NITRIDE SEMICONDUCTOR LASER DIODE AND METHOD FOR MANUFACTURING NITRIDE SEMICONDUCTOR LASER DIODE

Abstract

To provide a nitride semiconductor laser diode with excellent optical characteristics and a manufacturing method of the nitride semiconductor laser diode.SOLUTION: A nitride semiconductor laser diode includes a substrate, a first nitride semiconductor layer provided above the substrate and made of first conductivity type AlGaN, an active layer made of AlGaN formed above the first nitride semiconductor layer, and an upper AlGaN layer formed on the active layer, and a ridge semiconductor layer is formed by a part or all of the upper AlGaN layer, and the ridge semiconductor layer has a forward tapered shape in a cross-sectional view in which a region close to the substrate gradually widens toward the substrate, and has an inverse tapered shape in which a region far from the substrate has a gradually narrower width toward the substrate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to nitride semiconductor laser diodes and methods for manufacturing nitride semiconductor laser diodes. [Background technology]

[0002] Conventionally, in order to reduce the threshold current density, reduce power consumption, improve light emission efficiency, and extend the lifespan, the semiconductor laminate portion of a nitride semiconductor laser diode has been made into a ridge structure, or the side surfaces of the ridge structure have been made into a structure coated with a low refractive index material (for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 9-64467 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the nitride semiconductor laser diode having the above-described ridge structure, the light-emitting characteristics as a laser diode may not be sufficient.

[0005] An object of the present disclosure is to provide a nitride semiconductor laser diode with excellent light-emitting characteristics and a method for manufacturing the nitride semiconductor laser diode. [Means for solving the problem]

[0006] In order to solve the above problems, a nitride semiconductor laser diode according to one aspect of the present disclosure includes a substrate, a first nitride semiconductor layer formed above the substrate and made of AlGaN of a first conductivity type, an active layer formed above the first nitride semiconductor layer and made of AlGaN, and an upper AlGaN layer formed on the active layer, wherein a ridge semiconductor layer is formed by part or all of the upper AlGaN layer, and the ridge semiconductor layer has a forward tapered shape in cross section, where a region closer to the substrate gradually becomes wider toward the substrate, and a reverse tapered shape where a region farther from the substrate gradually becomes narrower toward the substrate.

[0007] In addition, a method for manufacturing a nitride semiconductor laser diode according to an aspect of the present disclosure includes forming, on a substrate, a first nitride semiconductor layer made of first-conductivity-type AlGaN, an active layer made of AlGaN or GaN, and an upper AlGaN layer made of AlGaN in that order to form a semiconductor laminate; covering a part of the top surface of the second nitride semiconductor layer with an etching mask made of a metal, and then removing by dry etching from the second nitride semiconductor layer to a part in the thickness direction of the upper guide layer of the semiconductor laminate that is not covered by the etching mask; and wet-etching with an alkaline solution the side surfaces of the second nitride semiconductor layer to the upper guide layer of the semiconductor laminate that remains after the dry etching, thereby forming a nitride semiconductor laser diode. the nitride semiconductor layer and the compositionally graded layer are formed in an inversely tapered shape that narrows toward the substrate in a cross-sectional view, and the upper guide layer is formed in a tapered shape that widens toward the substrate in a cross-sectional view to form a ridge semiconductor layer; a silicon oxide film is formed on at least the upper surface of the upper guide layer remaining after dry etching and on the upper surface of the mask, and then the etching mask and the silicon oxide film formed on the etching mask are removed with a solution that dissolves a metal that constitutes the etching mask; a second electrode is formed that is electrically connected to the second nitride semiconductor layer from which the etching mask has been removed, and then an upper second electrode is formed by electron beam evaporation to cover the second electrode and the sides of the ridge semiconductor layer via a cavity. [Effects of the Invention]

[0008] According to one aspect of the present disclosure, it is possible to provide a nitride semiconductor laser diode with excellent light-emitting characteristics and a method for manufacturing the nitride semiconductor laser diode. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view showing a configuration example of a nitride semiconductor laser diode according to a first embodiment of the present disclosure. [Figure 2] 1 is a cross-sectional view showing an example of a cross section of a ridge semiconductor layer of a nitride semiconductor laser diode according to a first embodiment of the present disclosure. [Figure 3A] FIG. 4 is a cross-sectional view showing another example of the cross section of the ridge semiconductor layer of the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 3B] FIG. 4 is a cross-sectional view showing another example of the cross section of the ridge semiconductor layer of the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 3C] FIG. 4 is a cross-sectional view showing another example of the cross section of the ridge semiconductor layer of the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4A] 3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4B] 3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4C] 3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4D] 3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4E] 3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4F] 3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4G]3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4H] 3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4I] 3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 4J] 3A to 3C are cross-sectional views showing an example of a method for manufacturing the nitride semiconductor laser diode according to the first embodiment of the present disclosure. [Figure 5] FIG. 2 is a cross-sectional view showing a configuration example of a nitride semiconductor laser diode according to a second embodiment of the present disclosure. [Figure 6] FIG. 4 is a cross-sectional view showing an example of a cross section of a ridge semiconductor layer of a nitride semiconductor laser diode according to a second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0010] The nitride semiconductor laser diode according to the present embodiment will be described below through embodiments, but the following embodiments do not limit the scope of the invention as claimed. Also, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

[0011] 1. First embodiment A nitride semiconductor laser diode 1 according to a first embodiment of the present disclosure will be described below with reference to FIGS. 1 to 3 and 4A to 4J. The nitride semiconductor laser diode 1 is a laser diode capable of emitting ultraviolet light. The nitride semiconductor laser diode 1 is capable of lasing ultraviolet light by current injection. The nitride semiconductor laser diode 1 can emit light in the UVB region with a wavelength of 280 nm to 320 nm, for example.

[0012] (1.1) Overall structure of nitride semiconductor laser diode The configuration of the nitride semiconductor laser diode 1 will be described with reference to FIGS. FIG. 1 is a cross-sectional view showing an example of the configuration of a nitride semiconductor laser diode 1, and FIG. 2 is an enlarged cross-sectional view showing in detail the structure of a ridge semiconductor layer of the nitride semiconductor laser diode 1. As shown in FIG.

[0013] As shown in FIG. 1, the nitride semiconductor laser diode 1 includes a substrate 10, a first nitride semiconductor layer 31 provided above the substrate 10, a nitride semiconductor active layer 322 constituting a light emitting section 32, and an upper AlGaN layer 36 formed on the nitride semiconductor active layer 322. In the nitride semiconductor laser diode 1, a ridge semiconductor layer 30 is formed by part or all of the upper AlGaN layer 36. The nitride semiconductor laser diode 1 also includes a silicon oxide film 40, which is an insulator, provided so as to cover a side surface 30A of the ridge semiconductor layer 30. In a cross-sectional view, the ridge semiconductor layer 30 has a forward tapered shape in which a region of the ridge semiconductor layer 30 closer to the substrate 10 gradually becomes wider toward the substrate 10, and a region farther from the substrate 10 has a reverse tapered shape gradually becoming narrower toward the substrate 10.

[0014] The upper AlGaN layer 36 is composed of a part of the upper guide layer 323 that constitutes the light emitting section 32, the compositionally graded layer 34, and the second nitride semiconductor layer 35. The upper AlGaN layer includes a compositionally graded layer in which the Al composition ratio decreases in a direction away from the nitride semiconductor active layer 322. The upper AlGaN layer also has the upper guide layer 323 on the active layer side of the upper AlGaN layer. The ridge semiconductor layer 30 is formed so that the bottom surface of the forward tapered region (the lower region close to the substrate 10) of the ridge semiconductor layer 30 is located in the upper guide layer 323.

[0015] The nitride semiconductor laser diode 1 includes a first electrode 51 electrically connected to the first nitride semiconductor layer 31. In this embodiment, the first electrode 51 includes, for example, a lower first electrode 51A and an upper first electrode 51B disposed on the lower first electrode 51A. The nitride semiconductor laser diode 1 also includes a second electrode 52 electrically connected to the upper AlGaN layer 36 (particularly, the second nitride semiconductor layer 35 provided on the upper surface 30B of the ridge semiconductor layer 30). In this embodiment, the second electrode 52 includes, for example, a lower second electrode 52A and an upper second electrode 52B disposed on the lower second electrode 52A. Here, the second electrode 52 (particularly, the upper second electrode 52B) is an example of a coating film. Here, the nitride semiconductor laser diode 1 is described as having a configuration including a buffer layer 20 provided on the substrate 10 and an electron blocking layer 33 provided on the light-emitting portion 32, but the buffer layer 20 and the electron blocking layer 33 do not necessarily have to be provided.

[0016] 2, the ridge semiconductor layer 30 has a forward tapered shape in cross section, where a region close to the substrate 10 (part of the upper guide layer 323 in this embodiment) gradually becomes wider toward the substrate 10. Also, the ridge semiconductor layer 30 has a reverse tapered shape in cross section, where a region farther from the substrate 10 (the electron block layer 33, the composition gradient layer 34, and the second nitride semiconductor layer 35) gradually becomes narrower toward the substrate 10. The second electrode 52 is provided to cover the ridge semiconductor layer 30 having such a shape. The second electrode 52 covers the side surface 30A of the ridge semiconductor layer 30 via a cavity formed on the side of the silicon oxide film 40. More specifically, in the nitride semiconductor laser diode 1, the second electrode 52 covers the side surface 30A of the ridge semiconductor layer 30 via the silicon oxide film 40 and the cavity 60 formed on the side of the silicon oxide film 40. Each layer constituting the nitride semiconductor laser diode 1 will be described in detail below.

[0017] <Substrate> Examples of the substrate 10 include Si, SiC, MgO, Ga2O3, Al2O3, ZnO, GaN, InN, AlN, or mixed crystals thereof. The substrate 10 supports the upper thin film, improves crystallinity, and dissipates heat to the outside. Therefore, it is preferable to use an AlN substrate as the substrate 10, which can grow AlGaN with high quality and has high thermal conductivity. The commonly used +c-plane AlN is suitable for the growth surface of the substrate because of its low cost, but it can also be -c-plane AlN, a semipolar plane substrate, or a nonpolar plane substrate. When using a compositionally graded structure that acts as a p-type semiconductor, +c-plane AlN is preferred from the perspective of maximizing the effect of polarization doping.

[0018] Substrate 10 preferably has a thin rectangular shape for ease of assembly, but is not limited to this configuration. In addition, the off-angle of substrate 10 is preferably greater than 0 degrees and less than 2 degrees from the viewpoint of growing high-quality crystals. The thickness of the substrate 10 is not particularly limited as long as the AlGaN layer is to be laminated thereon, but it is preferably 50 μm or more and 1 μm or less. There is also no particular limitation on the crystalline quality of the substrate 10, but it is preferable that the threading dislocation density is 1×10 9 cm -2 Preferably, it is 1×10 or less. 8 cm -2 It is more preferable that the following is true: This allows a thin-film element with high luminous efficiency to be formed above the substrate 10.

[0019] <Buffer layer> The buffer layer 20 is formed on the entire surface of the substrate 10. The buffer layer 20 is preferably formed of AlN, AlGaN, GaN, or the like. The buffer layer 20 has a small difference in lattice constant and thermal expansion coefficient from the first nitride semiconductor layer 31, and a nitride semiconductor layer with few defects can be grown on the buffer layer 20. Furthermore, the buffer layer 20 allows the first nitride semiconductor layer 31 to be grown under compressive stress, and can suppress the occurrence of cracks in the first nitride semiconductor layer 31. Therefore, even when the substrate 10 is formed of a nitride semiconductor such as AlN or AlGaN, a nitride semiconductor layer with few defects can be grown above the substrate 10 via the buffer layer 20.

[0020] The buffer layer 20 may contain impurities such as C, Si, Fe, and Mg. When AlN is used as the material for forming the substrate 10, the buffer layer 20 and the substrate 10 are formed of the same material, making the boundary between the buffer layer 20 and the substrate 10 unclear. In this embodiment, when the substrate 10 and the buffer layer 20 are formed of the same material (e.g., AlN), the substrate 10 is considered to constitute the substrate 10 and the buffer layer 20. For the purpose of improving the crystal quality, the buffer layer 20 may be formed as a thick film using a layer of AlN, AlGaN, GAN, or the like under different conditions for the initial growth and the later growth of the buffer layer 20.

[0021] The buffer layer 20 has a thickness of, for example, several μm (e.g., 1.6 μm), but is not limited to this value. Specifically, the thickness of the buffer layer 20 is preferably greater than 10 nm but less than 10 μm. When the buffer layer 20 is greater than 10 nm, the crystallinity of AlN is increased. Furthermore, when the buffer layer 20 is less than 10 μm, cracks are less likely to occur in the buffer layer 20 formed by crystal growth over the entire wafer surface. Furthermore, the buffer layer 20 is more preferably greater than 50 nm but less than 5 μm. When the buffer layer 20 is greater than 50 nm, AlN with high crystallinity can be produced with good reproducibility. Furthermore, when the buffer layer 20 is less than 5 μm, cracks are less likely to occur in the buffer layer 20.

[0022] The buffer layer 20 is formed thinner than the first nitride semiconductor layer 31, but this is not a limitation. When the buffer layer 20 is thinner than the first nitride semiconductor layer 31, the first nitride semiconductor layer 31 can be made thicker as long as cracks do not occur. In this case, the resistance in the plane direction of the thin film stack of the first nitride semiconductor layer 31 is reduced, making it possible to realize a nitride semiconductor laser diode 1 that can be driven at a low voltage. When the nitride semiconductor laser diode 1 can be driven at a low voltage, it becomes possible to further suppress breakdown due to heat generation under high current density driving.

[0023] <First nitride semiconductor layer> The first nitride semiconductor layer 31 is a layer provided on the surface of the light emitting section 32 including the nitride semiconductor active layer 322 opposite to the compositionally graded layer 34. The first nitride semiconductor layer 31 has a first stacked region 311 and a second stacked region 312 formed on a part of the upper surface of the first stacked region 311. Therefore, the upper surface of the first stacked region 311 includes a region where the second stacked region 312 is not formed and a region where the second stacked region 312 is formed. A first electrode 51 connected to the first stacked region 311 is provided in the region of the upper surface of the first stacked region 311 where the second stacked region 312 is not formed. The first nitride semiconductor layer 31 may not have the second stacked region 312 and may have a flat upper surface.

[0024] The first nitride semiconductor layer 31 is formed of AlGaN of the first conductivity type (n-type or p-type). The Al composition ratio of the first nitride semiconductor layer 31 can be specified by energy dispersive X-ray spectroscopy (EDX) of the cross-sectional structure. The cross-section of the first nitride semiconductor layer 31 can be observed by exposing a cross-section along the a-plane of AlGaN using a focused ion beam (FIB) apparatus. As a method for observing the cross-section, a transmission electron microscope is used. The magnification for observation is preferably changed according to the thickness of the layer to be measured, and the magnification is set so that the scale bars of the first nitride semiconductor layers 31 with different thicknesses are of the same degree. For example, when observing the first nitride semiconductor layer 31 with a thickness of 100 nm, the magnification is preferably about 100,000 times. Further, when the magnification for observing the first nitride semiconductor layer 31 with a thickness of 100 nm is about 100,000 times, the first nitride semiconductor layer 31 with a thickness of 1 μm is preferably observed at a magnification of about 10,000 times. Thereby, the first nitride semiconductor layers 31 with different thicknesses can be observed at the same scale.

[0025] The Al composition ratio can be defined as the ratio of the number of moles of Al to the sum of the number of moles of Al and Ga, and specifically, it can be defined using the values of the number of moles of Al and Ga analyzed and quantified from EDX. The first nitride semiconductor layer 31 is, for example, Al x5 Ga (1-x5) N (0 < x5 < 1). The first nitride semiconductor layer 31 may contain elements other than Al and Ga as group III elements (for example, B or In) in AlGaN, but since the formation of defects and changes in durability occur at locations containing B or In, it is preferable not to contain group III elements other than Al and Ga. Further, the first nitride semiconductor layer 31 may contain group V elements other than N such as P, As, or Sb, and impurities such as C, H, F, O, Mg, Si, etc. together with AlGaN.

[0026] In the present disclosure, the first nitride semiconductor layer 31 is, for example, an n-type semiconductor. When the first nitride semiconductor layer 31 is an n-type semiconductor, for example, Si is 1×10 19 cm -3 When the first nitride semiconductor layer 31 is a p-type semiconductor, for example, Mg is doped at a concentration of 3×10 19 cm -3 The impurity concentration may be uniform or non-uniform throughout the layer, or may be non-uniform only in the thickness direction or only in the direction horizontal to the substrate.

[0027] The first nitride semiconductor layer 31 and the first electrode 51 may be in direct contact with each other, or may be connected via different layers such as a tunnel junction. For example, when the first nitride semiconductor layer 31 made of an n-type semiconductor is tunnel-junctioned with the first electrode 51, a p-type semiconductor is provided between the first nitride semiconductor layer 31 and the first electrode 51. For this reason, the first electrode 51 is preferably formed of a material capable of forming an ohmic junction with a p-type semiconductor. The first electrode 51 is preferably, for example, a stacked electrode of Ni and Au or an electrode made of an alloyed metal.

[0028] From the viewpoint of fabricating a PN diode together with the compositionally graded layer 34, which will be described later, the first nitride semiconductor layer 31 has a conductivity type different from that of the compositionally graded layer 34. The compositionally graded layer 34 uses AlGaN, in which the Al composition ratio x decreases in the thickness direction of the compositionally graded layer 34. Therefore, when the first nitride semiconductor layer 31 is an n-type semiconductor and the thin film growth direction of the compositionally graded layer 34 relative to the substrate 10 is the Al plane, the compositionally graded layer 34 is likely to become p-type by polarization doping.

[0029] The thickness of the first nitride semiconductor layer 31 is not particularly limited, but for example, it is preferably 200 nm or more and 20 μm or less. When the thickness of the first nitride semiconductor layer 31 is 200 nm or more, the resistance of the first nitride semiconductor layer 31 is reduced. When the thickness of the first nitride semiconductor layer 31 is 20 μm or less, the generation of cracks during the formation of the first nitride semiconductor layer 31 is suppressed.

[0030] <Light emitting part> The light emitting part 32 includes a nitride semiconductor active layer 322, a lower guide layer 321 provided on one surface of the nitride semiconductor active layer 322, and an upper guide layer 323 provided on the other surface of the nitride semiconductor active layer 322. The lower guide layer 321 is provided between the first nitride semiconductor layer 31 and the nitride semiconductor active layer 322. The upper guide layer 323 is provided between the nitride semiconductor active layer 322 and the electron blocking layer 33.

[0031] (Lower guide layer) The lower guide layer 321 is formed on the first nitride semiconductor layer 31. The lower guide layer 321 provides a refractive index difference with the first nitride semiconductor layer 31 in order to confine the light emitted by the nitride semiconductor active layer 322 in the light emitting part 32. The lower guide layer 321 is formed, for example, by a mixed crystal of AlN and GaN. Specifically, the lower guide layer 321 is Al x7 Ga (1-x7) formed by N(0 < x7 < 1). In addition, the material forming the lower guide layer 321 may contain group V elements other than N such as P, As, or Sb, group III elements such as In or B, and impurities such as C, H, F, O, Si, Cd, Zn, or Be.

[0032] The Al composition ratio x7 of the lower guide layer 321 can be determined by energy dispersive X-ray analysis (EDX) of the cross-sectional structure. The Al composition ratio x7 can be defined as the ratio of the number of moles of Al to the sum of the number of moles of Al and Ga, and specifically, can be defined using the values ​​of the number of moles of Al and Ga analyzed and quantified by EDX. The Al composition ratio x7 of the lower guide layer 321 may be smaller than the Al composition ratio of the first nitride semiconductor layer 31. This makes the refractive index of the lower guide layer 321 larger than that of the first nitride semiconductor layer 31, making it possible to confine light emitted in the nitride semiconductor active layer 322 in the light emitting portion 32.

[0033] The lower guide layer 321 may be an undoped layer that does not contain Si as a dopant. The lower guide layer 321 may also have the same conductivity type as the first nitride semiconductor layer 31. When the lower guide layer 321 is an n-type semiconductor, the amount of Si as a dopant for AlGaN is 1×10 19 cm -3 When the lower guide layer 321 is a p-type semiconductor, the dopant Mg is doped at a concentration of 3×10 19 cm -3 By doping at this concentration, the lower guide layer 321 becomes p-type.

[0034] (Nitride semiconductor active layer) The nitride semiconductor active layer 322 is a light emitting portion from which light emission of the nitride semiconductor laser diode 1 is obtained. The nitride semiconductor active layer 322 is formed of, for example, AlGaN, GaN, or a mixed crystal thereof. More specifically, the nitride semiconductor active layer 322 is formed of, for example, Al x8 Ga (1-x8) N (0≦x8≦1). The Al composition ratio x8 in the nitride semiconductor active layer 322 is preferably smaller than the Al composition ratio x7 of the lower guide layer 321. This allows carriers injected from the first electrode 51 and the second electrode 52 to be efficiently confined in the light emitting part 32. The nitride semiconductor active layer 322 may contain impurities such as group V elements other than N, such as P, As, or Sb, group III elements, such as In or B, or C, H, F, O, Si, Cd, Zn, or Be.

[0035] When the nitride semiconductor active layer 322 is an n-type semiconductor, the dopant Si is 1×10 19 cm -3 When the nitride semiconductor active layer 322 is a p-type semiconductor, the dopant Mg is doped at a concentration of 3×10 19 cm -3 The nitride semiconductor active layer 322 becomes p-type by being doped at this concentration. The nitride semiconductor active layer 322 may be an undoped layer that does not contain Si or Mg as dopants.

[0036] The nitride semiconductor active layer 322 has a well layer and a barrier layer provided adjacent to the well layer. The nitride semiconductor active layer 322 may have a multiple quantum well (MQW) structure in which well layers and barrier layers are alternately stacked. The nitride semiconductor laser diode 1 has a nitride semiconductor active layer 322 with a single well structure, thereby increasing the carrier density in one well layer. On the other hand, the nitride semiconductor active layer 322 may have, for example, a quantum well structure such as "well layer / barrier layer / well layer," or a double or more quantum well structure (multiple quantum well structure). The nitride semiconductor laser diode 1 has a nitride semiconductor active layer 322 with a multiple quantum well structure, thereby improving the light emission efficiency and light emission intensity of the nitride semiconductor active layer 322. In the case of a double quantum well structure, the well layer may have a thickness of, for example, 4 nm, the barrier layer may have a thickness of, for example, 8 nm, and the nitride semiconductor active layer 322 may have a thickness of, for example, 16 nm.

[0037] The Al composition ratio of the well layer is smaller than the Al composition ratio of each of the lower guide layer 321 and the upper guide layer 323. The Al composition ratio of the well layer is also smaller than the Al composition ratio of the barrier layer. The Al composition ratio of the barrier layer may be the same as or different from the Al composition ratio of each of the lower guide layer 321 and the upper guide layer 323. The average Al composition ratio of the well layer and the barrier layer is the Al composition ratio of the entire nitride semiconductor active layer 322. The Al composition ratio of the well and barrier layers can be determined by energy dispersive X-ray analysis (EDX) of the cross-sectional structure. The Al composition ratio can be defined as the ratio of the number of moles of Al to the sum of the number of moles of Al and Ga, specifically, it can be defined using the number of moles of Al and Ga analyzed and quantified by EDX.

[0038] (Upper guide layer) The upper guide layer 323 is formed on the nitride semiconductor active layer 322. The upper guide layer 323 has a refractive index different from that of the upper layer (electron blocking layer 33 in FIG. 1) in order to confine light emitted in the nitride semiconductor active layer 322 in the light emitting section 32. A protruding region 324 having a narrower width in cross section than other regions is provided in a part of the upper guide layer 323. The protruding region 324 constitutes a part of the ridge semiconductor layer 30. The upper guide layer 323 is formed of, for example, AlN, GaN, or a mixed crystal thereof. Specifically, the upper guide layer 323 is made of Al x9 Ga (1-x9) N (0≦x9≦1). Furthermore, the material forming the upper guide layer 323 may contain impurities such as group V elements other than N, such as P, As, or Sb, group III elements such as In or B, or C, H, F, O, Si, Cd, Zn, or Be.

[0039] The Al composition ratio x9 of the upper guide layer 323 can be determined by energy dispersive X-ray analysis (EDX) of the cross-sectional structure. The Al composition ratio x9 can be defined as the ratio of the number of moles of Al to the sum of the number of moles of Al and Ga, and specifically, can be defined using the values ​​of the number of moles of Al and Ga analyzed and quantified by EDX. The Al composition ratio x9 of the upper guide layer 323 may be larger than the Al composition ratio of the well layer. This makes it possible to confine carriers in the nitride semiconductor active layer 322.

[0040] The upper guide layer 323 may be either an n-type semiconductor or a p-type semiconductor. When the upper guide layer 323 is an n-type semiconductor, for example, Si is 1×10 19 cm -3 When the upper guide layer 323 is a p-type semiconductor, for example, Mg is doped at a concentration of 3×10 for AlGaN. 19 cm -3 The upper guide layer 323 becomes p-type by being doped at this concentration. The upper guide layer 323 may be an undoped layer.

[0041] The thickness of the upper guiding layer 323 is preferably greater than 10 nm and less than 500 nm. By forming the upper guiding layer 323 in this thickness range, the optical confinement efficiency factor can be improved, and a laser diode with a lower oscillation threshold can be realized.

[0042] <Electron Blocking Layer> The electron blocking layer 33 is designed to have a larger band gap energy than the nitride semiconductor active layer 322. For example, when the nitride semiconductor active layer 322 is made of AlGaN, the electron blocking layer 33 can be made of AlGaN with a high AlN mixed crystal ratio. It is desirable that the electron blocking layer 33 is substantially flat in the plane direction of the substrate 10. The substantially flat electron blocking layer 33 serves to diffuse the carriers injected from the first electrode 51, which are locally concentrated in the composition gradient layer 34, in the plane direction within the electron blocking layer 33. This is because the electron blocking layer 33 has a large band gap energy, and therefore acts as a barrier to carrier diffusion. The material composition of the electron blocking layer 33 may be constant in the film thickness direction or may vary; however, it is preferable that the composition be constant in order to efficiently block electrons and to efficiently diffuse the above-mentioned carriers within the horizontal plane of the electron blocking layer 33.

[0043] The electron blocking layer 33 may be of the same conductivity type as the compositionally graded layer 34. When the electron blocking layer 33 is an n-type semiconductor, for example, Si is 1×10 19 cm -3 When the electron blocking layer 33 is a p-type semiconductor, for example, Mg is doped at a concentration of 3×10 19 cm -3 The electron blocking layer 33 becomes p-type by being doped at this concentration. The electron blocking layer 33 may also be an undoped layer.

[0044] <Composition gradient layer> The compositionally graded layer 34 is formed of AlGaN, in which the Al composition ratio decreases in the direction away from the nitride semiconductor active layer 322. The AlN mixed crystal ratio of the compositionally graded layer 34 is preferably the same as or lower than that of the electron blocking layer 33. This design makes it possible to efficiently transport carriers injected from the first electrode 51 to the nitride semiconductor active layer 322. Furthermore, because the Al composition ratio of the layer of the compositionally graded layer 34 close to the second nitride semiconductor layer 35 is low, the barrier with the AlGaN or GaN constituting the second nitride semiconductor layer 35 can be significantly reduced. This further reduces the resistance between the second compositionally graded region 342 and the second nitride semiconductor layer 35, lowers the Schottky barrier, and further improves carrier injection efficiency.

[0045] Furthermore, the composition gradient layer 34 has a higher Al composition ratio closer to the substrate 10. Therefore, when wet-etching the side surface of the composition gradient layer 34 remaining after dry etching with, for example, an alkaline solution to form the ridge portion semiconductor layer 30, the region of the ridge portion semiconductor layer 30 including the composition gradient layer 34 (the upper portion of the ridge portion semiconductor layer 30) can be formed in an inverted taper shape. This is because the region of the composition gradient layer 34 closer to the substrate 10 has a higher Al composition ratio and is etched at a higher etching rate (details will be described later).

[0046] More specifically, the composition gradient layer 34 is formed of Al x Ga (1-x) N, and the Al composition ratio x of the composition gradient layer 34 is preferably, for example, 0 < x ≦ 0.9. That is, the Al composition ratio x of the composition gradient layer 34 preferably changes from 0.9 to almost 0 in the direction away from the nitride semiconductor active layer 322. The Al composition ratio x in the composition gradient layer 34 may decrease at a constant rate of change throughout the thickness direction of the composition gradient layer 34. Also, the Al composition ratio x may have a configuration in which the Al composition ratio x changes in multiple steps by having a region where the Al composition ratio x becomes constant in the middle of the thickness direction of the composition gradient layer 34.

[0047] Also, as shown in FIGS. 1 and 2, the composition gradient layer 34 may include a first composition gradient region 341 and a second composition gradient region 342 formed of AlGaN. The second composition gradient region 342 is, for example, a region farther from the nitride semiconductor active layer 322 than the first composition gradient region 341. The first composition gradient region 341 and the second composition gradient region 342 are, for example, regions where the average Al composition ratio and the rate of change of the Al composition ratio are different from each other. In the present embodiment, the case where the composition gradient layer 34 includes the first composition gradient region 341 and the second composition gradient region 342 will be described. Hereinafter, each region will be described in detail.

[0048] (First AlGaN Region) The first compositionally graded region 341 is formed of AlGaN, in which the Al composition ratio decreases in the direction away from the nitride semiconductor active layer 322. The first compositionally graded region 341 is formed above the electron blocking layer 33. More specifically, the first compositionally graded region 341 is formed of AlGaN, in which the Al composition ratio decreases in the direction away from the nitride semiconductor active layer 322. x1 Ga (1-x1) The first composition gradient region 341 is made of N. From the viewpoint of efficiently confining light, the Al composition ratio x1 in the first composition gradient region 341 is preferably higher than that of the upper guide layer, and more preferably 0.6≦x1≦0.9. The Al composition ratio x1 in the first composition gradient region 341 decreases within the above-mentioned range in the direction away from the nitride semiconductor active layer 322.

[0049] The first compositionally graded region 341 may be of the same conductivity type as the second nitride semiconductor layer 35. When the first compositionally graded region 341 is an n-type semiconductor, for example, Si is 1×10 19 cm -3 When the first composition gradient region 341 is a p-type semiconductor, for example, Mg is doped at a concentration of 3×10 19 cm -3 By doping at this concentration, the first composition gradient region 341 becomes p-type. The first composition gradient region 341 may also be an undoped layer.

[0050] The first composition gradient region 341 preferably has a thickness of more than 0 nm and less than 400 nm. When the thickness of the first composition gradient region 341 is less than 400 nm, the resistance of the first composition gradient region 341 is low, which suppresses an increase in heat generation due to an increase in drive voltage, making the nitride semiconductor laser diode 1 less likely to be damaged. The thickness of the first composition gradient region 341 is preferably 150 nm or more and less than 400 nm, and more preferably 200 nm or more and less than 400 nm. The thickness of the first composition gradient region 341 is, for example, 320 nm.

[0051] (Second AlGaN region) The second composition inclined region 342 is formed of AlGaN in which the Al composition ratio decreases in a direction away from the nitride semiconductor active layer 322. The second composition inclined region 342 is formed on the first composition inclined region 341. More specifically, the second composition inclined region 342 is formed of Al x2 Ga (1-x2) N. The Al composition ratio x2 in the second composition inclined region 342 is preferably, for example, 0 < x2 ≦ 0.6. The Al composition ratio x2 of the second composition inclined region 342 decreases within the above-described range in a direction away from the nitride semiconductor active layer 322. In the second composition inclined region 342, it is formed so that the average Al composition ratio is lower than that of the first composition inclined region 341. Thereby, it is possible to efficiently flow current from the electrode to the active layer.

[0052] It is preferable that the Al composition ratio x2 continuously changes in the thickness direction of the region in the second composition inclined region 342. At this time, the slope rate (i.e., the change rate) of the Al composition ratio x2 may be constant or may continuously change. It is more preferable that the Al composition ratio continuously changes in both the first composition inclined region 341 and the second composition inclined region 342. Here, the continuous change of the Al composition in both the first composition inclined region 341 and the second composition inclined region 342 means that the Al composition ratio at the interface where the first composition inclined region 341 and the second composition inclined region 342 contact is the same. Further, the Al composition ratio at the interface where the first composition inclined region 341 and the second composition inclined region 342 contact means the Al composition ratio at the intersection of the regression line of the Al composition ratio x1 of the first composition inclined region 341 and the regression line of the Al composition ratio x2 of the second composition inclined region 342.

[0053] The AlGaN constituting the second composition inclined region 342 may contain a group V element other than N such as P, As, or Sb, a group III element such as In or B, or an impurity such as C, H, F, O, Si, Cd, Zn, or Be. Furthermore, when the AlGaN constituting the second composition gradient region 342 contains, for example, Mg as a dopant for a p-type semiconductor, the second composition gradient region 342 is a region in which the Al composition ratio x2 decreases continuously, and holes are generated in the second composition gradient region 342 due to polarization during +c-plane growth.

[0054] The second composition gradient region 342 may be an undoped layer that does not contain Mg as a dopant. By making the second composition gradient region 342 an undoped layer, it is possible to suppress light absorption caused by impurities, thereby reducing internal loss in a laser diode. Furthermore, in a light-emitting diode, suppressing light absorption improves light extraction efficiency and light emission efficiency. The second composition gradient region 342 may be in direct contact with the first composition gradient region 341. Furthermore, an AlGaN layer, which is a mixed crystal of AlN and GaN with a constant composition, may be included between the first composition gradient region 341 and the second composition gradient region 342.

[0055] The second composition gradient region 342 has a thickness of, for example, more than 0 nm and less than 130 nm. If the thickness of the second composition gradient region 342 is less than 130 nm, it is preferable because the nitride semiconductor laser diode 1 oscillates appropriately. The second composition gradient region 342 is formed to a thickness of, for example, 75 nm.

[0056] The first compositionally graded region 341 and the second compositionally graded region 342 are formed by thin film growth using, for example, an organic vapor phase epitaxy (MOVPE) apparatus. The compositionally graded layer 34 is grown by continuously increasing the flow rate of the source gas TMG (trimethylgallium) and continuously decreasing the flow rate of TMA (trimethylaluminum), while simultaneously flowing ammonia gas. This allows the formation of a compositionally graded layer 34 in which the Al composition ratio of AlGaN varies. At this time, by simultaneously flowing Cp2Mg (cyclopentadienylmagnesium) and ammonia gas, Mg can be added to the AlGaN as an impurity.

[0057] <Second nitride semiconductor layer> The second nitride semiconductor layer 35 is a layer that is in a region farther from the nitride semiconductor active layer 322 than the second composition gradient region 342 and covers the entire surface of the second composition gradient region 342. The second nitride semiconductor layer 35 is preferably formed of AlGaN or GaN having an Al composition ratio lower than that of the second composition gradient region 342. That is, the second nitride semiconductor layer 35 is Al x3 Ga (1-x3) N (0 ≦ x3 < x2).

[0058] The second nitride semiconductor layer 35 is formed of a semiconductor having a second conductivity type (n-type or p-type) different from the first conductivity type. When the second nitride semiconductor layer 35 is an n-type semiconductor, for example, when Si is doped at a concentration of 1 × 10 19 cm -3 the second nitride semiconductor layer 35 becomes n-type. When the second nitride semiconductor layer 35 is a p-type semiconductor, for example, when Mg is doped at a concentration of 3 × 10 19 cm -3 the second nitride semiconductor layer 35 becomes p-type. The concentration of the dopant may be constant or non-uniform in the film thickness direction of the substrate 10. Also, the concentration of the dopant may be constant or non-uniform in the in-plane direction of the substrate 10.

[0059] When the uppermost layer of the second nitride semiconductor layer 35 is p-type GaN (p-GaN), the contact resistance with the second electrode 52 disposed on the second nitride semiconductor layer 35 can be reduced, and the wavelength range of ultraviolet light that the nitride semiconductor laser diode 1 can handle becomes wider. This is because when p-GaN is used as the second nitride semiconductor layer 35, the Al composition ratio of the AlGaN in the second composition gradient region 342 can be widely designed. The second nitride semiconductor layer 35 may have a structure in which a plurality of layers are stacked. In this case, the Al composition ratio of the second nitride semiconductor layer 35 described above indicates the composition ratio at the outermost layer, that is, the surface in contact with the second electrode 52.

[0060] The second nitride semiconductor layer 35 may have a structure in which the Al composition ratio of AlGaN is graded. For example, the second nitride semiconductor layer 35 may have a layer structure in which the Al composition ratio of AlGaN decreases continuously or stepwise from the minimum value of the Al composition ratio in the compositionally graded layer 34. When the second nitride semiconductor layer 35 has a layer structure, the second nitride semiconductor layer 35 may be an undoped layer. The second nitride semiconductor layer 35 may have a laminated structure further including a highly doped layer as the uppermost layer. The second nitride semiconductor layer 35 may have a laminated structure of two or more layers. In this case, it is preferable that the Al composition ratio decreases toward the upper layer in order to efficiently transport carriers to the nitride semiconductor active layer 322.

[0061] The second nitride semiconductor layer 35 preferably has a thickness of more than 10 nm and less than 10 μm, more preferably 200 nm or more and less than 10 μm, and even more preferably 500 nm or more and 5 μm or less. When the thickness of the second nitride semiconductor layer 35 exceeds 10 nm, adhesion between the compositionally graded layer 34 and the second nitride semiconductor layer 35 provided on the upper surface of the compositionally graded layer 34 is improved. Specifically, the formation of voids at the interface between the compositionally graded layer 34 and the second nitride semiconductor layer 35 can be suppressed. This can improve the current density. Furthermore, local current concentration when carriers are injected from the second electrode 52 can be suppressed, and current can be injected uniformly from the upper surface of the second compositionally graded region 342 (the surface facing the second nitride semiconductor layer 35). Furthermore, when the thickness of the second nitride semiconductor layer 35 exceeds 10 nm, the compositionally graded layer 34 and the second electrode 52 are connected with low resistance via the second nitride semiconductor layer 35. Furthermore, if the thickness of the second nitride semiconductor layer 35 is less than 10 μm, cracks are less likely to occur when the composition gradient layer 34 is formed, which is preferable. Furthermore, when the thickness of second nitride semiconductor layer 35 is within this range, it is possible to suppress three-dimensional growth due to lattice relaxation during the growth of second nitride semiconductor layer 35, and to flatten the surface of second nitride semiconductor layer 35. This stabilizes the contact between second nitride semiconductor layer 35 and second electrode 52, and realizes a nitride semiconductor laser diode 1 with a low drive voltage.

[0062] <Ridge Semiconductor Layer> The ridge semiconductor layer 30 is composed of a part of the upper guide layer 323 that constitutes the light emitting portion 32, a compositionally graded layer 34, and a second nitride semiconductor layer 35. In the nitride semiconductor laser diode 1 of this embodiment, the ridge semiconductor layer 30 also includes an electron blocking layer 33. By forming the ridge semiconductor layer 30, it is possible to remove layers made of materials with high refractive indexes that are formed above the light emitting portion 32, and to confine light from the light emitting portion 32 in the plane direction within the light emitting portion 32.

[0063] In the nitride semiconductor laser diode 1 of this embodiment, the upper portion of the ridge semiconductor layer 30 has a reverse tapered shape, and the lower portion has a forward tapered shape. The inversely tapered upper portion of the ridge semiconductor layer 30 increases the contact area between the ridge semiconductor layer 30 and the second electrode, thereby reducing resistance and driving voltage, and constricts the current to improve current density, thereby reducing the threshold current. Furthermore, the inversely tapered upper portion of the ridge semiconductor layer 30 increases the electrode area, improving the degree of freedom in designing the electrode pattern. Furthermore, in a flip-chip bonding type laser diode, the bonding area can be increased, improving heat dissipation and increasing the degree of freedom in designing the bonding pattern in flip-chip bonding.

[0064] Furthermore, by forming the lower portion of the ridge semiconductor layer 30 in a forward tapered shape, excessive light confinement in the light emitting portion 32 is suppressed, the characteristics of light intensity L relative to current I are stabilized, and kinks are prevented. Furthermore, degradation of the light emission characteristics of the nitride semiconductor laser diode 1 due to deterioration of the crystallinity of the side surface 30A of the ridge semiconductor layer 30 can be suppressed. Furthermore, the distance between the light emission point inside the nitride semiconductor active layer 322 where the current is confined and the outside (cavity 60 in FIGS. 1 and 2 ) can be increased. This has the effect of suppressing chemical reactions between the outside air (water and oxygen) and the AlGaN of the nitride semiconductor active layer 322, which are accelerated by current and heat and cause degradation, and thereby extending the life of the nitride semiconductor laser diode 1. Furthermore, the current in the portion of the above-mentioned inverted tapered portion where the current is most concentrated (the side portion of the ridge semiconductor layer 30 where the inverted tapered portion and the forward tapered portion meet) can be partially diffused in the forward tapered portion before this current reaches the nitride semiconductor active layer 322, thereby suppressing damage to the element due to current concentration and extending the life of the element. This improves the light confinement effect on the side surfaces of the ridge semiconductor layer, thereby improving the light emission efficiency of the nitride semiconductor laser diode 1. Furthermore, the nitride semiconductor laser diode 1 can achieve a high current density and reduce the laser oscillation threshold.

[0065] As described above, the role of the ridge semiconductor layer 30 is to concentrate current and confine light to the light emitting portion 32. For this reason, the ridge semiconductor layer 30 does not necessarily have to be formed of only a part of the upper guide layer 323, the composition gradient layer 34, and the second nitride semiconductor layer 35. The inclusion of the compositionally graded layer 34 in the upper AlGaN layer 36 makes it particularly easy to form the inverted tapered shape described above. This is because, when an alkaline solution is used in a wet etching process, the side etching progresses more easily as the Al composition of the AlGaN increases, making it easier to form an inverted tapered structure in a compositionally graded structure in which the Al composition changes continuously.

[0066] The bottom surface of the forward tapered region of the ridge semiconductor layer 30 is preferably located in the upper guide layer 323. This is because, in the processing step for forming the shape of the ridge semiconductor layer 30, the nitride semiconductor surface on the same plane as the bottom surface of the forward tapered region is farther away from the nitride semiconductor active layer 322 by the thickness of the upper guide layer 323, thereby reducing the possibility that processing damage in the processing step will reach the nitride semiconductor active layer 322.

[0067] In this case, the position of the bottom surface of the reverse tapered region of the ridge semiconductor layer 30, i.e., the location where the forward tapered shape switches between the forward tapered shape and the reverse tapered shape, may be in the upper guide layer 323, or may be in a region of the upper AlGaN layer 36 other than the upper guide layer 323. That is, the forward tapered region of the ridge semiconductor layer 30 may be formed in at least one or both of the upper guide layer 323 and the lower region of the upper AlGaN layer 36 other than the upper guide layer 323 (electron block layer 33, composition change layer 34, second nitride semiconductor layer 35), and more preferably, it is formed in both the upper guide layer 323 and the upper AlGaN layer 36 other than the upper guide layer 323. Furthermore, the reverse tapered region of the ridge semiconductor layer 30 may be formed in the upper AlGaN layer 36 other than the forward tapered region.

[0068] More specifically, the position of the bottom surface of the inverted tapered region of the ridge semiconductor layer 30 is preferably at the following location. Note that the "position of the bottom surface" may be the bottom surface or top surface of each layer, i.e., the position may be at the boundary between layers. 1) Upper guide layer 323 2) Electron Blocking Layer 33 3) Composition gradient layer 34

[0069] 3A, when the bottom surface of the inversely tapered region of the ridge semiconductor layer 30 is located in the upper guide layer 323, the effect of suppressing light leakage in the horizontal direction is particularly large, and the effects of controlling the horizontal spread of the laser and reducing the oscillation threshold are enhanced. Furthermore, because the inversely tapered region is formed up to the upper guide layer 323, the horizontal spread of current in the forward tapered region can be suppressed, and the effects of controlling the mode and reducing the oscillation threshold are further enhanced.

[0070] 3B, when the bottom surface of the reverse-tapered region of the ridge semiconductor layer 30 is located in the electron blocking layer 33, the reproducibility of forming a forward tapered region and a reverse-tapered region by etching is high. This is because, for example, in the case of an AlGaN material, the etching rate increases as the Al / (Al+Ga) composition ratio increases, and the electron blocking layer 33 typically uses a material with a band gap larger than that of the upper guide layer 323 and the compositionally graded layer 34 (in the case of AlGaN, the Al / (Al+Ga) composition ratio is the highest), making it easy to form the transition point between the forward tapered shape and the reverse tapered shape in the electron blocking layer 33. In other words, the manufacturing yield is high in the manufacturing process of the nitride semiconductor laser diode 1 having characteristics that fall within the standard range.

[0071] As shown in Figure 3C, when the bottom surface of the inverted tapered region of the ridge semiconductor layer 30 is located in the compositionally graded layer 34, the surface area of ​​the inverted tapered portion is smaller than in the cases shown in Figures 3A and 3B. In the nitride semiconductor laser diode 1, when a p-type semiconductor that becomes the compositionally graded layer 34 is grown on the +c plane, the moisture resistance and corrosion resistance of this inverted tapered region (where the -c plane is partially exposed) are low. If structural degradation in this portion progresses during power supply or storage, the lifetime of the device will be shortened. However, in the structure shown in Figure 3C, by reducing the area of ​​this inverted tapered region, a device with a long lifetime can be realized. 3A to 3C, the case where the bottom surface of the inversely tapered region of the ridge semiconductor layer 30 is located in the upper guide layer 323, the electron block layer 33, and the compositionally graded layer 34 has been described, but the present invention is not limited to this configuration. The bottom surface of the inversely tapered region of the ridge semiconductor layer 30 may be located at the interface between the layers.

[0072] It is also preferable that a cavity 60 covered with a coating film is formed in the lateral direction of the upper AlGaN layer 36. This is because the air (refractive index 1) in the cavity 60 has a low refractive index, and therefore the light confinement efficiency in the axial direction perpendicular to both the extension direction of the ridge semiconductor portion 30 and the film thickness direction of the thin film is improved during light resonance. Furthermore, since the cavity 60 is covered, the device surface in contact with the cavity 60 can be protected from external influences on the nitride semiconductor laser diode 1 (vibration, water, heat, electromagnetic waves, etc.), and a highly reliable nitride semiconductor laser diode 1 can be realized. Alternatively, an insulating film such as a silicon oxide film may be used as the covering film, i.e., the cavity 60 may be covered with an insulating film. In this case, the cavity 60 is formed on the side of the ridge semiconductor layer 30 whose side surface 30A is covered with the silicon oxide film 40, and the cavity 60 is covered with an insulating film (not shown).

[0073] As described above, the ridge semiconductor layer 30 is disposed biased toward the first electrode 51. By bringing the ridge semiconductor layer 30 closer to the first electrode 51, the current path through the nitride semiconductor laser diode 1 becomes shorter, thereby reducing the resistance of the current path formed in the nitride semiconductor laser diode 1 and enabling the driving voltage of the nitride semiconductor laser diode 1 to be lowered. However, from the viewpoint of lithography reproducibility, it is preferable that the ridge semiconductor layer 30 be separated from the end of the mesa structure (the end of the light emitting portion 32) by 1 μm or more. The ridge semiconductor layer 30 may be disposed in the center of the mesa structure or may be formed biased toward the end of the mesa structure.

[0074] <Resonator surface> The formation of a cavity facet is required for the nitride semiconductor laser diode 1. The cavity facet 70 is configured as the same plane formed by the side surfaces of the second stacked region 312 of the first nitride semiconductor layer 31, the light emitting portion 32, the compositionally graded layer 34, and the second nitride semiconductor layer 35. The cavity facet 70 is the surface whose outline is shown by a bold line in FIG. The back cavity facet (not shown in FIG. 1) is an end face facing the cavity facet 70, and is configured as a single plane formed by the end faces of the second stacked region 312 of the first nitride semiconductor layer 31, the light emitting portion 32, the compositionally graded layer 34, and the second nitride semiconductor layer 35.

[0075] The cavity facets 70 and the rear cavity facet are provided for the purpose of reflecting light emitted from the light emitting section 32. The cavity facets 70 and the rear cavity facet are provided as a pair to confine light reflected by the cavity facets 70 and the rear cavity facet within the light emitting section 32. The cavity facets 70 are, for example, the light emission side of the nitride semiconductor laser diode 1. In order to reflect light emitted from the light emitting section 32 at the cavity facets 70 and the rear cavity facet, the cavity facets 70 and the rear cavity facet may be flat and perpendicular to the contact surface between the nitride semiconductor active layer 322 and the upper guide layer 323. However, the cavity facets 70 and the rear cavity facet may have inclined or uneven portions entirely or partially.

[0076] An insulating protective film such as a dielectric multilayer film and a reflective film may be formed on the surfaces of the cavity facet 70 and the back cavity facet. Specifically, the insulating protective film may be formed of SiO2, or may be formed of Al2O3, SiN, SnO2, ZrO, HfO2, or the like. The insulating protective film may also have a structure in which these materials are laminated. The insulating protective film may be formed on both the cavity facet 70, which is the light-emitting side of the nitride semiconductor laser diode 1, and the back cavity facet, which is the reflecting side and not the light-emitting side. The insulating protective film formed on the cavity facet 70 on the light-emitting side and the insulating protective film formed on the back cavity facet on the light-reflecting side may have the same structure or different structures.

[0077] <Silicon oxide film> The silicon oxide film 40 is an example of an insulator, and is provided so as to cover at least the side surface 30A of the ridge semiconductor layer 30. The silicon oxide film 40 may be provided on the upper surface of the upper guide layer 323 from which the upper layer has been removed. This prevents the side surface 30A of the ridge semiconductor layer 30 from coming into direct contact with the air in the cavity 60, and makes it possible to prevent deterioration due to chemical changes caused by heat and current when the device is operating between the oxygen, carbon, water, carbon dioxide, etc. remaining in the cavity 60 and the side surface 30A of the ridge semiconductor layer 30.

[0078] <First electrode> The first electrode 51 is formed on the first nitride semiconductor layer 31. The first electrode 51 includes, for example, a lower first electrode 51A and an upper first electrode 51B, but may be composed of only the lower first electrode 51A.

[0079] When the first electrode 51 is an n-type electrode, the first electrode 51 is made of a general n-type electrode material for nitride semiconductor light-emitting laser diodes, provided that the first electrode 51 is used for the purpose of injecting electrons into the first nitride semiconductor layer 31. For example, the first electrode 51 is made of Ti, Al, Ni, Au, Cr, V, Zr, Hf, Nb, Ta, Mo, W, or an alloy thereof, ITO, or the like. When the first electrode 51 is a p-type electrode, if the first electrode 51 is used for the purpose of injecting holes into a nitride semiconductor light-emitting laser diode, the first electrode 51 is formed of a general p-type electrode material for nitride semiconductor light-emitting laser diodes. For example, the first electrode 51 is formed of Ni, Au, Pt, Ag, Rh, Pd, Cu, or an alloy thereof, or ITO, and in particular, Ni, Au, or an alloy thereof, or ITO is preferable. This is because the contact resistance between the first electrode 51 and the first nitride semiconductor layer 31 is small. In this embodiment, the first electrode 51 is formed to be, for example, an n-type electrode.

[0080] The first electrode 51 may have a pad electrode (upper first electrode) 51B above the lower first electrode 51A for the purpose of diffusing current evenly over the entire area of ​​the first electrode 51. The upper first electrode 51B is formed of, for example, Au, Al, Cu, Ag, or W, and is preferably formed of Au from the viewpoint of conductivity. The first electrode 51 is formed to a thickness of, for example, 60 nm. In the present disclosure, the first electrode 51 is formed to a thickness different from that of the second electrode 52, but may be formed to the same thickness as that of the second electrode 52.

[0081] <Second electrode> The second electrode 52 is formed on the ridge semiconductor layer 30, i.e., on the second nitride semiconductor layer 35, which is the uppermost layer of the upper AlGaN layer 36. The second electrode 52 is preferably electrically connected to the upper AlGaN layer 36, and the covering film is preferably composed of the second electrode 52. This structure allows the cavity 60 to be covered simultaneously when the second electrode 52 is formed on the upper surface 30B of the ridge semiconductor layer 30, thereby simplifying the process. Furthermore, the second electrode 52 preferably includes a lower second electrode 52A in contact with the ridge semiconductor layer 30 and an upper second electrode 52B disposed on the lower second electrode 52A, and the covering film is preferably composed of the upper second electrode 52B of the second electrode 52. This structure allows different metal materials to be used for the electrode (lower second electrode 52A) formed on the upper surface 30B of the ridge semiconductor layer 30 and the electrode (upper second electrode 52B) covering the cavity 60, thereby enabling different processing conditions to be applied. For example, after forming the lower second electrode 52A, a heat treatment is performed to form an alloy at the interface with the second nitride semiconductor layer 35, and then the upper second electrode 52B is covered, thereby forming the cavity 60. With this structure, it is possible to prevent the shape of the second electrode 52 covering the cavity 60 from changing when the lower second electrode 52A is processed. The second electrode 52 includes, for example, a lower second electrode 52A and an upper second electrode 52B disposed on the lower second electrode 52A, but may be composed of only the lower second electrode 52A.

[0082] The second electrode 52, which is a coating film, may have a through-path that penetrates from the inner surface to the outer surface of the second electrode 52. The through-path is a minute through-hole that connects the inside (cavity 60) of the nitride semiconductor laser diode 1 to the outside. By supplying oxygen via this through-path, it becomes possible to maintain the formation of an oxide film (silicon oxide film 40) that is formed on the side surface 30A of the ridge semiconductor layer 30 by passivation. This serves to suppress the promotion of oxidation from the side surface 30A of the ridge semiconductor layer 30 to the inside, which would otherwise be caused by cracks in the silicon oxide film 40 due to heat and current during use of the nitride semiconductor laser diode 1, by repeatedly passivating it.

[0083] When the second electrode 52 is a p-type electrode, if the second electrode 52 is used for the purpose of injecting holes into a nitride semiconductor light-emitting laser diode, the second electrode 52 is formed of a general p-type electrode material for nitride semiconductor light-emitting laser diodes. For example, the second electrode 52 is formed of Ni, Au, Pt, Ag, Rh, Pd, Cu, or an alloy thereof, or ITO, etc., and Ni, Au, or an alloy thereof, or ITO is particularly preferred. This is because the contact resistance between the second electrode 52 and the ridge semiconductor layer 30 is reduced. When the second electrode 52 is an n-type electrode, the second electrode 52 is made of a general n-type electrode material for nitride semiconductor light-emitting laser diodes, provided that the second electrode 52 is used for the purpose of injecting electrons into the ridge semiconductor layer 30. For example, the second electrode 52 is made of Ti, Al, Ni, Au, Cr, V, Zr, Hf, Nb, Ta, Mo, W, or an alloy thereof, ITO, or the like. In this embodiment, the second electrode 52 is formed to be, for example, a p-type electrode.

[0084] The second electrode 52 has a pad electrode (upper second electrode) 52B above the lower second electrode 52A for the purposes of diffusing current evenly throughout the entire area of ​​the second electrode 52 and forming a cavity 60 on the side of the ridge semiconductor layer 30. The upper second electrode 52B can be made of the same material and have the same configuration as the upper first electrode 51B. Alternatively, instead of the upper second electrode 52B, a lower second electrode 52A made of an alloy of Ni and Au may be formed on the ridge semiconductor layer 30, and the upper second electrode 52B made of Au may be formed on the lower second electrode 52A. The second electrode 52 is formed to a thickness of, for example, 240 nm.

[0085] The second electrode 52 preferably has a rectangular shape with short sides of less than 10 μm and long sides of 1000 μm or less, and is stacked on the second nitride semiconductor layer 35. The surface of the second electrode 52 facing the ridge semiconductor layer 30 preferably has substantially the same shape as the upper surface 30B of the ridge semiconductor layer 30. By having the same shape at the contact surfaces of the second electrode 52 and the ridge semiconductor layer 30, carriers injected from the second electrode 52 are prevented from diffusing in the surface direction of the substrate 10 within the ridge semiconductor layer 30, and light emission in the nitride semiconductor active layer 322 can be controlled.

[0086] (1.2) Manufacturing method of nitride semiconductor laser diode 4A to 4J, a method for manufacturing nitride semiconductor laser diode 1 will be described. Note that the following manufacturing method will be described for a case where first nitride semiconductor layer 31 is formed of an n-type semiconductor and second nitride semiconductor layer 35 is formed of a p-type semiconductor. Also, the following manufacturing method will be described for a nitride semiconductor laser diode 1 that includes buffer layer 20 on substrate 10 and electron blocking layer 33 between light emitting portion 32 and composition gradient layer 34.

[0087] 4A , a buffer layer 20, a first nitride semiconductor layer 31A, a lower guide layer 321A, a nitride semiconductor active layer 322A, an upper guide layer 323A, an electron block layer 33A, a compositionally graded layer 34A, and a second nitride semiconductor layer 35A are formed in this order on a substrate 10, such as a sapphire substrate, to form a semiconductor stack 301. The compositionally graded layer 34A is composed of a first compositionally graded region 341A and a second compositionally graded region 342A. Here, the first nitride semiconductor layer 31A, the lower guide layer 321A, the nitride semiconductor active layer 322A, the upper guide layer 323A, the electron block layer 33A, the compositionally graded layer 34A, and the second nitride semiconductor layer 35A are nitride semiconductor layers that will later become the first nitride semiconductor layer 31, the lower guide layer 321, the nitride semiconductor active layer 322, the upper guide layer 323, the electron block layer 33, the compositionally graded layer 34, and the second nitride semiconductor layer 35, respectively. Subsequently, the substrate 10 on which the semiconductor laminated portion has been formed is heat-treated to activate the impurities. The heat treatment is performed, for example, in an environment of 500° C. to 600° C. for 5 minutes to 20 minutes.

[0088] 4B, a part of the upper surface of the second nitride semiconductor layer 35A is covered with an etching mask 100 made of metal. The etching mask 100 is formed by depositing a metal material such as Ni in a region where the ridge semiconductor layer 30 (see FIG. 1) will be formed later. Next, as shown in FIG. 4C , dry etching using Cl gas is performed to remove the semiconductor laminate 301 from the second nitride semiconductor layer 35A to a portion of the upper guide layer 323A in the thickness direction in a region not covered by the etching mask 100. That is, dry etching is performed from the second nitride semiconductor layer 35A to a portion of the upper guide layer 323A in the thickness direction, and then the dry etching is terminated. If the dry etching process is anisotropic etching, which is widely used, the end face 302A of the semiconductor laminate 302 formed as the dry etching progresses becomes difficult to dry etch. Therefore, when the portion of the semiconductor laminate 301 not covered by the etching mask 100 is removed, the cross-sectional shape of the remaining semiconductor laminate 302 covered by the etching mask 100 becomes forward tapered. The remaining semiconductor laminate 302 includes the upper guide layer 323B, the electron block layer 33B, the compositionally graded layer 34B having the first compositionally graded region 341B and the second compositionally graded region 342B, and the second nitride semiconductor layer 35B.

[0089] When forming the upper AlGaN layer 36, it is preferable to form an upper guide layer 323 on the nitride semiconductor active layer 322 and terminate the dry etching after performing dry etching partway through the thickness direction of the upper guide layer 323. By performing this step, it is possible to suppress dry etching damage to the nitride semiconductor active layer 323.

[0090] Next, as shown in FIG. 4D, the side surfaces of the second nitride semiconductor layer 35B to the upper guide layer 323B of the semiconductor laminate 302 remaining after the dry etching are wet-etched using, for example, an alkaline solution. At this time, for example, tetramethylammonium hydroxide (TMAH) is used as the alkaline solution. As a result, the side surfaces of the semiconductor laminate 302 are removed by wet etching. This is because the etching rates of the a-plane and m-plane of nitride semiconductors are faster than the etching rate of the c-plane. Wet etching is performed until the upper part of the semiconductor laminate 302 has an inverse tapered shape.

[0091] At this time, because the side surface (end surface 302A) of the semiconductor laminate 302 remaining after dry etching has a forward tapered shape, as etching of the end surface 302A progresses, the cross-sectional shape of the semiconductor laminate 302 becomes a structure that combines a forward tapered shape and a reverse tapered shape, as shown in FIG. 4D. When the structure shown in FIG. 4D is achieved, the wet etching is stopped. Note that, to form the structure shown in FIG. 4D, the wet etching time can be set taking into account the etching rate of the end surface 302A. Furthermore, at this time, the higher the Al composition ratio, the faster the etching rate. Therefore, the side surface of the electron blocking layer 33B, which has the highest Al composition ratio, is removed the most, and the side surface of the second nitride semiconductor layer 35B, which has the lowest Al composition ratio, is removed the least. Furthermore, in the composition gradient layer 34B, in which the Al composition ratio decreases in the direction away from the nitride semiconductor active layer 322, the region closer to the electron blocking layer 33B is removed more, and the region closer to the second nitride semiconductor layer 35B is removed less. This results in the formation of the ridge semiconductor layer 30, which is a semiconductor laminated portion having a forward tapered lower portion and a reverse tapered upper portion. In other words, a device having a thin-film structure such as the nitride semiconductor laser diode 1 has the advantage that the Al composition ratio can be adjusted relatively easily, and therefore the structure shown in FIG. 4D can be easily formed. In the ridge semiconductor layer 30, the second nitride semiconductor layer 35, the composition gradient layer 34, and the electron blocking layer 33 are formed in an inverse tapered shape that narrows toward the substrate 10 in a cross-sectional view, and a portion of the upper guide layer 323 has a tapered shape that widens toward the substrate 10 in a cross-sectional view.

[0092] 4E, a silicon oxide film 40A, which is an insulator, is formed by sputtering so as to cover the upper surface of the upper guide layer 323, the side surface 30A of the ridge semiconductor layer 30, and the etching mask 100 that remain after dry etching. The sputtering causes the material to spread to the side surface 30A of the ridge semiconductor layer 30, and the silicon oxide film 40A is formed so as to cover not only the upper surface of the upper guide layer 323 and the upper surface of the etching mask 100 but also the side surface 30A of the ridge semiconductor layer 30. At this time, cracks, pinholes, etc. (hereinafter referred to as cracks) C1 and C2 are generated in the silicon oxide film 40A, starting from portions P1 and P2 where the shape of the side surface 30A of the ridge semiconductor layer 30 changes abruptly in cross-sectional view (mutation points where the slope of the line contacting the side surface 30A of the ridge semiconductor layer 30 changes abruptly). The crack C1 is formed at the boundary between the silicon oxide film 40A formed on the upper surface of the etching mask 100 and the silicon oxide film 40 formed on the side surface 30A of the ridge semiconductor layer 30.

[0093] At this time, after forming the silicon oxide film 40A on the upper surface of the upper guide layer 323 and the side surface 30A of the ridge semiconductor layer 30, the silicon oxide film 40A may be formed under formation conditions in which the coating material is coarser than the silicon oxide film 40A, for example, by lowering the film formation temperature. This makes it possible to form the silicon oxide film 40A on the upper surface of the etching mask 100, which is made of crystals that are coarser than the silicon oxide film 40A formed on the upper surface of the upper guide layer 323 and the side surface 30A of the ridge semiconductor layer 30. This causes a crack (corresponding to the crack C1 shown in FIG. 4E ) between the silicon oxide film 40A formed on the upper surface of the upper guide layer 323 and the side surface 30A of the ridge semiconductor layer 30 and the silicon oxide film 40A formed on the upper surface of the etching mask 100.

[0094] Next, as shown in FIG. 4F , the region where the silicon oxide film 40A is formed is treated with a solution that dissolves the metal (e.g., Ni) that constitutes the etching mask 100. Here, for example, nitric acid is used as the solution that dissolves the metal. As a result, the nitric acid penetrates into the silicon oxide film 40A through the cracks C1 and C2, and the nitric acid that penetrates from the crack C1 close to the etching mask 100 dissolves the etching mask 100. As a result, the etching mask 100 and the silicon oxide film 40A formed on the etching mask 100 are removed, and the upper surface 30B (second nitride semiconductor layer 35) of the ridge semiconductor layer 30 is exposed. That is, in this embodiment, in order to expose the second nitride semiconductor layer 35, the ridge semiconductor layer 30 is formed so that its upper portion has a reverse tapered shape and its lower portion has a forward tapered shape, and a crack C1 is generated in the silicon oxide film 40A. Here, there is no region made of metal near the crack C2 formed in the lower part of the ridge semiconductor layer 30. Therefore, the silicon oxide film 40A formed on the upper surface of the upper guide layer 323 and on the side surface 30A of the ridge semiconductor layer 30 remains as the silicon oxide film 40 even after the nitric acid treatment. This method allows the side surface 30A of the ridge semiconductor layer 30 to be covered with the silicon oxide film 40 and the upper surface 30B of the ridge semiconductor layer 30 to be exposed, thereby increasing the contact area between the second electrode 52 and the upper surface 30B of the ridge semiconductor layer 30.

[0095] 4G, the region excluding the region where the ridge semiconductor layer 30 is formed and its surrounding region is removed to form a mesa structure 37. At this time, an etching mask is formed of a metal (Ni, etc.) on the top surface of the region where the ridge semiconductor layer 30 is formed and its surrounding region, and the region where the etching mask (not shown in FIG. 4G) is not formed is removed by dry etching using Cl gas. Thereafter, the etching mask is removed by nitric acid treatment.

[0096] Next, as shown in FIG. 4H, a first electrode 51 is formed by electron beam (EB) deposition on the upper surface of the first nitride semiconductor layer 31 from which the upper layer was removed when forming the mesa structure 37, and a second electrode 52 is formed on the upper surface of the second nitride semiconductor layer 35. 4I, a resist mask (not shown in FIG. 4H) covering the ridge semiconductor layer 30 and the first electrode 51 is formed by photolithography, a silicon oxide film 40B (not shown in FIG. 4H) is formed, and then the resist mask and the silicon oxide film 40B formed on the resist mask are removed to form the silicon oxide film 40 covering the side surface 30A of the ridge semiconductor layer 30 and the upper surface of the first nitride semiconductor layer 31. Note that FIG. 4I shows only the silicon oxide film 40 after the silicon oxide film 40B on the resist mask covering the first electrode 51 has been removed.

[0097] Finally, as shown in FIG. 4J , an upper first electrode 51B covering the first electrode 51 and an upper second electrode 52B covering the second electrode 52 are formed by electron beam evaporation. The upper first electrode 51B and the upper second electrode 52B are formed simultaneously, for example, by electron beam evaporation. Therefore, a coating material is deposited in a direction substantially perpendicular to the substrate 10 to form the upper second electrode 52B. That is, a metal material that will be used as the coating material for the upper second electrode 52B, which is a pad electrode, is evaporated substantially linearly from above the ridge semiconductor layer 30 toward the substrate 10 to form the upper second electrode 52B. Therefore, the second electrode 52 serves as a mask, and metal is not evaporated on the side surface 30A of the ridge semiconductor layer 30, forming a cavity 60 between the side surface of the ridge semiconductor layer 30 and the upper second electrode 52B. In this manner, the upper second electrode 52B, which is a coating film that covers the side surface of the ridge semiconductor layer 30 via the cavity 60, is formed. Here, "a direction approximately perpendicular to the substrate 10" refers not only to a direction strictly perpendicular to the substrate 10, but also to a direction that is not strictly perpendicular to the substrate 10 but is such that a coating material does not deposit by electron beam evaporation on the side surface 30A of the ridge semiconductor layer 30 formed at an angle to the thickness direction of the substrate 10. At this time, by increasing the degree of vacuum in the deposition environment, fine particles that hinder the straightness of the electron beam deposition material in the deposition environment can be reduced, and the straightness can be further improved. Thereafter, the second stacked region 312 of the first nitride semiconductor layer 31, the light emitting portion 32, the composition gradient layer 34, and the second nitride semiconductor layer 35 are etched in the direction of light oscillation to form a cavity facet (not shown), thereby obtaining the nitride semiconductor laser diode 1 shown in FIG.

[0098] (1.3) Effects of the First Embodiment The nitride semiconductor laser diode according to the first embodiment has the following effects. (1) In the nitride semiconductor laser diode according to this embodiment, a ridge semiconductor layer is formed by a portion of the compositionally graded layer of the upper guide layer formed on the substrate, the compositionally graded layer, and the second nitride semiconductor layer, and the ridge semiconductor layer has a forward tapered shape in a region close to the substrate in a cross-sectional view. This increases the contact area between the ridge semiconductor layer and the second electrode, lowering the resistance and driving voltage, while constricting the current to improve current density and reduce the threshold current. It also increases the electrode area, improving the degree of freedom in designing the electrode pattern. Furthermore, in a flip-chip bonding laser diode, it increases the bonding area, improving heat dissipation and increasing the degree of freedom in designing the bonding pattern in flip-chip bonding.

[0099] (2) In the nitride semiconductor laser diode according to this embodiment, the region of the ridge semiconductor layer far from the substrate has an inverse tapered shape. This forward tapered shape of the lower portion of the ridge semiconductor layer suppresses excessive optical confinement, stabilizes the characteristics of light intensity L versus current I, and prevents kinks from occurring. It also suppresses degradation of the light-emitting characteristics of the nitride semiconductor laser diode due to deterioration of the crystallinity of the side surfaces of the ridge semiconductor layer. It also increases the distance between the light-emitting point inside the nitride semiconductor active layer, where the current is confined, and the outside (cavity). This effectively suppresses chemical reactions that cause deterioration of nitride semiconductor laser diodes, thereby extending the life of the nitride semiconductor laser diode. Furthermore, the forward tapered portion can partially diffuse the current in the area where the most current concentrates in the reverse tapered portion of the ridge semiconductor layer (the side surface of the ridge semiconductor layer where the reverse tapered portion and the forward tapered portion meet) before it reaches the nitride semiconductor active layer, thereby suppressing device destruction due to current concentration and extending the life of the device.

[0100] (3) In the nitride semiconductor laser diode according to this embodiment, the upper second electrode covers the second electrode electrically connected to the second nitride semiconductor layer and the side surfaces of the ridge semiconductor layer. The ridge semiconductor layer having such a shape is covered with the upper second electrode via a cavity, which is a layer of air with a low refractive index, between the upper second electrode and the side surfaces of the ridge semiconductor layer. This improves the light confinement effect on the side surfaces of the ridge semiconductor layer, thereby improving the light emission efficiency of the nitride semiconductor laser diode.

[0101] (4) The nitride semiconductor laser diode according to this embodiment has a compositionally graded layer including a region formed of AlGaN in which the Al composition ratio at the surface on the upper guide layer side is larger than the Al composition ratio in the upper guide layer, and in which the Al composition ratio continuously decreases in the direction away from the nitride semiconductor active layer. This enables carriers to be efficiently transported to the nitride semiconductor active layer 322, and also improves carrier injection efficiency by significantly lowering the barrier between the composition gradient layer and the second nitride semiconductor layer 35. Furthermore, the ridge semiconductor layer structure for forming a cavity between the upper second electrode and the side surface of the ridge semiconductor layer can be easily formed by wet etching.

[0102] 2. Second embodiment 5 and 6, the nitride semiconductor laser diode 2 according to the second embodiment will be described. The nitride semiconductor laser diode 2 is a laser diode capable of emitting ultraviolet light, similar to the nitride semiconductor laser diode 1.

[0103] (2.1) Overall structure of nitride semiconductor laser diode 4, the nitride semiconductor laser diode 2 includes a substrate 10, a first nitride semiconductor layer 31 provided on the substrate 10, a light emitting portion 32, a compositionally graded layer 34, and a second nitride semiconductor layer 35. In the nitride semiconductor laser diode 1, a ridge semiconductor layer 30 is formed by a part of the upper guide layer 323 constituting the light emitting portion 32, the compositionally graded layer 34, and the second nitride semiconductor layer 35. As shown in FIG. 2, the ridge semiconductor layer 30 has a forward tapered shape in a cross-sectional view in a region closer to the substrate 10 and a reverse tapered shape in a region farther from the substrate 10, similar to the nitride semiconductor laser diode 1. In the nitride semiconductor laser diode 1, the upper second electrode 52B covers the side surface 30A of the ridge semiconductor layer 30 only via a cavity 60. The nitride semiconductor laser diode 2 includes a silicon oxide film 140 provided around the ridge semiconductor layer 30 so as not to cover the side surface 30A of the ridge semiconductor layer 30 but to cover only the top surface of the upper guide layer 323. That is, the nitride semiconductor laser diode 2 differs from the nitride semiconductor laser diode 1 according to the first embodiment in that the side surface 30A of the ridge semiconductor layer 30 is not covered with the silicon oxide film .

[0104] It is also preferable that the covering film covers the side surface 30A of the ridge semiconductor layer 30 only via the cavity 60. This is because, particularly when realizing a highly efficient laser diode, by placing air, which has a low refractive index, directly adjacent to the side surface 30A of the ridge semiconductor layer 30, the above-mentioned optical confinement effect can be maximized and high efficiency can be achieved.

[0105] Hereinafter, a method for manufacturing the nitride semiconductor laser diode 2 in which the side surface 30A of the ridge semiconductor layer 30 is not covered with a silicon oxide film will be described. Note that each part of the nitride semiconductor laser diode 2 has the same configuration as each part of the nitride semiconductor laser diode 1, and therefore description thereof will be omitted.

[0106] (2.2) Manufacturing method of nitride semiconductor laser diode First, the ridge semiconductor layer 30 is formed by the same method as in the nitride semiconductor laser diode 1 (see FIGS. 4A to 4D). Next, a silicon oxide film 40A is formed by electron beam evaporation on the upper surface of the upper guide layer 323 remaining after the dry etching and on the upper surface of the etching mask 100. At this time, since electron beam evaporation has a high linearity of material, electron beam evaporation is performed from above the ridge semiconductor layer 30 toward the substrate 10, so that the silicon oxide film 40A is formed on the upper surface of the upper guide layer 323 and on the upper surface of the etching mask 100 while leaving the side surface 30A of the ridge semiconductor layer 30 exposed.

[0107] Thereafter, the region where the silicon oxide film 40A is formed is treated with a solution (e.g., nitric acid) that dissolves the metal constituting the etching mask 100, thereby dissolving the etching mask 100. As a result, the etching mask 100 and the silicon oxide film 40A formed on the etching mask 100 are removed, and the upper surface 30B of the ridge semiconductor layer 30 is exposed. In this way, the nitride semiconductor laser diode 2 is obtained in which the upper second electrode 52B is formed so as to cover the side surface 30A of the ridge semiconductor layer 30 only via the cavity 60. The nitride semiconductor laser diode 2 includes a silicon oxide film 140 provided around the ridge semiconductor layer 30 so as not to cover the side surface 30A of the ridge semiconductor layer 30 but to cover the top surface of the upper guide layer 323.

[0108] (2.3) Effects of the Second Embodiment The nitride semiconductor laser diode according to the second embodiment has the following effects. (1) In the nitride semiconductor laser diode according to this embodiment, the upper second electrode covers the side surface of the ridge semiconductor layer only through a cavity, i.e., only a cavity, which is a layer of air with a low refractive index, exists between the upper second electrode and the side surface of the ridge semiconductor layer. This makes it possible to further improve the light confinement effect on the side surface of the ridge semiconductor layer and improve the light emission efficiency of the nitride semiconductor laser diode, compared to when a silicon oxide film is present between the upper second electrode and the side surface of the ridge semiconductor layer.

[0109] The scope of the present disclosure is not limited to the exemplary embodiments shown and described, but also includes all embodiments that achieve equivalent effects to those intended by the present disclosure. Furthermore, the scope of the present disclosure is not limited to the combination of inventive features defined by the claims, but may be defined by any desired combination of specific features from among all the respective disclosed features. [Explanation of symbols]

[0110] 1,2 Nitride semiconductor laser diode 10 Substrate 20 Buffer Layer 30 Ridge semiconductor layer 31, 31A First nitride semiconductor layer 32 Light-emitting part 321,321A Lower guide layer 322,322A Nitride semiconductor active layer 323, 323A, 323B Upper guide layer 33, 33A, 33B Electron blocking layer 34,34A,34B Composition gradient layer 341,341B First composition gradient region 342,342B Second composition gradient region 35,35A,35B Second nitride semiconductor layer 36 Upper AlGaN layer 40, 40A, 40B Silicon oxide film 51 First electrode 51A Lower first electrode 51B Upper first electrode 52 Second electrode 52A Lower second electrode 52B Upper second electrode 60 hollow 100 Etching Mask 300, 301, 302 Semiconductor laminated section C1, C2 cracks

Claims

1. A substrate; a first nitride semiconductor layer formed of first conductivity type AlGaN and provided above the substrate; an active layer made of AlGaN formed above the first nitride semiconductor layer; an upper AlGaN layer formed on the active layer; Equipped with a ridge semiconductor layer is formed by a part or all of the upper AlGaN layer, the ridge portion semiconductor layer has a forward tapered shape in a cross-sectional view, in which a region closer to the substrate gradually becomes wider toward the substrate, and a reverse tapered shape in which a region farther from the substrate gradually becomes narrower toward the substrate, the upper AlGaN layer has, on the active layer side in the upper AlGaN layer, an upper guide layer having a higher Al composition ratio than the active layer, and a composition gradient layer provided above the upper guide layer, the Al composition ratio of the composition gradient layer decreases in a direction away from the active layer, The forward tapered region of the ridge semiconductor layer is formed to include at least one or both of the upper guide layer and the lower region of the composition gradient layer, and the reverse tapered region is formed of the upper AlGaN layer other than the forward tapered region. Nitride semiconductor laser diode.

2. 2. The nitride semiconductor laser diode according to claim 1, wherein the forward tapered region of the ridge semiconductor layer is formed by both the upper guide layer and a lower region of the upper AlGaN layer other than the upper guide layer, and the reverse tapered region is formed by an upper region of the upper AlGaN layer other than the upper guide layer.

3. The bottom surface of the forward tapered region of the ridge semiconductor layer is located in the upper guide layer.

3. The nitride semiconductor laser diode according to claim 1.

4. 4. The nitride semiconductor laser diode according to claim 1, wherein a cavity covered with a coating film is formed in a lateral direction of the upper AlGaN layer.

5. a second electrode electrically connected to the upper AlGaN layer; The coating film is made of the second electrode.

5. The nitride semiconductor laser diode according to claim 4.

6. the second electrode includes a lower second electrode in contact with the ridge portion semiconductor layer and an upper second electrode disposed on the lower second electrode, The coating film is formed on the upper second electrode of the second electrode.

6. The nitride semiconductor laser diode according to claim 5.

7. an insulator provided so as to cover a side surface of the ridge semiconductor layer, The covering film covers the side surface of the ridge semiconductor layer via the cavity formed on the side of the insulator.

7. The nitride semiconductor laser diode according to claim 4.

8. The covering film covers the side surface of the ridge semiconductor layer only through the cavity.

7. The nitride semiconductor laser diode according to claim 4.

9. The coating film has a through-path that penetrates from the inner surface to the outer surface of the coating film.

8. The nitride semiconductor laser diode according to claim 4.

10. forming a semiconductor laminated portion by sequentially forming a first nitride semiconductor layer made of AlGaN of a first conductivity type, an active layer made of AlGaN or GaN, and an upper AlGaN layer made of AlGaN on a substrate; a part of an upper surface of the upper AlGaN layer is covered with an etching mask, and then a part or all of the upper AlGaN layer in the thickness direction of the semiconductor laminate portion that is not covered with the etching mask is removed by dry etching; The side surfaces of the upper AlGaN layer of the semiconductor laminate remaining after the dry etching are wet-etched so that a region of the upper AlGaN layer close to the substrate has a forward tapered shape that widens toward the substrate in cross-sectional view, and a region of the upper AlGaN layer farther from the substrate has a reverse tapered shape that narrows toward the substrate in cross-sectional view, and then the wet etching is stopped, thereby forming a ridge semiconductor layer. A method for manufacturing a nitride semiconductor laser diode.

11. When forming the upper AlGaN layer, a composition gradient layer is formed in which the Al composition ratio decreases in a direction away from the active layer. The method for manufacturing the nitride semiconductor laser diode according to claim 10.

12. When forming the upper AlGaN layer, an upper guide layer is formed above the active layer; Dry etching is completed after the upper guide layer is dry-etched halfway through its thickness.

12. The method for manufacturing the nitride semiconductor laser diode according to claim 10.

13. A coating material is deposited in a direction substantially perpendicular to the substrate, thereby forming a coating film that covers the sides of the ridge semiconductor layer via a cavity. A method for manufacturing the nitride semiconductor laser diode according to any one of claims 10 to 12.

14. The coating material is a metal that will be used as an electrode material. The method for manufacturing the nitride semiconductor laser diode according to claim 13 .

15. The coating material is a metal that will be used as the pad electrode material.

15. The method for manufacturing the nitride semiconductor laser diode according to claim 13 or 14.

16. An insulator is formed so as to cover the side surface of the ridge semiconductor layer and the etching mask, and the covering film is formed after the insulator is formed. A method for manufacturing the nitride semiconductor laser diode according to any one of claims 13 to 15.

17. An insulator is used as the coating material. A method for manufacturing the nitride semiconductor laser diode according to any one of claims 13 to 15.

18. When forming the covering film, after the covering film is formed on the side of the ridge semiconductor layer, the covering film is formed under forming conditions in which the covering material is coarser than the covering film formed on the side of the ridge semiconductor layer, thereby covering the cavity. A method for manufacturing the nitride semiconductor laser diode according to any one of claims 13 to 17.

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