Semiconductor laser element

Abstract

The present invention provides a semiconductor laser element which is capable of unifying or almost unifying the vertical modes of an oscillation wavelength. The present invention specifically provides a semiconductor laser element which is provided with a nitride semiconductor multilayer body that has a first end face, a second end face and an optical waveguide, wherein: the nitride semiconductor multilayer body comprises a first n-side nitride semiconductor layer, a second n-side nitride semiconductor layer, an active layer and a p-side nitride semiconductor layer; the first n-side nitride semiconductor layer comprises a diffraction grating part that is provided with a periodic structure in which the refractive index periodically changes in the resonance direction of the optical waveguide, and a non-diffraction grating part that is positioned between the diffraction grating part and the first end face, and is not provided with a periodic structure; the active layer comprises an n-side well layer and an n-side barrier layer; the second n-side nitride semiconductor layer is a nitride semiconductor layer that comprises In and Ga; and the thickness of the second n-side nitride semiconductor layer is larger than the thickness of the n-side barrier layer.

Description

[Technical field]

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

[0002] Semiconductor laser elements partially provided with a diffraction grating are known. For example, Patent Document 1 describes an EA (Electro-Absorption) modulator integrated DFB laser element having an oscillator section provided with a diffraction grating layer and a modulator section not provided with a diffraction grating layer. The semiconductor laser element described in Patent Document 1 has an InGaAsP active layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2011-151088 A Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present disclosure is to provide a semiconductor laser element having a nitride semiconductor layer, which is capable of unifying or approaching a single longitudinal mode of the oscillation wavelength. [Means for solving the problem]

[0005] The present disclosure includes the following aspects. a nitride semiconductor stack having a first end face, a second end face, and an optical waveguide connecting the first end face and the second end face, the nitride semiconductor stack having a first n-side nitride semiconductor layer, a second n-side nitride semiconductor layer arranged above the first n-side nitride semiconductor layer, an active layer having one or more well layers and one or more barrier layers arranged above the second n-side nitride semiconductor layer, and a p-side nitride semiconductor layer arranged above the active layer, the first n-side nitride semiconductor layer having a periodic structure in which a refractive index changes periodically along a resonance direction of the optical waveguide; and a non-diffraction grating portion located between the diffraction grating portion and the first end face and not provided with the periodic structure, the active layer having an n-side well layer located closest to the second n-side nitride semiconductor layer among the one or more well layers, and an n-side barrier layer located between the n-side well layer and the second n-side nitride semiconductor layer among the one or more barrier layers, the second n-side nitride semiconductor layer being a nitride semiconductor layer containing In and Ga, and a thickness of the second n-side nitride semiconductor layer being greater than a thickness of the n-side barrier layer. Effect of the Invention

[0006] According to the above-mentioned semiconductor laser device, the longitudinal mode of the oscillation wavelength can be made single or close to single. [Brief description of the drawings]

[0007] [Figure 1] 1 is a schematic top view showing a semiconductor laser element according to one embodiment of the present invention; [Diagram 2] FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Diagram 3] FIG. 2 is a cross-sectional view taken along line III-III in FIG. [Figure 4] FIG. 2 is a schematic diagram illustrating a first n-side nitride semiconductor layer. [Diagram 5] FIG. 2 is a schematic diagram for explaining a first contact layer and a second contact layer. [Figure 6] 4 is a graph showing the spectrum of the semiconductor laser device of Example 1. [Figure 7] 1 is a Z-contrast image of a portion of the semiconductor laser element of Example 1. [Figure 8] 13 is a graph showing the spectrum of the semiconductor laser device of Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the drawings, the same elements are denoted by the same reference numerals.

[0009] FIG. 1 is a schematic top view showing the semiconductor laser element of this embodiment. FIG. 2 is a cross-sectional view taken along line II-II in FIG. 1. FIG. 3 is a cross-sectional view taken along line III-III in FIG. 1. FIG. 4 is a schematic view for explaining the first n-side nitride semiconductor layer. As shown in FIGS. 1 to 4, the semiconductor laser element 100 of this embodiment includes a nitride semiconductor laminate 20. The nitride semiconductor laminate 20 has a first end face 20a, a second end face 20b, and an optical waveguide 10 connecting the first end face 20a and the second end face 20b. The nitride semiconductor laminate 20 includes a first n-side nitride semiconductor layer 31, a second n-side nitride semiconductor layer 32 disposed above the first n-side nitride semiconductor layer 31, an active layer 40 having one or more well layers 41 and one or more barrier layers 42 disposed above the second n-side nitride semiconductor layer 32, and a p-side nitride semiconductor layer 50 disposed above the active layer 40. In this embodiment, the direction from the n-side nitride semiconductor layer 30 toward the p-side nitride semiconductor layer 50 is described as the upward direction. This upward direction does not necessarily have to coincide with the upward direction of a light emitting device to which the semiconductor laser device 100 is fixed.

[0010] (Substrate 60) The substrate 60 is, for example, a semiconductor substrate. The substrate 60 is, for example, a nitride semiconductor substrate such as a GaN substrate. For example, a nitride semiconductor substrate may be used as the substrate 60, and the upper surface of the substrate may be a +c-plane (i.e., a (0001) plane). In this embodiment, the c-plane is not limited to a plane that strictly coincides with the (0001) plane, but also includes a plane having an off-angle within a range of ±0.03 to 1 degree. The semiconductor laser device 100 may not have the substrate 60. The upper surface of the substrate may be a nonpolar plane (M-plane or A-plane), or a semipolar plane having an off-angle within a range of ±0.03 to 25 degrees from the nonpolar plane.

[0011] (Nitride semiconductor laminate 20) The nitride semiconductor laminate 20 has a plurality of nitride semiconductor layers. The nitride semiconductor constituting the nitride semiconductor laminate 20 is, for example, a group III nitride semiconductor. Examples of group III nitride semiconductors include GaN, InGaN, AlGaN, InN, AlN, and InAlGaN. The nitride semiconductor laminate 20 has an n-side nitride semiconductor layer 30, an active layer 40, and a p-side nitride semiconductor layer 50. The active layer 40 is disposed between the n-side nitride semiconductor layer 30 and the p-side nitride semiconductor layer 50. The n-side nitride semiconductor layer 30, the active layer 40, and the p-side nitride semiconductor layer 50 may be in direct contact with each other, or another semiconductor layer may be disposed between them. The nitride semiconductor laminate 20 has, in this order, a first n-side nitride semiconductor layer 31, a second n-side nitride semiconductor layer 32, an active layer 40, and a p-side nitride semiconductor layer 50. The nitride semiconductor stack 20 is, for example, epitaxially grown on a substrate 60. The primary surface of the nitride semiconductor stack 20 is, for example, the +c plane (that is, the (0001) plane).

[0012] In Figs. 1 to 3, the resonance direction is the resonance direction D1, and the direction perpendicular to the resonance direction is the vertical direction D2. When the length of the optical waveguide 10 in the vertical direction D2 is the width of the optical waveguide 10, the width of the optical waveguide 10 is, for example, 1 µm or more. The width of the optical waveguide 10 is preferably 10 µm or more. This can improve the optical output of the semiconductor laser element 100. The width of the optical waveguide 10 is more preferably 50 µm or more, and may be 80 µm or more. The width of the optical waveguide 10 can be, for example, 400 µm or less. When the nitride semiconductor laminate 20 has a ridge 20c as shown in Figs. 1 to 3, the width of the ridge 20c may be regarded as the width of the optical waveguide 10. Alternatively, when a current confinement structure other than the ridge 20c is present, the width of the current confinement structure in the vertical direction D2 may be regarded as the width of the optical waveguide 10.

[0013] The nitride semiconductor laminate 20 has a first end face 20a and a second end face 20b. The first end face 20a and the second end face 20b are surfaces non-parallel to the main surface of the active layer 40. The first end face 20a and the second end face 20b are, for example, surfaces perpendicular to the main surface of the active layer 40. The first end face 20a and the second end face 20b are surfaces intersecting the resonance direction D1 of the optical waveguide 10, for example, surfaces perpendicular to the resonance direction D1. The first end face 20a is one of a light emitting end face and a light reflecting end face, and the second end face 20b is the other of the light emitting end face and the light reflecting end face.

[0014] (n-side nitride semiconductor layer 30) The n-side nitride semiconductor layer 30 has one or more nitride semiconductor layers containing n-type impurities. Examples of n-type impurities include Si and Ge. The n-side nitride semiconductor layer 30 may have an undoped layer that is not intentionally doped with impurities. The n-side nitride semiconductor layer 30 includes a first n-side nitride semiconductor layer 31 and a second n-side nitride semiconductor layer 32. The n-side nitride semiconductor layer 30 may have other layers. The semiconductor laser element 100 shown in FIGS. 1 to 3 has a third n-side nitride semiconductor layer 33, a fourth n-side nitride semiconductor layer 34, and a fifth n-side nitride semiconductor layer 35. The n-side nitride semiconductor layer 30 does not have to have all of these layers. The n-side nitride semiconductor layer 30 may have other layers.

[0015] (First n-side nitride semiconductor layer 31) The first n-side nitride semiconductor layer 31 has a diffraction grating portion 311 having a periodic structure in which the refractive index changes periodically along the resonance direction D1, and a non-diffraction grating portion 312 located between the diffraction grating portion 311 and the first end face 20a and not having a periodic structure.

[0016] The semiconductor laser element 100 can unify or approach a single longitudinal mode of the oscillation wavelength by including the first n-side nitride semiconductor layer 31 having the diffraction grating portion 311. Considering a small amount of spontaneous emission light, there is no state of a single longitudinal mode in the strict sense. Therefore, a case where the output of one mode is sufficiently stronger than the output of another mode is considered to be a single longitudinal mode or a longitudinal mode close to a single longitudinal mode. The semiconductor laser element 100 has not only the diffraction grating portion 311 but also the non-diffraction grating portion 312. An increase in absorption loss may occur due to damage, defects, impurities, or the like that occurs when forming the diffraction grating by etching. Since the semiconductor laser element 100 has the non-diffraction grating portion 312, an increase in absorption loss due to the diffraction grating and / or a decrease in light confinement to the active layer 40 do not occur in the non-diffraction grating portion 312. Therefore, the light emission efficiency of the semiconductor laser element 100 can be improved compared to a semiconductor laser element having a periodic structure provided from the first end face 20a to the second end face 20b.

[0017] The diffraction grating portion 311 and the non-diffraction grating portion 312 are arranged to be aligned along the resonance direction D1. In the diffraction grating portion 311, a periodic structure is provided directly below the optical waveguide 10. The periodic structure has a refractive index that changes periodically along the shortest direction connecting the first end face 20a and the second end face 20b. In the non-diffraction grating portion 312, a periodic structure is not provided at least directly below the optical waveguide 10. The non-diffraction grating portion 312 may be a region where no periodic structure is provided anywhere. The non-diffraction grating portion 312 can function as a gain region that does not have a diffraction grating. For this reason, the length of the diffraction grating portion 311 along the resonance direction D1 is preferably smaller than the length of the non-diffraction grating portion 312 along the resonance direction D1. In other words, the length of the non-diffraction grating portion 312 along the resonance direction D1 is preferably larger than the length of the diffraction grating portion 311 along the resonance direction D1. This can improve the optical output of the semiconductor laser element 100.

[0018] The length of the non-diffraction grating portion 312 in the resonance direction D1 may be, for example, 100 μm or more. As the distance from the periodic structure provided in the diffraction grating portion 311 to the active layer 40 increases, the coupling efficiency between the light from the active layer 40 and the periodic structure decreases, and the reflectance at the wavelength corresponding to the periodic structure decreases. By setting the length of the diffraction grating portion 311 in the resonance direction D1 to 100 μm or more, it is possible to set the reflectance corresponding to the periodic structure to a value high enough to enable the semiconductor laser element 100 to oscillate. If the diffraction grating portion 311 is made too long, the absorption loss increases due to regrowth damage, and the reflectance due to the resonator structure increases too much, making it difficult to obtain optical output, but by increasing the length of the non-diffraction grating portion 312, the actual gain of the semiconductor laser element 100 increases, so that the decrease in optical output can be suppressed. For this reason, the length of the non-diffraction grating portion 312 in the resonance direction D1 is preferably 500 μm or more. The length of the non-diffraction grating portion 312 in the resonance direction D1 may be 1500 μm or more. The length of the non-diffraction grating portion 312 in the resonance direction D1 can be set to 4000 μm or less.

[0019] In nitride semiconductors, the activation rate of n-type impurities (e.g., Si) tends to be higher than that of p-type impurities (e.g., Mg). Therefore, the n-type impurity concentration of the n-side nitride semiconductor layer 30 can be made lower than the p-type impurity concentration of the p-side nitride semiconductor layer 50. The periodic structure is formed, for example, by forming a concave-convex structure in one semiconductor layer and then filling the concave-convex structure with another semiconductor layer, and the lower the impurity concentration, the easier it is to fill more densely. Therefore, it is suitable for the diffraction grating portion 311 provided with the periodic structure to be disposed as a part of the n-side nitride semiconductor layer 30. The periodic structure of the diffraction grating portion 311 of the first n-side nitride semiconductor layer 31 has an average refractive index higher than that of the fifth n-side nitride semiconductor layer 35, for example. Alternatively, the first n-side nitride semiconductor layer 31 may also serve as the n-side cladding layer.

[0020] If the periodic structure is disposed close to the active layer 40, the electric field strength of the p-side nitride semiconductor layer 50 becomes relatively high, which may increase absorption loss and / or reduce light confinement to the active layer 40. As a result, the threshold current at which the semiconductor laser element 100 oscillates may increase. For this reason, the first n-side nitride semiconductor layer 31 is provided at a position away from the active layer 40. For example, as shown in FIG. 2, the second n-side nitride semiconductor layer 32 is disposed between the first n-side nitride semiconductor layer 31 and the active layer 40. This makes it possible to reduce absorption loss and / or improve light confinement to the active layer 40 by relatively lowering the electric field strength of the p-side nitride semiconductor layer 50. This makes it possible to reduce the threshold current of the semiconductor laser element 100. By reducing the threshold current, it is possible to reduce the current density during laser oscillation and reduce the probability of higher-order modes appearing in the longitudinal mode. Furthermore, since the electric field strength in the p-side nitride semiconductor layer 50 is relatively low, the absorption loss due to the p-type impurity can be suppressed, and the slope efficiency of the semiconductor laser device 100 can be improved.

[0021] In this embodiment, the periodic structure provided in the diffraction grating portion 311 is a diffraction grating. The size of the periodic structure can be appropriately adjusted depending on the wavelength of the laser light to be obtained, the composition of the semiconductor to be used, and the like. The cross-sectional shape of the unevenness constituting the periodic structure along the resonance direction D1 of the optical waveguide 10 can be, for example, sawtooth, sinusoidal, rectangular, trapezoidal, inverted trapezoidal, and the like. The cross-sectional shape of the convex portion of the unevenness constituting the periodic structure is rectangular in FIG. 2, but is not limited thereto, and may be a shape having an inclined side such as a trapezoid whose width becomes narrower as it approaches the active layer 40. By having a shape having an inclined side, it is easy to grow a semiconductor layer filling the unevenness, and the thickness of the semiconductor layer can be reduced. Each of the convex portions of the unevenness can have an upper surface. This upper surface is, for example, a surface parallel to the main surface of the active layer 40. Each of the concave portions of the unevenness shown in FIG. 2 has a bottom surface. This bottom surface is, for example, a surface parallel to the main surface of the active layer 40. Each concave portion of the unevenness may have a shape without a bottom surface, such as a U-shape or a V-shape.

[0022] The period (pitch) of the projections and recesses constituting the periodic structure can be determined by the wavelength to be oscillated and the effective refractive index. The projections and recesses pitch (one period) can be, for example, 40 nm or more and 140 nm or less. The width of the projections and recesses in the direction along the resonance direction D1 of the optical waveguide 10 may be the same or different. When a diffraction structure of a higher mode is provided, the width can be 120 nm or more and 420 nm or less for third-order diffraction, or 400 nm or more and 2000 nm or less for tenth-order diffraction or more. It is preferable that the width of one of the projections and recesses is in the range of 1 / 2 to 2 times the width of the other.

[0023] The height H of the projections and recesses constituting the periodic structure can be set to 300 nm or less, and may be set to 200 nm or less. gratingincreases, so that the coupling coefficient k can be increased. For this reason, the height H of the irregularities is preferably 50 nm or more. The height of the irregularities constituting the periodic structure is, for example, the shortest distance between a line parallel to the principal surface of the active layer 40 that passes through the portion of the irregularities closest to the active layer 40 and a line parallel to the principal surface of the active layer 40 that passes through the portion of the irregularities farthest from the active layer 40, in a cross section perpendicular to the principal surface of the active layer 40 and parallel to the resonance direction D1 of the optical waveguide 10. Such a cross section can be observed, for example, by a transmission electron microscope (TEM). The cross section may also be observed by a scanning transmission electron microscope (STEM).

[0024] The diffraction grating portion 311 has a plurality of first portions and a plurality of second portions having a refractive index higher than that of the first portions. The periodic structure is formed by alternately arranging the plurality of first portions and the plurality of second portions along the resonance direction D1.

[0025] In the diffraction grating portion 311 shown in FIG. 4, the multiple first portions are connected to one common portion, and the multiple first portions and the one common portion constitute one first semiconductor portion 31a. Similarly, the multiple second portions are connected to one common portion, and the multiple second portions and the one common portion constitute one second semiconductor portion 31b. In other words, the first semiconductor portion 31a has multiple first portions protruding upward from one common portion, and the second semiconductor portion 31b has multiple second portions protruding downward from one common portion, and the first portions and the second portions are alternately arranged along the resonance direction D1. A part of the first semiconductor portion 31a and the second semiconductor portion 31b constitutes the diffraction grating portion 311, and the other part constitutes the non-diffraction grating portion 312. The first n-side nitride semiconductor layer 31 may be composed of only the first semiconductor portion 31a and the second semiconductor portion 31b.

[0026] The composition of the common portion of the first semiconductor portion 31a is the same as the composition of the first portion. The composition of the common portion of the second semiconductor portion 31b is the same as the composition of the second portion. The composition being the same means that the compositions are obtained by forming the semiconductor portions without intentionally making them different compositions, and may include errors that occur during manufacturing. Here, it has been described that the first semiconductor portion 31a includes a plurality of first portions and the second semiconductor portion 31b includes a plurality of second portions, but the first semiconductor portion 31a may include a plurality of second portions and the second semiconductor portion 31b may include a plurality of first portions.

[0027] The first semiconductor portion 31a can be obtained, for example, by forming a first semiconductor layer that will become the first semiconductor portion 31a, and then removing a portion of the first semiconductor layer by dry etching or the like. If the first semiconductor layer is removed down to its lower surface during the partial removal, a first semiconductor portion consisting of only a plurality of first portions without a common portion can be formed. If the first semiconductor layer is removed to a depth that does not reach the lower surface thereof, taking into consideration the accuracy of the removal depth, the first semiconductor portion 31a consisting of one common portion and a plurality of first portions can be formed.

[0028] The second semiconductor portion 31b can be obtained, for example, by forming the second semiconductor portion 31b on the first semiconductor portion 31a. The second semiconductor portion 31b is filled between the multiple first portions of the first semiconductor portion 31a. The second semiconductor portion 31b can be formed, for example, under growth conditions that promote lateral growth more than the first semiconductor layer. When forming the second semiconductor portion 31b in this manner, the lower the impurity concentration of the second semiconductor portion 31b, the more likely it is that a gap will occur between the second semiconductor portion 31b and the first semiconductor portion 31a. For this reason, the n-type impurity concentration of the second semiconductor portion 31b is set to 1×10 20 / cm 3 The n-type impurity concentration of the first semiconductor portion 31a is preferably 1×10 17 / cm 3 More than 1×10 20 / cm 3The n-type impurity concentration of the first semiconductor portion 31a may be greater than the n-type impurity concentration of the second semiconductor portion 31b. The concentration of impurities other than the n-type impurity of the second semiconductor portion 31b may be less than the detection limit. The second semiconductor portion 31b is preferably made of GaN, which can reduce the possibility of a gap occurring between the second semiconductor portion 31b and the first semiconductor portion 31a. In FIG. 2, the upper surface of the second semiconductor portion 31b is at the same height from the diffraction grating portion 311 to the non-diffraction grating portion 312, but this is not limited to this. For example, the upper surface of the second semiconductor portion 31b of the diffraction grating portion 311 having a recess in the first semiconductor portion 31a may be lower than the upper surface of the second semiconductor portion 31b of the non-diffraction grating portion 312.

[0029] At least one of both ends in the vertical direction D2 of either the multiple first portions or the multiple second portions may be located inside the nitride semiconductor stack 20. Both ends of either the multiple first portions or the multiple second portions may be rephrased as both ends of the periodic structure. When a method in which the formation area is proportional to the operation time, such as electron beam lithography, is used to form the periodic structure, the formation time of the periodic structure can be shortened by making the width of the periodic structure smaller than the width of the semiconductor laser element 100 in this manner.

[0030] In FIG. 2, the first semiconductor part 31a has a plurality of concave shapes recessed in a direction away from the active layer 40. The portions of the first semiconductor part 31a sandwiching the plurality of concave shapes along the resonance direction D1 are either the first portion or the second portion. The plurality of concave shapes are filled by the second semiconductor part 31b, and the portion filling the plurality of concave shapes is the other of the first portion or the second portion. The first semiconductor part 31a may have a plurality of convex shapes protruding toward the active layer 40. In this case, the plurality of convex portions of the first semiconductor part 31a are either the first portion or the second portion, and the portions of the second semiconductor part 31b sandwiching the plurality of convex shapes along the resonance direction D1 are the other of the first portion or the second portion. It is considered that the concave or convex shape can be stably formed easily by at least one of both ends in the vertical direction D2 of the concave or convex portion of the first semiconductor part 31a being located inside the nitride semiconductor stack 20. This is because the strength of the first semiconductor portion 31a can be expected to improve as the width of the concave or convex shape in the vertical direction D2 becomes narrower. Preferably, the concave or convex shape is a shape in which both ends of the concave or convex shape in the vertical direction D2 are located inside the nitride semiconductor stack 20. The width of the concave or convex shape in the vertical direction D2 is equal to or greater than the width of the optical waveguide 10, and may be greater than the width of the optical waveguide 10. The width of the concave or convex shape in the vertical direction D2 may be a value obtained by adding 5 μm or more on one side to the width of the optical waveguide 10, and a total of 10 μm or more. The width of the concave or convex shape in the vertical direction D2 may be smaller than the maximum width of the p-electrode 82 in the vertical direction D2. When a method in which the formation area and the work time are proportional, such as electron beam lithography, is used to form a periodic structure, the formation time of the first semiconductor portion 31a having a plurality of concave shapes can be shortened compared to the formation of the first semiconductor portion 31a having a plurality of convex shapes. Furthermore, it is expected that the strength of the first semiconductor portion 31a will be improved by forming the first semiconductor portion 31a having a plurality of concave shapes, as compared to forming the first semiconductor portion 31a having a plurality of convex shapes.

[0031] For example, the first portion is made of a nitride semiconductor containing Ga, and the second portion is made of a nitride semiconductor containing In and Ga. For example, the first portion is made of GaN, and the second portion is made of In.X Ga 1-X It consists of N(0 < X < 1). The In composition ratio of the second part can be 0.001 ≤ X ≤ 0.1. In this case, an n-side cladding layer can be provided as a layer different from the first n-side nitride semiconductor layer 31, and the first n-side nitride semiconductor layer 31 can be disposed between the n-side cladding layer and the active layer 40. Thereby, the threshold current can be reduced and the light confinement can be improved. When the first part consists of GaN, it is preferable that the first semiconductor part 31a includes a plurality of second parts and the second semiconductor part 31b includes a plurality of first parts. Thereby, since the unevenness of the first semiconductor part 31a can be filled by the second semiconductor part 31b, the probability of generating a gap between the first part and the second part can be reduced. Such a first semiconductor part 31a and second semiconductor part 31b may have a gentler contrast change at the bottom than at the side surface of the concave part of the first semiconductor part 31a in, for example, a Z-contrast image (ZC image) obtained by STEM. The Z-contrast image is a contrast image based on atomic weight.

[0032] As another example of the materials of the first part and the second part, the first part may consist of a nitride semiconductor containing Al and Ga, and the second part may consist of a nitride semiconductor containing Ga. For example, the first part is Al Y Ga 1-Y It consists of N(0 < Y < 1), and the second part consists of GaN. The Al composition ratio of the first part can be 0.001 ≤ Y ≤ 0.2. When the first n-side nitride semiconductor layer 31 includes a nitride semiconductor containing Al and Ga in at least a part thereof, the first n-side nitride semiconductor layer 31 may be a layer that functions as an n-side cladding layer.

[0033] The average refractive index of the periodic structure provided in the diffraction grating part 311 can be the average value of the refractive indices in the resonance direction D1 of the minimum unit of the periodic repetition constituting the periodic structure. For example, when the minimum unit of the periodic repetition constituting the periodic structure is one first part and one second part, the average refractive index n ave of the periodic structure provided in the diffraction grating part 311 is represented by the following formula (1).

[0034] n ave ={(n1×t1)+(n2×t2)} / (t1+t2) (1) Here, n1 is the refractive index of the first portion, n2 is the refractive index of the second portion, t1 is the length of one first portion in the resonance direction D1, and t2 is the length of one second portion in the resonance direction D1. When the lengths of the first and second portions in the resonance direction D1 vary depending on the height, the lengths of the first and second portions at half the thickness of the periodic structure are set as t1 and t2, respectively, to obtain the average refractive index of the periodic structure. The refractive index is the refractive index at the peak wavelength of the laser light oscillated by the semiconductor laser element 100. The refractive index of each material may be a known refractive index value of each material. The refractive index of a semiconductor can be calculated from the composition ratio of the semiconductor.

[0035] The distance from the first n-side nitride semiconductor layer 31 to the well layer 41 is preferably greater than 300 nm. This can reduce the threshold current of the semiconductor laser element 100. Also, the slope efficiency of the semiconductor laser element 100 can be improved. The distance from the first n-side nitride semiconductor layer 31 to the well layer 41 can be, for example, 800 nm or less, and is preferably 500 nm or less. This makes it easier to obtain a desired coupling efficiency. The distance from the first n-side nitride semiconductor layer 31 to the active layer 40 may be within these numerical ranges. Also, the distance from the periodic structure provided in the diffraction grating portion 311 of the first n-side nitride semiconductor layer 31 to the well layer 41 may be within these numerical ranges, and the distance from the periodic structure to the active layer 40 may be within these numerical ranges. The distance from the periodic structure provided in the diffraction grating portion 311 to the well layer 41 (n-side well layer) may be 320 nm or more and 800 nm or less, or 400 nm or more and 800 nm or less.

[0036] The thickness of the first n-side nitride semiconductor layer 31 is preferably 50 nm or more, and more preferably 100 nm or more. This makes it easy to form a periodic structure in the first n-side nitride semiconductor layer 31. The thickness of the first n-side nitride semiconductor layer 31 can be 1000 nm or less, and may be 500 nm or less.

[0037] The thickness of the periodic structure, i.e., the length of the periodic structure in a direction perpendicular to the primary surface of active layer 40, is equal to or smaller than the thickness of the first n-side nitride semiconductor layer 31. The difference between the thickness of the first n-side nitride semiconductor layer 31 and the thickness of the periodic structure can be not less than 0 nm and not more than 1000 nm.

[0038] When the semiconductor laser device 100 has a ridge 20c, the refractive index of the periodic structure provided in the diffraction grating portion 311 changes periodically along the extension direction of the ridge 20c. The periodic structure is disposed at least directly below the ridge 20c.

[0039] (Second n-side nitride semiconductor layer 32) The second n-side nitride semiconductor layer 32 is disposed between the first n-side nitride semiconductor layer 31 and the active layer 40 .

[0040] As the distance between the periodic structure and the active layer 40 decreases, the electric field strength in the p-side nitride semiconductor layer 50 increases, leading to an increase in absorption loss and / or a tendency for light confinement in the active layer 40 to decrease. By providing the second n-side nitride semiconductor layer 32, it is possible to increase the distance between the first n-side nitride semiconductor layer 31, in which the periodic structure is partially provided, and the active layer 40, compared to the case where the second n-side nitride semiconductor layer 32 is not provided. This makes it possible to reduce the electric field strength in the p-side nitride semiconductor layer 50, leading to a reduction in absorption loss, and / or to improve light confinement in the active layer 40. As a result, it is possible to reduce the threshold current of the semiconductor laser device 100.

[0041] The second n-side nitride semiconductor layer 32 is preferably a nitride semiconductor layer containing In and Ga. The thickness of the second n-side nitride semiconductor layer 32 is preferably greater than the thickness of the n-side barrier layer described below. By providing these configurations, it is possible to reduce absorption loss and / or improve light confinement in the active layer 40.

[0042] The refractive index of the second n-side nitride semiconductor layer 32 is preferably higher than the average refractive index of the periodic structure of the diffraction grating portion 311. The thickness of the second n-side nitride semiconductor layer 32 is preferably larger than the thickness of the periodic structure of the diffraction grating portion 311. By providing these configurations, it is possible to reduce absorption loss and / or improve light confinement in the active layer 40. By providing that the refractive index of the material constituting the second n-side nitride semiconductor layer 32 is higher than any of the refractive indexes of the materials constituting the periodic structure of the diffraction grating portion 311, it may be said that the refractive index of the second n-side nitride semiconductor layer 32 is higher than the average refractive index of the periodic structure provided in the diffraction grating portion 311. It is more preferable that the thickness of the second n-side nitride semiconductor layer 32 is larger than the thickness of the first n-side nitride semiconductor layer 31.

[0043] The refractive index of the second n-side nitride semiconductor layer 32 is preferably higher than that of the n-side barrier layer. The n-side barrier layer has a band gap energy larger than that of the well layer in order to function as a barrier layer, but such an n-side barrier layer tends to have a relatively low refractive index. For this reason, by providing the second n-side nitride semiconductor layer 32 having a refractive index higher than that of the n-side barrier layer, it is possible to reduce the absorption loss and / or improve the light confinement in the active layer 40. When the n-side barrier layer is composed of multiple layers, the refractive index of the second n-side nitride semiconductor layer 32 is preferably higher than the average refractive index of the n-side barrier layer, and may be higher than the refractive index of any of the multiple layers constituting the n-side barrier layer.

[0044] The second n-side nitride semiconductor layer 32 is, for example, In Z Ga 1-ZIt consists of N(0 < Z < 1). The In composition ratio of the second n-side nitride semiconductor layer 32 can be 0.001 ≤ Z ≤ 0.2. The second n-side nitride semiconductor layer 32 may be a compositionally graded layer. For example, the second n-side nitride semiconductor layer 32 can be an InGaN throughout, and a compositionally graded layer in which the In composition ratio increases as it approaches the active layer 40. Such a compositionally graded layer can also be referred to as a nitride semiconductor layer having In and Ga. When forming a compositionally graded layer with GaN at the portion farthest from the active layer 40, InGaN at the portion closest to the active layer 40, and the In composition ratio increasing as it approaches the active layer 40, the remaining portion of the compositionally graded layer excluding the portion farthest from the active layer 40 can be used as the second n-side nitride semiconductor layer 32.

[0045] The thickness of the second n-side nitride semiconductor layer 32 can be 150 nm or more, and preferably 200 nm or more. Thereby, absorption loss can be reduced and / or light confinement in the active layer 40 can be improved. The thickness of the second n-side nitride semiconductor layer 32 may be greater than the thickness of the fourth n-side nitride semiconductor layer 34. The thickness of the second n-side nitride semiconductor layer 32 can be 500 nm or less. From the relationship between the light intensity in the diffraction grating, the light confinement in the well layer 41, and the leakage light to the p-side nitride semiconductor layer 50, the thickness of the second n-side nitride semiconductor layer 32 can be 170 nm or more and 500 nm or less, can be 230 nm or more and 500 nm or less, or can be 300 nm or more and 500 nm or less.

[0046] (The third n-side nitride semiconductor layer 33) The third n-side nitride semiconductor layer 33 is disposed on the side opposite to the active layer 40 of the first n-side nitride semiconductor layer 31. The first n-side nitride semiconductor layer 31 is located between the third n-side nitride semiconductor layer 33 and the active layer 40.

[0047] The third n-side nitride semiconductor layer 33 may be disposed between the n-side cladding layer and the first n-side nitride semiconductor layer 31. This arrangement is possible in order to prevent the distance from the periodic structure of the first n-side nitride semiconductor layer 31 to the active layer 40 from being too large. The n-side cladding layer is disposed on the opposite side of the first n-side nitride semiconductor layer 31 to the active layer 40. The third n-side nitride semiconductor layer 33 has a refractive index between the refractive index of the n-side cladding layer and the average refractive index of the periodic structure of the diffraction grating portion 311 of the first n-side nitride semiconductor layer 31. For example, the refractive index of the third n-side nitride semiconductor layer 33 is higher than the refractive index of the n-side cladding layer and lower than the average refractive index of the periodic structure of the diffraction grating portion 311. Since the refractive index of the material constituting the third n-side nitride semiconductor layer 33 is lower than any of the refractive indexes of the materials constituting the periodic structure of the diffraction grating portion 311, it may be said that the refractive index of the third n-side nitride semiconductor layer 33 is lower than the average refractive index of the periodic structure provided in the diffraction grating portion 311.

[0048] 2, the third n-side nitride semiconductor layer 33 is disposed between the first n-side nitride semiconductor layer 31 and a fifth n-side nitride semiconductor layer 35 described later. The third n-side nitride semiconductor layer 33 may have a refractive index between the refractive index of the fifth n-side nitride semiconductor layer 35 and the average refractive index of the periodic structure of the diffraction grating portion 311 of the first n-side nitride semiconductor layer 31. For example, the fifth n-side nitride semiconductor layer 35 is an n-side cladding layer.

[0049] Providing the third n-side nitride semiconductor layer 33 can reduce light leaking to the substrate 60 and the like located below. For example, when the first n-side nitride semiconductor layer 31 has a periodic structure in which GaN and InGaN are periodically arranged, the refractive index of the first n-side nitride semiconductor layer 31 increases compared to when the first n-side nitride semiconductor layer 31 does not have a periodic structure and is made of only GaN. In this way, when the refractive index of the first n-side nitride semiconductor layer 31 is relatively high, it is particularly preferable to provide the third n-side nitride semiconductor layer 33 to reduce light leakage.

[0050] The third n-side nitride semiconductor layer 33 is, for example, an AlGaN layer. The third n-side nitride semiconductor layer 33 may contain n-type impurities. The third n-side nitride semiconductor layer 33 may have a thickness of not less than 100 nm and not more than 1000 nm.

[0051] (4th n-side nitride semiconductor layer 34) The fourth n-side nitride semiconductor layer 34 is disposed between the second n-side nitride semiconductor layer 32 and the first n-side nitride semiconductor layer 31. The refractive index of the fourth n-side nitride semiconductor layer 34 may be lower than the refractive index of the second n-side nitride semiconductor layer 32 and higher than the average refractive index of the periodic structure of the diffraction grating portion 311 of the first n-side nitride semiconductor layer 31. The refractive index of the material constituting the fourth n-side nitride semiconductor layer 34 is lower than both of the refractive indexes of the materials constituting the periodic structure of the diffraction grating portion 311, so that it may be said that the refractive index of the fourth n-side nitride semiconductor layer 34 is lower than the average refractive index of the periodic structure provided in the diffraction grating portion 311.

[0052] By providing the fourth n-side nitride semiconductor layer 34, it is possible to improve the light confinement in the active layer 40. For example, when the first n-side nitride semiconductor layer 31 has a periodic structure in which AlGaN and GaN are periodically arranged, the refractive index of the first n-side nitride semiconductor layer 31 is lower than when the first n-side nitride semiconductor layer 31 does not have a periodic structure and is made of only GaN. In this way, when the refractive index of the first n-side nitride semiconductor layer 31 is relatively low, it is preferable to improve the light confinement in the active layer 40 by providing the fourth n-side nitride semiconductor layer 34. Alternatively, the fourth n-side nitride semiconductor layer 34 may not be provided, and instead, the thickness of the common portion of the second semiconductor portion 31b may be set to 50 nm or more. This makes it possible to improve the light confinement in the active layer 40. The thickness of the common portion of the second semiconductor portion 31b may be 300 nm or less.

[0053] The fourth n-side nitride semiconductor layer 34 is, for example, an InGaN layer. The fourth n-side nitride semiconductor layer 34 may contain n-type impurities. The fourth n-side nitride semiconductor layer 34 may have a thickness of not less than 1 nm and not more than 500 nm.

[0054] (5th n-side nitride semiconductor layer 35) The fifth n-side nitride semiconductor layer 35 is disposed on the side of the first n-side nitride semiconductor layer 31 opposite to the active layer 40. The fifth n-side nitride semiconductor layer 35 is disposed between the first n-side nitride semiconductor layer 31 and the substrate 60. The fifth n-side nitride semiconductor layer 35 is, for example, an n-side cladding layer. The fifth n-side nitride semiconductor layer 35 is, for example, the layer having the largest band gap energy among the n-side nitride semiconductor layer 30. The fifth n-side nitride semiconductor layer 35 is, for example, an AlGaN layer containing n-type impurities.

[0055] (active layer 40) The active layer 40 is disposed between the n-side nitride semiconductor layer 30 and the p-side nitride semiconductor layer 50. The active layer 40 may have a multiple quantum well structure or a single quantum well structure. The active layer 40 has one or more well layers 41 and one or more barrier layers 42.

[0056] The active layer 40 has an n-side well layer, among the one or more well layers 41, that is located closest to the second n-side nitride semiconductor layer 32, and an n-side barrier layer, among the one or more barrier layers 42, that is located between the n-side well layer and the second n-side nitride semiconductor layer 32.

[0057] The active layer 40 is preferably provided from above the diffraction grating portion 311 to above the non-diffraction grating portion 312. Furthermore, each layer above the first n-side nitride semiconductor layer 31 including the active layer 40 is preferably provided from above the diffraction grating portion 311 to above the non-diffraction grating portion 312. For example, a method for forming the diffraction grating portion and the non-diffraction grating portion includes a method of forming a first epitaxial structure including the non-diffraction grating portion and the active layer, removing a part of the first epitaxial structure to expose the end face of the active layer, and then re-growing a second epitaxial structure including the diffraction grating portion and the active layer. In such a method, impurities are taken into the interface between the exposed end face and the re-grown second epitaxial structure during the regrowth, and there is a concern that the impurities may increase the light absorption loss and the carrier trap. In the semiconductor laser element 100 shown in FIG. 2, the diffraction grating portion 311 and the non-diffraction grating portion 312 are formed in the first n-side nitride semiconductor layer 31. This allows the semiconductor laser device 100 to have a structure in which the regrowth interface is only in the first n-side nitride semiconductor layer 31. Since the electric field strength in the first n-side nitride semiconductor layer 31 when the semiconductor laser device 100 is driven is smaller than the electric field strength in the active layer 40, such an arrangement can reduce the influence caused by impurities introduced during regrowth.

[0058] By providing the periodic structure of the diffraction grating portion 311, the upper surface of the first n-side nitride semiconductor layer 31 may be lower in the diffraction grating portion 311 than in the non-diffraction grating portion 312. When there is a step on the upper surface of the first n-side nitride semiconductor layer 31 like this, if the thickness of the layer provided thereon is less than the step, the layer may be divided by the step. However, even in this state, it can be said that the layer is provided from above the diffraction grating portion 311 to above the non-diffraction grating portion 312.

[0059] When multiple semiconductor layers are present between the n-side well layer and the second n-side nitride semiconductor layer 32, it is preferable that the thickness of the second n-side nitride semiconductor layer 32 is greater than the thickness of the thickest layer among them. This can reduce the absorption loss and / or improve the light confinement to the active layer 40. Furthermore, it is preferable that the thickness of the second n-side nitride semiconductor layer 32 is greater than the total thickness of the multiple semiconductor layers located between the n-side well layer and the second n-side nitride semiconductor layer 32. This can further reduce the absorption loss and / or improve the light confinement to the active layer 40.

[0060] The active layer 40 may be formed with a composition capable of emitting light with a wavelength of, for example, 400 nm or more and 600 nm or less. The one or more well layers 41 may be made of, for example, InGaN. The In composition ratio of the InGaN constituting the one or more well layers 41 may be, for example, 0.05 or more and 0.50 or less. The In composition ratio of the InGaN constituting the one or more well layers 41 may be 0.15 or more.

[0061] (p-side nitride semiconductor layer 50) The p-side nitride semiconductor layer 50 has one or more nitride semiconductor layers containing p-type impurities. Examples of p-type impurities include Mg. The p-side nitride semiconductor layer 50 may have an undoped layer that is not intentionally doped with impurities. The p-side nitride semiconductor layer 50 may have a contact layer. The p-side nitride semiconductor layer 50 may have one or more of a p-side light guide layer, an electron block layer, and a p-side cladding layer. The p-side nitride semiconductor layer 50 may have all of these layers, or may have layers other than these.

[0062] In nitride semiconductors, the activation rate of p-type impurities is lower than that of n-type impurities. Therefore, the p-type impurity concentration of the p-side nitride semiconductor layer 50 tends to be higher than the n-type impurity concentration of the n-side nitride semiconductor layer 30. For example, the maximum value of the p-type impurity concentration in the p-side nitride semiconductor layer 50 is higher than the maximum value of the n-type impurity concentration in the n-side nitride semiconductor layer 30.

[0063] (n electrode 81) The semiconductor laser device 100 has an n-electrode 81. The n-electrode 81 is provided on the lower surface of the substrate 60. Examples of materials for the n-electrode 81 include a single layer or multilayer film of a conductive oxide containing at least one selected from metals or alloys such as Ni, Rh, Cr, Au, W, Pt, Ti, and Al, and Zn, In, and Sn. Examples of conductive oxides include ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), and GZO (Gallium-doped Zinc Oxide).

[0064] (p electrode 82) The semiconductor laser element 100 has a p-electrode 82. The p-electrode 82 is provided in contact with a part of the p-side nitride semiconductor layer 50. The p-electrode 82 is provided in contact with, for example, the upper surface of the ridge 20c. In the semiconductor laser element 100, the p-electrode 82 may have a pad electrode. Examples of materials for the p-electrode 82 include a single layer or multilayer film of a conductive oxide containing at least one selected from metals or alloys such as Ni, Rh, Cr, Au, W, Pt, Ti, and Al, and Zn, In, and Sn. Examples of the conductive oxide include ITO, IZO, and GZO.

[0065] As shown in FIG. 1, the semiconductor laser element 100 can have a first p electrode 821 and a second p electrode 822 as the p electrode 82. The first p electrode 821 is provided on the upper surface of the p-side nitride semiconductor layer 50 and above the diffraction grating portion 311. The second p electrode 822 is provided on the upper surface of the p-side nitride semiconductor layer 50 and above the non-diffraction grating portion 312, away from the first p electrode 821. By having such a first p electrode 821 and a second p electrode 822, it is possible to independently control the current for each of the diffraction grating portion 311 and the non-diffraction grating portion 312, and it is possible to pass a current of a suitable magnitude for each. For example, the current passed through the non-diffraction grating portion 312 is set to a magnitude according to the desired optical output, and the current passed through the diffraction grating portion 311 is set to a magnitude that reduces optical absorption in the diffraction grating portion 311. When no current is passed through the diffraction grating portion 311, the well layer 41 becomes a highly absorbing layer, causing a rising kink, but this can be suppressed by passing a current through the diffraction grating portion 311. The current passed through the diffraction grating portion 311 may be smaller than the current passed through the non-diffraction grating portion 312. In addition, the wavelength may be tuned by changing the magnitude of the current passed through the diffraction grating portion 311. The first p-electrode 821 is disposed away from the second p-electrode 822.

[0066] 5 is a schematic diagram for explaining the first contact layer 511 and the second contact layer 512. When the first p electrode 821 and the second p electrode 822 are provided, as shown in FIG. 5, the p-side nitride semiconductor layer 50 may have, as contact layers, the first contact layer 511 in contact with the lower surface of the first p electrode 821 and the second contact layer 512 in contact with the lower surface of the second p electrode 822. This makes it possible to more reliably control the diffraction grating portion 311 and the non-diffraction grating portion 312 independently. The first contact layer 511 is disposed away from the second contact layer 512.

[0067] The first p electrode 821 can have a first conductive oxide film 823 provided on the upper surface of the p-side nitride semiconductor layer 50, and a first metal film 824 arranged above the first conductive oxide film 823. The second p electrode 822 can have a second conductive oxide film 825 provided on the upper surface of the p-side nitride semiconductor layer 50, and a second metal film 826 arranged above the second conductive oxide film 825. In a top view, it is preferable that one or both of the light emission side end of the first conductive oxide film 823 and the light reflection side end of the second conductive oxide film 825 are located between the light emission side end of the first metal film 824 and the light reflection side end of the second metal film 826. In the top view, it is preferable that one or both of the end of the first conductive oxide film 823 on the first end face 20a side and the end of the second conductive oxide film 825 on the second end face 20b side are located between the end of the first metal film 824 on the first end face 20a side and the end of the second metal film 826 on the second end face 20b side. This makes it possible to reduce the distance from the first p electrode 821 to the second p electrode 822 compared to the case where the first p electrode 821 and the second p electrode 822 are formed only of metal. In each film constituting the first p electrode 821 and the second p electrode 822, the end closer to the first end face 20a among both ends in the resonance direction of the optical waveguide is the end on the first end face 20a side, and the end closer to the second end face 20b is the end on the second end face 20b side. The end on the first end face 20a side is the end on the light emitting side, and the end on the second end face 20b side is the end on the light reflecting side. Alternatively, the end on the first end face 20a side may be the end on the light reflecting side, and the end on the second end face 20b side may be the end on the light emitting side. It is more preferable that both the end on the first end face 20a side of the first conductive oxide film 823 and the end on the second end face 20b side of the second conductive oxide film 825 are located between the end on the first end face 20a side of the first metal film 824 and the end on the second end face 20b side of the second metal film 826. It is more preferable that both the end on the light emitting side of the first conductive oxide film 823 and the end on the light reflecting side of the second conductive oxide film 825 are located between the end on the light emitting side of the first metal film 824 and the end on the light reflecting side of the second metal film 826. No current is injected between the first p electrode 821 and the second p electrode 822, and the greater the distance between them, the greater the light absorption.By using the first conductive oxide film 823 and the second conductive oxide film 825, the region into which no current is injected can be made smaller, thereby reducing light absorption. The distance from the first conductive oxide film 823 to the second conductive oxide film 825 can be set to, for example, 1 μm or more and 30 μm or less.

[0068] In top view, the end of the first conductive oxide film 823 on the first end face 20a side protrudes from the end of the first metal film 824 on the first end face 20a side toward the second p electrode 822. In top view, the end of the first conductive oxide film 823 on the light emission side protrudes from the end of the first metal film 824 on the light emission side toward the second p electrode 822. In top view, the distance from the first end face 20a to the first conductive oxide film 823 is smaller than the distance from the first end face 20a to the first metal film 824. In top view, the end of the second conductive oxide film 825 on the second end face 20b side protrudes from the end of the second metal film 826 on the second end face 20b side toward the first p electrode 821. In top view, the end of the second conductive oxide film 825 on the light reflection side protrudes from the end of the second metal film 826 on the light reflection side toward the first p electrode 821. In the top view, the distance from the second end face 20b to the second conductive oxide film 825 is smaller than the distance from the second end face 20b to the second metal film 826. The first conductive oxide film 823 and the second conductive oxide film 825 have at least a portion overlapping with the optical waveguide in the top view. In FIG. 1, the first conductive oxide film 823 and the second conductive oxide film 825 have at least a portion overlapping with the ridge 20c. The portions of the first conductive oxide film 823 and the second conductive oxide film 825 protruding from the first metal film 824 and the second metal film 826 at least partially overlap with the optical waveguide in the top view. In FIG. 1, both the first p electrode 821 and the second p electrode 822 have a conductive oxide film, but a structure in which only one of them has a conductive oxide film may be used.

[0069] (First protective film 71, second protective film 72) The semiconductor laser element 100 may have a first protective film 71 and a second protective film 72. The first protective film 71 is provided on the first end face 20a of the nitride semiconductor stack 20. The second protective film 72 is provided on the second end face 20b of the nitride semiconductor stack 20. One or both of the first protective film 71 and the second protective film 72 may not be provided. Each of the first protective film 71 and the second protective film 72 can have one or more dielectric films.

[0070] When the first end face 20a, which is the end face on the side where the non-diffraction grating portion 312 is located, is used as the light emitting end face, the reflectance of the first protective film 71 can be, for example, 0.1% or more, and is preferably 5% or more. This makes it possible to reduce the threshold current compared to when the first protective film 71 is an AR (anti-reflective) coating. By using the first end face 20a as the light emitting end face, the diffraction grating portion 311 can be made part of the reflective film on the light reflecting side, and the reflectance of the light reflecting side can be increased, thereby increasing the light output.

[0071] In order to further suppress the increase in threshold current, the reflectance of the first protective film 71 may be 18% or more, and more preferably 30% or more. When the semiconductor laser element 100 is an element that emits laser light with a peak wavelength of 500 nm or more, the gain inside the resonator tends to be lower than when the semiconductor laser element 100 emits laser light with a peak wavelength of less than 500 nm. For this reason, in the case of the semiconductor laser element 100 that emits laser light with a peak wavelength of 500 nm or more, the reflectance of the first protective film 71 is preferably 30% or more and less than the reflectance of the second protective film 72. This can reduce the threshold current. When a periodic structure is provided in the nitride semiconductor stack 20 and the longitudinal mode of the oscillation wavelength is unified or made close to unified by the periodic structure, the confinement factor related to laser oscillation is lowered compared to when this is not the case. The higher the reflectance of the first protective film 71, the higher the confinement factor can be. The reflectance of the first protective film 71 may be 60% or more, or may be 80% or more. The upper limit of the reflectance of the first protective film 71 can be set to be less than the reflectance of the second protective film 72 .

[0072] The reflectance of the second protective film 72 is higher than the reflectance of the first protective film 71. The reflectance of the second protective film 72 can be, for example, 95% or more, and may be 98% or more. The reflectance of the second protective film 72 can be, for example, 100% or less. The reflectance of the second protective film 72 may be 100%. The reflectance of the first protective film 71 and the reflectance of the second protective film 72 refer to the reflectance at the peak wavelength of the laser light oscillated by the semiconductor laser device 100.

[0073] The second end face 20b, which is the end face on the side where the diffraction grating portion 311 is located, may be used as the light emitting end face. In this case, the reflectance of the second protective film 72 is lower than that of the first protective film 71. The reflectance of the first protective film 71 may be, for example, 85% or more, or may be 90% or more. The reflectance of the first protective film 71 may be, for example, 100% or less. The second protective film 72 may be an AR coating. The reflectance of the second protective film 72 may be 0.1% or more, or may be 5% or more. The second protective film 72 may not be provided. By using the second end face 20b as the light emitting end face, the second protective film 72 can be made unnecessary. By using the second end face 20b as the light emitting end face, the light output can be stabilized. This is because an effective refractive index difference occurs between the concave and convex portions of the diffraction grating portion 311, and an effective reflectance in the laser diode is obtained. By forming the diffraction grating portion 311 with a length in the resonance direction D1 of 100 μm or more, it is possible to obtain a sufficient reflectance required for laser oscillation even without the second protective film 72.

[0074] (Insulating film 73) The semiconductor laser device 100 can have an insulating film 73 provided on a part of the surface of the p-side nitride semiconductor layer 50. The insulating film 73 is a single layer or a multilayer film of an oxide or nitride of, for example, Si, Al, Zr, Ti, Nb, Ta, or the like.

[0075] Example 1 As Example 1, the semiconductor laser element 100 shown below was fabricated. An MOCVD apparatus was used to fabricate an epitaxial wafer that would become the semiconductor laser element 100. As raw materials, trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH3), silane gas, and bis(cyclopentadienyl)magnesium (Cp2Mg) were appropriately used.

[0076] On a c-plane GaN substrate (substrate 60), a Si-containing Al 0.016 Ga 0.984 The N layer was grown to a thickness of 1.8 μm. Next, Al containing Si 0.08 Ga 0.92 The N layer was grown to a thickness of 200 nm. Next, the In containing Si 0.04 Ga 0.96 The N layer was grown to a thickness of 150 nm. Next, Al containing Si 0.08 Ga 0.92 The N layer (n-side cladding layer) was grown to a thickness of 650 nm. Next, a Si-containing GaN layer was grown to a thickness of 100 nm. Next, the In containing Si 0.03 Ga 0.97 An N layer (a first semiconductor layer that becomes the first semiconductor portion 31a) was grown to a thickness of 150 nm.

[0077] The epitaxial wafer on which the above layers were formed was removed from the MOCVD apparatus, and a periodic uneven shape (periodic structure) was fabricated using an electron beam lithography apparatus, reactive ion etching (RIE), and sputtering. The depth of the recess was 83 nm, the width of the recess was 39 nm, and the diffraction grating period Λ (one period of the unevenness) was 112 nm. The periodic structure was formed only in the diffraction grating portion 311. The length of the diffraction grating portion 311 along the resonance direction D1 was 300 μm, and the length of the non-diffraction grating portion 312 along the resonance direction D1 was 500 μm.

[0078] After the uneven shape was created, a GaN layer containing Si (second semiconductor portion 31b) was grown to a thickness of 150 nm in an MOCVD apparatus. The thickness of 150 nm is the thickness from the top of the protrusion of the uneven shape to the upper surface of the GaN layer containing Si (second semiconductor portion 31b).

[0079] Next, undoped In 0.03 Ga 0.97 The N layer (second n-side nitride semiconductor layer 32) was grown to a thickness of 240 nm. 0.016 Ga 0.984 The n-side nitride semiconductor layer 30 extends from the N layer to this layer.

[0080] Next, a 1 nm thick Si-doped GaN layer, an 8 nm thick Si-doped In layer, and 0.05 Ga 0.95 The n-side barrier layer (barrier layer 42) is made up of three layers: an n-type layer, a 1 nm-thick Si-doped GaN layer, and an undoped In layer with a thickness of 2.1 nm. 0.25 Ga 0.75 The N layer (well layer 41), an undoped GaN layer (barrier layer 42) having a thickness of 3.3 nm, and an undoped In layer having a thickness of 2.1 nm are arranged in a GaN well layer 42. 0.25 Ga 0.75 An active layer 40 including, in this order, an N layer (well layer 41) and a 2.3 nm-thick undoped GaN layer (barrier layer 42) was grown.

[0081] Next, an undoped compositionally graded layer was grown to a thickness of 150 nm. 0.05 Ga 0.95 The In composition was grown by substantially monotonically decreasing from the growth end of the GaN crystal to a GaN crystal such that the composition gradient was nearly linear. Next, Mg-containing Al 0.10 Ga 0.90 N layer and Al 0.16 Ga 0.84 The N layers were grown to thicknesses of 3 nm and 7 nm, respectively. Next, undoped Al 0.015 Ga 0.985 The N layer was grown to a thickness of 125 nm. Next, Mg-containing Al 0.015 Ga0.985 The N layer was grown to a thickness of 325 nm. Next, a GaN layer containing Mg was grown to a thickness of 15 nm. The p-side nitride semiconductor layer 50 is formed from the undoped compositionally graded layer to this layer.

[0082] Then, the epitaxial wafer on which the above layers were formed was removed from the MOCVD apparatus, and the ridge 20c, the p-electrode 82, the n-electrode 81, and the like were formed by photolithography, RIE, and sputtering. As shown in Fig. 1 and Fig. 2, a first conductive oxide film 823, a first metal film 824, a second conductive oxide film 825, and a second metal film 826 were formed as the p-electrode 82. As shown in Fig. 4, the p-side nitride semiconductor layer 50 was structured to have a first contact layer 511 and a second contact layer 512 by photolithography and RIE.

[0083] Thereafter, the semiconductor laser device 100 was obtained by forming a first protective film 71 on the first end facet 20a and a second protective film 72 on the second end facet 20b, thereby obtaining a semiconductor laser device 100. The reflectance of the first protective film 71 was 90%, and the reflectance of the second protective film 72 was 97%. That is, in the semiconductor laser device 100 of Example 1, the first end facet 20a is the light emitting end face. The semiconductor laser device 100 had a ridge width of 2 μm, a cavity length of 800 μm, and an element width of 200 μm.

[0084] In the semiconductor laser device 100 of Example 1, when a voltage was applied between the n-electrode 81 and the second metal film 826 provided on the non-diffraction grating portion 312 and a current of 500 mA was injected, the semiconductor laser device 100 of Example 1 oscillated a laser beam with a peak wavelength of about 528 nm. No voltage was applied between the n-electrode 81 and the first metal film 824 provided on the diffraction grating portion 311. The spectrum of the semiconductor laser device 100 of Example 1 is shown in FIG. 6. The horizontal axis of FIG. 6 is the wavelength. The vertical axis of FIG. 6 represents the light intensity as an intensity ratio (unit: dB), and the side mode suppression ratio (SMSR) was estimated with the largest peak set to 0 dB. The side mode suppression ratio of the semiconductor laser device 100 of Example 1 was greater than 20 dB. The spectral width of the semiconductor laser device 100 of Example 1 was about 3 pm. Since the resolution of the spectrum analyzer used was about 3 pm, it can be said that an extremely narrow spectral width was obtained. The spacing between adjacent longitudinal modes calculated from the effective refractive index and the cavity length is about 0.06 nm to 0.07 nm. However, no large peaks that may be due to adjacent longitudinal modes are observed, and the side mode suppression ratio is at least greater than 20 dB. Therefore, it can be said that the semiconductor laser element 100 of Example 1 oscillated in a single longitudinal mode.

[0085] FIG. 7 shows a Z-contrast image obtained by STEM for a part of the semiconductor laser element of Example 1. FIG. 7 is a Z-contrast image of a cross section of a portion including the first semiconductor portion 31a and the second semiconductor portion 31b in the diffraction grating portion 311. FIG. 7 is a cross section along the resonance direction D1. In the Z-contrast image, the difference in composition can be observed as a difference in display density in the image. In FIG. 7, the portion having a recessed portion recessed downward is the first semiconductor portion 31a, and the portion provided thereon filling the recess is the second semiconductor portion 31b. In FIG. 7, the first semiconductor portion 31a and the second semiconductor portion 31b are shown with different display densities, and it can be seen that they have different compositions.

[0086] Example 2 As Example 2, a semiconductor laser element similar to that of Example 1 was fabricated, except that the second protective film 72 was not provided and the reflectance of the first protective film 71 was set to 90%. That is, in the semiconductor laser element of Example 2, the second end facet 20b is a light-emitting end facet.

[0087] In the semiconductor laser device of Example 2, a voltage was applied between n-electrode 81 and second metal film 826 provided on non-diffraction grating portion 312 to inject a current of 500 mA, and a voltage was also applied between n-electrode 81 and first metal film 824 provided on diffraction grating portion 311 to inject a current of 100 mA to 300 mA. The spectrum of the semiconductor laser device of Example 2 is shown in FIG. 8. The horizontal axis of FIG. 8 is wavelength. The vertical axis of FIG. 8 represents the optical intensity as an intensity ratio (unit: dB), and the side mode suppression ratio (SMSR) was estimated with the largest peak set to 0 dB. The five peaks in FIG. 8 are spectra when the currents applied between n-electrode 81 and first metal film 824 were set to 100 mA, 150 mA, 200 mA, 250 mA, and 300 mA, in order from the left. The side mode suppression ratio of the semiconductor laser device of Example 2 was greater than 20 dB. The semiconductor laser device 100 of Example 2 oscillated laser light with a peak wavelength of approximately 528.5 nm by injecting a current of 100 mA between the n-electrode 81 and the first metal film 824. The semiconductor laser device 100 of Example 2 oscillated laser light with a peak wavelength of approximately 530 nm by injecting a current of 300 mA between the n-electrode 81 and the first metal film 824. It can be said that the semiconductor laser device of Example 2 oscillated in a single longitudinal mode, and further, wavelength control was possible by changing the injected current.

[0088] Through the contents described so far in this specification, the following technical matters are disclosed. (Section 1) a nitride semiconductor laminate having a first end face, a second end face, and an optical waveguide connecting the first end face and the second end face; The nitride semiconductor stack is A first n-side nitride semiconductor layer; a second n-side nitride semiconductor layer disposed above the first n-side nitride semiconductor layer; an active layer having one or more well layers and one or more barrier layers disposed above the second n-side nitride semiconductor layer; a p-side nitride semiconductor layer disposed above the active layer; having the first n-side nitride semiconductor layer has a diffraction grating portion provided with a periodic structure in which a refractive index changes periodically along a resonance direction of the optical waveguide, and a non-diffraction grating portion located between the diffraction grating portion and the first end face and not provided with the periodic structure, the active layer includes an n-side well layer, among the one or more well layers, that is located closest to the second n-side nitride semiconductor layer, and an n-side barrier layer, among the one or more barrier layers, that is located between the n-side well layer and the second n-side nitride semiconductor layer, the second n-side nitride semiconductor layer is a nitride semiconductor layer containing In and Ga, a thickness of the second n-side nitride semiconductor layer is greater than a thickness of the n-side barrier layer. (Section 2) Item 2. The semiconductor laser element according to item 1, wherein a length of the diffraction grating portion along the resonance direction is smaller than a length of the non-diffraction grating portion along the resonance direction. (Section 3) a first p-electrode provided on an upper surface of the p-side nitride semiconductor layer and above the diffraction grating portion; 3. The semiconductor laser element according to item 1 or 2, further comprising: a second p-electrode provided on an upper surface of the p-side nitride semiconductor layer, above the non-diffraction grating portion, and spaced apart from the first p-electrode. (Section 4) the first p-electrode has a first conductive oxide film provided on an upper surface of the p-side nitride semiconductor layer, and a first metal film disposed above the first conductive oxide film; the second p-electrode has a second conductive oxide film provided on an upper surface of the p-side nitride semiconductor layer, and a second metal film disposed above the second conductive oxide film; 4. The semiconductor laser element according to item 3, wherein, in a top view, one or both of an end of the first conductive oxide film on a first end face side and an end of the second conductive oxide film on a second end face side are located between an end of the first metal film on the first end face side and an end of the second metal film on the second end face side. (Section 5) a refractive index of the second n-side nitride semiconductor layer is higher than an average refractive index of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer, 5. The semiconductor laser element according to any one of items 1 to 4, wherein a thickness of the second n-side nitride semiconductor layer is greater than a thickness of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer. (Section 6) 6. The semiconductor laser element according to any one of items 1 to 5, wherein the distance from the first n-side nitride semiconductor layer to the well layer is greater than 300 nm. (Section 7) the first n-side nitride semiconductor layer has a plurality of first portions made of a nitride semiconductor containing Ga and a plurality of second portions made of a nitride semiconductor containing In and Ga, 7. The semiconductor laser element according to any one of items 1 to 6, wherein the periodic structure of the diffraction grating portion is configured by alternately arranging the multiple first portions and the multiple second portions along the resonance direction. (Section 8) The nitride semiconductor stack is an n-side cladding layer disposed on a side of the first n-side nitride semiconductor layer opposite to the active layer; and a third n-side nitride semiconductor layer disposed between the n-side cladding layer and the first n-side nitride semiconductor layer, and having a refractive index between a refractive index of the n-side cladding layer and an average refractive index of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer. (Section 9) the nitride semiconductor stack includes a fourth n-side nitride semiconductor layer disposed between the second n-side nitride semiconductor layer and the first n-side nitride semiconductor layer, Item 9. The semiconductor laser element according to item 8, wherein a refractive index of the fourth n-side nitride semiconductor layer is lower than a refractive index of the second n-side nitride semiconductor layer and is higher than an average refractive index of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer. (Section 10) a nitride semiconductor laminate having a first end face, a second end face, and an optical waveguide connecting the first end face and the second end face; a first p-electrode; and a second p-electrode; The nitride semiconductor stack is A first n-side nitride semiconductor layer; a second n-side nitride semiconductor layer disposed above the first n-side nitride semiconductor layer; an active layer having one or more well layers and one or more barrier layers disposed above the second n-side nitride semiconductor layer; a p-side nitride semiconductor layer disposed above the active layer; having the first n-side nitride semiconductor layer has a diffraction grating portion provided with a periodic structure in which a refractive index changes periodically along a resonance direction of the optical waveguide, and a non-diffraction grating portion located between the diffraction grating portion and the first end face and not provided with the periodic structure, the first p-electrode has a first conductive oxide film provided on an upper surface of the p-side nitride semiconductor layer and above the diffraction grating portion, and a first metal film disposed above the first conductive oxide film; the second p-electrode includes a second conductive oxide film provided on the top surface of the p-side nitride semiconductor layer and above the non-grating portion, the second conductive oxide film being spaced apart from the first conductive oxide film, and a second metal film disposed above the second conductive oxide film; a semiconductor laser element, wherein, in a top view, one or both of an end of the first conductive oxide film on the first end face side and an end of the second conductive oxide film on the second end face side are located between the first metal film and the second metal film. [Explanation of symbols]

[0089] 10 Optical waveguide 20. Nitride semiconductor laminate 20a 1st end face 20b Second end face 20c Ridge 30 n-side nitride semiconductor layer 31 First n-side nitride semiconductor layer 31a First semiconductor part 31b Second semiconductor part 311 Diffraction grating part 312 Non-diffraction grating part 32 Second n-side nitride semiconductor layer 33 Third n-side nitride semiconductor layer 34 Fourth n-side nitride semiconductor layer 35 Fifth n-side nitride semiconductor layer 40 Active layer 41 Well layer 42 Barrier layer 50 p-side nitride semiconductor layer 511 First contact layer 512 Second contact layer 60 Substrate 71 First protective film 72 Second protective film 73 Insulating film 81 n-electrode 82 p-electrode 821 First p-electrode 822 Second p-electrode 823 First conductive oxide film 824 First metal film 825 Second conductive oxide film 826 Second metal film 100 Semiconductor laser element

Claims

1. a nitride semiconductor laminate having a first end face, a second end face, and an optical waveguide connecting the first end face and the second end face; The nitride semiconductor stack is A first n-side nitride semiconductor layer; a second n-side nitride semiconductor layer disposed above the first n-side nitride semiconductor layer; an active layer having one or more well layers and one or more barrier layers disposed above the second n-side nitride semiconductor layer; a p-side nitride semiconductor layer disposed above the active layer; having the first n-side nitride semiconductor layer has a diffraction grating portion provided with a periodic structure in which a refractive index changes periodically along a resonance direction of the optical waveguide, and a non-diffraction grating portion located between the diffraction grating portion and the first end face and not provided with the periodic structure, the active layer includes an n-side well layer, among the one or more well layers, that is located closest to the second n-side nitride semiconductor layer, and an n-side barrier layer, among the one or more barrier layers, that is located between the n-side well layer and the second n-side nitride semiconductor layer, the second n-side nitride semiconductor layer is a nitride semiconductor layer containing In and Ga, a thickness of the second n-side nitride semiconductor layer is greater than a thickness of the n-side barrier layer.

2. 2. The semiconductor laser device according to claim 1, wherein a length of said diffraction grating portion along the resonance direction is smaller than a length of said non-diffraction grating portion along the resonance direction.

3. a first p-electrode provided on an upper surface of the p-side nitride semiconductor layer and above the diffraction grating portion; 2. The semiconductor laser device according to claim 1, further comprising: a second p-electrode provided on an upper surface of said p-side nitride semiconductor layer above said non-grating portion and spaced apart from said first p-electrode.

4. the first p-electrode has a first conductive oxide film provided on an upper surface of the p-side nitride semiconductor layer, and a first metal film disposed above the first conductive oxide film; the second p-electrode has a second conductive oxide film provided on an upper surface of the p-side nitride semiconductor layer, and a second metal film disposed above the second conductive oxide film; 4. The semiconductor laser element according to claim 3, wherein, in a top view, one or both of an end of the first conductive oxide film on the first end face side and an end of the second conductive oxide film on the second end face side are located between an end of the first metal film on the first end face side and an end of the second metal film on the second end face side.

5. a refractive index of the second n-side nitride semiconductor layer is higher than an average refractive index of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer, The semiconductor laser device according to claim 1 , wherein a thickness of the second n-side nitride semiconductor layer is greater than a thickness of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer.

6. The semiconductor laser device according to claim 1 , wherein a distance from said first n-side nitride semiconductor layer to said well layer is greater than 300 nm.

7. the first n-side nitride semiconductor layer has a plurality of first portions made of a nitride semiconductor containing Ga and a plurality of second portions made of a nitride semiconductor containing In and Ga, 2. The semiconductor laser device according to claim 1, wherein the periodic structure of the diffraction grating portion is configured by arranging the plurality of first portions and the plurality of second portions alternately along the resonance direction.

8. The nitride semiconductor stack is an n-side cladding layer disposed on a side of the first n-side nitride semiconductor layer opposite to the active layer; 8. The semiconductor laser element according to claim 1, further comprising: a third n-side nitride semiconductor layer disposed between the n-side cladding layer and the first n-side nitride semiconductor layer, the third n-side nitride semiconductor layer having a refractive index between a refractive index of the n-side cladding layer and an average refractive index of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer.

9. the nitride semiconductor stack includes a fourth n-side nitride semiconductor layer disposed between the second n-side nitride semiconductor layer and the first n-side nitride semiconductor layer, 9. The semiconductor laser element according to claim 8, wherein a refractive index of the fourth n-side nitride semiconductor layer is lower than a refractive index of the second n-side nitride semiconductor layer and is higher than an average refractive index of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer.

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