92 an InGaN layer with a thickness of 150 nm doped with Si, 0.04 Ga 0.96 a GaN layer with a thickness of 10 nm doped with Si, an AlGaN layer with a thickness of 650 nm doped with Si, 0.08 Ga 0.92 an n-type semiconductor layer 22), a GaN layer with a thickness of 200 nm doped with Si (n-side intermediate part 23), and an undoped InGaN layer with a thickness of 230 nm, 0.03 Ga 0.97 an N layer, and is formed to have the above.

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

[0080] The p-side semiconductor layer 4 is, in order from the side of the n-type GaN substrate, an undoped first p-side composition gradient layer 411 with a thickness of 180 nm, an undoped intermediate layer 413 with a thickness of 50 nm, an undoped second p-side composition gradient layer 412 with a thickness of 100 nm, an undoped intermediate layer 414 made of AlGaN with a thickness of 200 nm, an electron barrier layer 42 made of AlGaN with a thickness of 10.9 nm doped with Mg, a lower p-type semiconductor layer made of AlGaN with a thickness of 100 nm doped with Mg, and an upper p-type semiconductor layer made of GaN with a thickness of 18 nm doped with Mg, and is formed to have the above. The first p-side composition gradient layer 411 has a growth start end at InGaN, 0.04 Ga 0.96 N, 0.05 Ga 0.95Let N be the starting point of growth and GaN be the ending point of growth. The second p-side composition gradient layer 412 has an AlGaN with an Al composition ratio of 1% or less at the starting point of growth and Al 0.04 Ga 0.96 N at the ending point of growth.

[0081] Then, the epitaxial wafer on which the above layers are formed is taken out from 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 are formed. Reflective films are formed on the light-emitting end face and the light-reflecting end face, respectively, and the semiconductor laser element 100 is obtained by singulation. The depth of the ridge 4a was set to about 270 nm. That is, the ridge 4a was formed such that its lower end is located in the intermediate layer 414. Also, as the p-electrode 6, an ITO film with a thickness of 200 nm was formed. The peak wavelength of the laser light emitted by the semiconductor laser element 100 according to Example 2 was about 527 nm.

[0082] (Example 3) The semiconductor laser element 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 are a GaN layer (n-side intermediate portion 23) with a thickness of 250 nm to which Si is added and an undoped In 0.03 Ga 0.97 N layer. In the p-side semiconductor layer 4, the thickness of the second p-side composition gradient layer 412 was set to 300 nm, and the intermediate layer 414 was eliminated. The undoped intermediate layer 413 with a thickness of 50 nm was left as it was. The depth of the ridge 4a was kept at about 270 nm. That is, the ridge 4a was formed such that its lower end is located in the second p-side composition gradient layer 412. The peak wavelength of the laser light emitted by the semiconductor laser element 100 according to 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 composition gradient layer 412 were not formed, and an undoped GaN layer with a thickness of 120 nm was formed at that position. The depth of the ridge 4a was kept at about 270 nm. That is, the ridge 4a was formed such that its lower end was located in the undoped GaN layer with 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 Example 2, except for the following points. The last two layers in the n-side semiconductor layer 2 were a GaN layer with a thickness of 250 nm doped with Si and an undoped In 0.03 Ga 0.97 GaN layer with a thickness of 180 nm. In the p-side semiconductor layer 4, the second p-side composition gradient layer 412 was not formed, and an undoped GaN layer with a thickness of 150 nm was formed at that position. The peak wavelength of the laser light emitted by the semiconductor laser device according to Comparative Example 2 was 527 nm.

[0085] (I-L characteristics) The I-L characteristics of the semiconductor laser devices of Example 1 and Comparative Example 1 are shown in FIG. 8. In the graph of FIG. 8, the horizontal axis represents current and the vertical axis represents optical output. In FIG. 8, the solid line represents Example 1 and the dashed line represents Comparative Example 1. The I-L characteristics of the semiconductor laser devices of Example 2, Example 3, and Comparative Example 2 are shown in FIG. 9. In the graph of FIG. 9, the horizontal axis represents current and the vertical axis represents optical output. In FIG. 9, the solid line represents Example 2, the dash-dotted line represents Example 3, and the dashed line represents Comparative Example 2. As shown in FIGS. 8 and 9, it was confirmed that by providing the first p-side composition gradient layer 411, the intermediate layer 413, and the second p-side composition gradient layer 412, the slope efficiency was improved and the optical output was improved. This is considered to be the effect of reducing the carrier loss in the p-side semiconductor layer 4.

[0086] Through the content described so far in this specification, the following technical matters are disclosed. (Item 1) A semiconductor laser device having an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each made of a nitride semiconductor, in this order upward, The p-side semiconductor layer has one or more semiconductor layers, a first portion that is undoped, an electron barrier layer having a larger bandgap energy than the first portion and containing a p-type impurity, and a second portion having one or more p-type semiconductor layers containing a p-type impurity, in this order upward, The first portion is made of In x Ga 1-x N, and is a first p-side composition gradient layer in which the In composition ratio x decreases in the range of 0 or more and less than 1 upward, is disposed between the first p-side composition gradient layer and the electron barrier layer, and is made of Al y Ga 1-y N, and is a second p-side composition gradient layer in which the Al composition ratio y increases in the range of more than 0 and less than 1 upward, is disposed between the first p-side composition gradient layer and the second p-side composition gradient layer, and is made of Al z Ga 1-z N, and has an intermediate layer in which the Al composition ratio z is more than 0 and less than y. A semiconductor laser device. (Item 2) The first p-side composition gradient layer changes in composition at a first rate so that the bandgap energy increases upward, The second p-side composition gradient layer changes in composition at a second rate smaller than the first rate so that the bandgap energy increases upward. The semiconductor laser device according to Item 1. (Item 3) The Al composition ratio y at the upper end of the second p-side composition gradient layer is 0.1 or less. The semiconductor laser device according to Item 1 or 2. (Item 4) The thickness of the second p-side composition gradient layer is 2 nm or more. The semiconductor laser device according to any one of Items 1 to 3. (Item 5) The Al composition ratio z of the intermediate layer is more than 0 and 0.01 or less. The semiconductor laser device according to any one of Items 1 to 4. (Item 6) In the semiconductor laser device according to any one of Items 1 to 5, the Al composition ratio y at the lower end of the second p-side composition gradient layer is more than 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 first portion has a second intermediate layer disposed between the second p-side composition gradient layer and the electron barrier layer and having the same bandgap energy as the upper end of the second p-side composition gradient layer or the bandgap energy between the upper end of the second p-side composition gradient layer and the electron barrier layer. The semiconductor laser device according to any one of Items 1 to 6. (Item 8) In the semiconductor laser device according to Item 7, the ratio of the thickness of the second p-side composition gradient layer to the sum of the thicknesses of the second p-side composition gradient layer and the second intermediate layer is 0.5 or less. (Item 9) The n-side semiconductor layer An n-side composition gradient layer disposed in contact with the lower surface of the active layer and having a composition that changes so that the bandgap energy increases downward, and An n-type semiconductor layer disposed below the n-side composition gradient layer and having a bandgap energy greater than that of any layer constituting the first portion of the p-side semiconductor layer and containing n-type impurities, and An n-side intermediate portion disposed between the n-side composition gradient layer and the n-type semiconductor layer, and has In the semiconductor laser device according to any one of Items 1 to 8, the distance from the n-side composition gradient layer to the n-type semiconductor layer is greater than the distance from the first p-side composition gradient layer to the electron barrier layer. (Item 10) The active layer 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, and An intermediate barrier layer located between the first well layer and the second well layer, and has In the semiconductor laser device according to any one of Items 1 to 9, the thickness of the intermediate barrier layer is smaller than the thickness of the p-side barrier layer.

Explanation of Reference Numerals

[0087] 100 semiconductor laser element 1 substrate 2 n-side semiconductor layers 21 n-side composition gradient layer 22 n-type semiconductor layer 23 n-side intermediate part 231 intermediate layer 232 composition gradient layer 233 intermediate layer 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 first part 411 first p-side composition gradient layer 411a, 411b, 411c, 411y, 411z sub-layers 412 second p-side composition gradient layer 412a, 412b, 412c, 412y, 412z sub-layers 413 intermediate layer 414 intermediate layer 42 electron barrier layer 43 second part 4a ridge 5 insulating film 6 p-electrode 7 p-side pad electrode 8 n-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. 3. 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. 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. 5. 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. 6. 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 0.05 or less.

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, 9. 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; 10. The semiconductor laser device according to claim 1, wherein a thickness of the intermediate barrier layer is smaller than a thickness of the p-side barrier layer.

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