阅读说明：本技术 半导体激光元件及其制造方法 (Semiconductor laser element and method for manufacturing the same ) 是由 中津嘉隆 于 2020-01-16 设计创作，主要内容包括：提供半导体激光元件,能够降低吸收损失从而能够提高效率。半导体激光元件朝向上方依次具有各自由氮化物半导体构成的n侧半导体层、活性层和p侧半导体层,在p侧半导体层设有向上方突出的脊。p侧半导体层具有：未掺杂的第一部分,其与活性层的上表面相接地配置,并具有一个以上的半导体层；电子屏障层,其与第一部分的上表面相接地配置,带隙能量比第一部分大,并含有p型杂质；第二部分,其与电子屏障层的上表面相接地配置,并具有一个以上的p型半导体层,p型半导体层含有p型杂质。第一部分具有：未掺杂的p侧成分倾斜,其层带隙能量随着朝向上方而变大；未掺杂的p侧中间层,其配置于p侧成分倾斜层的上方；脊的下端位于p侧中间层。(Provided is a semiconductor laser element capable of reducing absorption loss and improving efficiency. The semiconductor laser element includes an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each of which is composed of a nitride semiconductor, in this order from the upper side, and the p-side semiconductor layer is provided with a ridge protruding upward. The p-side semiconductor layer has: an undoped first portion which is disposed in contact with the upper surface of the active layer and has one or more semiconductor layers; an electron barrier layer which is disposed so as to be in contact with the upper surface of the first portion, has a band gap energy larger than that of the first portion, and contains a p-type impurity; and a second portion which is disposed so as to be grounded on the upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities. The first part has: the undoped p-side component is inclined, and the layer band gap energy of the component is increased along with the upward direction; an undoped p-side intermediate layer disposed above the p-side component inclined layer; the lower end of the ridge is located in the p-side intermediate layer.)

1. A semiconductor laser element comprising, in order upward, an n-side semiconductor layer, an active layer and a p-side semiconductor layer each comprising a nitride semiconductor, wherein the p-side semiconductor layer is provided with a ridge projecting upward,

the p-side semiconductor layer has:

an undoped first portion which is disposed so as to be grounded on the upper surface of the active layer and has one or more semiconductor layers;

an electron barrier layer which is disposed so as to be grounded on the upper surface of the first portion, has a band gap energy larger than that of the first portion, and contains a p-type impurity;

a second portion which is disposed so as to be grounded on an upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities;

the first portion has:

an undoped p-side component gradient layer whose band gap energy increases upward;

an undoped p-side intermediate layer disposed above the p-side component inclined layer;

the lower end of the ridge is located at the p-side intermediate layer.

2. The semiconductor laser device according to claim 1,

the second portion has:

an upper p-type semiconductor layer constituting an upper surface of the ridge;

and a lower p-type semiconductor layer which is disposed between the upper p-type semiconductor layer and the electron barrier layer and has a band gap energy larger than that of the upper p-type semiconductor layer.

3. The semiconductor laser element according to claim 1 or 2,

the first portion has, as the p-side intermediate layer:

an undoped first layer having a band gap energy greater than an average band gap energy of the p-side composition-inclined layer and less than a band gap energy of the electron barrier layer;

an undoped second layer having a band gap energy greater than a band gap energy of the first layer and less than a band gap energy of the electronic barrier layer.

4. The semiconductor laser device according to claim 3,

the first layer and the second layer are each a single component layer, and the lower end of the ridge is located in the first layer or the second layer.

5. The semiconductor laser element according to claim 3 or 4,

the first layer is a GaN layer.

6. The semiconductor laser element according to any one of claims 3 to 5,

the second layer is an AlGaN layer.

7. The semiconductor laser element according to any one of claims 1 to 6,

the p-side composition gradient layer is composed of a plurality of sub-layers having different compositions from each other,

the lowermost sublayer of the p-side composition gradient layer is composed of In_aGa_1-aN(0<a<1) The structure of the utility model is that the material,

the uppermost sublayer of the p-side composition gradient layer is composed of In_zGa_1-zN(0≦z<a) And (4) forming.

8. A semiconductor laser element comprising, in order upward, an n-side semiconductor layer, an active layer and a p-side semiconductor layer each comprising a nitride semiconductor, wherein the p-side semiconductor layer is provided with a ridge projecting upward,

the p-side semiconductor layer has:

an undoped first portion which is disposed so as to be grounded on the upper surface of the active layer and has one or more semiconductor layers;

an electron barrier layer which is disposed so as to be grounded on the upper surface of the first portion, has a band gap energy larger than that of the first portion, and contains a p-type impurity;

a second portion which is disposed so as to be grounded on an upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities;

the second portion has a thickness thinner than a thickness of the first portion,

the lower end of the ridge is located at the first portion.

9. The semiconductor laser element according to any one of claims 1 to 8,

the first portion has a thickness of 400nm or more.

10. The semiconductor laser element according to any one of claims 1 to 9,

the shortest distance from the bottom surface of the ridge to the electronic barrier layer is larger than the shortest distance from the upper surface of the ridge to the electronic barrier layer.

11. The semiconductor laser element according to any one of claims 1 to 10,

the semiconductor laser element has a p-electrode provided on an upper surface of the ridge,

the p-electrode is a transparent conductive film having a refractive index smaller than that of the second portion.

12. The semiconductor laser element according to any one of claims 1 to 11,

the semiconductor laser element can oscillate laser light having a wavelength of 530nm or more.

13. The semiconductor laser element according to any one of claims 1 to 12,

the first portion has an uppermost layer contiguous with a lower surface of the electronic barrier layer,

the second portion has a lowermost layer in contact with an upper surface of the electronic barrier layer,

the band gap energy of the lowermost layer is smaller than that of the uppermost layer.

14. A method for manufacturing a semiconductor laser device, comprising:

forming an n-side semiconductor layer on a substrate;

forming an active layer on the n-side semiconductor layer;

forming a first portion having one or more semiconductor layers on an upper surface of the active layer without doping;

forming an electron barrier layer having a band gap energy larger than that of the first portion by doping a p-type impurity on an upper surface of the first portion;

forming a second portion having one or more p-type semiconductor layers formed by doping a p-type impurity on an upper surface of the electron barrier layer;

forming a ridge protruding upward by removing a part of the p-side semiconductor layer including the first portion, the electron barrier layer, and the second portion;

the step of forming the first portion in an undoped manner includes:

forming a p-side component-inclined layer in which band gap energy increases upward without doping;

forming a p-side intermediate layer on the p-side inclined component layer without doping;

in the step of forming the ridge, a part of the p-side semiconductor layer is removed so that a lower end of the ridge is positioned in the p-side intermediate layer.

15. A method for manufacturing a semiconductor laser device, comprising:

forming an n-side semiconductor layer on a substrate;

forming an active layer on the n-side semiconductor layer;

forming a first portion having one or more semiconductor layers on an upper surface of the active layer without doping;

forming an electron barrier layer having a band gap energy larger than that of the first portion by doping a p-type impurity on an upper surface of the first portion;

forming a second portion having one or more p-type semiconductor layers formed by doping a p-type impurity on an upper surface of the electron barrier layer;

forming a ridge protruding upward by removing a part of the p-side semiconductor layer including the first portion, the electron barrier layer, and the second portion;

forming the second portion having a thickness thinner than that of the first portion in the step of forming the second portion,

in the step of forming the ridge, a part of the p-side semiconductor layer is removed so that a lower end of the ridge is positioned at the first portion.

Technical Field

The present invention relates to a semiconductor laser element and a method for manufacturing the same.

Background

In these days, a semiconductor laser device having a nitride semiconductor (hereinafter, also referred to as "nitride semiconductor laser device") can oscillate light from an ultraviolet region to green, and can be used for various applications other than a light source for an optical disk. As such a semiconductor laser element, a structure is known in which an n-side cladding layer, an n-side light guide layer, an active layer, a p-side light guide layer, and a p-side cladding layer are provided in this order on a substrate (for example, patent documents 1, 2, and 3).

Disclosure of Invention

Technical problem to be solved by the invention

A p-type impurity such as Mg is added to a semiconductor layer on the p-side of the nitride semiconductor laser element, but the p-type impurity generates a deep level and causes light absorption. Therefore, the absorption loss increases as the light intensity in the p-type impurity-containing layer increases, and the efficiency such as the slope efficiency decreases. Therefore, the present disclosure proposes a semiconductor laser element capable of reducing absorption loss and improving efficiency.

Means for solving the problems

A first aspect of the semiconductor laser device of the present disclosure is a semiconductor laser device including, in order upward, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer each made of a nitride semiconductor, the p-side semiconductor layer being provided with a ridge projecting upward,

the p-side semiconductor layer has:

an undoped first portion which is disposed so as to be grounded on the upper surface of the active layer and has one or more semiconductor layers;

an electron barrier layer which is disposed so as to be grounded on the upper surface of the first portion, has a band gap energy larger than that of the first portion, and contains a p-type impurity;

a second portion which is disposed so as to be grounded on an upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities;

the first portion has:

an undoped p-side component gradient layer whose band gap energy increases upward;

an undoped p-side intermediate layer disposed above the p-side component inclined layer;

the lower end of the ridge is located at the p-side intermediate layer.

A second aspect of the semiconductor laser device of the present disclosure is a semiconductor laser device including, in order upward, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer each made of a nitride semiconductor, the p-side semiconductor layer being provided with a ridge projecting upward,

the p-side semiconductor layer has:

an undoped first portion which is disposed so as to be grounded on the upper surface of the active layer and has one or more semiconductor layers;

an electron barrier layer which is disposed so as to be grounded on the upper surface of the first portion, has a band gap energy larger than that of the first portion, and contains a p-type impurity;

a second portion which is disposed so as to be grounded on an upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities;

the second portion has a thickness thinner than a thickness of the first portion,

the lower end of the ridge is located at the first portion.

A first aspect of the method for manufacturing a semiconductor laser element of the present disclosure includes:

forming an n-side semiconductor layer on a substrate;

forming an active layer on the n-side semiconductor layer;

forming a first portion having one or more semiconductor layers on an upper surface of the active layer without doping;

forming an electron barrier layer having a band gap energy larger than that of the first portion by doping a p-type impurity on an upper surface of the first portion;

forming a second portion having one or more p-type semiconductor layers formed by doping a p-type impurity on an upper surface of the electron barrier layer;

forming a ridge protruding upward by removing a part of the p-side semiconductor layer including the first portion, the electron barrier layer, and the second portion;

the step of forming the first portion in an undoped manner includes:

forming a p-side component-inclined layer in which band gap energy increases upward without doping;

forming a p-side intermediate layer on the p-side inclined component layer without doping;

in the step of forming the ridge, a part of the p-side semiconductor layer is removed so that a lower end of the ridge is positioned in the p-side intermediate layer.

A second aspect of the method for manufacturing a semiconductor laser device of the present disclosure includes:

forming an n-side semiconductor layer on a substrate;

forming an active layer on the n-side semiconductor layer;

forming a first portion having one or more semiconductor layers on an upper surface of the active layer without doping;

forming an electron barrier layer having a band gap energy larger than that of the first portion by doping a p-type impurity on an upper surface of the first portion;

forming a second portion having one or more p-type semiconductor layers formed by doping a p-type impurity on an upper surface of the electron barrier layer;

forming a ridge protruding upward by removing a part of the p-side semiconductor layer including the first portion, the electron barrier layer, and the second portion;

forming the second portion having a thickness thinner than that of the first portion in the step of forming the second portion,

in the step of forming the ridge, a part of the p-side semiconductor layer is removed so that a lower end of the ridge is positioned at the first portion.

Effects of the invention

According to such a semiconductor laser element, absorption loss can be reduced, and efficiency can be improved.

Drawings

Fig. 1 is a schematic cross-sectional view of a semiconductor laser device according to an embodiment of the present invention.

Fig. 2A is a diagram schematically illustrating an example of the layer structure of the p-side semiconductor layer of the semiconductor laser element of fig. 1.

Fig. 2B is a diagram schematically illustrating another example of the layer structure of the p-side semiconductor layer of the semiconductor laser element of fig. 1.

Fig. 2C is a diagram schematically showing another example of the layer structure of the p-side semiconductor layer of the semiconductor laser element of fig. 1.

Fig. 2D is a diagram schematically illustrating another example of the layer structure of the p-side semiconductor layer of the semiconductor laser element of fig. 1.

Fig. 3A is a diagram schematically showing an example of the relationship of the band gap energies between the uppermost layer of the first portion, the electronic barrier layer, and the lowermost layer of the second portion.

Fig. 3B is a diagram schematically showing another example of the relationship of the band gap energies between the uppermost layer of the first portion, the electronic barrier layer, and the lowermost layer of the second portion.

Fig. 3C is a diagram schematically showing another example of the relationship of the band gap energies between the uppermost layer of the first portion, the electronic barrier layer, and the lowermost layer of the second portion.

Fig. 4 is a diagram schematically showing an example of the layer structure of the n-side semiconductor layer of the semiconductor laser element of fig. 1.

Fig. 5 is a partially enlarged view schematically showing the p-side component gradient layer and its vicinity in the semiconductor laser device of fig. 1.

Fig. 6A is a flowchart illustrating a method for manufacturing a semiconductor laser device according to an embodiment of the present invention.

Fig. 6B is a flowchart showing an example of the step S103.

Fig. 7 is a graph showing a relationship between the thickness of the first portion and the proportion of light leakage to the second portion of calculation examples 1 to 5.

Fig. 8 is a graph showing the I-L characteristics of the semiconductor laser elements of comparative examples 1 to 4.

Fig. 9A is a graph showing the I-L characteristics of the semiconductor laser elements of examples 1 to 3.

Fig. 9B is a graph showing the I-V characteristics of the semiconductor laser elements of examples 1 to 3.

Fig. 10A is a graph showing the I-L characteristics of the semiconductor laser elements of examples 3 to 5.

Fig. 10B is a graph showing the I-V characteristics of the semiconductor laser elements of examples 3 to 5.

Fig. 11A is a graph showing I-L characteristics of the semiconductor laser devices of examples 3 and 6.

Fig. 11B is a graph showing the I-V characteristics of the semiconductor laser devices of examples 3 and 6.

Description of the reference numerals

100 semiconductor laser element

1 substrate

2 n-side semiconductor layer

21 substrate layer

22 first n-side cladding layer

23 anti-cracking layer

24 intermediate layer

25 second n-side cladding layer

26 first n-side light-guiding layer

27 second n-side light-guiding layer

28 hole blocking layer

281 first hole blocking layer

282 second hole blocking layer

3 active layer

31 n side barrier layer

32 well layer

34 p side barrier layer

4 p-side semiconductor layer

41 first part

411 p-side component inclined layer

412 p side intermediate layer

412A first layer

412B second layer

42 electronic barrier layer

42A first electronic barrier layer

42B second electronic barrier layer

43 second part

431 lower p-type semiconductor layer

432 upper p-type semiconductor layer

4a ridge

5 insulating film

6 p electrode

7 p side pad electrode

8 n electrode

411a, 411b, 411c, 411y, 411z sublayers

Detailed Description

Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the embodiments described below are embodiments illustrating a method for embodying the technical idea of the present invention, and the present invention is not limited to the embodiments described below. In the following description, the same names and reference numerals are used for the same or similar members, and detailed description thereof will be omitted as appropriate.

Fig. 1 is a schematic cross-sectional view of a semiconductor laser element 100 according to the present embodiment, showing a cross-section of the semiconductor laser element 100 in a direction perpendicular to a resonator direction. Fig. 2A, 2B, 2C, and 2D are diagrams each schematically showing an example of the layer structure of the p-side semiconductor layer 4, and each shows a different example. Fig. 2A to 2D are diagrams schematically showing the relationship between the magnitude of the band gap energy of each of the active layer 3 and the p-side semiconductor layer 4 of the semiconductor laser device 100. In fig. 2A to 2D, the position of the bottom surface of the ridge 4a is indicated by a one-dot chain line. The bottom surface of the ridge 4a is a surface connecting the lowermost edges of the two side surfaces of the ridge 4a to each other. Fig. 4 is a diagram schematically illustrating an example of the layer structure of the n-side semiconductor layer 2.

As shown in fig. 1, the semiconductor laser element 100 includes, in order from the top, an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4 each made of a nitride semiconductor. The p-side semiconductor layer 4 is provided with a ridge 4a projecting upward. Note that in this specification, a direction from the n-side semiconductor layer 2 toward the p-side semiconductor layer 4 is referred to as "up" or "upper", and an opposite direction thereof is referred to as "down" or "lower".

The p-side semiconductor layer 4 has a first portion 41, an electron barrier layer 42, and a second portion 43, the first portion 41 is disposed so as to be in contact with the upper surface of the active layer 3, and has one or more semiconductor layers, the first portion 41 is undoped, the electron barrier layer 42 is disposed so as to be in contact with the upper surface of the first portion 41, the electron barrier layer 42 has a larger band gap energy than the first portion 41, and contains a p-type impurity, the second portion 43 is disposed so as to be in contact with the upper surface of the electron barrier layer, the second portion 43 has one or more p-type semiconductor layers containing a p-type impurity, and the lower end of the ridge 4a is located in the first portion 41, that is, the ridge 4a is constituted of a part of the first portion 41, the electron barrier layer 42, and the second portion 43¹⁷/cm³However, since the first portion 41 is in contact with the electron barrier layer 42 having a high p-type impurity concentration, there is a case where the p-type impurity is detected in the analysis result even if the first portion 41 is formed so as not to be doped intentionally with the p-type impurity, the concentration of the p-type impurity detected in this case is preferably not 1 × 10¹⁸/cm³. In addition, when the first portion 41 is formed so as not to be doped, there is a case where an unintended impurity such as H or C is containedHowever, this case can also be referred to as undoped. In the present specification, the film thickness or thickness of a certain layer or portion means the shortest distance from the lowermost surface to the uppermost surface of the layer or portion. In the case where the lowermost surface and/or the uppermost surface has local concave portions and/or convex portions such as V-shaped pits, the shortest distance between flat portions having no concave portions and/or convex portions in the lowermost surface and/or the uppermost surface may be set as the film thickness or the thickness of the layer or the portion.

The semiconductor laser element 100 has the following structures (1) to (3). (1) The first portion 41 has: an undoped p-side component-inclined layer 411 whose band gap energy becomes larger toward the upper side; and an undoped p-side intermediate layer 412 disposed above the p-side component inclined layer 411. The lower end of the ridge is located at the p-side intermediate layer 412. (2) The thickness of the second portion 43 is thinner than the thickness of the first portion 41, and the lower end of the ridge 4a is located at the first portion 41. (3) The thickness of the first portion 41 is 400nm or more, and the lower end of the ridge 4a is positioned at the first portion 41. The semiconductor laser element 100 may have only one of the structures (1) to (3), or two or more of the structures may be satisfied at the same time.

First, description is made of (1). As shown in fig. 2A, the p-side component gradient layer 411 is a layer in which the band gap energy increases upward. With such a structure, light confinement to the active layer 3 can be enhanced. The p-side intermediate layer 412 is a layer different from the p-side component inclined layer 411. By providing not only the p-side component gradient layer 411 but also the p-side intermediate layer 412, the film thickness of the first portion 41 can be made thick. This makes it possible to separate the peak of the light intensity from the p-type impurity-containing portions of the electron barrier layer 42 and the second portion 43. By having these structures, the light intensity in the second portion 43 can be reduced, and the absorption loss of light can be reduced. This can improve the efficiency of the semiconductor laser element 100. The efficiency of the semiconductor laser element 100 may be a slope efficiency which is a slope in a characteristic diagram of a current value equal to or greater than a threshold current and an optical output.

In addition, the lower end of the ridge 4a is located deeper than the electron barrier layer 42 and is located in the p-side intermediate layer 412. thus, even if the first portion 41 having a large film thickness is provided, the distance between the lower end of the ridge 4a and the active layer 3 can be shortened, and therefore, the lateral light confinement can be enhanced as compared with the case where the lower end of the ridge 4a is located above the first portion 41. in the case where the lateral light confinement is weak, the horizontal lateral mode of the semiconductor laser element 100 becomes unstable, and kink may occur in the I-L characteristic indicating the relationship between the current and the light output.a formation of the ridge 4a such that the lower end thereof is located in the p-side intermediate layer 412 can enhance the lateral light confinement and stabilize the horizontal lateral mode, and therefore, the probability of kink occurring in the I-L characteristic can be reduced.

Next, (2) will be described. By making the film thickness of the first portion 41 thick, the peak of the light intensity can be made distant from the p-type impurity-containing layer as in (1), and the loss due to free carrier absorption in the p-type impurity-containing layer can be reduced. Therefore, the slope efficiency equivalent ratio of the semiconductor laser element 100 can be improved. In addition, by making the second portion 43 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 lowered by making the film thickness of the portion containing the p-type impurity thin is that: in a nitride semiconductor, a p-type impurity such as Mg has a lower activation rate than an n-type impurity such as Si, and a p-type impurity-containing layer has a higher resistance. The first portion 41 is located between the electron barrier layer 42 and the active layer 3 although undoped, and thus tends to exhibit n-type conductivity rather than complete insulation due to electron overflow or the like. Therefore, it is considered that by making the thickness of the second portion 43 containing the p-type impurity having a high resistance thin and lowering the driving voltage, an effect of suppressing the increase in the driving voltage due to making the thickness of the undoped first portion 41 thick can be obtained. This effect was confirmed in experimental result 2 relating to examples 1 to 3 described later. Further, similarly to (1), since the film thickness of the first portion 41 is thick, the lower end of the ridge 4a is preferably provided in the first portion 41, whereby the light confinement in the lateral direction can be enhanced.

Next, (3) will be described. By making the thickness of the first portion 41 thick, 400nm or more, the loss in the p-type impurity-containing layer can be reduced and the efficiency can be improved as in (1) and (2). Further, by disposing the lower end of the ridge 4a in the first portion 41, the light confinement in the lateral direction can be enhanced as in (1) and (2).

In calculation example 1, a structure In which only a p-side component inclined layer 411 having a film thickness OF 260nm is provided as a first portion 41, and In calculation examples 2 to 5, a structure In which a p-side component inclined layer 411 and a p-side intermediate layer 412 are provided as a first portion 41, and film thicknesses OF the p-side intermediate layer 412 In calculation examples 2 to 5 are respectively set to 50nm, 100nm, 200nm, and 400nm, that is, the thickness OF the first portion 41 In calculation examples 1 to 5 is set to 260nm, 310nm, 360nm, 460nm, and 660nm, respectively, and the structure OF the layer other than the first portion 41 is slightly different from that In example 1, and the second light-side light-emitting layer 27 is substantially different from that In the semiconductor element 100, and the second light-side light-emitting layer 27 is substantially different from that In example 1, and the thickness OF the second light-emitting layer 27 is set to be substantially the same as that In the structure OF the semiconductor element 1, and the second light-emitting device_0.05Ga_0.95A composition gradient layer of N, and the like. The layer structures other than the first portion 41 are the same as in calculation examples 1 to 3, and calculation examples 4 and 5 are the same as in calculation examples 1 to 3 except that the film thickness of the first n-side light guide layer 26 is set to two thirds. The reason why the film thickness of the first n-side light-guiding layer 26 in calculation examples 4 and 5 is made thin is: in order to correct the shift of the peak value of the electric field intensity from the active layer 3 caused by making the first portion 41 thick.

For the calculation examples 1 to 5, the relationship between the thickness of the first portion 41 and the proportion of light leakage to the second portion 43 is shown in fig. 7. As shown in fig. 7, the thicker the thickness of the first portion 41 is, the less light leakage to the second portion 43 is, and the degree of reduction becomes gradually smaller from the vicinity of the thickness 400 nm. Therefore, the thickness of the first portion 41 is preferably 400nm or more. This can reduce light leakage to the second portion 43 to, for example, less than 3%.

< semiconductor laser element 100>

As shown in fig. 1, the semiconductor laser element 100 includes a substrate 1, and an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4 provided thereon. The semiconductor laser element 100 is an end-face light emitting laser element having a light emitting end face and a light reflecting end face intersecting with a main face of a semiconductor layer such as the active layer 3. A ridge 4a is provided on the upper side of the p-side semiconductor layer 4. The ridge 4a is of mesa configuration. The ridge 4a has a shape in plan view which is long in a direction connecting the light emitting end and the light reflecting end, and is, for example, a rectangular shape in which a direction parallel to the light reflecting end face is a short side and a direction perpendicular to the light reflecting end face is a long side. The portion of the active layer 3 immediately below the ridge 4a and the vicinity thereof are optical waveguide regions. The 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 4 a. The substrate 1 is made of, for example, an n-type semiconductor, and an n-electrode 8 is provided on the lower surface thereof. 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 provided thereon.

The semiconductor laser element 100 may have a structure in which laser light in a long wavelength region is oscillated. The semiconductor laser device 100 can oscillate a laser beam of a green wavelength band, for example, a laser beam having a wavelength of 530nm or more. That is, a laser having a peak wavelength of 530nm or more can be oscillated. As the oscillation wavelength increases from the blue wavelength band to the green wavelength band, light leakage from the light guide layer to the outside increases due to the influence of wavelength dispersion of the refractive index. As a result, the threshold current increases, and the current density during laser oscillation increases. Further, as the current density increases, the effective transition interval is expanded by shielding of local energy levels and band filling, and the oscillation wavelength is shifted to a short wavelength. By providing the p-side component-inclined layer 411, the oscillation threshold current density can be reduced as described later, and the shift to a short wavelength can be suppressed. In addition, since the semiconductor laser device of the green wavelength band hardly causes a difference in refractive index between the cladding layer and the active layer due to the influence of wavelength dispersion of the refractive index, the threshold current density is still higher and the slope efficiency is lower than that of the semiconductor laser device of the blue wavelength band. Therefore, if the structure of the present embodiment is adopted for a semiconductor laser element in the green wavelength band, the efficiency improvement effect due to the reduction of the light absorption loss in the p-type semiconductor layer can be expected. Note that light absorption loss in the p-type semiconductor layer may occur regardless of the wavelength of laser light oscillated by the semiconductor laser element 100. Therefore, the wavelength of the laser light oscillated by the semiconductor laser element 100 is not limited to the green wavelength band, and may be, for example, a blue wavelength band.

< substrate 1>

As the substrate 1, for example, a nitride semiconductor substrate made of GaN or the like can be used. Examples of the n-side semiconductor layer 2, the active layer 3, and the p-side semiconductor layer 4 grown on the substrate 1 include semiconductors grown substantially along the c-axis direction. For example, each semiconductor layer can be grown on the + c plane using a GaN substrate having the + c plane ((0001) plane) as a main surface. Here, the case where the + c plane is a main surface may include a case where the deviation angle is within ± 1 degree. By using a substrate having the + c plane as the main surface, the advantage of excellent mass productivity can be obtained.

< n-side semiconductor layer 2>

The n-side semiconductor layer 2 may have a multilayer structure made of nitride semiconductors such as GaN, InGaN, and AlGaN. The n-side semiconductor layer 2 includes one or more n-type semiconductor layers. Examples of the n-type semiconductor layer include a layer made of a nitride semiconductor containing an n-type impurity such as Si or Ge. The n-side semiconductor layer 2 may have an n-side cladding layer and an n-side light-conductive layer, and may include other layers. The n-side cladding layer has a larger band gap energy than the n-side optical waveguide layer. Although not as good as the p-type impurity, the n-type impurity is also a cause of light absorption, and therefore the n-side light-guiding layer is preferably undoped or the n-type impurity concentration is smaller than the n-side cladding layer.

An example of the layer structure of the n-side semiconductor layer 2 is shown in fig. 4. The n-side semiconductor layer 2 shown in fig. 4 has, in order from the substrate 1 side, a substrate layer 21, a first n-side cladding layer 22, an anti-cracking layer 23, an intermediate layer 24, a second n-side cladding layer 25, a first n-side light-guiding layer 26, a second n-side light-guiding layer 27, and a hole-blocking layer 28. The hole blocking layer 28 includes a first hole blocking layer 281 and a second hole blocking layer 282.

N-type impurities are added from the substrate layer 21 to the first n-side light guide layer 26. The substrate layer 21 is, for example, an n-type AlGaN layer. The first n-side clad layer 22 is, for example, a layer having a larger band gap energy than the substrate layer 21 and added with an n-type impurity. The crack prevention layer 23 is made of, for example, InGaN, and has a smaller band gap energy than the well layer in the active layer 3. By providing the crack prevention layer 23, the probability of occurrence of cracks can be reduced. The intermediate layer 24 has a lattice constant between the crack prevention layer 23 and the second n-side cladding layer 25, and is composed of GaN, for example. In the case where the crack prevention layer 23 is an InGaN layer, it is preferable to form the intermediate layer 24 of a GaN layer before growing the second n-side clad layer 25. If the second n-side cladding layer 25 grows in contact with the upper surface of the crack prevention layer 23, part of the crack prevention layer 23 may be decomposed to affect the growth of the active layer 3, but the probability of occurrence of such decomposition can be reduced by providing the intermediate layer 24. The intermediate layer 24 is provided to have a film thickness smaller than that of the crack prevention layer 23, for example. The second n-side clad layer 25 may be the same as the first n-side clad layer 22, for example, a layer having a larger band gap energy than the base layer 21. The first n-side clad layer 22 and the second n-side clad layer 25 are made of AlGaN, for example. One or both of the first n-side clad layer 22 and the second n-side clad layer 25 may have the largest band gap energy in the n-side semiconductor layer 2. The composition and/or n-type impurity concentration of the first n-side clad layer 22 and the second n-side clad layer 25 may also be the same. The n-side cladding layer may also be a single layer, in which case the crack-preventing layer may also not be provided, or a crack-preventing layer may also be provided above or below the n-side cladding layer.

The first n-side light-guiding layer 26 is a layer having a smaller band gap energy and a smaller n-type impurity concentration than the first n-side cladding layer 22 and the second n-side cladding layer 25. First n-side light guide layer 26 is made of GaN, for example. The band gap energy of the second n-side light-guiding layer 27 is larger than the band gap energy of the well layer in the active layer 3 and smaller than the band gap energy of the first n-side light-guiding layer 26. Since second n-side light guide layer 27 is located closer to active layer 3 than first n-side light guide layer 26, the n-type impurity concentration is preferably lower than first n-side light guide layer 26 in order to reduce light absorption loss. Second n-side light guide layer 27 is made of, for example, undoped InGaN.

The second n-side light guide layer 27 may be a composition-inclined layer having smaller band gap energy as it approaches the active layer 3. In the case where a component-inclined layer is provided as the n-side light-guiding layer, the component is changed stepwise so that the refractive index increases as the component approaches the active layer 3. This allows the n-side component-inclined layer to continuously form a barrier of the optical waveguide, thereby enhancing light confinement to the active layer 3. As a reference for determining the relationship between the band gap energy and the impurity concentration of the component gradient layer and the size of the other layer, an average value of the component gradient layer can be used. The average value of the composition gradient layer is a value obtained by dividing the total value of the product of the bandgap energy and the like of each sublayer constituting the composition gradient layer and the film thickness by the total film thickness. When a composition-gradient layer having a larger lattice constant is provided in the n-side semiconductor layer 2 closer to 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 may be formed of a plurality of sub-layers having gradually different compositions. Therefore, in the component gradient layer, even if the component change rate is small, it is difficult to avoid the generation of fixed charges. Since the fixed charges can be shielded by adding the n-type impurity, the voltage rise due to the generation of the fixed charges can be reduced.

The hole blocking layer 28 preferably contains an n-type impurity at least in a part thereof. This enables holes to be blocked more effectively. For example, the first hole blocking layer 281 is composed of GaN, and the second hole blocking layer 282 is composed of InGaN.

< active layer 3>

The active layer 3 may have a multilayer structure made of a nitride semiconductor such as GaN or InGaN. The active layer 3 has a single quantum well configuration or a multiple quantum well configuration. It is considered that a sufficient gain can be obtained more easily in the multiple quantum well configuration than in the single quantum well configuration. When the active layer 3 has a multiple quantum well structure, it includes a plurality of well layers and an intermediate barrier layer sandwiched between the well layers. For example, the active layer 3 includes a well layer, an intermediate barrier layer, and a well layer in this order from the n-side semiconductor layer 2 side. The n-side barrier layer 31 may be provided between the well layer closest to the n-side semiconductor layer 2 and the n-side semiconductor layer 2. The n-side barrier layer 31 may also function as a part of the hole blocking layer 28. The n-side barrier layer 31 may be omitted, and the hole blocking layer 28 and the n-side light guide layer (second n-side light guide layer 27) may function as an n-side barrier layer. Similarly, a p-side barrier layer may be provided between the well layer closest to the p-side semiconductor layer 4 and the p-side semiconductor layer 4, and when the p-side barrier layer is not provided or is thin, a part of the p-side semiconductor layer 4 may function as a p-side barrier layer. When the p-side barrier layer is provided in the active layer 3, the film thickness of the p-side barrier layer is, for example, 5nm or less. In other words, the shortest distance between the p-side semiconductor layer 4 and the well layer in the active layer 3 is, for example, 5nm or less. In addition, as described above, since the light absorption loss increases when the p-type impurity is added, the active layer 3 is preferably formed without adding the p-type impurity. Each layer of the active layer 3 is, for example, an undoped layer.

In the case of using a semiconductor laser element having an oscillation wavelength of 530nm or more_xGa_1-XThe In composition ratio x of the N well layer is slightly increased or decreased depending on the layer structure other than the active layer 3, and is, for example, 0.25 or more. The upper limit of the In composition ratio x of the well layer is, for example, 0.50 or less. In this case, the oscillation wavelength of the semiconductor laser element is considered to be about 600nm or less.

< p-side semiconductor layer 4>

The p-side semiconductor layer 4 may have a multilayer structure made of nitride semiconductors such as GaN, InGaN, and AlGaN. The p-side semiconductor layer 4 may have a p-side cladding layer and a p-side light-conducting 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, and therefore the cladding layer may not be provided 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 an n-type impurity such as Si, free carrier absorption loss due to the p-type impurity increases in the p-type semiconductor layer. The greater the absorption lossThe slope efficiency of the semiconductor laser element 100 decreases, and generally, α is internally lost_iIncluding free carrier absorption loss α_fcAbsorption of free carriers to loss α_fcThe internal loss other than α_intThe threshold mode gain required for laser oscillation can be expressed by the following model equation attached to the free carrier absorption loss α herein_fc、α_iAnd α_mFree carrier absorption loss, average internal loss, and mirror loss were used. Note that, for convenience, the mode distributions are marked evenly regardless of the mode distribution. Is the light confinement coefficient in the active region, g_thThe threshold gain of the laser oscillation is marked.

g_th＝α_fc+α_int+α_m

The free carrier absorption loss here also includes losses other than the active layer 3. For example, in the p-type semiconductor layer, the p-type impurity concentration n and the coefficient σ reflecting the free carrier absorption cross-sectional area can be set to be_fcAverage light leakage to p-type semiconductor layer_pThat is, even if the impurity concentration of the p-type semiconductor layer is the same, when light leakage to the p-type semiconductor layer increases, free carrier absorption loss α_fcAlso, even if the light leakage to the p-type semiconductor layer is the same, free carrier absorption loss α increases when the impurity concentration of the p-type semiconductor layer increases_fcThere is a concern that the driving voltage rises significantly when the p-type impurity concentration falls, and therefore α is intended to reduce the free carrier absorption loss_fcIt is particularly effective to reduce light leakage to a layer having a high p-type impurity concentration.

a_fc＝n×σ_fc×_p

As can be understood from the above equation, when light leakage into the p-type cladding layer increases, the free carrier absorption loss increases, and therefore the threshold gain g increases_thAnd (4) increasing. In laser oscillation, g is equal to g in the laser resonator_thIn a steady state. In such a steady state, the mode gain is monotonically dependent on the carrier density, and therefore the laser oscillation threshold current is set to be equal toUpper carrier density is clamped at a threshold carrier density N_thThe higher the injected carrier density, the more the local energy level is shielded, the more the substantial band gap energy is easily increased, and the more easily the laser oscillation wavelength is shifted to the short wavelength side, and the free carrier absorption loss α is caused_fcDecrease, reach threshold gain g at a lower current_thCan make the threshold current density j_thAnd threshold carrier density N_thGoing low together. This reduces the density of injected carriers, suppresses the shielding of local energy levels, and enables laser oscillation on the long wavelength side. Therefore, from this point of view, it is particularly preferable to reduce free carrier absorption loss in the semiconductor laser element 100 capable of oscillating laser light of a long wavelength, for example, 530nm or longer. In addition, it can be said that there are advantages in the following points: even in a semiconductor laser element on a shorter wavelength side than this, a laser light source with a low threshold current can be obtained.

Note that although the suppression of the shielding of the local energy level is described here, the suppression of the band filling effect is also the same, that is, the band filling effect in which the quasi-fermi level is moved away from the band end by the current injection to expand the effective transition gap also causes the generation of the short-wavelength shift, but the free carrier absorption loss α is caused_fcThis problem can be suppressed by lowering the threshold carrier density.

< first part 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 is a portion that does not contain the p-type semiconductor layer, and if the concentration and film thickness of the p-type impurity are such that the degree of free carrier absorption loss is not affected, the p-type impurity-containing layer may be contained in a part of the first portion 41, however, when Mg that is necessary for making p-type is doped, 1 × 10 is necessary¹⁸/cm³In this case, the p-type impurity of the above degree is highly likely to increase the free carrier absorption loss. Therefore, it is preferable that the first portion 41 is a portion not including the p-type semiconductor layer. Preferably, the p-type impurity concentration of the entire first portion 41 is lowTo the extent that p-type impurities are not detected by SIMS or the like analysis. For example, the entire first portion 41 is intentionally formed without adding p-type impurities at the time of manufacturing. As described above, the thicker the thickness of the first portion 41 is, the more the light leakage to the second portion 43 can be reduced, and therefore the thickness of the first portion 41 is preferably 400nm or more. The upper limit of the thickness of the first portion 41 can be set to a level that does not interfere with the supply of holes from the second portion 43. As shown in experimental result 3 described later, the thicker the thickness of the first portion 41 is, the more the electrons that overflow, and therefore, from this viewpoint, it is preferable that the thickness of the first portion 41 is thin. The thickness of the first portion 41 can be set to 660nm or less, for example. In addition, the first portion 41 has a band gap difference between the first portion 41 and the electron barrier layer 42, so that the probability of occurrence of electron overflow can be reduced. Therefore, it is preferable that the first portion 41 has a layer having a smaller band gap energy than the electron barrier layer 42 as a layer in contact with the electron barrier layer 42.

Note that when the first portion 41 is not undoped but doped at a low concentration, the entire portion is preferably made to have a p-type impurity concentration lower than the p-type impurity concentration of the electron barrier layer 42, and the entire portion is preferably made to have a p-type impurity concentration lower than the p-type impurity concentration of either the electron barrier layer 42 or the second portion 43, and the n-type impurity concentration of the first portion 41 is preferably less than 2 × 10¹⁸/cm³. Preferably, the first portion 41 is set to an n-type impurity concentration (that is, of a background value) as low as the n-type impurity is not detected by SIMS or the like analysis. In other words, it is preferable that the first portion 41 contains substantially no n-type impurity.

< p-side inclined component layer 411, p-side intermediate layer 412>

As shown in fig. 2A, the first portion 41 may have a p-side component inclined layer 411 and a p-side intermediate layer 412. The p-side intermediate layer 412 is provided above the p-side component inclined layer 411. The p-side intermediate layer 412 may be disposed so as to be in contact with the upper surface of the p-side component gradient layer 411, or may be disposed so as to be in contact with the lower surface of the electron barrier layer 42. The structure of the first portion 41 is not limited to the structure having both the p-side component gradient layer 411 and the p-side intermediate layer 412, but as described above, the light confinement to the active layer 3 can be enhanced by having the p-side component gradient layer 411, and the thickness of the first portion 41 can be further increased by having the p-side intermediate layer 412. The confinement of light to the active layer 3 is enhanced by the p-side component-inclined layer 411, whereby the laser oscillation threshold current density can be reduced. This can suppress the local energy level from being masked, and can suppress the oscillation wavelength from shifting to a shorter wavelength with an increase in current injection, which is advantageous for increasing the oscillation wavelength.

The p-side component gradient layer 411 is a layer whose band gap energy increases upward. The P-side component inclined layer 411 has an upper surface and a lower surface, and the band gap energy thereof becomes larger from the lower surface toward the upper surface. The band gap energy is smaller on the lower surface side than on the upper surface side. In fig. 2A, the p-side component-inclined layer 411 is illustrated in a ramp shape, and as described later, it can be said that it is an aggregate of a plurality of sublayers whose components are different from each other, and therefore, in the p-side component-inclined layer 411, it can be said that the band gap energy increases stepwise from the lower surface toward the upper surface. The n-side semiconductor layer 2 may be provided with an n-side component gradient layer paired with the p-side component gradient layer 411. As such an n-side component gradient layer, a layer in which the band gap energy becomes smaller as it approaches the active layer 3 can be cited. For example, the p-side component gradient layer 411 and the n-side component gradient layer are formed symmetrically with the active layer 3 interposed therebetween. By providing the component-inclined layers on both sides of the active layer 3 in this manner, light can be confined to the active layer 3 in a balanced manner from both sides. In order to enhance the light confinement effect by the p-side component inclined layer 411, the p-side component inclined layer 411 is preferably disposed in the vicinity of the active layer 3. Therefore, the p-side component gradient layer 411 is preferably disposed so as to be in contact with the active layer 3. Further, the shortest distance between the p-side component inclined layer 411 and the well layer 32 in the active layer 3 is preferably 5nm or less.

The p-side component inclined layer 411 functions as, for example, a p-side light guide layer. The thickness of the p-side component inclined layer 411 is larger than the thickness of the well layer 32, and when the p-side barrier layer 34 is present, the thickness of the p-side component inclined layer 411 is larger than the thickness of the p-side barrier layer 34. In order to enhance the light confinement effect, the p-side component inclined layer 411 preferably has a film thickness of 200nm or more. The thickness of the p-side component-inclined layer 411 may be 500nm or less, preferably 350nm or less, and more preferably 300nm or less. In the case where the p-side barrier layer 34 is provided, the band gap energy of the lower end of the p-side component inclined layer 411 is preferably smaller than that of the p-side barrier layer 34. The band gap energy at the upper end of the p-side component inclined layer 411 may be the same as that of the p-side barrier layer 34 or may have a band gap energy above this level. In order to suppress electron overflow while bringing light closer to the active layer 3, the p-side component inclined layer 411 preferably has a structure in which the refractive index decreases monotonically from the active layer 3 side toward the electron barrier layer 42 side, and the band gap energy increases monotonically from the active layer 3 side toward the electron barrier layer 42 side.

As shown in fig. 5, the p-side component gradient layer 411 may be said to be composed of a plurality of sublayers 411a, 411b, 411c, 411y, and 411z having different components. Fig. 5 is a partial enlarged view of the p-side component-inclined layer 411 and its vicinity, and a plurality of sublayers not explicitly shown are present between the sublayer 411c and the sublayer 411 y. In the case where the p-side composition gradient layer 411 is formed of InGaN or GaN, the lowermost sublayer 411a of the p-side composition gradient layer 411 is formed of In_aGa_1-aN(0<a<1) The uppermost sublayer 411z of the p-side component-inclined layer 411 is composed of In_zGa_1-zN(0≦z<a) And (4) forming. The upper limit of the In composition ratio a is, for example, 0.25. In consideration of suppression of deterioration of crystallinity, it is preferable that the In content ratio a is 0.1 or less. In addition, the lattice constant difference between adjacent sublayers is preferably small. Thereby, strain can be reduced. For this reason, the p-side component-inclined layer 411 preferably has a small thickness and a small change in the component. Specifically, it is preferable that the p-side component inclined layer 411 decreases In the In component ratio once per 25nm or less film thickness from the lower surface to the upper surface. That is, the sub-layers 411a, 411b, 411c, 411y, and 411z preferably have a film thickness of 25nm or less. More preferably, the sub-layers 411a, 411b, 411c, 411y, and 411z have a film thickness of 20nm or less. The lower limit of the film thickness of each of the sublayers 411a, 411b, 411c, 411y, and 411z is, for example, about one atomic layer (about 0.25 nm). In addition, it is preferable that the difference between the In composition ratios of the adjacent sublayers (for example, the sublayer 411a and the sublayer 411b) is 0.005 or less. More preferably 0.001 or less. The lower limit value is, for example, about 0.00007.

It is preferable that such a range is satisfied in the entire p-side component-inclined layer 411. That is, it is preferable that all the sublayers are within such a range. For example, In the p-side composition gradient layer 411 having a film thickness of 260nm, In is the lowermost sub-layer 411a_0.05Ga_0.95N, and GaN is used as the uppermost seed layer 411z, the growth is performed under a manufacturing condition in which the composition gradually changes in 120 steps. In the p-side component-inclined layer 411, the number of times of component change is preferably 90 or more. The composition change rate of the p-side composition-sloped layer 411 (i.e., the difference between the composition ratios of adjacent sublayers) may be constant or may vary throughout the p-side composition-sloped layer 411. Preferably, the component change rate of the p-side component gradient layer 411 is 0.001 or less in the entire p-side component gradient layer 411. When the component-inclined layer is provided on the n-side, the preferable ranges of the component, the rate of change of the component, and the film thickness can be the same as those of the p-side component-inclined layer 411.

It is preferable that the lower end of the ridge 4a is not located on the p-side component inclined layer 411. This is because if the lower end of the ridge 4a is disposed in the p-side component gradient layer 411, the difference in effective refractive index between the inside and outside of the ridge 4a is greatly varied due to the variation in depth of the ridge 4a, whereas if the lower end of the ridge 4a is located in a layer of a single component, the difference in effective refractive index between the inside and outside of the ridge 4a can be made smaller than this. Therefore, when the p-side component inclined layer 411 is provided, it is preferable to provide a single-component layer for arranging the lower end of the ridge 4 a. Preferably, the single-component layer has a film thickness larger than the variation in depth when the ridge 4a is formed. Thus, even if the depth of the ridge 4a varies, the lower end of the ridge 4a can be positioned in the single-component layer, and therefore variation in the difference in effective refractive index between the inside and outside of the ridge 4a can be made small. The thickness of the single-component layer for disposing the lower end of the ridge 4a is preferably thicker than the sub-layers constituting the component gradient layer, and may be thicker than 25nm, for example. The thickness of the single-component layer can be, for example, 600nm or less. Note that a layer having a single component refers to a layer formed without intentionally changing the component.

The p-side intermediate layer 412 may be provided as such a single-component layer. The p-side intermediate layer 412 may have a multilayer structure. When the p-side intermediate layer 412 has a multilayer structure, at least the layer at the lower end of the ridge 4a in the p-side intermediate layer 412 may be a single-component layer. As shown in fig. 2B, in the case where the p-side intermediate layer 412 has a multilayer configuration, as the p-side intermediate layer 412, there may be a first layer 412A and a second layer 412B. The first layer 412A has a band gap energy larger than the average band gap energy of the p-side component-inclined layer 411 and smaller than the band gap energy of the electron barrier layer 42. The second layer 412B has a band gap energy larger than that of the first layer 412A and smaller than that of the electron barrier layer 42. The first layer 412A and the second layer 412B are undoped. In addition, as the relation of the refractive index, the average refractive index of the p-side component-inclined layer 411, the refractive index of the first layer 412A, and the refractive index of the second layer 412B may be made smaller in this order. Note that in this specification, the average bandgap energy is a value obtained by dividing the total value of the product of the bandgap energy and the film thickness of each layer by the total film thickness. In the composition gradient layer, the band gap energy of each sub-layer constituting the composition gradient layer is multiplied by the film thickness, and the value obtained by dividing the total value by the total film thickness is set as the average band gap energy of the composition gradient layer. The average refractive index and the average composition ratio are also the same.

By providing the second layer 412B, light leakage into the second portion 43 can be reduced, and free carrier absorption loss occurring in the second portion 43 can be reduced. By providing the second layer 412B having a lower refractive index than the first layer 412A as the p-side intermediate layer 412, the film thickness of the p-side intermediate layer 412 required to obtain the same degree of light confinement effect can be made thinner than in the case where the p-side intermediate layer 412 is constituted by only the first layer 412A. As described above, the thickness of the first portion 41 is preferably large in order to reduce light leakage to the second portion 43, and it is effective to make the thickness of the first portion 41 small in order to further reduce the voltage. By providing the p-side component inclined layer 411 and the second layer 412B, voltage rise can be suppressed while light leakage to the second portion 43 is reduced.

In the case where the first layer 412A and the second layer 412B are each a single-component layer, the lower end of the ridge 4a is preferably located at the first layer 412A or the second layer 412B. Thus, even if the position of the lower end of the ridge 4a varies during manufacturing as described above, the variation in the effective refractive index difference between the inside and outside of the ridge 4a can be made small. The lower end of the ridge 4a may be located in the first layer 412A as shown in fig. 2B and 2C, or may be located in the second layer 412B as shown in fig. 2D. The first layer 412A is, for example, a GaN layer. The second layer 412B is, for example, an AlGaN layer. In this case, the Al composition ratio of the second layer 412B may be set to, for example, 0.01% or more and 10% or less. The film thickness of the second layer 412B may be 1nm or more and 600nm or less. When the refractive index of the second layer 412B is smaller than the refractive index of the first layer 412A, the film thickness of the second layer 412B is preferably larger than the film thickness of the first layer 412A. This can further enhance the light confinement to the active layer 3. For example, the thickness of the second layer 412B is set to be 50nm or more thicker than the thickness of the first layer 412A. In this case, since the first layer 412A has less influence on optical confinement than the p-side component inclined layer 411 and the second layer 412B, by making the film thickness of the first layer 412A thin, it is possible to reduce light leakage to the second portion 43 and reduce electrons overflowing from the active layer 3. This can improve the slope efficiency of the semiconductor laser element. From this viewpoint, the film thickness of the first layer 412A is preferably 100nm or less, and more preferably 50nm or less. The thickness of the first layer 412A is preferably equal to or less than half, more preferably equal to or less than one-fourth, the thickness of the second layer 412B. The first layer 412A may have a film thickness of 1nm or more.

As shown in fig. 2B, the second layer 412B may have a band gap energy larger than that of a layer (the lower p-type semiconductor layer 431 in fig. 2B) in the second portion 43 in contact with the electron barrier layer 42. Thus, by providing the layer having a larger band gap energy in the first portion 41, the following advantages can be considered: light leakage to the second portion 43, which is a portion having a larger absorption loss than the first portion 41, can be reduced. In this case, it is preferable to use a material that functions as a clad layer for the p-electrode 6. Thus, the second portion 43 does not need to be provided with a p-type clad layer, and thus the voltage at the time of applying a bias voltage can be reduced. These will be described in detail below.

First, in the case where the p-electrode 6 functions as a cladding layer, a light-transmissive material is used as the p-electrode 6, but even a light-transmissive material may cause absorption loss. Therefore, if light leakage to the p-electrode 6 is large and it is desired to reduce this, a layer functioning as a p-type clad layer, for example, a p-type layer containing Al, is provided in the second portion 43. The Al composition ratio is preferably large in order to function as a p-type clad layer, but on the other hand, the activation energy required to activate the p-type impurity of the layer increases as the Al composition ratio increases. When the p-type of the second portion 43 is insufficient, the series resistance increases and the voltage increases when a bias is applied, so that when the p-type clad layer is provided in the second portion 43, the acceptor concentration increases by, for example, increasing the amount of p-type impurity added. However, as described above, if the amount of the p-type impurity added increases, the light absorption loss increases, and the light output decreases. In the structure in which an undoped AlGaN layer is provided between the active layer 3 and the electron barrier layer 42 as shown in fig. 2B, the AlGaN layer does not need to be made p-type, and thus the voltage is less likely to increase even if the Al composition ratio is increased. In addition, since the AlGaN layer is undoped, the light absorption loss is also hard to increase. Note that the undoped layer is generally a high-resistance layer, and when this is provided, the voltage tends to rise, but the provision of the undoped AlGaN layer in the first portion 41 is different from this general tendency. This is believed to be due to: when a bias is applied to the undoped AlGaN layer disposed between the active layer 3 and the electron barrier layer 42, the donor is a main body, and an acceptor having an activation energy larger than that of the donor acts as a minority carrier. Therefore, an AlGaN layer having a relatively large Al composition can be provided in the first portion 41 as in the second layer 412B. In other words, a layer having a larger band gap energy may be provided in the first portion 41. Since light leakage to the second portion 43 can be reduced by providing such a layer, the p-type clad layer may not be provided in the second portion 43. That is, the Al composition ratio of the second portion 43 can be set small. This can reduce the series resistance of the second portion 43, and can reduce the voltage of the semiconductor laser element 100.

Fig. 3A to 3C schematically show examples of the relationship of the band gap energy among the uppermost layer of the first portion 41, the electron barrier layer 42, and the lowermost layer of the second portion 43. The uppermost layer of the first portion 41 is in contact with the lower surface of the electronic barrier layer 42, and the lowermost layer of the second portion 43 is in contact with the upper surface of the electronic barrier layer 42. In fig. 3A, the band gap energy of the lowermost layer is smaller than that of the uppermost layer. In fig. 3B, the band gap energy of the lowermost layer is equal to that of the uppermost layer. In fig. 3C, the band gap energy of the lowermost layer is larger than that of the uppermost layer. For the above reasons, as shown in fig. 3A, it is preferable that the band gap energy of the lowermost layer (for example, the lower p-type semiconductor layer 431) of the second portion 43 is smaller than the band gap energy of the uppermost layer (for example, the second layer 412B) of the first portion 41. This can reduce light leakage to the second portion 43, and is suitable for a structure in which a material functioning as a cladding layer is used for the p-electrode 6. With this structure, the voltage of the semiconductor laser element 100 can be reduced when a bias is applied. As for the magnitude relation of the band gap energy, when the uppermost layer and/or the lowermost layer is a layer in which the band gap energy is not constant, such as a superlattice layer or a composition-inclined layer, the magnitude relation may be compared by using the average band gap energy thereof. In the superlattice layer, the average bandgap energy of the superlattice layer is defined as a value obtained by multiplying the thickness of each of the sublayers constituting the superlattice layer by the bandgap energy of each of the sublayers constituting the superlattice layer and dividing the total value by the total thickness of the superlattice layer. In the case where the uppermost layer and the lowermost layer are AlGaN layers, the magnitude relationship of the band gap energies can be changed to the magnitude relationship of the Al composition ratio.

The lowermost layer may be an AlGaN layer having an Al composition ratio of 4% or less. The lowermost layer may be a GaN layer having an Al composition ratio of substantially zero. The lowermost layer may be a p-type semiconductor layer containing a p-type impurity such as Mg, for example. The lowermost layer may be a quaternary layer such as AlInGaN. In order to lower the voltage, it is preferable that the average Al composition ratio of the second portion 43 including the lowermost layer is 4% or less. The uppermost layer provided on the first portion 41 preferably contains 0.01% or more of Al in a part or the whole. More preferably, the average Al composition ratio of the uppermost layer is set to more than 4%. The uppermost layer may have a band gap energy larger than that of each layer (may be a single layer) constituting the second portion 43. The uppermost layer may be a superlattice layer or a composition gradient layer including AlGaN or AlInGaN. Note that one or more layers connecting the uppermost layer to the active layer 3 may be each a layer having a band gap energy smaller than that of the uppermost layer. The one or more layers are, for example, the first layer 412A and the p-side component gradient layer 411 shown in fig. 2B, but these layers may not be provided.

The semiconductor laser device 100 may have the following structure. The semiconductor device has an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4, each of which is composed of a nitride semiconductor, in this order from the upper side, and the p-side semiconductor layer 4 is provided with a ridge 4a protruding upward. The p-side semiconductor layer 4 has: an undoped first portion 41 which is disposed in contact with the upper surface of the active layer 3 and has one or more semiconductor layers; an electron barrier layer 42 which is disposed so as to be in contact with the upper surface of the first portion 41, has a larger band gap energy than the first portion 41, and contains a p-type impurity; and a second portion 43 which is arranged to be grounded on the upper surface of the electron barrier layer 42 and has one or more p-type semiconductor layers containing p-type impurities. The first portion 41 has an uppermost layer contacting the lower surface of the electronic barrier layer 42, and the second portion 43 has a lowermost layer contacting the upper surface of the electronic barrier layer 42, the lowermost layer having a smaller band gap energy than the uppermost layer. The lower end of the ridge 4a is located at the first portion 41.

< electronic Barrier layer 42>

The electron barrier layer 42 contains p-type impurities such as Mg. The band gap energy of the electron barrier layer 42 is larger than that of the first portion 41. In the case where the first portion 41 has a multilayer structure as described above, the electron barrier layer 42 is formed as a layer having a larger band gap energy than any of the layers constituting the first portion 41. By providing the electron barrier layer 42 with such a large band gap energy, the electron barrier layer 42 can function as a barrier against electrons overflowing from the active layer 3. Preferably, the band gap energy difference between the electron barrier layer 42 and the uppermost layer of the first portion 41 is 0.1eV or more. The difference in band gap energy between them can be set to, for example, 1eV 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 have a film thickness smaller than that of the p-side component gradient layer 411. The electron barrier layer 42 may have a multilayer structure. In this case, the electron barrier layer 42 has a layer having a band gap energy larger than that of any one of the layers constituting the first portion 41. For example, as shown in fig. 2A and the like, a first electron barrier layer 42A and a second electron barrier layer 42B may be provided. Note that, in the case where the first portion 41 and the electron barrier layer 42 have a superlattice layer, the magnitude relationship is compared by using the average bandgap energy of the superlattice layer, instead of using the bandgap energy of each layer constituting the superlattice layer. The electron barrier layer 42 is made of AlGaN, for example. When the electron barrier layer 42 is AlGaN, the Al composition ratio thereof may be set to 8 to 30%. The thickness of the electron barrier layer 42 may be, for example, 5nm or more and 100nm or less.

As shown in fig. 2C and 2D, it is preferable that the shortest distance from the bottom surface of the ridge 4a to the electronic barrier layer 42 is longer than the shortest distance from the upper surface of the ridge 4a to the electronic barrier layer 42, and by such a disposition, the peak of the light intensity can be kept away from the portion containing the p-type impurity such as the electronic barrier layer 42, and the distance between the lower end of the ridge 4a and the active layer 3 can be shortened, and the shortest distance from the bottom surface of the ridge 4a to the electronic barrier layer 42, when viewed in cross section as shown in fig. 1, refers to the shortest distance from a virtual straight line connecting the lower end and the lower end of the ridge 4a to the lower surface of the electronic barrier layer 42.

< second portion 43>

The second portion 43 has one or more p-type semiconductor layers containing p-type impurities, and the concentration of the p-type impurities in the p-type semiconductor layer of the second portion 43 can be set to, for example, 1 × 10¹⁸/cm³Above, and can be set to 1 × 10²²/cm³The following. As described above, the driving voltage can be reduced by making the thickness of the second portion 43 thin, and therefore the thickness of the second portion 43 is preferably 260nmThe following. The thickness of the second portion 43 can be set to 10nm or more. The second portion 43 may include an undoped layer, but when the undoped layer is present in the second portion 43, the resistance of the second portion 43 becomes higher, and therefore, it is preferable that the entire second portion 43 contains a p-type impurity. In the case of a superlattice layer, since the average p-type impurity concentration thereof 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 stacked structure of an undoped layer and a p-type impurity-containing layer.

As shown in fig. 2A, the second portion 43 may have an upper p-type semiconductor layer 432 and a lower p-type semiconductor layer 431. The upper p-type semiconductor layer 432 constitutes the upper surface of the ridge 4 a. That is, the upper p-type semiconductor layer 432 is the uppermost layer of the second portion 43 and is the uppermost layer of the ridge 4 a. The upper p-type semiconductor layer 432 functions as a p-side contact layer. The lower p-type semiconductor layer 431 is disposed between the upper p-type semiconductor layer 432 and the electron barrier layer 42, and has a larger band gap energy than the upper p-type semiconductor layer 432.

The lower p-type semiconductor layer 431 is made of AlGaN, for example. The upper p-type semiconductor layer 432 is made of GaN, for example. Preferably, the lower p-type semiconductor layer 431 has a band gap energy between the electron barrier layer 42 and the upper p-type semiconductor layer 432. AlGaN containing a p-type impurity is more likely to have higher resistance than GaN containing a p-type impurity, and therefore, it is preferable that the upper p-type semiconductor layer 432 be a GaN layer to which a p-type impurity is added. Further, by providing the lower p-type semiconductor layer made of AlGaN below the upper p-type semiconductor layer 432, light confinement to the active layer 3 can be enhanced as compared with the case where the second portion 43 is formed only of a GaN layer. Further, by making the Al composition ratio of the lower p-type semiconductor layer 431 smaller than that of the electron barrier layer 42, the resistance of the lower p-type semiconductor layer 431 can be made lower than that of the electron barrier layer 42. The lower p-type semiconductor layer 431 may function as a p-side cladding layer. The lower p-type semiconductor layer 431 may be a p-type GaN layer, and thus the resistance of the second portion 43 may be set to be lower. In this case, the p-electrode is preferably formed using a material such as ITO which functions as a cladding layer.

The thickness of the upper p-type semiconductor layer 432 can be set to 5 to 30nm, for example. The film thickness of the lower p-type semiconductor layer 431 can be set to 1 to 260nm, for example. The film thickness of the lower p-type semiconductor layer 431 may be thinner than the film thickness of the p-side intermediate layer 412, and may be thinner than the film thickness of the second layer 412B. Both the lower p-type semiconductor layer 431 and the second layer 412B may be AlGaN layers, and the Al composition ratios thereof may be the same. The film thickness of the lower p-type semiconductor layer 431 is thicker than the film thickness of the electron barrier layer 42, for example. Therefore, in order to reduce the free carrier absorption loss, it is preferable that the p-type impurity concentration of the lower p-type semiconductor layer 431 is lower than that of the electron barrier layer 42.

< insulating film 5, n-electrode 8, p-electrode 6, p-side pad electrode 7>

The insulating film 5 may be formed of a single-layer film or a multi-layer film of an oxide, nitride, or the like of Si, Al, Zr, Ti, Nb, Ta, or the like, for example. The n-electrode 8 is provided on substantially the entire lower surface of the n-type substrate 1, for example. The p-electrode 6 is provided on the upper surface of the ridge 4 a. When the p-electrode 6 is narrow, the p-side pad electrode 7 having a width larger than that of the p-electrode 6 may be provided on the p-electrode 6, and a lead or the like may be connected to the p-side pad electrode 7. Examples of the material of each electrode include a single-layer film or a multi-layer film of a metal or an alloy such as Ni, Rh, Cr, Au, W, Pt, Ti, and Al, a conductive oxide containing at least one substance selected from Zn, In, and Sn, and the like. Examples of the conductive Oxide include ito (indium Tin Oxide), izo (indium Zinc Oxide), GZO (Gallium-doped Zinc Oxide), and the like. The thickness of the electrode is generally sufficient if it functions as an electrode of the semiconductor element. For example, it is about 0.1 μm to 2 μm.

Preferably, the p-electrode 6 is a transparent conductive film having a refractive index smaller than that of the active layer 3. This can function as a coating layer. More preferably, the p-electrode 6 is a transparent conductive film having a refractive index smaller than that of the second portion 43. Thereby, the light confinement effect can be more obtained. In the case where the p-side clad layer is provided in the second portion 43, for example, the AlGaN layer having a high Al composition ratio and a p-type impurity added thereto is used as the p-side clad layer. If the p-electrode 6 is caused to function as a clad layer, the p-side clad layer may not be provided in the second portion 43, or the Al composition ratio thereof may be set low even if the p-side clad layer is provided. This can reduce the resistance and the drive voltage of the semiconductor laser element 100. The p-electrode 6 functioning as the clad layer is, for example, a p-electrode 6 made of ITO.

< production method >