Semiconductor laser element

Abstract

Provided is a semiconductor laser element having a periodic structure in which a threshold current is reduced. A semiconductor laser element comprising a nitride semiconductor stack having an optical waveguide, the nitride semiconductor stack comprising, in this order: a first n-side nitride semiconductor layer (31) having a periodic structure in which the refraction index is periodically changed along a resonance direction of the optical waveguide; a second n-side nitride semiconductor layer (32); an active layer (40) having one or more well layers and one or more barrier wall layers; and a p-side nitride semiconductor layer (50). The active layer (40) includes, among the one or more well layers, an n-side well layer positioned closest to the second n-side nitride semiconductor layer (32), and, among the one or more barrier wall layers, an n-side barrier wall layer positioned between the n-side well layer and the second n-side nitride semiconductor layer (32). The second n-side nitride semiconductor layer (32) is a nitride semiconductor layer comprising In and Ga. The thickness of the second n-side nitride semiconductor layer (32) is greater than the thickness of the n-side barrier wall layer.

Description

semiconductor laser element

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

[0002] Today, semiconductor laser elements having nitride semiconductors are capable of emitting light from the ultraviolet region to green, and are used in a variety of applications, such as light sources for optical discs and projectors, medical light sources, and automotive headlights. Applications such as spectroscopic light sources and visible light communications may require a narrow wavelength spectral width and high wavelength controllability. Distributed feedback (DFB) laser elements are expected to be used in such applications. For example, Patent Document 1 describes a DFB laser element having a diffraction grating.

[0003] International Publication No. 2019 / 146321

[0004] Among semiconductor laser elements having nitride semiconductors, DFB laser elements provided with a diffraction grating tend to have a larger threshold current than Fabry-Perot type semiconductor laser elements not provided with a diffraction grating.

[0005] The present disclosure includes the following invention (1): (1) A semiconductor laser device comprising: a nitride semiconductor stack having an optical waveguide, the nitride semiconductor stack including: a 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, a second n-side nitride semiconductor layer, an active layer having one or more well layers and one or more barrier layers, in this order, and a p-side nitride semiconductor layer, the active layer including an n-side well layer of the one or more well layers located nearest 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, 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 invention, the threshold current can be reduced in a semiconductor laser device having a periodic structure.

[0007] 1 is a schematic top view showing a semiconductor laser element according to an embodiment of the present invention, a cross-sectional view taken along line II-II in FIG. 1, and a cross-sectional view taken along line III-III in FIG. 1. In calculation example 1, the thickness of the second n-side nitride semiconductor layer and the light intensity ratio Γ of the portion coupled to the diffraction grating grating 10 is a graph showing the relationship between the thickness of the second n-side nitride semiconductor layer and the optical confinement Γ of the well layer in Calculation Example 1. well 10 is a graph showing the relationship between the thickness of the second n-side nitride semiconductor layer and the ratio Γ of leakage light to the p-side nitride semiconductor layer in Calculation Example 1. ｐ 1 is a graph showing the relationship between. FIG. 1 is a graph showing the results of calculating a normalized coupling coefficient kL in calculation example 1. FIG. 2 is a graph showing the IL characteristics of the semiconductor laser elements of Example 1 and Comparative Example 1. FIG. 3 is a graph showing the wavelength spectra of the semiconductor laser elements of Example 1 and Comparative Example 1. FIG. 4 is a graph showing the side mode suppression ratio of the semiconductor laser element of Example 1. FIG. 5 is a graph showing the IL characteristics of the semiconductor laser elements of Example 2 and Comparative Example 2. FIG. 6 is a graph showing the wavelength spectrum of the semiconductor laser element of Example 2. FIG. 7 is a graph showing the wavelength spectrum of the semiconductor laser element of Comparative Example 2. FIG. 8 is a graph showing the side mode suppression ratio of the semiconductor laser element of Example 2. FIG. 9 is a Z-contrast image of a portion 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 . As shown in FIGS. 1 to 3 , the semiconductor laser device 100 of this embodiment includes a nitride semiconductor stack 20 having an optical waveguide 10. The semiconductor laser device 100 of this embodiment includes a substrate 60, on which the nitride semiconductor stack 20 is disposed. The nitride semiconductor stack 20 includes an n-side nitride semiconductor layer 30, an active layer 40, and a p-side nitride semiconductor layer 50. In this embodiment, the direction from the n-side nitride semiconductor layer 30 toward the p-side nitride semiconductor layer 50 will be described as the upward direction. This upward direction does not necessarily 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, 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 direction D1, and the direction perpendicular to the resonance direction is defined as direction D2. The width W of the optical waveguide 10 in the direction perpendicular to the resonance direction (direction D2) １０ is, for example, 1 μm or more. １０ is preferably 10 μm or more. This makes it possible to improve the optical output of the semiconductor laser device 100, and by providing the first n-side nitride semiconductor layer 31 having a periodic structure, the longitudinal mode of the oscillation wavelength can be made single or close to single. Considering the small amount of spontaneous emission light, a state of single longitudinal mode does not exist in the strict sense. Therefore, a case where the output of one mode is sufficiently stronger than the output of other modes is considered to be a single longitudinal mode or a longitudinal mode close to single. The width W of the optical waveguide 10 １０ The width W of the optical waveguide 10 is more preferably 50 μm or more, and may be 80 μm or more. １０ can be 400 μm or less.

[0013] When the nitride semiconductor laminate 20 has a ridge 20c as shown in FIGS. 1 to 3, the width of the ridge 20c is set to be equal to the width W of the optical waveguide 10.１０ Alternatively, when a current confinement structure other than the ridge 20c is provided, the width of the current confinement structure in the direction D2 can be regarded as the width W of the optical waveguide 10. １０ It can be considered as such.

[0014] Length L in the resonance direction (direction D1) of the optical waveguide 10 １０ The longer the distance from the active layer 40 to the periodic structure of the first n-side nitride semiconductor layer 31, the lower the coupling efficiency between the light from the active layer 40 and the periodic structure. １０ By increasing the length L of the optical waveguide 10, it is possible to suppress a decrease in the optical output of the semiconductor laser device 100. １０ The length L of the optical waveguide 10 is preferably 1000 μm or more. １０ The length L of the optical waveguide 10 may be 1500 μm or more. １０ The length L of the optical waveguide 10 can be set to 3000 μm or less. １０ is equal to the cavity length.

[0015] The nitride semiconductor stack 20 has a light-emitting end face 20a and a light-reflecting end face 20b. The light-emitting end face 20a and the light-reflecting end face 20b are surfaces non-parallel to the major surface of the active layer 40. The light-emitting end face 20a and the light-reflecting end face 20b are, for example, surfaces perpendicular to the major surface of the active layer 40. The light-emitting end face 20a and the light-reflecting end face 20b are surfaces that intersect with the resonance direction (direction D1) of the optical waveguide 10, and are, for example, surfaces perpendicular to direction D1.

[0016] (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.

[0017] (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.

[0018] (Third n-side nitride semiconductor layer 33 ) The third n-side nitride semiconductor layer 33 is disposed between the fifth n-side nitride semiconductor layer 35 and the first n-side nitride semiconductor layer 31 .

[0019] The third n-side nitride semiconductor layer 33 may have a refractive index between that of the fifth n-side nitride semiconductor layer 35 and the average refractive index of the first n-side nitride semiconductor layer 31. For example, if the volume ratio of the first semiconductor portion 31a to the second semiconductor portion 31b in the first n-side nitride semiconductor layer 31 is 1:1, the average refractive index of the first n-side nitride semiconductor layer 31 can be half the sum of the refractive index of the first semiconductor portion 31a and the refractive index of the second semiconductor portion 31b. Alternatively, if the refractive index of the third n-side nitride semiconductor layer 33 is lower than both the refractive index of the first semiconductor portion 31a and the refractive index of the second semiconductor portion 31b, 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 first n-side nitride semiconductor layer 31. The refractive index of each semiconductor can be estimated from the composition of the semiconductor.

[0020] Providing the third n-side nitride semiconductor layer 33 can reduce light leakage into the fifth n-side nitride semiconductor layer 35 and the substrate 60. 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.

[0021] 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.

[0022] (First n-side nitride semiconductor layer 31) The first n-side nitride semiconductor layer 31 has a periodic structure in which the refractive index changes periodically along the resonance direction (direction D1) of the optical waveguide 10. In nitride semiconductors, the activation rate of n-type impurities (e.g., Si) tends to be higher than the activation rate 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, it is suitable to provide the periodic structure in the n-side nitride semiconductor layer 30. The periodic structure of the first n-side nitride semiconductor layer 31 has a 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.

[0023] If the periodic structure is located 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 having a periodic structure is located away from the active layer 40. For example, as shown in FIG. 2 , the second n-side nitride semiconductor layer 32 is located 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, thereby reducing the threshold current of the semiconductor laser device 100. Reducing the threshold current reduces the current density during laser oscillation and the probability of higher-order longitudinal modes appearing. Furthermore, improving optical confinement in the active layer 40 improves the slope efficiency of the semiconductor laser device 100. The following explanation will be given using calculation results.

[0024] First, in a distributed feedback (DFB) laser or a distributed Bragg reflector (DBR) laser, a diffraction grating is formed along an optical waveguide, and by coupling a forward wave and a backward wave in the optical waveguide, feedback becomes strong near the Bragg frequency, resulting in frequency selectivity. This enables laser oscillation in a single longitudinal mode or a longitudinal mode close to a single mode. For example, if light propagates parallel to the diffraction grating and Bragg reflection occurs, the forward wave and the backward wave must be in phase, so the diffraction grating period Λ is determined by the mode order m and the effective refractive index n. ｅｆｆ and wavelength λ, it is expressed by equation (1).

[0025]

[0026] However, when there is reflection at the end face, the DFB mode changes depending on the phase of the diffraction grating, so a phase shift can be imparted by changing the spacing of part of the diffraction grating. Examples include a λ / 4 shift type, an equivalent phase shift type with a flat section, a pitch modulation shift type that changes the pitch period of the diffraction grating, and a multi-phase shift type. In addition to a one-dimensional periodic structure in the horizontal direction, a two-dimensional periodic structure can also be considered. Even in the case of a one-dimensional periodic structure, a structure that does not have periodicity in another in-plane direction relative to the direction of the periodic structure, or a structure with a shifted period, can also be considered.

[0027] In a DFB laser, the coupling coefficient k indicates the degree to which propagating light is diffracted by a diffraction grating and coupled with other light per unit length. For a general shape, the coupling coefficient k of the TE mode (Transverse Electric mode) is expressed by Equation (2) and Equation (3). where β is the propagation constant, E ｙ is the electric field of the TE mode, k ０ = 2π / λ, where λ is the wavelength and n(x, z) is the refractive index. For example, if the diffraction grating is rectangular and has a low height, and the electric field strength can be considered to be constant in the diffraction grating region, the coupling coefficient k can be approximately expressed by the following equation (4). where n １ is the refractive index of the convex part of the diffraction grating, n ２is the refractive index of the recesses of the diffraction grating, n ｅｆｆ is the effective refractive index, Γ grating is the fraction of light intensity coupled to the diffraction grating, Λ １ is the width of the convex portion of the diffraction grating. From these equations, it can be seen that the greater the difference in refractive index between the concave and convex portions and the higher the proportion of the electric field that couples to the diffraction grating, the higher the coupling coefficient.

[0028] Gamma grating can be calculated by equivalent refractive index calculation. In the calculations shown below, the refractive index of each layer was calculated based on the composition ratio of the nitride semiconductor constituting that layer using the formula described in M.J. Bergmann, et. Al., JOURNAL OF APPLIED PHYSICS vol. 84 (1998) pp. 1196-1203. As Calculation Example 1, the refractive index of each layer was calculated based on the composition ratio of the nitride semiconductor constituting that layer using the semiconductor laser device of Example 1 described later and the second n-side nitride semiconductor layer 32 (undoped In ０．０３ Ga ０．９７ 4 to 6 were calculated using the same structure except for varying the thickness of the second n-side nitride semiconductor layer 32 (N layer). The thicknesses of the second n-side nitride semiconductor layer 32 were set to 15 nm, 50 nm, 100 nm, 150 nm, 200 nm, 300 nm, and 400 nm. That is, in Calculation Example 1, the distances from the second n-side nitride semiconductor layer 32 to the active layer 40 were set to 215 nm, 250 nm, 300 nm, 350 nm, 400 nm, 500 nm, and 600 nm.

[0029] In the calculation example 1, the ratio Γ of the light intensity of the portion coupled to the diffraction grating to the thickness of the second n-side nitride semiconductor layer 32 grating and the optical confinement Γ of the well layer 41 well and the ratio Γ of light leaking into the p-side nitride semiconductor layer 50 ｐ The relationships between the second n-side nitride semiconductor layer 32 and the optical intensity at the diffraction grating are shown in Figures 4, 5, and 6, respectively. As shown in Figure 4, the optical intensity at the diffraction grating decreases as the thickness of the second n-side nitride semiconductor layer 32 increases, and the degree of decrease becomes gentler from a thickness of approximately 400 nm. As mentioned above, when the proportion of the electric field coupled to the diffraction grating is high, the coupling coefficient k is high. Therefore, at first glance, it seems that a thinner second n-side nitride semiconductor layer 32 is preferable. However, as shown in Figure 5, the optical confinement Γ of the well layer 41 wellis maximum at 300 nm and gradually decreases around that point. In the DFB laser, as in a typical Fabry-Perot laser, the threshold current increases as the optical confinement Γ in the active layer decreases. Furthermore, as shown in FIG. 6, the thinner the second n-side nitride semiconductor layer 32 is, the lower the ratio Γ of light leaking into the p-side nitride semiconductor layer 50 becomes. ｐ increases. ｐ An increase in ρ causes an increase in free carrier absorption loss in the p-side nitride semiconductor layer 50, and an increase in loss inside the resonator causes an increase in threshold current and a decrease in slope efficiency.

[0030] Here, the free carrier absorption loss is, for example, the leakage light Γ into the p-type semiconductor layer in the case of a p-type semiconductor layer. ｐ and the coefficient σ reflecting the impurity concentration n of the p-type semiconductor layer and the free carrier absorption cross section ｆｃ This can be approximately explained by the product of α ｆｃ = n × σ ｆｃ ×Γ p (5)

[0031] In other words, even if the impurity concentration of the p-type semiconductor layer is the same, if the leakage light into the p-type semiconductor layer increases, the free carrier absorption loss α ｆｃ increases. An example of the p-type semiconductor layer is a layer made of a nitride semiconductor containing p-type impurities such as Mg. Since the activation rate of p-type impurities in nitride semiconductors is lower than that of n-type impurities such as Si, a relatively large amount of p-type impurities is required in the p-type semiconductor layer, and the free carrier absorption loss due to the p-type impurities increases. From the above formula, it can be understood that the more light leaked into the p-side nitride semiconductor layer 50, the more the free carrier absorption loss increases, which in turn increases the loss inside the resonator, and therefore the threshold current increases. Similarly, even if the amount of light leaked into the p-side nitride semiconductor layer 50 remains the same, the free carrier absorption loss α ｆｃ increases, and the loss inside the resonator increases, so the threshold current increases.

[0032] The calculation results in FIG. 6 indicate that a larger thickness of the second n-side nitride semiconductor layer 32 is preferable for fabricating a DFB laser with a low threshold current and high slope efficiency. On the other hand, the coupling coefficient k is important for obtaining a stable single-longitudinal-mode or near-single-longitudinal-mode DFB laser. The calculation results in FIG. 4 indicate that the coupling coefficient k decreases as the thickness of the second n-side nitride semiconductor layer 32 increases. Therefore, focusing only on the coupling coefficient k, it is considered that increasing the thickness of the second n-side nitride semiconductor layer 32 is disadvantageous for obtaining a stable single-longitudinal-mode or near-single-longitudinal-mode DFB laser. However, the parameter that actually affects the coupling between the diffraction grating and the propagating light is not only the coupling coefficient k, but also the product kL of the coupling coefficient k and the region length L of the diffraction grating. kL is also called the normalized coupling coefficient.

[0033] 7 shows the calculation results of the normalized coupling coefficient kL. In FIG. 7, the coupling coefficient k used in the calculation is the same as in FIG. 4, and the region lengths L of the circles, squares, triangles, and crosses are 300 μm, 600 μm, 1000 μm, and 2000 μm, respectively. It can be seen from FIG. 7 that the desired normalized coupling coefficient kL can be obtained by adjusting the cavity length. In other words, it is possible to increase the film thickness of the second n-side nitride semiconductor layer 32 to reduce free carrier absorption loss and to obtain the desired normalized coupling coefficient kL at the same time.

[0034] In this embodiment, the periodic structure of the first n-side nitride semiconductor layer 31 is a diffraction grating. By providing the first n-side nitride semiconductor layer 31 with a periodic structure, the semiconductor laser device 100 can be a DFB laser device. The size of the periodic structure can be adjusted as appropriate depending on the wavelength of the laser light to be obtained, the composition of the semiconductor used, etc.

[0035] The cross-sectional shape of the irregularities constituting the periodic structure along the resonance direction (direction D1) of the optical waveguide 10 can be, for example, sawtooth, sinusoidal, rectangular, trapezoidal, inverted trapezoidal, etc. While the cross-sectional shape of the convex portions of the irregularities constituting the periodic structure is rectangular in FIG. 2 , it is preferable that the cross-sectional shape be a trapezoidal shape with inclined sides that narrow as they approach the active layer 40. This facilitates growth of the semiconductor layer filling the irregularities, allowing the thickness of the semiconductor layer to be reduced. 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 of the concave portions of the irregularities may have a shape without a bottom surface, such as a V-shape.

[0036] The period (pitch) of the irregularities constituting the periodic structure can be determined by the desired oscillation wavelength and the 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 (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 one of the convex portions and the width of the concave portions be in the range of 1 / 2 to 2 / 3 of the width of the other. The number of concave portions and the number of convex portions of the irregularities constituting the periodic structure can be the same or different. For example, when a concave portion starts and ends with a concave portion from the light-emitting end face 20a to the light-reflecting end face 20b, the number of concave portions is one more than the number of convex portions. 2 schematically shows 11 recesses and 10 protrusions, but the number of recesses and protrusions is not limited to this. For example, when the cavity length is 300 μm and the period is 110 nm, the number of periods of the recesses and protrusions constituting the periodic structure is approximately 2727. In this case, the number of recesses and protrusions constituting the periodic structure of one semiconductor laser element 100 is 2727 or 2728, respectively.

[0037] The height of the projections and recesses constituting the periodic structure can be 300 nm or less, and may be 200 nm or less. grating Since the coupling coefficient k increases, the height of the concaves and convexes is preferably 50 nm or more. The height of the concaves and convexes 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 concaves and convexes 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 concaves and convexes farthest from the active layer 40 in a cross section that is perpendicular to the major surface of the active layer 40 and parallel to the resonance direction 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).

[0038] The irregularities constituting the periodic structure may be formed continuously from the light-emitting end face 20a to the light-reflecting end face 20b. As shown in Fig. 2, irregularities may not be formed near the light-emitting end face 20a and / or the light-reflecting end face 20b. When the periodic structure is formed continuously from the light-emitting end face 20a to the light-reflecting end face 20b, the region length L of the periodic structure is equal to the length L of the optical waveguide 10. １０ Alternatively, the region length L of the periodic structure is equal to the length L of the optical waveguide 10. １０ The region length L of the periodic structure may be less than the length L of the optical waveguide 10. １０ The region length L of the periodic structure is preferably 200 μm or more and 3000 μm or less, and more preferably 300 μm or more and 1500 μm or less. This makes it easier to obtain the desired coupling efficiency. It is also possible to provide both a region with a periodic structure layer formed by a diffraction grating and a region without a diffraction grating. In this case, the region length L of the periodic structure is set to the length L of the optical waveguide 10. １０The length L of the periodic structure layer can be set to 10% of the length L of the optical waveguide 10, and is preferably set to 30% or more. In this case, the remaining region can function as a gain region without a diffraction grating and / or as a phase shift region that shifts the phase by applying a bias. It is also possible to provide a region with a periodic structure layer using a diffraction grating and a region with a complex function that does not have a diffraction grating. In this case, the length L of the periodic structure region is set to 10% of the length L of the optical waveguide 10. １０ In this case, an optical amplifier (SOA: Semiconductor Optical Amplifier), an electroabsorption modulator (EAM: Electroabsorption Modulator), a Mach-Zehnder type intensity modulator, etc. can be integrated into the device.

[0039] The first n-side nitride semiconductor layer 31 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.

[0040] In the first n-side nitride semiconductor layer 31 shown in FIGS. 1 to 3 , 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 direction D1. 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. "Having the same composition" means that the compositions are obtained by forming the semiconductor portions without intentionally making them different in composition, and may include errors that occur during manufacturing. Here, it has been described that the first semiconductor part 31a includes a plurality of first portions and the second semiconductor part 31b includes a plurality of second portions, but the first semiconductor part 31a may include a plurality of second portions and the second semiconductor part 31b may include a plurality of first portions.

[0041] 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.

[0042] 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 ３ 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 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 can reduce the possibility of a gap occurring between the second semiconductor portion 31b and the first semiconductor portion 31a.

[0043] At least one of both ends of either the plurality of first portions or the plurality of second portions in a direction perpendicular to the resonance direction 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.

[0044] 2 and 3 , 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 (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 portion that fills 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, 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 (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 ends of the concave or convex portions of the first semiconductor portion 31a in the direction (direction D2) perpendicular to the resonance direction being located inside the nitride semiconductor stack 20. This is because the strength of the first semiconductor portion 31a can be expected to improve as the width of the concave or convex shape in the direction D2 becomes narrower. Preferably, the concave or convex shape is a shape in which both ends of the concave or convex shape in the direction D2 are located inside the nitride semiconductor stack 20. The width of the concave or convex shape in the direction D2 is set to be smaller than the width W of the optical waveguide 10. １０ The width W of the optical waveguide 10 is １０ The width of the concave or convex shape in the direction D2 is the width W of the optical waveguide 10. １０ The width of the concave or convex shape in the direction D2 may be smaller than the width of the pad electrode 83 in the direction D2. When a method in which the formation area and the working time are proportional is used to form the periodic structure, such as electron beam lithography, 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, forming the first semiconductor portion 31a having a plurality of concave shapes is expected to improve the strength of the first semiconductor portion 31a compared to forming the first semiconductor portion 31a having a plurality of convex shapes.

[0045] 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 is made of GaN (0<X<1). The In composition ratio of the second portion can be 0.001≦X≦0.1. In this case, an n-side cladding layer can be provided as a layer separate 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. 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 probability of gaps occurring between the first and second portions. In such first and second semiconductor portions 31a and 31b, for example, as shown in FIG. 15 (described later) in a Z-contrast image (ZC image) obtained by STEM, the contrast change is more gradual at the bottom of the recess in the first semiconductor portion 31a than at the side. Such a change in contrast can be confirmed, for example, by observing a cross section perpendicular to the major surface of the active layer 40 and along the resonance direction of the optical waveguide 10. A Z-contrast image is a contrast image based on atomic weight.

[0046] For example, the first portion is made of a nitride semiconductor containing Al and Ga, and the second portion is made of a nitride semiconductor containing Ga. For example, the first portion is 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.

[0047] The periodic structure of the first n-side nitride semiconductor layer 31 has a refractive index that changes periodically along the extension direction of the ridge 20 c. The refractive index of the periodic structure changes periodically along the shortest direction connecting the light-emitting end face 20 a and the light-reflecting end face 20 b. The periodic structure is disposed at least directly below the ridge 20 c.

[0048] 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. If the distance from the first n-side nitride semiconductor layer 31 to the well layer 41 is 300 nm or less, the threshold current increases, and laser oscillation may not occur even when a current of, for example, 400 mA is injected. 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. This makes it easier to obtain the 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 of the first n-side nitride semiconductor layer 31 to the well layer 41 may be within these numerical ranges, and the distance from the periodic structure to the active layer 40 may also be within these numerical ranges. The distance from the periodic structure of the first n-side nitride semiconductor layer 31 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.

[0049] 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.

[0050] 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.

[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 first n-side nitride semiconductor layer 31. The average refractive index of the first n-side nitride semiconductor layer 31 may be determined from the volume ratio of the multiple semiconductor portions that make up the first n-side nitride semiconductor layer 31. Alternatively, the refractive index of the fourth n-side nitride semiconductor layer 34 being higher than any of the refractive indexes of the multiple semiconductor portions may be considered to be higher than the average refractive index of the first n-side nitride semiconductor layer 31.

[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] (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 .

[0055] As the distance between the first n-side nitride semiconductor layer 31 having a periodic structure and the active layer 40 decreases, the electric field strength in the p-side nitride semiconductor layer 50 increases, resulting in increased absorption loss and / or decreased light confinement in the active layer 40. By providing the second n-side nitride semiconductor layer 32, the electric field strength in the p-side nitride semiconductor layer 50 can be reduced, reducing absorption loss and / or improving light confinement in the active layer 40. Therefore, the threshold current of the semiconductor laser device 100 can be reduced.

[0056] 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.

[0057] The refractive index of the second n-side nitride semiconductor layer 32 is preferably higher than the average refractive index of the first n-side nitride semiconductor layer 31. The thickness of the second n-side nitride semiconductor layer 32 is preferably greater than the thickness of the first n-side nitride semiconductor layer 31. By providing these configurations, absorption loss can be reduced and / or light confinement in the active layer 40 can be improved.

[0058] The average refractive index of the first n-side nitride semiconductor layer 31 may be determined from the volume ratio of the plurality of semiconductor parts constituting the first n-side nitride semiconductor layer 31. Alternatively, if the refractive index of the second n-side nitride semiconductor layer 32 is higher than any of the refractive indexes of the plurality of semiconductor parts, it may be said that the refractive index of the second n-side nitride semiconductor layer 32 is higher than the average refractive index of the first n-side nitride semiconductor layer 31. The thickness of the second n-side nitride semiconductor layer 32 may be compared with the thickness of the periodic structure of the first n-side nitride semiconductor layer 31 instead of the thickness of the first n-side nitride semiconductor layer 31.

[0059] 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.

[0060] The second n-side nitride semiconductor layer 32 is, for example, In Z Ga １－Z 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. The second n-side nitride semiconductor layer 32 may be, for example, entirely made of InGaN, and 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.

[0061] The thickness of the second n-side nitride semiconductor layer 32 can 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 can be 500 nm or less. In consideration of the relationships 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 shown in FIGS. 4 , 5 , and 6 , 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.

[0062] (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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] (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 an optical guide layer, an electron blocking layer, and a cladding layer. The p-side nitride semiconductor layer 50 may have all of these layers, or may have layers other than these.

[0067] 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.

[0068] (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 light-emitting end face 20a of the nitride semiconductor stack 20. The second protective film 72 is provided on the light-reflecting end face 20b of the nitride semiconductor stack 20. One or both of the first protective film 71 and the second protective film 72 may not be provided. The first protective film 71 and the second protective film 72 can each have one or more dielectric films.

[0069] The first protective film 71 may be an AR (anti-reflective) coating. In this case, the reflectance of the first protective film 71 is preferably 1% or less, more preferably 0.1% or less, and 0.001% or more. However, using an AR coating for the first protective film 71 is appropriate when the gain inside the resonator is sufficiently high. If the gain inside the resonator is not sufficiently high, it is preferable to provide a first protective film 71 with a higher reflectance. To suppress an increase in threshold current, the reflectance of the first protective film 71 is preferably 0.1% or more, and more preferably 5% or more. In a semiconductor laser device 100 that emits laser light with a peak wavelength of 420 nm or more and less than 500 nm, the gain inside the resonator can be increased, and the reflectance of the first protective film 71 is preferably 25% or less, and more preferably 18% or less. This allows for an increase in slope efficiency and a higher output.

[0070] 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. A semiconductor laser device 100 emitting laser light with a peak wavelength of 500 nm or more tends to have a lower gain inside the resonator than a device with a peak wavelength of less than 500 nm. Therefore, for a 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 approaches unification using the periodic structure, the confinement factor associated with laser oscillation is reduced compared to when this is not the case. The higher the reflectance of the first protective film 71, the higher the confinement factor can be. The reflectance of the first protective film 71 may be 60% or more, or even 80% or more.

[0071] 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.

[0072] (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.

[0073] (n-electrode 81, p-electrode 82, pad electrode 83) The semiconductor laser device 100 has an n-electrode 81 and a p-electrode 82. The n-electrode 81 is provided on the lower surface of the substrate 60. 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 the upper surface of the ridge 20c, for example. The semiconductor laser device 100 may have a pad electrode 83 provided on the p-electrode 82. The pad electrode 83 is provided in contact with the p-electrode 82. Examples of materials for each electrode include a single-layer film or a multilayer 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 ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), GZO (Gallium-doped Zinc Oxide), etc. In this specification, the side of the active layer 40 on which the p-side nitride semiconductor layer 50 is located is referred to as the upper side, and the side on which the n-side nitride semiconductor layer 30 is located is referred to as the lower side.

[0074] (Semiconductor laser element 100) The semiconductor laser element 100 is, for example, a DFB laser element. The peak wavelength of the laser light emitted by the semiconductor laser element 100 can be, for example, 400 nm or more and 600 nm or less. The peak wavelength of the laser light emitted by the semiconductor laser element 100 is, for example, 500 nm or more. The semiconductor laser element 100 including the first n-side nitride semiconductor layer 31 has a periodic structure and can emit laser light with a peak wavelength of 500 nm or more.

[0075] The spectral width of the laser light emitted by the semiconductor laser element 100 can be 10 pm or less, for example, 3 pm or less. The spectral width of the laser light emitted by the semiconductor laser element 100 is, for example, 1 fm or more, and may be 10 fm or more. Alternatively, if the spectral linewidth is equal to or less than the measurement resolution, it may be said to be a single wavelength. The measurement resolution is, for example, on the order of pm.

[0076] The side mode suppression ratio (SMSR) of the laser light emitted by the semiconductor laser element 100 is, for example, 10 dB or more. The SMSR is the intensity ratio between the peak with the highest spectral intensity (main mode) and the second highest peak (side mode). The smaller the SMSR, the higher the monochromaticity of the spectrum of the oscillated laser light, i.e., the singleness of the longitudinal mode. The SMSR of the laser light emitted by the semiconductor laser element 100 may be, for example, 60 dB or less. Alternatively, if the SMSR is equal to or higher than the background level, it can be said that the longitudinal mode is single. The background level is, for example, about 20 to 40 dB.

[0077] Example 1 As Example 1, the following semiconductor laser device was fabricated. An MOCVD apparatus was used to fabricate an epitaxial wafer that would become the semiconductor laser device. The raw materials were trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH ３ ), silane gas, bis(cyclopentadienyl)magnesium (Cp ２Mg) was used as appropriate.

[0078] 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 (first semiconductor layer that becomes the first semiconductor portion 31a) was grown to a thickness of 650 nm.

[0079] The epitaxial wafer with the above layers formed was removed from the MOCVD apparatus, and a periodic concave-convex shape (periodic structure) was fabricated using an electron beam lithography apparatus, reactive ion etching (RIE), and sputtering. The concave portions had a depth of 200 nm, a width of 80 nm, and a diffraction grating period Λ (one period of the concave-convex structure) of 110 nm.

[0080] After the formation of the concave-convex shape, a GaN layer containing Si (second semiconductor portion 31b) was grown to a thickness of 200 nm in an MOCVD apparatus. 200 nm 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). Next, undoped In ０．０３ Ga ０．９７ The N layer (second n-side nitride semiconductor layer 32) was grown to a thickness of 240 nm. ０．０１６ Ga ０．９８４ The layers from the N layer to this layer are the n-side nitride semiconductor layer 30. Next, a 1 nm thick Si-doped GaN layer, an 8 nm thick Si-doped In layer, and a 1 nm thick Si-doped In layer are formed. 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 2.9 nm, and an undoped In layer having a thickness of 2.1 nm. ０．２５ Ga ０．７５An active layer 40 including an N layer (well layer 41) and an undoped GaN layer (barrier layer 42) having a thickness of 2.9 nm was grown in this order. Next, an undoped compositionally graded layer was grown to a thickness of 150 nm. The compositionally graded layer was grown by starting the growth from In. ０．０５ Ga ０．９５ The growth was performed by starting with N and terminating with GaN, and the In composition was substantially monotonically decreased in 120 steps so that the composition gradient was nearly 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 An N layer was grown to a thickness of 450 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.

[0081] The epitaxial wafer on which the above layers were formed was then removed from the MOCVD apparatus, and ridge 20c, p-electrode 82, pad electrode 83, n-electrode 81, etc. were formed using photolithography, RIE, and sputtering. The wafer was then singulated, and first and second protective films 71 and 72 were formed on the end faces to obtain semiconductor laser devices. The reflectance of the first protective film 71 was set to 80%, and the reflectance of the second protective film 72 was set to 98%. The semiconductor laser devices had a ridge width of 2 μm, a cavity length of 300 μm, and an element width of 200 μm.

[0082] Example 2 The following semiconductor laser element was fabricated as the semiconductor laser element of Example 2. The semiconductor laser element of Example 2 differs from the semiconductor laser element of Example 1 mainly in that the layer forming the periodic uneven shape (the first semiconductor layer that becomes the first semiconductor portion 31 a) is an InGaN layer, and that the ridge width is 15 μm.

[0083] 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 1.25 μm. Next, the Si-containing Al ０．０８ Ga ０．９２The N layer was grown to a thickness of 250 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 (fifth n-side nitride semiconductor layer 35) was grown to a thickness of 650 nm. Next, a Si-containing GaN layer (third n-side nitride semiconductor layer 33) 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 200 nm.

[0084] The epitaxial wafer with the above layers formed was removed from the MOCVD apparatus, and a periodic concave-convex shape (periodic structure) was fabricated using an electron beam lithography apparatus, reactive ion etching (RIE), and sputtering. The concave portions had a depth of 150 nm, a width of 50 nm, and a diffraction grating period Λ (one period of the concave-convex structure) of 115 nm.

[0085] After the formation of the concave-convex shape, a GaN layer containing Si (second semiconductor portion 31b) was grown to a thickness of 100 nm in an MOCVD apparatus. The thickness of 100 nm was measured 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). Next, undoped In ０．０３ Ga ０．９７ An N layer (second n-side nitride semiconductor layer 32) was grown to a thickness of 230 nm. Next, a Si-doped GaN layer with a thickness of 1 nm and a Si-doped In layer with a thickness of 44 nm were grown. 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 ０．７５An active layer 40 including an N layer (well layer 41) and an undoped GaN layer (barrier layer 42) having a thickness of 2.2 nm was grown in this order. Next, an undoped compositionally graded layer was grown to a thickness of 180 nm. The compositionally graded layer was grown by starting the growth from In. ０．０５ Ga ０．９５ The In composition was grown in 120 steps, with the starting point being GaN and the end point being GaN, so that the composition gradient was nearly linear. Next, an undoped composition gradient layer was grown to a thickness of 150 nm. The starting point of the composition gradient layer was GaN, and the end point of the growth was Al. ０．０４ Ga ０．９６ The Al composition was increased substantially monotonically in 70 steps so that the composition gradient was nearly linear. ０．０４ Ga ０．９６ The N layer was grown to a thickness of 200 nm. ０．１０ Ga ０．９０ N layer and Al ０．１９ Ga ０．８１ The N layer was grown to a thickness of 3.9 nm and 7 nm, respectively. ０．０４ Ga ０．９６ The N layer was grown to a thickness of 100 nm, followed by a GaN layer containing Mg to a thickness of 15 nm.

[0086] The epitaxial wafer on which the above layers were formed was then removed from the MOCVD apparatus, and photolithography, RIE, and sputtering were used to form the ridge 20c, p-electrode 82, pad electrode 83, n-electrode 81, etc. After that, the wafer was singulated, and a first protective film 71 and a second protective film 72 were formed on the end faces to obtain semiconductor laser elements. The reflectance of the first protective film 71 was 90%, and the reflectance of the second protective film 72 was 98%. The semiconductor laser elements had a ridge width of 15 μm, a cavity length of 300 μm, and an element width of 200 μm.

[0087] Comparative Example 1 As a semiconductor laser element of Comparative Example 1, an Al containing Si was used. ０．０８ Ga ０．９２ A semiconductor laser device was fabricated having the same structure as the semiconductor laser device of Example 1, except that no irregularities were formed on the N layer (first semiconductor layer).

[0088] Comparative Example 2 As a semiconductor laser element of Comparative Example 2, an In containing Si was used. ０．０３ Ga ０．９７ A semiconductor laser device was fabricated having the same structure as the semiconductor laser device of Example 2, except that no irregularities were formed on the N layer (first semiconductor layer).

[0089] (Evaluation) The IL characteristics of the semiconductor laser devices of Example 1 and Comparative Example 1 are shown in FIG. 8. In FIG. 8, the horizontal axis represents current and the vertical axis represents optical output. The wavelength spectra of the semiconductor laser devices of Example 1 and Comparative Example 1 are shown in FIG. 9. In FIG. 9, the horizontal axis represents wavelength and the vertical axis represents intensity normalized by area. The side mode suppression ratio (SMSR) of the semiconductor laser device of Example 1 is shown in FIG. 10. The semiconductor laser device of Example 1 oscillated laser light with a peak wavelength of approximately 512 nm. The side mode suppression ratio of the semiconductor laser device of Example 1 was 23.4 dB. The spectral width of the semiconductor laser device of Example 1 was 4 pm or less. The semiconductor laser device of Comparative Example 1 oscillated, but the wavelength spectrum was multiple, that is, it oscillated in a longitudinal multi-mode.

[0090] The threshold current of the semiconductor laser element of Example 1 was 34 mA. The threshold current of the semiconductor laser element of Comparative Example 1 was 28 mA. The difference in threshold current between Example 1 and Comparative Example 1 was 6 mA, and it can be said that the semiconductor laser element of Example 1 was able to suppress an increase in threshold current due to the provision of a periodic structure.

[0091] FIG. 11 shows the IL characteristics of the semiconductor laser devices of Example 2 and Comparative Example 2. In FIG. 11, the horizontal axis represents current and the vertical axis represents optical output. FIGS. 12 and 13 show the wavelength spectra of the semiconductor laser devices of Example 2 and Comparative Example 2, respectively. In FIGS. 12 and 13, the horizontal axis represents wavelength and the vertical axis represents intensity normalized by area. FIG. 14 shows the side mode suppression ratio (SMSR) of the semiconductor laser device of Example 2. The semiconductor laser device of Example 2 oscillated laser light with a peak wavelength of approximately 532 nm. The SMSR of the semiconductor laser device of Example 2 was 15 dB. The spectral width of the semiconductor laser device of Example 2 was 7 pm or less. The semiconductor laser device of Comparative Example 2 oscillated, but the wavelength spectrum was multiple, i.e., it oscillated in a longitudinal multimode.

[0092] The threshold current of the semiconductor laser element of Example 2 was 65 mA. The threshold current of the semiconductor laser element of Comparative Example 2 was 60 mA. The difference in threshold current between Example 2 and Comparative Example 2 was 5 mA, and it can be said that the semiconductor laser element of Example 2 was able to suppress an increase in threshold current due to the provision of a periodic structure.

[0093] FIG. 15 shows a Z-contrast image obtained by STEM of a portion of the semiconductor laser device of Example 2. FIG. 15 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 Z-contrast image, differences in composition can be observed as differences in display density in the image. In FIG. 15, the first semiconductor portion 31a and the second semiconductor portion 31b are displayed with different display densities, indicating that they have different compositions. FIG. 15 also shows that the change in contrast from the first semiconductor portion 31a to the second semiconductor portion 31b is more gradual at the bottom of the recess in the first semiconductor portion 31a than at the side of the recess. Since STEM also collects information about the depth of the sample, the change in contrast at the boundary between semiconductor layers or semiconductor portions tends to be gradual. However, the particularly gradual change at the bottom of the recess in the first semiconductor portion 31a may be due to the depth of the recess varying in the depth direction, or it may be due to a gradual change in composition from the first semiconductor portion 31a to the second semiconductor portion 31b.

[0094] The present disclosure includes the following inventions. (2) The semiconductor laser device according to (1), in which the refractive index of the second n-side nitride semiconductor layer is higher than the average refractive index of the first n-side nitride semiconductor layer, and the thickness of the second n-side nitride semiconductor layer is larger than the thickness of the first n-side nitride semiconductor layer. (3) The semiconductor laser device according to (1) or (2), in which the refractive index of the second n-side nitride semiconductor layer is higher than the refractive index of the n-side barrier layer. (4) The semiconductor laser device according to any one of (1) to (3), in which the thickness of the second n-side nitride semiconductor layer is 200 nm or more. (5) The semiconductor laser device according to any one of (1) to (4), in which the distance from the first n-side nitride semiconductor layer to the well layer is greater than 300 nm. (6) The semiconductor laser device according to any one of (1) to (5), in which the thickness of the first n-side nitride semiconductor layer is 50 nm or more. (7) The semiconductor laser element according to any one of (1) to (6), wherein at least one of both ends of the periodic structure in a direction perpendicular to the resonance direction is located inside the nitride semiconductor stack. (8) The semiconductor laser element according to any one of (1) to (7), wherein the first n-side nitride semiconductor layer has a plurality of first portions and a plurality of second portions having a refractive index higher than that of the plurality of first portions, and the periodic structure is configured by the plurality of first portions and the plurality of second portions being alternately arranged along the resonance direction. (9) The semiconductor laser element according to (8), wherein at least one of both ends of either the plurality of first portions or the plurality of second portions in a direction perpendicular to the resonance direction is located inside the nitride semiconductor stack. (10) The semiconductor laser element according to (8) or (9), wherein the plurality of first portions are made of a nitride semiconductor containing Ga, and the plurality of second portions are made of a nitride semiconductor containing In and Ga. (11) The semiconductor laser element according to (10), comprising: an n-side cladding layer disposed on the side of the first n-side nitride semiconductor layer opposite to the active layer; and a third n-side nitride semiconductor layer disposed between the n-side cladding layer and the first n-side nitride semiconductor layer and having a refractive index between a refractive index of the n-side cladding layer and an average refractive index of the first n-side nitride semiconductor layer.(12) The semiconductor laser element according to (8) or (9), wherein the plurality of first portions are made of a nitride semiconductor containing Al and Ga, and the plurality of second portions are made of a nitride semiconductor containing Ga. (13) The semiconductor laser element according to (12), further comprising a fourth n-side nitride semiconductor layer disposed between the second n-side nitride semiconductor layer and the first n-side nitride semiconductor layer, wherein the refractive index of the fourth n-side nitride semiconductor layer is lower than the refractive index of the second n-side nitride semiconductor layer and higher than the average refractive index of the first n-side nitride semiconductor layer. (14) The semiconductor laser element according to (13), wherein the thickness of the second n-side nitride semiconductor layer is greater than the thickness of the fourth n-side nitride semiconductor layer. (15) The semiconductor laser element according to any of (1) to (14), wherein the width of the optical waveguide in a direction perpendicular to the resonance direction is 10 μm or more. (16) The semiconductor laser element according to any one of (1) to (15), wherein the length of the optical waveguide in the resonance direction is 1000 μm or more. (17) The nitride semiconductor stack has a light-emitting end face and a light-reflecting end face, the semiconductor laser element has a first protective film provided on the light-emitting end face and a second protective film provided on the light-reflecting end face, and the reflectivity of the first protective film is 30% or more and less than the reflectivity of the second protective film. The above describes embodiments of the present invention with reference to specific examples. However, the present invention is not limited to these specific examples. All embodiments that can be implemented by a person skilled in the art by appropriately modifying the design based on the above-described embodiments of the present invention fall within the scope of the present invention, as long as they encompass the gist of the present invention. Furthermore, a person skilled in the art may conceive of various modifications and alterations within the scope of the concept of the present invention, and it is understood that these modifications and alterations also fall within the scope of the present invention.

[0095] 10 Optical waveguide 20 Semiconductor laminate 20a Light emitting end face 20b Light reflecting end face 20c Ridge 30 n-side semiconductor layer 31 1st semiconductor layer 31a 1st semiconductor portion 31b 2nd semiconductor portion 32 2n-side semiconductor layer 33 3n-side semiconductor layer 34 4n-side semiconductor layer 35 5n-side semiconductor layer 40 Active layer 41 Isotope layer 42 Barrier layer 50 p-side semiconductor layer 60 Substrate 71 1st protective film 72 2nd protective film 73 Insulating film 81 n-electrode 82 p-electrode 83 Padd electrode 100 Semiconductor layer

Claims

1. A nitride semiconductor laminate having an optical waveguide, wherein the nitride semiconductor laminate has a first n-side nitride semiconductor layer having a periodic structure in which the refractive index changes periodically along the resonance direction of the optical waveguide, a second n-side nitride semiconductor layer, an active layer having one or more well layers and one or more barrier layers, a p-side nitride semiconductor layer, and has them in this order, 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 having In and Ga, the thickness of the second n-side nitride semiconductor layer is larger than the thickness of the n-side barrier layer, a semiconductor laser element.

2. The refractive index of the second n-side nitride semiconductor layer is higher than the average refractive index 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 first n-side nitride semiconductor layer, the semiconductor laser element according to claim 1.

3. The refractive index of the second n-side nitride semiconductor layer is higher than the refractive index of the n-side barrier layer, the semiconductor laser element according to claim 1.

4. The thickness of the second n-side nitride semiconductor layer is 200 nm or more, the semiconductor laser element according to claim 1.

5. The distance from the first n-side nitride semiconductor layer to the well layer is larger than 300 nm, the semiconductor laser element according to claim 1.

6. The thickness of the first n-side nitride semiconductor layer is 50 nm or more, the semiconductor laser element according to claim 1.

7. At least one of both ends in the direction perpendicular to the resonance direction of the periodic structure is located inside the nitride semiconductor laminate, the semiconductor laser element according to claim 1.

8. The first n-side nitride semiconductor layer has a plurality of first portions and a plurality of second portions having a refractive index higher than that of the plurality of first portions, the periodic structure 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 claim 1.

9. Either one of the plurality of first portions or the plurality of second portions has at least one of both ends in the direction perpendicular to the resonance direction located inside the nitride semiconductor laminate, the semiconductor laser element according to claim 8.

10. The plurality of first portions are made of a nitride semiconductor containing Ga. The semiconductor laser device according to claim 8, wherein the plurality of second portions are made of a nitride semiconductor containing In and Ga.

11. An n-side cladding layer disposed on a 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 first n-side nitride semiconductor layer. The semiconductor laser device according to claim 10.

12. The plurality of first portions are made of a nitride semiconductor containing Al and Ga. The semiconductor laser device according to claim 8, wherein the plurality of second portions are made of a nitride semiconductor containing Ga.

13. Having 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 semiconductor laser device according to claim 12, wherein the refractive index of the fourth n-side nitride semiconductor layer is lower than the refractive index of the second n-side nitride semiconductor layer and higher than the average refractive index of the first n-side nitride semiconductor layer.

14. The semiconductor laser device according to claim 13, wherein the thickness of the second n-side nitride semiconductor layer is larger than the thickness of the fourth n-side nitride semiconductor layer.

15. The semiconductor laser device according to claim 1, wherein the width in a direction perpendicular to the resonance direction of the optical waveguide is 10 μm or more.

16. The semiconductor laser device according to claim 1, wherein the length in the resonance direction of the optical waveguide is 1000 μm or more.

17. The nitride semiconductor laminate has a light emitting end face and a light reflecting end face. The semiconductor laser device has a first protective film provided on the light emitting end face and a second protective film provided on the light reflecting end face. The semiconductor laser device according to any one of claims 1 to 16, wherein the reflectance of the first protective film is 30% or more and less than the reflectance of the second protective film.