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

semiconductor laser element

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

[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.

[0003] JP 2011-151088 A

[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.

[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 including 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.

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

[0007] 1 is a schematic top view showing a semiconductor laser element of one embodiment of the present invention. 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 a first n-side nitride semiconductor layer. FIG. 5 is a schematic view for explaining a first contact layer and a second contact layer. FIG. 6 is a graph showing the spectrum of the semiconductor laser element of Example 1. FIG. 7 is a Z-contrast image of a portion of the semiconductor laser element of Example 1. FIG. 8 is a graph showing the spectrum of the semiconductor laser element of Example 2.

[0008] An embodiment of the present invention will now be described with reference to the drawings, in which like elements are designated by like reference numerals.

[0009] FIG. 1 is a schematic top view showing a semiconductor laser device 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 illustrating a first n-side nitride semiconductor layer. As shown in FIGS. 1 to 4 , the semiconductor laser device 100 of this embodiment includes a nitride semiconductor stack 20. The nitride semiconductor stack 20 has a first end facet 20a, a second end facet 20b, and an optical waveguide 10 connecting the first end facet 20a and the second end facet 20b. The nitride semiconductor stack 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 disposed above the second n-side nitride semiconductor layer 32 and having one or more well layers 41 and one or more barrier layers 42, 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 or the like to which the semiconductor laser element 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 can be used as the substrate 60, with its upper surface being the +c-plane (i.e., the (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 does not necessarily have to have the substrate 60. The upper surface of the substrate may be a non-polar plane (M-plane or A-plane) or a semi-polar plane having an off-angle within a range of ±0.03 to 25 degrees from the non-polar plane.

[0011] (Nitride Semiconductor Stack 20) ​​The nitride semiconductor stack 20 has a plurality of nitride semiconductor layers. The nitride semiconductors constituting the nitride semiconductor stack 20 are, for example, Group III nitride semiconductors. Examples of Group III nitride semiconductors include GaN, InGaN, AlGaN, InN, AlN, and InAlGaN. The nitride semiconductor stack 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 stack 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 (i.e., the (0001) plane).

[0012] 1 to 3 , the resonance direction is defined as the resonance direction D1, and the direction perpendicular to the resonance direction is defined as the vertical direction D2. When the length of the optical waveguide 10 in the vertical direction D2 is defined as 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 allows the optical output of the semiconductor laser device 100 to be improved. 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 stack 20 has a ridge 20 c as shown in FIGS. 1 to 3 , the width of the ridge 20 c may be considered to be the width of the optical waveguide 10. Alternatively, when a current confinement structure other than the ridge 20 c is included, the width of the current confinement structure in the vertical direction D2 may be considered to be the width of the optical waveguide 10.

[0013] The nitride semiconductor stack 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 major surface of the active layer 40. The first end face 20a and the second end face 20b are, for example, surfaces perpendicular to the major surface of the active layer 40. The first end face 20a and the second end face 20b are surfaces that intersect with the resonance direction D1 of the optical waveguide 10, for example, surfaces perpendicular to the resonance direction D1. The first end face 20a is either a light-emitting end face or 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 device 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 need 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 provided with 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 20 a and not provided with a periodic structure.

[0016] By including the first n-side nitride semiconductor layer 31 having the diffraction grating portion 311, the semiconductor laser device 100 can achieve a single or nearly single longitudinal mode of its oscillation wavelength. Considering the small amount of spontaneous emission, a single longitudinal mode does not exist in the strict sense. Therefore, a single or nearly single longitudinal mode is defined as a state in which the output of one mode is significantly stronger than the output of another mode. The semiconductor laser device 100 includes not only the diffraction grating portion 311 but also the non-diffraction grating portion 312. Increased absorption loss can occur due to damage, defects, impurities, or the like that occur when forming the diffraction grating by etching. By including the non-diffraction grating portion 312 in the semiconductor laser device 100, the non-diffraction grating portion 312 does not increase absorption loss due to the diffraction grating and / or reduce optical confinement in the active layer 40. Therefore, the light emission efficiency of the semiconductor laser device 100 can be improved compared to a semiconductor laser device having a periodic structure extending from the first end facet 20a to the second end facet 20b.

[0017] The diffraction grating portion 311 and the non-diffraction grating portion 312 are arranged side by side 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 varies periodically along the shortest direction connecting the first end face 20a and the second end face 20b. In the non-diffraction grating portion 312, no periodic structure is provided at least directly below the optical waveguide 10. The non-diffraction grating portion 312 may be a region that does not have a periodic structure anywhere. The non-diffraction grating portion 312 can function as a gain region without a diffraction grating. For this reason, the length of the diffraction grating portion 311 along the resonance direction D1 is preferably shorter 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 longer than the length of the diffraction grating portion 311 along the resonance direction D1. This allows the optical output of the semiconductor laser device 100 to be improved.

[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, resulting in a decrease in reflectance at a wavelength corresponding to the periodic structure. By setting the length of the diffraction grating portion 311 in the resonance direction D1 to 100 μm or more, it is possible to achieve a high reflectance corresponding to the periodic structure that allows the semiconductor laser device 100 to oscillate. If the diffraction grating portion 311 is too long, absorption loss due to regrowth damage increases, and the reflectance due to the resonator structure increases too much, making it difficult to obtain optical output. However, by increasing the length of the non-diffraction grating portion 312, the actual gain of the semiconductor laser device 100 is increased, thereby suppressing a decrease in optical output. Therefore, 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 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. The lower the impurity concentration, the easier it is to fill the concave-convex structure densely. Therefore, the diffraction grating portion 311 having the periodic structure is suitably disposed as 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 an n-side cladding layer.

[0020] If the periodic structure is disposed close to the active layer 40, the electric field strength in the p-side nitride semiconductor layer 50 becomes relatively high, which may increase absorption loss and / or reduce optical confinement in the active layer 40. These factors may increase the threshold current at which the semiconductor laser device 100 oscillates. For this reason, the first n-side nitride semiconductor layer 31 is disposed 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 relatively reduces the electric field strength in the p-side nitride semiconductor layer 50, thereby reducing absorption loss and / or improving optical confinement in the active layer 40. This allows the threshold current of the semiconductor laser device 100 to be reduced. Reducing the threshold current reduces the current density during laser oscillation and reduces the probability of higher-order longitudinal modes appearing. Furthermore, since the electric field strength in the p-side nitride semiconductor layer 50 is relatively low, absorption loss due to p-type impurities 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 adjusted appropriately depending on the wavelength of the laser light to be obtained, the composition of the semiconductor used, and the like. The cross-sectional shape of the irregularities constituting the periodic structure along the resonance direction D1 of the optical waveguide 10 can be, for example, sawtooth, sinusoidal, rectangular, trapezoidal, inverted trapezoidal, etc. The cross-sectional shape of the convex portions of the irregularities constituting the periodic structure is rectangular in FIG. 2 , but is not limited thereto. They may also have a shape with inclined sides, such as a trapezoid, whose width narrows as they approach the active layer 40. A shape with inclined sides facilitates the growth of a semiconductor layer filling the irregularities, thereby reducing the thickness of the semiconductor layer. Each of the convex portions of the irregularities can have an upper surface. This upper surface is, for example, a surface parallel to the major surface of the active layer 40. Each of the concave portions of the irregularities shown in FIG. 2 has a bottom surface. This bottom surface is, for example, a surface parallel to the major surface of the active layer 40. Each recess 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 irregularities constituting the periodic structure can be determined by the desired wavelength and effective refractive index. The irregularity pitch (one period of the irregularities) can be, for example, 40 nm or more and 140 nm or less. The width of the convex portions and the width of the concave portions in the direction along the resonance direction D1 of the optical waveguide 10 can be the same or different. Furthermore, 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 higher. It is preferable that the width of either the convex portions or the concave portions 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. gratingSince the coupling coefficient k increases, 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 major 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 major 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 major 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, using a transmission electron microscope (TEM). The cross section may also be observed using 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 the one common portion, and the second semiconductor portion 31b has multiple second portions protruding downward from the one common portion, and the first portions and second portions are arranged alternately along the resonance direction D1. Parts of the first semiconductor portion 31a and the second semiconductor portion 31b constitute the diffraction grating portion 311, and other parts constitute the non-diffraction grating portion 312. The first n-side nitride semiconductor layer 31 may be composed only of 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. "Same composition" 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, the first semiconductor portion 31a has been described as including multiple first portions and the second semiconductor portion 31b as including multiple second portions, but the first semiconductor portion 31a may also include multiple second portions and the second semiconductor portion 31b may also include multiple 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 removal of the portion extends to the underside of the first semiconductor layer, a first semiconductor portion consisting only of multiple first portions and no common portion can be formed. If the removal is performed to a depth that does not reach the underside of the first semiconductor layer, taking into account the accuracy of the removal depth, the first semiconductor portion 31a consisting of one common portion and multiple 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 way, 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 1×10 ２０ / cm ３ The n-type impurity concentration of the first semiconductor portion 31a is preferably 1×10 １７ / cm ３ 1x10 or more ２０ / cm ３The concentration of n-type impurities in the first semiconductor portion 31a may be greater than the concentration of n-type impurities in the second semiconductor portion 31b. The concentration of impurities other than n-type impurities in the second semiconductor portion 31b may be less than the detection limit. Furthermore, the second semiconductor portion 31b is preferably made of GaN, which reduces the possibility of a gap occurring between the second semiconductor portion 31b and the first semiconductor portion 31a. In FIG. 2 , the top 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 necessarily the case. For example, the top surface of the second semiconductor portion 31b in the diffraction grating portion 311, which has a recess in the first semiconductor portion 31a, may be lower than the top surface of the second semiconductor portion 31b in the non-diffraction grating portion 312.

[0029] At least one of both ends in the vertical direction D2 of either the plurality of first portions or the plurality of second portions may be located inside the nitride semiconductor stack 20. The both ends of either the plurality of first portions or the plurality of second portions may also be rephrased as both ends of the periodic structure. When a method in which the formation area and the working time are proportional, 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 portion 31a has a plurality of concave shapes recessed in a direction away from the active layer 40. Portions of the first semiconductor portion 31a that sandwich 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 with the second semiconductor portion 31b, and the portions that fill the plurality of concave shapes are the other of the first portion or the second portion. The first semiconductor portion 31a may have a plurality of convex shapes that protrude toward the active layer 40. In this case, the plurality of convex portions of the first semiconductor portion 31a are either the first portion or the second portion, and the portions of the second semiconductor portion 31b that sandwich the plurality of convex shapes along the resonance direction D1 are the other of the first portion or the second portion. It is believed that the concave or convex shape can be easily and stably formed by at least one of the two ends in the vertical direction D2 of the concave or convex portion of the first semiconductor portion 31a being located inside the nitride semiconductor stack 20. This is because the narrower the width of the concave or convex shape in the vertical direction D2, the more improved the strength of the first semiconductor portion 31a can be expected. 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 working time are proportional, such as electron beam lithography, is used to form a periodic structure, forming the first semiconductor portion 31a having a plurality of concave shapes can shorten the formation time of the periodic structure compared to forming the first semiconductor portion 31a having a plurality of convex shapes. Furthermore, it is expected that forming the first semiconductor portion 31a having a plurality of concave shapes will improve the strength of the first semiconductor portion 31a 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 The second portion may be made of In (0<X<1). The In composition ratio of the second portion may be 0.001≦X≦0.1. In this case, an n-side cladding layer may be provided as a layer separate from the first n-side nitride semiconductor layer 31, and the first n-side nitride semiconductor layer 31 may be disposed between the n-side cladding layer and the active layer 40. This reduces the threshold current and improves optical confinement. When the first portion is made of GaN, it is preferable that the first semiconductor portion 31a includes multiple second portions and the second semiconductor portion 31b includes multiple first portions. This allows the second semiconductor portion 31b to fill in the irregularities in the first semiconductor portion 31a, thereby reducing the likelihood of gaps occurring between the first and second portions. In a Z-contrast image (ZC image) obtained by STEM, for example, the first semiconductor portion 31a and the second semiconductor portion 31b may have a more gradual change in contrast at the bottom of the recess in the first semiconductor portion 31a than at the side. The Z-contrast image is a contrast image based on atomic weight.

[0032] As another example of the materials of the first portion and the second portion, the first portion may be made of a nitride semiconductor containing Al and Ga, and the second portion may be made of a nitride semiconductor containing Ga. For example, the first portion may be made of Al Ｙ Ga １－Ｙ The first portion is made of GaN (0<Y<1), and the second portion is made of GaN. The Al composition ratio of the first portion can be 0.001≦Y≦0.2. When the first n-side nitride semiconductor layer 31 at least partially contains a nitride semiconductor containing Al and Ga, 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 portion 311 can be set to the average value of the refractive index in the resonance direction D1 of the smallest unit of periodic repetition that constitutes the periodic structure. For example, when the smallest unit of periodic repetition that constitutes the periodic structure is one first portion and one second portion, the average refractive index n ａｖｅ is expressed by the following equation (1).

[0034] n ａｖｅ = {(n１ ×t １ ) + (n ２ ×t ２ ) / (t １ +t ２ ) (1) where n １ is the refractive index of the first portion, n ２ is the refractive index of the second part, t １ is the length of one first portion in the resonance direction D1, t ２ is the length of one second portion in the resonance direction D1. When the lengths of the first portion and the second portion in the resonance direction D1 vary depending on the height, the lengths of the first portion and the second portion at the position halfway through the thickness of the periodic structure are respectively defined as t １ and t ２ The average refractive index of the periodic structure can be calculated by using the formula: . The refractive index is the refractive index at the peak wavelength of the laser light emitted by the semiconductor laser element 100. The refractive index of each material may be a known refractive index value for 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 reduces the threshold current of the semiconductor laser device 100. Furthermore, the slope efficiency of the semiconductor laser device 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. Furthermore, 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, or 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, which 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 the direction perpendicular to the major surface of the 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 0 nm or more and 1000 nm or less.

[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, which increases absorption loss and / or decreases light confinement in the active layer 40. By providing the second n-side nitride semiconductor layer 32, the distance between the active layer 40 and the first n-side nitride semiconductor layer 31, on which the periodic structure is partially provided, can be increased compared to when the second n-side nitride semiconductor layer 32 is not provided. This reduces the electric field strength in the p-side nitride semiconductor layer 50, which reduces absorption loss and / or improves light confinement in the active layer 40. Therefore, the threshold current of the semiconductor laser device 100 can be reduced.

[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. These configurations can reduce absorption loss and / or improve light confinement in the active layer 40. When the refractive index of the material constituting the second n-side nitride semiconductor layer 32 is higher than the refractive index of any of the materials constituting the periodic structure of the diffraction grating portion 311, it can 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. The thickness of the second n-side nitride semiconductor layer 32 is more preferably 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 bandgap 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. Therefore, providing the second n-side nitride semiconductor layer 32 with a refractive index higher than that of the n-side barrier layer can reduce absorption loss and / or improve optical 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－ＺN (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. The second n-side nitride semiconductor layer 32 may be, for example, entirely made of InGaN, and may be a compositionally graded layer in which the In composition ratio increases toward the active layer 40. Such a compositionally graded layer may also be referred to as a nitride semiconductor layer containing In and Ga. When a compositionally graded layer is formed in which the portion farthest from the active layer 40 is made of GaN and the portion closest to the active layer 40 is made of InGaN, and the In composition ratio increases toward the active layer 40, the remaining portion of the compositionally graded layer excluding the portion farthest from the active layer 40 may be the second n-side nitride semiconductor layer 32.

[0045] The thickness of the second n-side nitride semiconductor layer 32 may be 150 nm or more, and preferably 200 nm or more. This can reduce absorption loss and / or improve light confinement in the active layer 40. 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 may be 500 nm or less. In consideration of the relationship between the light intensity in the diffraction grating, the light confinement in the well layer 41, and the leakage light into the p-side nitride semiconductor layer 50, the thickness of the second n-side nitride semiconductor layer 32 may be 170 nm or more and 500 nm or less, 230 nm or more and 500 nm or less, or 300 nm or more and 500 nm or less.

[0046] (Third n-side nitride semiconductor layer 33) The third n-side nitride semiconductor layer 33 is disposed on the side of the first n-side nitride semiconductor layer 31 opposite to the active layer 40. 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 becoming too large. The n-side cladding layer is disposed on the side of the first n-side nitride semiconductor layer 31 opposite 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 can 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 leakage 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 thickness of the third n-side nitride semiconductor layer 33 may be not less than 100 nm and not more than 1000 nm.

[0051] (Fourth 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. If 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, 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] Providing the fourth n-side nitride semiconductor layer 34 can improve optical 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, providing the fourth n-side nitride semiconductor layer 34 is particularly preferable to improve optical confinement in the active layer 40. 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 can improve optical 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 thickness of the fourth n-side nitride semiconductor layer 34 may be not less than 1 nm and not more than 500 nm.

[0054] (Fifth n-side nitride semiconductor layer 35) The fifth n-side nitride semiconductor layer 35 is disposed on the opposite side of the first n-side nitride semiconductor layer 31 from 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 with 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, of 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, of 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 forming a first epitaxial structure including the non-diffraction grating portion and the active layer, removing a portion of the first epitaxial structure to expose the end face of the active layer, and then regrowing a second epitaxial structure including the diffraction grating portion and the active layer. In this method, impurities are likely to be introduced into the interface between the exposed end face and the regrown second epitaxial structure during regrowth, which may increase optical absorption loss and carrier traps due to the impurities. In the semiconductor laser device 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 located 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 operating 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 in this way, if the thickness of a 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 be greater than the thickness of the thickest layer among them. This can reduce absorption loss and / or improve light confinement in the active layer 40. Furthermore, it is preferable that the thickness of the second n-side nitride semiconductor layer 32 be 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 absorption loss and / or improve light confinement in the active layer 40.

[0060] The active layer 40 can be formed with a composition that can emit light with a wavelength of, for example, 400 nm or more and 600 nm or less. The one or more well layers 41 can be made of, for example, InGaN. The In composition ratio of the InGaN that constitutes the one or more well layers 41 can be, for example, 0.05 or more and 0.50 or less. The In composition ratio of the InGaN that constitutes 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 optical guide layer, an electron blocking 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 in the p-side nitride semiconductor layer 50 tends to be higher than the n-type impurity concentration in 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 film or a multi-layer film of a metal or alloy such as Ni, Rh, Cr, Au, W, Pt, Ti, or Al, or a conductive oxide containing at least one element selected from Zn, In, and Sn. Examples of conductive oxides include indium tin oxide (ITO), indium zinc oxide (IZO), and gallium-doped zinc oxide (GZO).

[0064] (p-electrode 82) The semiconductor laser device 100 has a p-electrode 82. The p-electrode 82 is provided in contact with a portion 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 device 100, the p-electrode 82 may have a pad electrode. Examples of materials for the p-electrode 82 include metals or alloys such as Ni, Rh, Cr, Au, W, Pt, Ti, and Al, and single-layer or multilayer films of conductive oxides containing at least one selected from Zn, In, and Sn. Examples of conductive oxides include ITO, IZO, and GZO.

[0065] As shown in FIG. 1 , the semiconductor laser device 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, 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, above the non-diffraction grating portion 312, and spaced apart from the first p-electrode 821. By providing such a first p-electrode 821 and a second p-electrode 822, independent current control is possible for the diffraction grating portion 311 and the non-diffraction grating portion 312, allowing currents of appropriate magnitudes to flow through them. For example, the current flowing through the non-diffraction grating portion 312 is set to a magnitude corresponding to the desired optical output, and the current flowing through the diffraction grating portion 311 is set to a magnitude sufficient to reduce 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. Furthermore, 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 apart from the second p-electrode 822.

[0066] 5 is a schematic diagram illustrating 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, 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, as shown in FIG. 5 . 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 apart 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-emitting side end of the first conductive oxide film 823 and the light-reflecting side end of the second conductive oxide film 825 are located between the light-emitting side end of the first metal film 824 and the light-reflecting side end of the second metal film 826. In a top view, one or both of the end of the first conductive oxide film 823 on the first end facet 20a side and the end of the second conductive oxide film 825 on the second end facet 20b side are preferably located between the end of the first metal film 824 on the first end facet 20a side and the end of the second metal film 826 on the second end facet 20b side. This allows the distance from the first p electrode 821 to the second p electrode 822 to be shorter than when the first p electrode 821 and the second p electrode 822 are formed solely from metal. In each film constituting the first p electrode 821 and the second p electrode 822, the end closer to the first end facet 20a of both ends in the resonance direction of the optical waveguide is the end closer to the first end facet 20a, and the end closer to the second end facet 20b is the end closer to the second end facet 20b. The end closer to the first end facet 20a is the end on the light-emitting side, and the end closer to the second end facet 20b is the end on the light-reflecting side. Alternatively, the end on the first end facet 20a side may be the end on the light-reflecting side, and the end on the second end facet 20b side may be the end on the light-emitting side. It is more preferable that both the end of the first conductive oxide film 823 on the first end facet 20a side and the end of the second conductive oxide film 825 on the second end facet 20b side are located between the end of the first metal film 824 on the first end facet 20a side and the end of the second metal film 826 on the second end facet 20b side. 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.The use of the first conductive oxide film 823 and the second conductive oxide film 825 can reduce the area into which no current is injected, 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 a top view, the end of the first conductive oxide film 823 on the first end face 20 a side protrudes from the end of the first metal film 824 on the first end face 20 a side toward the second p electrode 822. In a 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 a top view, the distance from the first end face 20 a to the first conductive oxide film 823 is shorter than the distance from the first end face 20 a to the first metal film 824. In a top view, the end of the second conductive oxide film 825 on the second end face 20 b side protrudes from the end of the second metal film 826 on the second end face 20 b side toward the first p electrode 821. In a 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 a top view, the distance from the second end face 20b to the second conductive oxide film 825 is shorter 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 at least partially overlap with the optical waveguide in a top view. In FIG. 1, the first conductive oxide film 823 and the second conductive oxide film 825 at least partially overlap with the ridge 20c. The portions of the first conductive oxide film 823 and the second conductive oxide film 825 that protrude from the first metal film 824 and the second metal film 826 at least partially overlap with the optical waveguide in a 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 also be used.

[0069] (First Protective Film 71, Second Protective Film 72) The semiconductor laser device 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 facet 20a of the nitride semiconductor stack 20. The second protective film 72 is provided on the second end facet 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. The first protective film 71 and the second protective film 72 can each have one or more dielectric films.

[0070] When the first end facet 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 facet, the reflectance of the first protective film 71 can be, for example, 0.1% or more, and preferably 5% or more. This allows the threshold current to be reduced compared to when the first protective film 71 is an AR (anti-reflective) coating. By using the first end facet 20a as the light-emitting end facet, the diffraction grating portion 311 can be made part of the reflective film on the light-reflecting side, increasing the reflectance on the light-reflecting side and thereby increasing the light output.

[0071] To further suppress an increase in threshold current, the reflectance of the first protective film 71 may be 18% or more, preferably 30% or more. When the semiconductor laser device 100 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 device 100 emits laser light with a peak wavelength of less than 500 nm. Therefore, for the semiconductor laser device 100 emitting 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 reduces 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 close to unified by the periodic structure, the confinement factor associated with laser oscillation is lower than when this periodic structure is not provided. The confinement factor can be increased by increasing the reflectance of the first protective film 71. 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 lower 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%. Note that the reflectances of the first protective film 71 and 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 facet 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 facet. In this case, the reflectivity of the second protective film 72 is set lower than that of the first protective film 71. The reflectivity of the first protective film 71 may be, for example, 85% or more, or even 90% or more. The reflectivity of the first protective film 71 may be, for example, 100% or less. The second protective film 72 may be an AR coating. The reflectivity of the second protective film 72 may be 0.1% or more, or even 5% or more. The second protective film 72 may not be provided. By using the second end facet 20b as the light-emitting end facet, the second protective film 72 can be eliminated. Using the second end facet 20b as the light-emitting end facet stabilizes the optical output. This is because an effective refractive index difference occurs between the concave and convex portions of the diffraction grating portion 311, thereby achieving an effective reflectivity in the laser diode. By forming the diffraction grating portion 311 having a length of 100 μm or more in the resonance direction D1, 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 film or a multi-layer film made of, for example, an oxide or nitride of Si, Al, Zr, Ti, Nb, Ta, or the like.

[0075] Example 1 As Example 1, the semiconductor laser device 100 shown below was fabricated. An MOCVD apparatus was used to fabricate an epitaxial wafer that would become the semiconductor laser device 100. The raw materials were trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH３ ), silane gas, bis(cyclopentadienyl)magnesium (Cp ２ Mg) was used as appropriate.

[0076] On a c-plane GaN substrate (substrate 60), an Al containing Si was deposited. ０．０１６ Ga ０．９８４ The N layer was grown to a thickness of 1.8 μm. Next, the Si-containing Al ０．０８ Ga ０．９２ The N layer was grown to a thickness of 200 nm. ０．０４ Ga ０.９６ The N layer was grown to a thickness of 150 nm. ０．０８ Ga ０．９２ An 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, a Si-containing In layer was grown. ０．０３ Ga ０．９７ An N layer (first semiconductor layer that will become the first semiconductor portion 31a) was grown to a thickness of 150 nm.

[0077] The epitaxial wafer with the above layers formed thereon 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 recesses had a depth of 83 nm, a width of 39 nm, and a diffraction grating period Λ (one period of the unevenness) of 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 concave-convex shape was formed, a GaN layer containing Si (second semiconductor portion 31b) was grown in an MOCVD apparatus to a thickness of 150 nm, which is the thickness from the top of the convex portion of the concave-convex 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 ０．９８４The 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 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 An 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. ０．２５ Ga ０．７５ An active layer 40 including an N layer (well layer 41) and an undoped GaN layer (barrier layer 42) having a thickness of 2.3 nm, in this order, was grown.

[0081] Next, an undoped compositionally graded layer was grown to a thickness of 150 nm. ０．０５ Ga ０．９５ The growth was performed by using N as the growth terminal and GaN as the growth terminal, and the In composition was substantially monotonically decreased so that the composition gradient was almost linear. ０．１０ Ga ０．９０ N layer and Al ０．１６ Ga ０．８４ The N layer was grown to a thickness of 3 nm and 7 nm, respectively. ０．０１５ Ga ０．９８５ The N layer was grown to a thickness of 125 nm. ０．０１５ Ga ０．９８５ 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] The epitaxial wafer on which the above layers were formed was then removed from the MOCVD apparatus, and ridge 20c, p-electrode 82, n-electrode 81, etc. were formed using photolithography, RIE, and sputtering. As shown in FIGS. 1 and 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 p-electrode 82. As shown in FIG. 4 , p-side nitride semiconductor layer 50 was formed using photolithography and RIE to have a structure including first contact layer 511 and second contact layer 512.

[0083] The semiconductor laser device 100 was then singulated, and a first protective film 71 was formed on the first end facet 20a, and a second protective film 72 was formed on the second end facet 20b, thereby obtaining the 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-grating portion 312 and a current of 500 mA was injected, the semiconductor laser device 100 of Example 1 oscillated laser light with a peak wavelength of approximately 528 nm. No voltage was applied between the n-electrode 81 and the first metal film 824 provided on the 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 represents wavelength. The vertical axis of FIG. 6 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 SMSR 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 approximately 3 pm. Since the resolution of the spectrum analyzer used was approximately 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, but 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 device 100 of Example 1 oscillated in a single longitudinal mode.

[0085] FIG. 7 shows a Z-contrast image obtained by STEM for a portion of the semiconductor laser element of Example 1. FIG. 7 is a Z-contrast image of a cross section of a portion of the diffraction grating portion 311 including the first semiconductor portion 31a and the second semiconductor portion 31b. FIG. 7 shows a cross section along the resonance direction D1. In the Z-contrast image, differences in composition can be observed as differences 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 above the recess, filling it, 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, indicating that they have different compositions.

[0086] Example 2 As Example 2, a semiconductor laser element similar to 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 was the light-emitting end facet.

[0087] In the semiconductor laser device of Example 2, a voltage was applied between the n-electrode 81 and the second metal film 826 provided on the non-grating portion 312 to inject a current of 500 mA, and a voltage was also applied between the n-electrode 81 and the first metal film 824 provided on the 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 represents 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 represent the spectra when the currents flowing between the n-electrode 81 and the first metal film 824 were, from left to right, 100 mA, 150 mA, 200 mA, 250 mA, and 300 mA. The SMSR 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. (Item 1) 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, wherein the nitride semiconductor stack comprises: 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, wherein 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, and the active layer has an n-side well layer of the one or more well layers located closest to the second n-side nitride semiconductor layer, and an n-side barrier layer of the one or more barrier layers located between the n-side well layer and the second n-side nitride semiconductor layer, a first p-electrode provided on the top surface of the p-side nitride semiconductor layer above the diffraction grating portion, and a second p-electrode provided on the top surface of the p-side nitride semiconductor layer above the non-diffraction grating portion, spaced apart from the first p-electrode;(Item 4) The semiconductor laser device according to item 3, wherein the first p-electrode includes a first conductive oxide film provided on the upper surface of the p-side nitride semiconductor layer and a first metal film arranged above the first conductive oxide film, the second p-electrode includes a second conductive oxide film provided on the upper surface of the p-side nitride semiconductor layer and a second metal film arranged above the second conductive oxide film, and when viewed from above, one or both of an end on the first end face side of the first conductive oxide film and an end on the second end face side of the second conductive oxide film are located between an end on the first end face side of the first metal film and an end on the second end face side of the second metal film. (Item 5) The semiconductor laser device according to any one of items 1 to 4, wherein 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, and 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. (Item 6) The semiconductor laser device according to any one of Items 1 to 5, wherein a distance from the first n-side nitride semiconductor layer to the well layer is greater than 300 nm. (Item 7) The semiconductor laser device according to any one of Items 1 to 6, wherein 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, and the periodic structure of the diffraction grating portion is configured by alternately arranging the plurality of first portions and the plurality of second portions along the resonance direction. (Item 8) The semiconductor laser device according to any one of Items 1 to 7, wherein the nitride semiconductor stack includes: an n-side cladding layer arranged on the side of the first n-side nitride semiconductor layer opposite to the active layer, and a third n-side nitride semiconductor layer arranged 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.(Item 9) The semiconductor laser element according to Item 8, wherein 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, and 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 higher than an average refractive index of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer. (Item 10) 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, a first p-electrode, and a second p-electrode, wherein the nitride semiconductor stack has: 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; and a p-side nitride semiconductor layer disposed above the active layer, wherein 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 the upper surface of the p-side nitride semiconductor layer above the diffraction grating portion, and a first metal film arranged above the first conductive oxide film; the second p-electrode has a second conductive oxide film provided on the upper surface of the p-side nitride semiconductor layer above the non-diffraction grating portion and spaced apart from the first conductive oxide film, and a second metal film arranged above the second conductive oxide film; and 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 in top view.

[0089] 10 Optical waveguide 20 Carbon dioxide semiconductor laminate 20a First end face 20b Second end face 20c Ridge 30 n-side carbon dioxide semiconductor layer 31 1st n-side carbon dioxide semiconductor layer 31a First semiconductor portion 31b Second semiconductor portion 311 Reflected lattice portion 312 Non-reflected lattice portion 32 2n-side carbon dioxide semiconductor layer 33 3n-side carbon dioxide semiconductor layer 34 4n-side carbon dioxide semiconductor layer 35 5n-side carbon dioxide semiconductor layer 40 Active layer 41 Isotope layer 42 Barrier layer 50 p-side carbon dioxide semiconductor layer 511 First contagion layer 512 Second contagion 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 layer 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; 2. The semiconductor laser element according to claim 1, 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.

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.