Semiconductor laser element

Abstract

To provide a semiconductor laser element that can make a longitudinal mode of an oscillation wavelength a single mode or close to the single mode.SOLUTION: A semiconductor laser element includes a nitride semiconductor multilayer body including a first end surface, a second end surface, and an optical waveguide. The nitride semiconductor multilayer body includes 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 includes a diffraction grating part including a periodic structure in which the refractive index periodically changes along a resonance direction of the optical waveguide, and a non-diffraction grating part existing between the diffraction grating part and the first end surface and not including the periodic structure. The active layer includes an n-side well layer and an n-side barrier layer. The second n-side nitride semiconductor layer is a nitride semiconductor layer containing In and Ga. The thickness of the second n-side nitride semiconductor layer is larger than that of the n-side barrier layer.SELECTED DRAWING: Figure 2

Description

Technical Field

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

Background Art

[0002] A semiconductor laser device in which a diffraction grating is partially provided is known. For example, Patent Document 1 describes an EA (Electro-Absorption) modulator integrated DFB laser device having an oscillator section provided with a diffraction grating layer and a modulator section not provided with a diffraction grating layer. The semiconductor laser device described in Patent Document 1 has an InGaAsP active layer.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present disclosure aims to provide a semiconductor laser device having a nitride semiconductor layer, which can simplify or approximate the longitudinal mode of the oscillation wavelength to a single one.

Means for Solving the Problems

[0005] The present disclosure includes the following aspects. A semiconductor laser device includes 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 laminate includes 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. The first n-side nitride semiconductor layer has a diffraction grating portion provided with a periodic structure in which the refractive index periodically changes along the 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 has 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 is a nitride semiconductor layer containing 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.

Advantages of the Invention

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

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

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Figure 8

Embodiments for Carrying Out the Invention

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

[0009] FIG. 1 is a schematic top view showing the semiconductor laser device of the present embodiment. FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1. FIG. 3 is a cross-sectional view taken along line III-III of FIG. 1. FIG. 4 is a schematic diagram for explaining the first n-side nitride semiconductor layer. As shown in FIGS. 1 to 4, the semiconductor laser device 100 of the present 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 has 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 the present embodiment, the direction from the n-side nitride semiconductor layer 30 to the p-side nitride semiconductor layer 50 will be 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 device 100 is fixed.

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

[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 the group III nitride semiconductor 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 therebetween. The nitride semiconductor laminate 20 has 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 in this order. The nitride semiconductor laminate 20 is, for example, epitaxially grown on the substrate 60. The main surface of the nitride semiconductor laminate 20 is, for example, the +c plane (i.e., the (0001) plane).

[0012] In FIGS. 1 to 3, the resonance direction is defined as resonance direction D1, and the direction perpendicular to the resonance direction is defined as perpendicular direction D2. When the length of the optical waveguide 10 in the perpendicular direction D2 is taken 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. Thereby, the light output of the semiconductor laser element 100 can 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 laminate 20 has the 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 having a current constriction structure other than the ridge 20c, the width of the current constriction structure in the perpendicular 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 non-parallel to the main face of the active layer 40. The first end face 20a and the second end face 20b are, for example, perpendicular to the main face of the active layer 40. The first end face 20a and the second end face 20b are faces that intersect the resonance direction D1 of the optical waveguide 10, and are, for example, perpendicular to the resonance direction D1. The first end face 20a is one of the light emitting end face or the light reflecting end face, and the second end face 20b is the other of the light emitting end face or 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 an n-type impurity. Examples of the n-type impurity include Si and Ge. The n-side nitride semiconductor layer 30 may have an undoped layer that is not intentionally doped with an impurity. 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 provided with a periodic structure in which the refractive index periodically changes along the resonance direction D1, and a non-diffraction grating portion 312 that is located between the diffraction grating portion 311 and the first end face 20a and is 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 element 100 can make the longitudinal mode of its oscillation wavelength single or approach single. Considering a small amount of spontaneous emission light, there is no state of a strictly single longitudinal mode in a strict sense. Therefore, a case where the output of a certain mode is sufficiently stronger than the output of other modes is regarded as a single longitudinal mode or a longitudinal mode close to single. The semiconductor laser element 100 has not only the diffraction grating portion 311 but also the non-diffraction grating portion 312. When forming a diffraction grating by etching, an increase in absorption loss may occur due to damage, defects, or impurities generated. 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. For this reason, improvement in the light emission efficiency of the semiconductor laser element 100 is possible as compared with a semiconductor laser element provided with a periodic structure extending from the first end face 20a to the second end face 20b.

[0017] The diffractive grating section 311 and the non-diffractive grating section 312 are arranged so as to be aligned along the resonance direction D1. In the diffractive grating section 311, a periodic structure is provided directly below the optical waveguide 10. In the periodic structure, the refractive index varies periodically along the direction connecting the first end face 20a and the second end face 20b in the shortest manner. In the non-diffractive grating section 312, at least directly below the optical waveguide 10, no periodic structure is provided. The non-diffractive grating section 312 may be a region where no periodic structure is provided anywhere. The non-diffractive grating section 312 can function as a gain region having no diffractive grating. For this reason, it is preferable that the length of the diffractive grating section 311 along the resonance direction D1 is smaller than the length of the non-diffractive grating section 312 along the resonance direction D1. In other words, it is preferable that the length of the non-diffractive grating section 312 along the resonance direction D1 is larger than the length of the diffractive grating section 311 along the resonance direction D1. Thereby, the optical output of the semiconductor laser element 100 can be improved.

[0018] The length of the non-diffractive grating section 312 in the resonance direction D1 may be, for example, 100 μm or more. As the distance from the periodic structure provided in the diffractive grating section 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 reflectivity at the wavelength corresponding to the periodic structure decreases. By setting the length of the diffractive grating section 311 in the resonance direction D1 to 100 μm or more, it is possible to set the reflectivity corresponding to the periodic structure to a high value to such an extent that the semiconductor laser element 100 can oscillate. If the diffractive grating section 311 is made too long, the absorption loss due to regrowth damage increases, and the reflectivity due to the resonator structure increases too much, making it difficult to obtain optical output. However, by increasing the length of the non-diffractive grating section 312, the actual gain of the semiconductor laser element 100 increases, so that a decrease in optical output can be suppressed. For this reason, it is preferable that the length of the non-diffractive grating section 312 in the resonance direction D1 is 500 μm or more. The length of the non-diffractive grating section 312 in the resonance direction D1 may be 1500 μm or more. The length of the non-diffractive grating section 312 in the resonance direction D1 can be 4000 μm or less.

[0019] In a nitride semiconductor, 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 an uneven structure in one semiconductor layer and then filling the uneven structure with another semiconductor layer. The lower the impurity concentration, the easier it is to fill densely. Therefore, the diffraction grating portion 311 provided with the periodic structure is suitably arranged as a part of the n-side nitride semiconductor layer 30. The periodic structure of the diffraction grating portion 311 included in the first n-side nitride semiconductor layer 31 has, for example, an average refractive index higher than that of the fifth n-side nitride semiconductor layer 35. Alternatively, the first n-side nitride semiconductor layer 31 may also serve as the n-side cladding layer.

[0020] When the periodic structure is arranged close to the active layer 40, an increase in absorption loss due to a relatively high electric field strength of the p-side nitride semiconductor layer 50 and / or a decrease in light confinement in the active layer 40 may occur. Due to these, the threshold current at which the semiconductor laser element 100 oscillates in laser may increase. Therefore, 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, a second n-side nitride semiconductor layer 32 is arranged between the first n-side nitride semiconductor layer 31 and the active layer 40. Thereby, the electric field strength of the p-side nitride semiconductor layer 50 becomes relatively low, so that it is possible to reduce the absorption loss and / or improve the light confinement in the active layer 40. Thereby, it is possible to reduce the threshold current of the semiconductor laser element 100. By reducing the threshold current, the current density during laser oscillation can be reduced, and the probability of appearance of higher-order modes in the longitudinal mode can be reduced. Further, since the electric field strength of the p-side nitride semiconductor layer 50 becomes relatively low, absorption loss due to p-type impurities can be suppressed, and the slope efficiency of the semiconductor laser element 100 can be improved.

[0021] In this embodiment, the periodic structure provided in the diffraction grating section 311 is a diffraction grating. The size of the periodic structure can be appropriately adjusted according to the wavelength of the laser light to be obtained, the composition of the semiconductor 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, serrated, sinusoidal, rectangular, trapezoidal, inverted trapezoidal, or the like. The cross-sectional shape of the convex portion among 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 whose width narrows as it approaches the active layer 40, such as a trapezoidal shape. By having a shape with an inclined side, it is easy to grow a semiconductor layer that fills the unevenness, and the thickness of the semiconductor layer can be reduced. Each convex portion 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 concave portion 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-shaped or V-shaped.

[0022] The period (pitch) of the unevenness constituting the periodic structure can be determined by the wavelength to be oscillated and the effective refractive index. The pitch of the unevenness (one period of the unevenness) can be, for example, 40 nm or more and 140 nm or less. The width of the convex portion and the width of the concave portion in the direction along the resonance direction D1 of the optical waveguide 10 may be the same or different from each other. Further, when providing a diffraction structure of a higher-order mode, it can be 120 nm or more and 420 nm or less for third-order diffraction, and 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 portion or the concave portion is in the range of 1 / 2 to 2 times the width of the other.

[0023] The height H of the unevenness constituting the periodic structure can be 300 nm or less, and may be 200 nm or less. By increasing the height of the unevenness, Γ gratingSince it increases, the coupling coefficient k can be increased. For this reason, the height H of the unevenness is preferably 50 nm or more. The height of the unevenness constituting the periodic structure is, for example, in a cross section perpendicular to the main surface of the active layer 40 and parallel to the resonance direction D1 of the optical waveguide 10, a line parallel to the main surface of the active layer 40 that passes through the portion of the unevenness closest to the active layer 40, and a line parallel to the main surface of the active layer 40 that passes through the portion of the unevenness farthest from the active layer 40. The shortest distance between them. Such a cross section can be observed, for example, by a transmission electron microscope (TEM). The cross section may 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 portion. The periodic structure is configured 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 plurality of first portions are connected to one common portion, and one first semiconductor portion 31a is formed by the plurality of first portions and one common portion. Similarly, the plurality of second portions are connected to one common portion, and one second semiconductor portion 31b is formed by the plurality of second portions and one common portion. In other words, the first semiconductor portion 31a has a plurality of first portions protruding upward from one common portion, the second semiconductor portion 31b has a plurality of 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 another part constitutes 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 part of the first semiconductor portion 31a is the same as that of the first part. The composition of the common part of the second semiconductor portion 31b is the same as that of the second part. Having the same composition means that it can be obtained without intentionally making different compositions, and may include manufacturing errors. Here, the description has been made assuming that the first semiconductor portion 31a includes a plurality of first parts and the second semiconductor portion 31b includes a plurality of second parts, but the first semiconductor portion 31a may include a plurality of second parts and the second semiconductor portion 31b may include a plurality of first parts.

[0027] The first semiconductor portion 31a can be obtained, for example, by forming a first semiconductor layer to be the first semiconductor portion 31a and then removing a part of the first semiconductor layer by dry etching or the like. If the lower surface of the first semiconductor layer is removed during the partial removal, a first semiconductor portion composed only of a plurality of first parts without a common part can be formed. Considering the accuracy of the removal depth, if the removal is performed at a depth that does not reach the lower surface of the first semiconductor layer, the first semiconductor portion 31a composed of one common part and a plurality of first parts 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 plurality of first parts that the first semiconductor portion 31a has. 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 lower the possibility of generating a gap 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 preferably 1×10 20 / cm 3 or less. The n-type impurity concentration of the first semiconductor portion 31a is preferably 1×10 17 / cm 3 or more and 1×10 20 / cm 3It may be as follows, or may be below the detection limit. The 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 impurities in the second semiconductor portion 31b may be below the detection limit. Further, the second semiconductor portion 31b is preferably made of GaN, whereby the possibility of a gap being generated between the second semiconductor portion 31b and the first semiconductor portion 31a can be reduced. In FIG. 2, the upper surface of the second semiconductor portion 31b is the same height from the diffraction grating portion 311 to the non-diffraction grating portion 312, but it is not limited to this. For example, the upper surface of the second semiconductor portion 31b of the diffraction grating portion 311 provided with a concave portion 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] Either one of the plurality of first portions or the plurality of second portions may have at least one of both ends in the vertical direction D2 located inside the nitride semiconductor laminate 20. Both ends of either one of the plurality of first portions or the plurality of second portions may be paraphrased 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 for the formation of 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 way.

[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. Among the first semiconductor portion 31a, a portion sandwiching the plurality of concave shapes along the resonance direction D1 is one of the first portion or the second portion. The plurality of concave shapes are filled by the second semiconductor portion 31b, and a portion filling the plurality of concave shapes is the other of the first portion or the second portion. The first semiconductor portion 31a may have a plurality of convex shapes protruding toward the active layer 40. In this case, a portion of the plurality of convex shapes of the first semiconductor portion 31a is one of the first portion or the second portion, and a portion of the second semiconductor portion 31b sandwiching the plurality of convex shapes along the resonance direction D1 is the other of the first portion or the second portion. By having at least one of both ends in the vertical direction D2 of the concave or convex portion of the first semiconductor portion 31a located inside the nitride semiconductor laminate 20, it is considered that the concave or convex shape can be stably formed. This is because it is expected that the strength of the first semiconductor portion 31a will improve as the width in the vertical direction D2 of the concave or convex shape becomes narrower. Preferably, the concave or convex shape is such that both ends in the vertical direction D2 of the concave or convex shape are located inside the nitride semiconductor laminate 20. The width in the vertical direction D2 of the concave or convex shape 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 in the vertical direction D2 of the concave or convex shape may be a value obtained by adding 5 μm or more to one side and a total of 10 μm or more to the width of the optical waveguide 10. The width in the vertical direction D2 of the concave or convex shape may be smaller than the maximum width in the vertical direction D2 of the p-electrode 82. When using a method in which the formation area and the working time are proportional, such as electron beam lithography, for forming the periodic structure, forming the first semiconductor portion 31a having a plurality of concave shapes can shorten the formation time of the periodic structure more than forming the first semiconductor portion 31a having a plurality of convex shapes. Also, it is expected that the strength of the first semiconductor portion 31a will improve more when forming the first semiconductor portion 31a having a plurality of concave shapes than when 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 InX 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 the 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 at least in a part thereof, the first n-side nitride semiconductor layer 31 may be a layer functioning 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 n ={(n1×t1)+(n2×t2)} / (t1+t2) (1) Here, n1 is the refractive index of the first part, n2 is the refractive index of the second part, t1 is the length of one first part in the resonance direction D1, and t2 is the length of one second part in the resonance direction D1. When the lengths of the first part and the second part in the resonance direction D1 differ depending on the height, the lengths of the first part and the second part at the position half the thickness of the periodic structure may be set as t1 and t2 respectively, and the average refractive index of the periodic structure may be obtained. Note that the refractive index shall be the refractive index at the peak wavelength of the laser light oscillated by the semiconductor laser element 100. As the value of the refractive index of each material, the known refractive index value of each material may be used. The refractive index of the semiconductor can be obtained by calculation 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. Thereby, the threshold current of the semiconductor laser element 100 can be reduced. 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 preferably 500 nm or less. Thereby, a desired coupling efficiency can be easily obtained. The distance from the first n-side nitride semiconductor layer 31 to the active layer 40 may be within those 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 those numerical ranges, and the distance from the periodic structure to the active layer 40 may be within those 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 may be 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. Thereby, it is 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 in the direction perpendicular to the main surface of the active layer 40 of the periodic structure, is the same as 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 element 100 has the ridge 20c, the refractive index of the periodic structure provided in the diffraction grating portion 311 changes periodically along the extending direction of the ridge 20c. The periodic structure is disposed at least directly under 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] The smaller the distance between the periodic structure and the active layer 40, the more likely the electric field strength of the p-side nitride semiconductor layer 50 increases and the absorption loss increases, and / or the light confinement to the active layer 40 tends to decrease. By providing the second n-side nitride semiconductor layer 32, the distance between the first n-side nitride semiconductor layer 31 in which the periodic structure is partially provided and the active layer 40 can be increased as compared with the case where it is not provided. Thereby, the electric field strength of the p-side nitride semiconductor layer 50 can be decreased to reduce the absorption loss, and / or the light confinement to the active layer 40 can be improved. Therefore, the threshold current of the semiconductor laser element 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 larger than the thickness of the n-side barrier layer described later. By providing these configurations, the absorption loss can be reduced, and / or the light confinement to the active layer 40 can be improved.

[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, absorption loss can be reduced and / or light confinement in the active layer 40 can be improved. The refractive index of the material constituting the second n-side nitride semiconductor layer 32 is higher than either of the refractive indices of the materials constituting the periodic structure of the diffraction grating portion 311, so 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. More preferably, 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 the refractive index of the n-side barrier layer. The n-side barrier layer has a bandgap energy larger than the bandgap energy 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, by providing the second n-side nitride semiconductor layer 32 having a refractive index higher than that of the n-side barrier layer, absorption loss can be reduced and / or light confinement in the active layer 40 can be improved. When the n-side barrier layer is composed of a plurality of 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 plurality of 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 a compositionally graded layer that is entirely InGaN and has an increasing In composition ratio 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 an increasing In composition ratio 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, 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. Such an arrangement can be made in order not to excessively increase the distance from the periodic structure of the first n-side nitride semiconductor layer 31 to the active layer 40. The n-side cladding layer is disposed on the side opposite to the active layer 40 of the first n-side nitride semiconductor layer 31. 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. 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 because the refractive index of the material constituting the third n-side nitride semiconductor layer 33 is lower than any of the refractive indices of the materials constituting the periodic structure of the diffraction grating portion 311.

[0048] In FIG. 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] By providing the third n-side nitride semiconductor layer 33, light leaking to the substrate 60 or the like located below it can be reduced. 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 as compared with the case where the first n-side nitride semiconductor layer 31 has no periodic structure and is composed only of GaN. When the refractive index of the first n-side nitride semiconductor layer 31 is relatively high in this way, it is particularly preferable to reduce light leakage by providing the third n-side nitride semiconductor layer 33.

[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 100 nm or more and 1000 nm or less.

[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. It can 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 in that the refractive index of the material constituting the fourth n-side nitride semiconductor layer 34 is lower than either of the refractive indices of the materials constituting the periodic structure of the diffraction grating portion 311.

[0052] By providing the fourth n-side nitride semiconductor layer 34, the light confinement in the active layer 40 can be improved. 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 decreases as compared with the case where the first n-side nitride semiconductor layer 31 has no periodic structure and is composed only of GaN. In such a case where the refractive index of the first n-side nitride semiconductor layer 31 is relatively low, it is particularly preferable to improve the light confinement in the active layer 40 by providing the fourth n-side nitride semiconductor layer 34. Alternatively, instead of providing the fourth n-side nitride semiconductor layer 34, the thickness of the common portion of the second semiconductor portion 31b may be 50 nm or more. Thereby, the light confinement in the active layer 40 can be improved. 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 1 nm or more and 500 nm or less.

[0054] (Fifth n-side nitride semiconductor layer 35) The fifth n-side nitride semiconductor layer 35 is disposed on the side opposite to the active layer 40 of the first n-side nitride semiconductor layer 31. 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 bandgap energy among the n-side nitride semiconductor layers 30. The fifth n-side nitride semiconductor layer 35 is, for example, an AlGaN layer containing an n-type impurity.

[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 can 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 located closest to the second n-side nitride semiconductor layer 32 among the one or more well layers 41, and an n-side barrier layer located between the n-side well layer and the second n-side nitride semiconductor layer 32 among the one or more barrier layers 42.

[0057] The active layer 40 is preferably provided from above the diffraction grating portion 311 to above the non-diffraction grating portion 312. Furthermore, it is preferable that each layer above the first n-side nitride semiconductor layer 31 including the active layer 40 is provided from above the diffraction grating portion 311 to above the non-diffraction grating portion 312. As a method for forming the diffraction grating portion and the non-diffraction grating portion, for example, there is a method in which a first epi-structure including the non-diffraction grating portion and the active layer is formed, a part of it is removed to expose the end face of the active layer, and then a second epi-structure including the diffraction grating portion and the active layer is regrown. In such a method, impurities are incorporated into the interface between the exposed end face and the second epi-structure to be regrown during regrowth, and there are concerns about an increase in optical absorption loss due to the impurities and an increase in carrier traps. 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. Thereby, the semiconductor laser element 100 can be made 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 of the first n-side nitride semiconductor layer 31 during driving of the semiconductor laser element 100 is smaller than the electric field strength of the active layer 40, such an arrangement can reduce the influence caused by impurities incorporated 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 the layer provided thereon is less than the step, it may be divided by the step, but such a state can also be said to be a state provided from above the diffraction grating portion 311 to above the non-diffraction grating portion 312.

[0059] When there are a plurality of semiconductor layers 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 larger than the thickness of the layer having the largest thickness among them. Thereby, absorption loss can be reduced and / or light confinement in the active layer 40 can be improved. Furthermore, it is preferable that the thickness of the second n-side nitride semiconductor layer 32 is larger than the total thickness of the plurality of semiconductor layers located between the n-side well layer and the second n-side nitride semiconductor layer 32. Thereby, further, absorption loss can be reduced and / or light confinement in the active layer 40 can be improved.

[0060] The active layer 40 can be formed of a composition capable of emitting light having a wavelength of, for example, 400 nm or more and 600 nm or less. The one or more well layers 41 are made of, for example, InGaN. The In composition ratio of the InGaN constituting 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 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 a p-type impurity. Examples of the p-type impurity include Mg. The p-side nitride semiconductor layer 50 may have an undoped layer that is not intentionally doped with an impurity. The p-side nitride semiconductor layer 50 can have a contact layer. The p-side nitride semiconductor layer 50 can 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 other layers.

[0062] In a nitride semiconductor, the activation rate of a p-type impurity is lower than that of an n-type impurity. For this reason, 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 larger 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 element 100 has an n-electrode 81. The n-electrode 81 is provided on the lower surface of the substrate 60. Examples of the material of the n-electrode 81 include a single-layer film or a multilayer film such as a metal or alloy of Ni, Rh, Cr, Au, W, Pt, Ti, Al, etc., or a conductive oxide containing at least one selected from Zn, In, Sn. Examples of the conductive oxide include ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), GZO (Gallium-doped Zinc Oxide), etc.

[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. The semiconductor laser element 100 can have a pad electrode for the p-electrode 82. Examples of the material of the p-electrode 82 include a single-layer film or a multilayer film such as a metal or alloy of Ni, Rh, Cr, Au, W, Pt, Ti, Al, etc., or a conductive oxide containing at least one selected from Zn, In, Sn. Examples of the conductive oxide include ITO, IZO, GZO, etc.

[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 above the diffraction grating portion 311 on the upper surface of the p - side nitride semiconductor layer 50. The second p - electrode 822 is provided above the non - diffraction grating portion 312 on the upper surface of the p - side nitride semiconductor layer 50, separated from the first p - electrode 821. By having such a first p - electrode 821 and a second p - electrode 822, independent current control can be performed for each of the diffraction grating portion 311 and the non - diffraction grating portion 312, and currents of appropriate magnitudes can be passed through each. 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 such that light absorption in the diffraction grating portion 311 is reduced. When no current is passed through the diffraction grating portion 311, the well layer 41 becomes a high - absorption layer and a rising kink occurs, but this can be suppressed by passing a current through the diffraction grating portion 311. The current flowing through the diffraction grating portion 311 may be smaller than the current flowing through the non - diffraction grating portion 312. Also, wavelength tuning may be performed by changing the magnitude of the current flowing through the diffraction grating portion 311. The first p - electrode 821 is arranged separated from the second p - electrode 822.

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

[0067] The first p - electrode 821 can include 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 disposed above the first conductive oxide film 823. The second p - electrode 822 can include 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 disposed above the second conductive oxide film 825. In a top view, 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 preferably 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 - face 20a side and the end of the second conductive oxide film 825 on the second end - face 20b side are preferably 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. Thereby, compared with the case where the first p - electrode 821 and the second p - electrode 822 are formed only of metal, the distance from the first p - electrode 821 to the second p - electrode 822 can be reduced. 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 light - emitting - side end, and the end on the second end - face 20b side is the light - reflecting - side end. Alternatively, the end on the first end - face 20a side may be the light - reflecting - side end, and the end on the second end - face 20b side may be the light - emitting - side end. More preferably, both 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. More preferably, both 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. No current is injected between the first p - electrode 821 and the second p - electrode 822, and the greater the distance therebetween, the greater the light absorption.By using the first conductive oxide film 823 and the second conductive oxide film 825, the region where no current is injected can be reduced, so that light absorption can be reduced. The distance from the first conductive oxide film 823 to the second conductive oxide film 825 can be, 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 side of the first end face 20a protrudes from the end of the first metal film 824 on the side of the first end face 20a toward the second p electrode 822. In a top view, the end of the first conductive oxide film 823 on the light-emitting side protrudes from the end of the first metal film 824 on the light-emitting side toward the second p electrode 822. In a 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 a top view, the end of the second conductive oxide film 825 on the side of the second end face 20b protrudes from the end of the second metal film 826 on the side of the second end face 20b toward the first p electrode 821. In a top view, the end of the second conductive oxide film 825 on the light-reflecting side protrudes from the end of the second metal film 826 on the light-reflecting 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 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 a 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 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 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 laminate 20. The second protective film 72 is provided on the second end face 20b of the nitride semiconductor laminate 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 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 reflectivity of the first protective film 71 can be, for example, 0.1% or more, and preferably 5% or more. Thereby, the threshold current can be reduced as compared with the case where the first protective film 71 is an AR (antireflection) coat. By using the first end face 20a as the light emitting end face, the diffraction grating portion 311 can be used as a part of the reflective film on the light reflection side, and the reflectivity on the light reflection side can be increased, so that the light output can be increased.

[0071] When it is desired to further suppress the increase in the threshold current, the reflectivity 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 having a peak wavelength of 500 nm or more, the gain inside the resonator tends to be lower than that of an element that emits laser light having a peak wavelength of less than 500 nm. For this reason, in the case of the semiconductor laser element 100 that emits laser light having a peak wavelength of 500 nm or more, it is preferable that the reflectivity of the first protective film 71 is 30% or more and less than the reflectivity of the second protective film 72. Thereby, the threshold current can be reduced. When a periodic structure is provided in the nitride semiconductor laminate 20 and the longitudinal mode of the oscillation wavelength is made single or closer to single by the periodic structure, the confinement factor related to laser oscillation decreases as compared with the case where it is not. The higher the reflectivity of the first protective film 71, the higher the confinement factor can be increased. The reflectivity of the first protective film 71 may be 60% or more, or may be 80% or more. The upper limit of the reflectivity of the first protective film 71 can be less than the reflectivity of the second protective film 72.

[0072] The reflectance of the second protective film 72 is higher than that 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 reflectances at the peak wavelength of the laser light oscillated by the semiconductor laser element 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 output end face. In this case, the reflectance of the second protective film 72 is made lower than that of the first protective film 71. The reflectance of the first protective film 71 can be, for example, 85% or more, and may be 90% or more. The reflectance of the first protective film 71 can be, for example, 100% or less. The second protective film 72 may be an AR coat. The reflectance of the second protective film 72 may be 0.1% or more, and may be 5% or more. The second protective film 72 may not be provided. By using the second end face 20b as the light output end face, the second protective film 72 can be made unnecessary. By using the second end face 20b as the light output 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 can be obtained. By forming the diffraction grating portion 311 with a length of 100 μm or more in the resonance direction D1, it is possible to sufficiently obtain the reflectance necessary for laser oscillation even without the second protective film 72.

[0074] (Insulating film 73) The semiconductor laser element 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, for example, a single-layer film or a multilayer film of oxides or nitrides such as Si, Al, Zr, Ti, Nb, Ta, etc.

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

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

[0077] The epitaxial wafer on which the above layers were formed was taken out 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 part 311. The length along the resonance direction D1 of the diffraction grating part 311 was set to 300 μm, and the length along the resonance direction D1 of the non-diffraction grating part 312 was set to 500 μm.

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

[0079] Next, an undoped In 0.03 Ga 0.97 N layer (second n-side nitride semiconductor layer 32) was grown to a thickness of 240 nm. From the Si-containing Al 0.016 Ga 0.984 N layer to this layer constitutes the n-side nitride semiconductor layer 30.

[0080] Next, an n-side barrier layer (barrier layer 42) composed of three layers of a 1-nm Si-doped GaN layer, an 8-nm Si-doped In 0.05 Ga 0.95 N layer, and a 1-nm Si-doped GaN layer, and an undoped In 0.25 Ga 0.75 N layer (well layer 41) with a thickness of 2.1 nm, a 3.3-nm undoped GaN layer (barrier layer 42), and an undoped In 0.25 Ga 0.75 N layer (well layer 41) with a thickness of 2.1 nm, and a 2.3-nm undoped GaN layer (barrier layer 42) were grown in this order to form the active layer 40.

[0081] Next, an undoped compositionally graded layer was grown to a thickness of 150 nm. The compositionally graded layer was grown with the starting point of growth as In 0.05 Ga 0.95 N and the ending point of growth as GaN, and the In composition was substantially monotonically decreased so that the compositional gradient was substantially linear. Next, an Mg-containing Al 0.10 Ga 0.90 N layer and an Al 0.16 Ga 0.84 N layer were each grown to a thickness of 3 nm and 7 nm. Next, an undoped Al 0.015 Ga 0.985 N layer was grown to a thickness of 125 nm. Next, an 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. From the undoped compositionally graded layer to this layer is the p-side nitride semiconductor layer 50.

[0082] Then, the epitaxial wafer on which the above layers were formed was taken out from the MOCVD apparatus, and the ridge 20c, the p electrode 82, the n electrode 81, etc. were formed using photolithography, RIE, and sputtering. As shown in FIGS. 1 and 2, as the p electrode 82, 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 shown in FIG. 4, the p-side nitride semiconductor layer 50 was formed into a structure having a first contact layer 511 and a second contact layer 512 using photolithography and RIE.

[0083] Thereafter, it was diced, a first protective film 71 was formed on the first end face 20a, and a second protective film 72 was formed on the second end face 20b to obtain the semiconductor laser element 100. The reflectance of the first protective film 71 was set to 90%, and the reflectance of the second protective film 72 was set to 97%. That is, in the semiconductor laser element 100 of Example 1, the first end face 20a is the light-emitting end face. The semiconductor laser element 100 had a ridge width of 2 μm, a resonator length of 800 μm, and an element width of 200 μm.

[0084] In the semiconductor laser element 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 element 100 of Example 1 oscillated laser light 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 element 100 of Example 1 is shown in FIG. 6. The horizontal axis in FIG. 6 is the wavelength. The vertical axis in 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 being 0 dB. The side mode suppression ratio of the semiconductor laser element 100 of Example 1 was greater than 20 dB. The spectral width of the semiconductor laser element 100 of Example 1 was about 3 pm. Since the resolution of the used spectral analyzer was about 3 pm, it can be said that an extremely narrow spectral width was obtained. The interval between adjacent longitudinal modes calculated from the effective refractive index and the resonator length is about 0.06 nm to 0.07 nm, but no large peak considered to be caused by adjacent longitudinal modes was observed, and since the side mode suppression ratio was at least greater than 20 dB, 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 in the direction along the resonance direction D1. In the Z-contrast image, the difference in composition can be observed as the difference in display density in the image. In FIG. 7, the portion having a recess recessed downward is the first semiconductor portion 31a, and the portion that fills the recess and is provided thereon 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 reflectivity of the first protective film 71 was set to 90%. That is, in the semiconductor laser element of Example 2, the second end face 20b is the light emitting end face.

[0087] In the semiconductor laser element of Example 2, a voltage was applied between the n electrode 81 and the second metal film 826 provided on the non-diffraction 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 diffraction grating portion 311 to inject a current of 100 mA to 300 mA. The spectrum of the semiconductor laser element of Example 2 is shown in FIG. 8. The horizontal axis in FIG. 8 is the wavelength. The vertical axis in FIG. 8 represents the light intensity as an intensity ratio (unit: dB), and the side mode suppression ratio (SMSR) was estimated with the largest peak being 0 dB. The five peaks in FIG. 8 are the spectra when the currents flowing between the n electrode 81 and the first metal film 824 are 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 element of Example 2 was greater than 20 dB. The semiconductor laser element 100 of Example 2 oscillated laser light with a peak wavelength of about 528.5 nm by injecting a current of 100 mA between the n electrode 81 and the first metal film 824. The semiconductor laser element 100 of Example 2 oscillated laser light with a peak wavelength of about 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 element of Example 2 oscillated in a single longitudinal mode, and the wavelength could be controlled by further changing the injection current.

[0088] Through the content described so far in this specification, the following technical matters are disclosed. (Item 1) A nitride semiconductor laminate including a first end face, a second end face, and an optical waveguide connecting the first end face and the second end face is provided. The nitride semiconductor laminate 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, and having, The first n-side nitride semiconductor layer has a diffraction grating portion provided with a periodic structure in which the refractive index periodically changes along the 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 has 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 is a nitride semiconductor layer containing In and Ga, A semiconductor laser element in which the thickness of the second n-side nitride semiconductor layer is larger than the thickness of the n-side barrier layer. (Item 2) The semiconductor laser element according to Item 1, wherein the length of the diffraction grating portion along the resonance direction is smaller than the length of the non-diffraction grating portion along the resonance direction. (Item 3) A first p-electrode provided on the upper surface of the p-side nitride semiconductor layer and above the diffraction grating portion, A second p-electrode provided on the upper surface of the p-side nitride semiconductor layer and separated from the first p-electrode above the non-diffraction grating portion, the semiconductor laser element according to Item 1 or 2. (Item 4) The first p-electrode has a first conductive oxide film provided on the 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 the upper surface of the p-side nitride semiconductor layer, and a second metal film disposed above the second conductive oxide film. In a top view, one or both of the end on the first end face side of the first conductive oxide film and the end on the second end face side of the second conductive oxide film are located between the end on the first end face side of the first metal film and the end on the second end face side of the second metal film. The semiconductor laser element according to item 3. (Item 5) The refractive index of the second n-side nitride semiconductor layer is higher than the average refractive index of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer. The thickness of the second n-side nitride semiconductor layer is larger than the thickness of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer. The semiconductor laser element according to any one of items 1 to 4. (Item 6) The distance from the first n-side nitride semiconductor layer to the well layer is greater than 300 nm. The semiconductor laser element according to any one of items 1 to 5. (Item 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. 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. The semiconductor laser element according to any one of items 1 to 6. (Item 8) The nitride semiconductor laminate An n-side cladding layer disposed on the side opposite to the active layer of the first n-side nitride semiconductor layer, 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 the refractive index of the n-side cladding layer and the average refractive index of the periodic structure of the diffraction grating portion of the first n-side nitride semiconductor layer. The semiconductor laser element according to any one of items 1 to 7. (Item 9) The nitride semiconductor laminate has a fourth n-side nitride semiconductor layer disposed between the second n-side nitride semiconductor layer and the first n-side nitride semiconductor layer. The refractive index of the 4n-side nitride semiconductor layer is lower than that of the 2n-side nitride semiconductor layer and higher than the average refractive index of the periodic structure of the diffraction grating portion of the 1n-side nitride semiconductor layer, the semiconductor laser device according to item 8. (Item 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, and a first p electrode and a second p electrode. The nitride semiconductor laminate 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 And has The first n-side nitride semiconductor layer has a diffraction grating portion provided with a periodic structure in which the refractive index periodically changes along the resonance direction of the optical waveguide, and is located between the diffraction grating portion and the first end face and has no periodic structure. And a non-diffraction grating portion. 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 disposed 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 separated from the first conductive oxide film, and a second metal film disposed above the second conductive oxide film. In a top view, one or both of the end on the first end face side of the first conductive oxide film and the end on the second end face side of the second conductive oxide film are located between the first metal film and the second metal film. A semiconductor laser device.

Explanation of symbols

[0089] 10 Optical waveguide 20 Nitride semiconductor laminate 20a First 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; 3 . The semiconductor laser device according to claim 1 , further comprising: a second p-electrode provided on an upper surface of the p-side nitride semiconductor layer above the non-grating portion and spaced apart from the 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, 5. The semiconductor laser element 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. 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, 7. The semiconductor laser element 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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