Semiconductor laser element

Abstract

The present invention provides a semiconductor laser element that can reduce absorption loss and improve efficiency. [Solution] The semiconductor laser element has an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each made of a nitride semiconductor, arranged in order from top to bottom, with a ridge protruding upward from the p-side semiconductor layer. The p-side semiconductor layer has a first portion that is undoped and is in contact with the upper surface of the active layer, and has one or more semiconductor layers; an electron barrier layer that is in contact with the upper surface of the first portion, has a larger band gap energy than the first portion, and contains p-type impurities; and a second portion that is in contact with the upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities. The first portion has a p-side composition gradient layer that is undoped and has a band gap energy that increases as it goes upward, and an undoped p-side intermediate layer that is located above the p-side composition gradient layer, and the lower end of the ridge is located in the p-side intermediate layer.

Description

Technical Field

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

Background Art

[0002] Today, a semiconductor laser device having a nitride semiconductor (hereinafter also referred to as a "nitride semiconductor laser device") can oscillate light from the ultraviolet region to green and is used in a wide variety of applications, not limited to the light source of an optical disk. As such a semiconductor laser device, a structure having an n-side cladding layer, an n-side optical guide layer, an active layer, a p-side optical guide layer, and a p-side cladding layer in this order on a substrate is known (for example, Patent Documents 1, 2, and 3).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the semiconductor layer on the p-side of a nitride semiconductor laser device, a p-type impurity such as Mg is added, but the p-type impurity creates a deep energy level and causes light absorption. Therefore, the greater the light intensity in the p-type impurity-containing layer, the greater the absorption loss and the lower the efficiency such as the slope efficiency. Therefore, the present disclosure proposes a semiconductor laser device capable of reducing absorption loss and improving efficiency.

Means for Solving the Problems

[0005] A first embodiment of the semiconductor laser element in this disclosure is a semiconductor laser element having, in order upward, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each made of a nitride semiconductor, and the p-side semiconductor layer is provided with an upwardly projecting ridge. The p-side semiconductor layer is Distended in contact with the upper surface of the active layer, having one or more semiconductor layers, and comprising a first portion which is undoped, An electron barrier layer is provided which is positioned in contact with the upper surface of the first portion, has a larger band gap energy than the first portion, and contains p-type impurities. A second portion is disposed in contact with the upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities, The above first part is, As you move upwards, the band gap energy increases, and the p-side composition gradient layer is undoped, The semiconductor laser element has a p-side intermediate layer which is undoped and positioned above the p-side composition gradient layer, and the lower end of the ridge is located in the p-side intermediate layer.

[0006] A second embodiment of the semiconductor laser element in this disclosure is a semiconductor laser element having, in order upward, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each made of a nitride semiconductor, and the p-side semiconductor layer is provided with an upwardly projecting ridge. The p-side semiconductor layer is Distended in contact with the upper surface of the active layer, having one or more semiconductor layers, and comprising a first portion which is undoped, An electron barrier layer is provided which is positioned in contact with the upper surface of the first portion, has a larger band gap energy than the first portion, and contains p-type impurities. A second portion is disposed in contact with the upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities, The thickness of the second part is thinner than the thickness of the first part. The lower end of the ridge is a semiconductor laser element located in the first portion.

[0007] A first aspect of the method for manufacturing a semiconductor laser element in this disclosure is: A process of forming an n-side semiconductor layer on a substrate, The process of forming an active layer on the n-side semiconductor layer, A step of forming a first portion having one or more semiconductor layers on the upper surface of the active layer by undoping, A step of forming an electron barrier layer having a band gap energy greater than the band gap energy of the first portion on the upper surface of the first portion by doping it with p-type impurities, A step of forming a second portion having one or more p-type semiconductor layers formed by doping p-type impurities on the upper surface of the electron barrier layer, The process includes a step of forming an upwardly protruding ridge by removing a portion of the p-side semiconductor layer, which includes the first portion, the electron barrier layer, and the second portion. The process of forming the aforementioned first portion by undoping is as follows: A process of forming a p-side composition gradient layer in which the band gap energy increases as you move upward by undoping, A method for manufacturing a semiconductor laser element, comprising the steps of forming a p-side intermediate layer by undoping above the p-side composition gradient layer, wherein in the step of forming the ridge, a portion of the p-side semiconductor layer is removed such that the lower end of the ridge is located in the p-side intermediate layer. A second aspect of the method for manufacturing a semiconductor laser element in this disclosure is: A process of forming an n-side semiconductor layer on a substrate, The process of forming an active layer on the n-side semiconductor layer, A step of forming a first portion having one or more semiconductor layers on the upper surface of the active layer by undoping, A step of forming an electron barrier layer having a band gap energy greater than the band gap energy of the first portion on the upper surface of the first portion by doping it with p-type impurities, A step of forming a second portion having one or more p-type semiconductor layers formed by doping p-type impurities on the upper surface of the electron barrier layer, The process includes a step of forming an upwardly protruding ridge by removing a portion of the p-side semiconductor layer, which includes the first portion, the electron barrier layer, and the second portion. In the process of forming the second portion, the second portion having a thickness thinner than the thickness of the first portion is formed. This is a method for manufacturing a semiconductor laser element, comprising removing a portion of the p-side semiconductor layer in the step of forming the ridge such that the lower end of the ridge is located in the first portion. [Effects of the Invention]

[0008] Such semiconductor laser elements can reduce absorption losses and improve efficiency. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a schematic cross-sectional view of a semiconductor laser device according to one embodiment of the present invention. [Figure 2A] Figure 2A is a schematic diagram showing an example of the layer structure of the p-side semiconductor layer of the semiconductor laser device shown in Figure 1. [Figure 2B] Figure 2B schematically shows another example of the layer structure of the p-side semiconductor layer of the semiconductor laser device shown in Figure 1. [Figure 2C] Figure 2C schematically shows another example of the layer structure of the p-side semiconductor layer of the semiconductor laser device shown in Figure 1. [Figure 2D] Figure 2D schematically shows another example of the layer structure of the p-side semiconductor layer of the semiconductor laser device shown in Figure 1. [Figure 3A] Figure 3A schematically shows an example of the relationship between the band gap energies of the top layer and electron barrier layer of the first part and the bottom layer of the second part. [Figure 3B] Figure 3B schematically shows another example of the relationship between the band gap energies of the top layer and electron barrier layer of the first part and the bottom layer of the second part. [Figure 3C] Figure 3C schematically shows another example of the relationship between the band gap energies of the top layer and electron barrier layer of the first part and the bottom layer of the second part. [Figure 4] Figure 4 is a schematic diagram showing an example of the layer structure of the n-side semiconductor layer of the semiconductor laser device shown in Figure 1. [Figure 5] Figure 5 is a magnified view of the p-side composition gradient layer and a portion of its vicinity in the semiconductor laser device shown in Figure 1. [Figure 6A] Figure 6A is a flowchart showing a method for manufacturing a semiconductor laser element according to one embodiment of the present invention. [Figure 6B] Figure 6B is a flowchart showing an example of process S103. [Figure 7] Figure 7 is a graph showing the relationship between the thickness of the first part and the percentage of light leakage into the second part in calculation examples 1 to 5. [Figure 8] Figure 8 is a graph showing the IL characteristics of the semiconductor laser devices of Comparative Examples 1 to 4. [Figure 9A] Figure 9A is a graph showing the IL characteristics of the semiconductor laser devices of Examples 1 to 3. [Figure 9B] Figure 9B is a graph showing the IV characteristics of the semiconductor laser elements of Examples 1 to 3. [Figure 10A] Figure 10A is a graph showing the IL characteristics of the semiconductor laser elements of Examples 3 to 5. [Figure 10B] Figure 10B is a graph showing the IV characteristics of the semiconductor laser elements of Examples 3 to 5. [Figure 11A] Figure 11A is a graph showing the IL characteristics of the semiconductor laser elements of Examples 3 and 6. [Figure 11B] Figure 11B is a graph showing the IV characteristics of the semiconductor laser elements of Examples 3 and 6. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below with reference to the drawings. However, the embodiments shown below are illustrative of methods for realizing the technical concept of the present invention and do not limit the present invention to these embodiments. Furthermore, in the following description, the same names and reference numerals indicate the same or identical components, and detailed explanations will be omitted as appropriate.

[0011] Figure 1 is a schematic cross-sectional view of the semiconductor laser element 100 according to this embodiment, showing a cross-section in a direction perpendicular to the resonator direction of the semiconductor laser element 100. Figures 2A, 2B, 2C, and 2D are schematic diagrams showing examples of the layer structure of the p-side semiconductor layer 4, each showing a different example. Figures 2A to 2D are also schematic diagrams showing the relative magnitudes of the bandgap energies of a part of the active layer 3 and each layer of the p-side semiconductor layer 4 of the semiconductor laser element 100. In Figures 2A to 2D, the position of the bottom surface of the ridge 4a is indicated by a dashed line. The bottom surface of the ridge 4a refers to the surface connecting the lowest edges of both sides of the ridge 4a. Figure 4 is a schematic diagram showing an example of the layer structure of the n-side semiconductor layer 2.

[0012] As shown in Figure 1, the semiconductor laser element 100 has an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4, all made of nitride semiconductors, arranged in this order from top to bottom. The p-side semiconductor layer 4 is provided with a ridge 4a that protrudes upward. In this specification, the direction from the n-side semiconductor layer 2 to the p-side semiconductor layer 4 is referred to as "up" or "upward," and the opposite direction is referred to as "down" or "downward."

[0013] The p-side semiconductor layer 4 has a first portion 41, an electron barrier layer 42, and a second portion 43. The first portion 41 is located in contact with the upper surface of the active layer 3 and has one or more semiconductor layers. The first portion 41 is undoped. The electron barrier layer 42 is located in contact with the upper surface of the first portion 41. The electron barrier layer 42 has a larger band gap energy than the first portion 41 and contains p-type impurities. The second portion 43 is located in contact with the upper surface of the electron barrier layer. The second portion 43 has one or more p-type semiconductor layers containing p-type impurities. The lower end of the ridge 4a is located in the first portion 41. That is, the ridge 4a is composed of a part of the first portion 41, the electron barrier layer 42, and the second portion 43. In this specification, "undoped" means intentionally not doping. An undoped state may refer to a concentration that does not exceed the detection limit in analytical results such as secondary ion mass spectrometry (SIMS). Alternatively, an impurity concentration of 1 × 10⁻⁶ 17 / cm 3 A state of being less than a certain threshold may be defined as undoped. For example, the first part 41 may be said to be undoped if the concentrations of p-type and n-type impurities are below the detection limit. However, since the first part 41 is in contact with the electron barrier layer 42 which has a high concentration of p-type impurities, p-type impurities may be detected in the analysis results even if the material is formed without intentionally doping with p-type impurities. In this case, the concentration of p-type impurities detected is 1 × 10⁻⁶. 18 / cm 3 It is preferable that it be less than [a certain value]. Also, when the first part 41 etc. is formed by undoping, unintended impurities such as H and C may be contained, but this can also be called undoping. Furthermore, in this specification, the film thickness or thickness of a layer or part refers to the shortest distance from the bottom surface to the top surface of that layer or part. If the bottom surface and / or top surface have partial recesses and / or protrusions such as V-pits, the shortest distance between flat parts of the bottom surface and / or top surface that do not have such recesses and / or protrusions may be used as the film thickness or thickness of that layer or part.

[0014] The semiconductor laser element 100 has the following structures (1) to (3): (1) The first part 41 has a p-side composition gradient layer 411 which is undoped and has a bandgap energy that increases as it goes upward, and a p-side intermediate layer 412 which is undoped and located above the p-side composition gradient layer 411. The lower end of the ridge is located in the p-side intermediate layer 412. (2) The thickness of the second part 43 is thinner than the thickness of the first part 41, and the lower end of the ridge 4a is located in the first part 41. (3) The thickness of the first part 41 is 400 nm or more, and the lower end of the ridge 4a is located in the first part 41. The semiconductor laser element 100 may have only one of these structures (1) to (3), or it may satisfy two or more simultaneously.

[0015] First, let's discuss (1). As shown in Figure 2A, the p-side composition gradient layer 411 is a layer in which the band gap energy increases as you move upward. Having this configuration strengthens the confinement of light to the active layer 3. The p-side intermediate layer 412 is a different layer from the p-side composition gradient layer 411. By providing not only the p-side composition gradient layer 411 but also the p-side intermediate layer 412, the first portion 41 can be made relatively thick. This allows the peak of light intensity to be moved away from the electron barrier layer 42 and the second portion 43, which contain p-type impurities. Having these configurations allows the light intensity in the second portion 43 to be reduced, thereby reducing the light absorption loss. Therefore, the efficiency of the semiconductor laser element 100 can be improved. The efficiency of the semiconductor laser element 100 can be expressed as slope efficiency, which is the slope in the characteristic graph of current and light output at current values ​​above the threshold current.

[0016] Furthermore, the lower end of the ridge 4a is located deeper than the electron barrier layer 42, in the p-side intermediate layer 412. This allows for a shorter distance between the lower end of the ridge 4a and the active layer 3, even when a relatively thick first portion 41 is provided. Therefore, lateral optical confinement can be strengthened compared to when the lower end of the ridge 4a is positioned above the first portion 41. If lateral optical confinement is weak, the horizontal transverse mode of the semiconductor laser element 100 becomes unstable, and kinking may occur in the IL characteristics, which show the relationship between current and optical output. By forming the ridge 4a such that its lower end is located in the p-side intermediate layer 412, lateral optical confinement can be strengthened and the horizontal transverse mode can be stabilized, thereby reducing the probability of kinking occurring in the IL characteristics. On the other hand, since electrical leakage may occur at the lower end of the ridge 4a due to etching damage during ridge 4a formation, it is preferable that the lower end of the ridge 4a is not too close to the active layer 3. For this reason, it is preferable that the lower end of the ridge 4a be located in the p-side intermediate layer 412 rather than the p-side composition gradient layer 411.

[0017] Next, we will discuss (2). Because the first portion 41 is a relatively thick film, similar to (1), the peak of light intensity can be moved away from the p-type impurity-containing layer, and the loss due to free carrier absorption in the p-type impurity-containing layer can be reduced. Therefore, the efficiency of the semiconductor laser element 100, such as the slope efficiency, can be improved. In addition, because the second portion 43 is thin, the driving voltage of the semiconductor laser element 100 can be reduced, and the efficiency can be improved. The reason why the voltage decreases when the film thickness of the portion containing p-type impurities is thin is that in nitride semiconductors, p-type impurities such as Mg have a lower activation rate than n-type impurities such as Si, and the p-type impurity-containing layer has relatively high resistance. The first portion 41 is undoped, but because it is located between the electron barrier layer 42 and the active layer 3, it tends to exhibit n-type conductivity rather than complete insulation due to factors such as electron overflow. From these points, it is thought that by thinning the thickness of the second portion 43 containing relatively high-resistance p-type impurities, the driving voltage can be reduced, and the increase in driving voltage caused by increasing the thickness of the undoped first portion 41 can be suppressed. This effect was confirmed in Experimental Results 2 for Examples 1 to 3, which will be described later. Also, similar to (1), since the first portion 41 is a relatively thick film, it is preferable to provide the lower end of the ridge 4a on the first portion 41, thereby strengthening lateral light confinement.

[0018] Next, we will discuss (3). Because the first portion 41 is relatively thick, with a thickness of 400 nm or more, it is possible to reduce losses in the p-type impurity-containing layer and improve efficiency, similar to (1) and (2). Furthermore, by positioning the lower end of the ridge 4a on the first portion 41, it is possible to enhance lateral light confinement, similar to (1) and (2).

[0019] Equivalent refractive index simulations were performed for multiple configurations of the first section 41. In these simulations, the refractive index of each layer was calculated based on the composition ratio of the nitride semiconductor constituting that layer, using the formula described in MJBergmann, et. Al., JOURNAL OF APPLIED PHYSICS vol.84 (1998) pp.1196-1203. Calculation Example 1 used a structure in which only a p-side composition gradient layer 411 with a film thickness of 260 nm was provided as the first section 41, while Calculation Examples 2 to 5 used a structure in which the p-side composition gradient layer 411 and a p-side intermediate layer 412 were provided as the first section 41. The film thickness of the p-side intermediate layer 412 in Calculation Examples 2 to 5 was set to 50 nm, 100 nm, 200 nm, and 400 nm, respectively. That is, the thickness of the first section 41 in Calculation Examples 1 to 5 was set to 260 nm, 310 nm, 360 nm, 460 nm, and 660 nm, respectively. The layer structure other than the first part 41 is generally the same as that of the semiconductor laser element 100 of Example 1 described later, except that the second n-side optical guide layer 27 is made of In 0.05 Ga 0.95 There are some minor differences in details, such as the composition gradient layer which changes to N. The layer structure other than the first part 41 is the same in calculation examples 1 to 3, and calculation examples 4 and 5 are the same as in calculation examples 1 to 3 except that the film thickness of the first n-side optical guide layer 26 is reduced to two-thirds. The reason for reducing the film thickness of the first n-side optical guide layer 26 in calculation examples 4 and 5 is to correct the shift of the electric field strength peak from the active layer 3 caused by increasing the thickness of the first part 41.

[0020] Figure 7 shows the relationship between the thickness of the first portion 41 and the percentage of light leakage into the second portion 43 for calculation examples 1 to 5. As shown in Figure 7, the thicker the first portion 41, the less light leakage into the second portion 43 occurs, and the degree of reduction becomes gradual from around a thickness of 400 nm. For this reason, it is preferable that the thickness of the first portion 41 be 400 nm or more. This makes it possible to reduce the amount of light leakage into the second portion 43 to, for example, less than 3%.

[0021] (Semiconductor laser element 100) As shown in Figure 2A, the semiconductor laser element 100 comprises a substrate 1, an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4, all located above the substrate 1. The semiconductor laser element 100 is an end-face emitting laser element having an optical emission end face and an optical reflection end face that intersect with the main surface of the semiconductor layers such as the active layer 3. A ridge 4a is provided on the upper side of the p-side semiconductor layer 4. The ridge 4a has a mesa structure. The top view shape of the ridge 4a is elongated in the direction connecting the optical emission end face and the optical reflection end face, and is, for example, a rectangle with the direction parallel to the optical reflection end face as the short side and the direction perpendicular to the optical reflection end face as the long side. The portion of the active layer 3 directly below the ridge 4a and its vicinity constitute the optical waveguide region. An insulating film 5 can be provided on the side surface of the ridge 4a and on the surface of the p-side semiconductor layer 4 that is continuous with the side surface of the ridge 4a. The substrate 1 is made of, for example, an n-type semiconductor, and an n-electrode 8 is provided on its lower surface. Furthermore, a p-electrode 6 is provided in contact with the upper surface of the ridge 4a, and a p-side pad electrode 7 is provided on top of it.

[0022] The semiconductor laser element 100 can have a structure that emits laser light in the long wavelength range. The semiconductor laser element 100 is capable of emitting laser light in the green wavelength band, for example, laser light with a wavelength of 530 nm or higher. That is, it is capable of emitting laser light with a peak wavelength of 530 nm or higher. As the oscillation wavelength lengthens from the blue wavelength band to the green wavelength band, leakage light outside the optical guide layer increases due to the effect of wavelength dispersion of the refractive index. As a result, the threshold current increases, and the current density during laser oscillation increases. The higher the current density, the greater the effective transition interval due to shielding of localized levels and band filling, and the oscillation wavelength shifts to a shorter wavelength. By providing the p-side composition gradient layer 411, the laser oscillation threshold current density can be reduced as described later, and the short-wavelength shift can be suppressed. Furthermore, in the green wavelength band semiconductor laser element, the refractive index difference between the cladding layer and the active layer is less pronounced due to the effect of wavelength dispersion of the refractive index, so the threshold current density is still higher and the slope efficiency is lower than that of the blue wavelength band semiconductor laser element. For this reason, if the configuration of this embodiment is adopted in the green wavelength band semiconductor laser element, a greater efficiency improvement effect due to the reduction of optical absorption loss in the p-type semiconductor layer can be expected. Furthermore, optical absorption loss in the p-type semiconductor layer can occur regardless of the wavelength of the laser light emitted by the semiconductor laser element 100. Therefore, the wavelength of the laser light emitted by the semiconductor laser element 100 is not limited to the green wavelength band, but may be, for example, the blue wavelength band.

[0023] (Circuit board 1) For substrate 1, a nitride semiconductor substrate made of GaN or the like can be used. The n-side semiconductor layer 2, active layer 3, and p-side semiconductor layer 4 grown on substrate 1 are semiconductors grown substantially in the c-axis direction. For example, a GaN substrate with the +c plane ((0001) plane) as the main plane can be used, and each semiconductor layer can be grown on the +c plane. Here, the term "main plane" may include those with an off-angle of approximately ±1 degree or less. By using a substrate with the +c plane as the main plane, the advantage of excellent mass productivity can be obtained.

[0024] (n-side semiconductor layer 2) The n-side semiconductor layer 2 can be a multilayer structure made of a nitride semiconductor such as GaN, InGaN, or AlGaN. The n-side semiconductor layer 2 includes one or more n-type semiconductor layers. Examples of n-type semiconductor layers include layers made of a nitride semiconductor containing n-type impurities such as Si and Ge. The n-side semiconductor layer 2 may have an n-side cladding layer and an n-side optical guide layer, and may also include other layers. The n-side cladding layer has a larger bandgap energy than the n-side optical guide layer. Since n-type impurities also contribute to light absorption, albeit to the same extent as p-type impurities, it is preferable that the n-side optical guide layer is undoped or has a lower n-type impurity concentration than the n-side cladding layer.

[0025] An example of the layer structure of the n-side semiconductor layer 2 is shown in Figure 4. The n-side semiconductor layer 2 shown in Figure 4 has, in order from the substrate 1 side, a base layer 21, a first n-side cladding layer 22, a crack prevention layer 23, an intermediate layer 24, a second n-side cladding layer 25, a first n-side optical guide layer 26, a second n-side optical guide layer 27, and a hole block layer 28. The hole block layer 28 has a first hole block layer 281 and a second hole block layer 282.

[0026] n-type impurities are added from the base layer 21 to the first n-side optical guide layer 26. The base layer 21 is, for example, an n-type AlGaN layer. The first n-side cladding layer 22 is, for example, a layer with a larger band gap energy than the base layer 21 and with added n-type impurities. The crack prevention layer 23 is, for example, made of InGaN, and its band gap energy is smaller than that of the well layer in the active layer 3. By providing the crack prevention layer 23, the probability of cracks occurring can be reduced. The intermediate layer 24 has a lattice constant between the crack prevention layer 23 and the second n-side cladding layer 25, and is, for example, made of GaN. When the crack prevention layer 23 is an InGaN layer, it is preferable to form the intermediate layer 24 of the GaN layer before growing the second n-side cladding layer 25. If the second n-side cladding layer 25 is grown in contact with the upper surface of the crack prevention layer 23, a part of the crack prevention layer 23 may decompose, which may affect the growth of the active layer 3, but by providing the intermediate layer 24, the probability of such decomposition occurring can be reduced. The intermediate layer 24 is provided with a thinner film thickness than, for example, the crack prevention layer 23. The second n-side cladding layer 25 is, for example, a layer with a larger bandgap energy than the base layer 21, and may be the same as the first n-side cladding layer 22. The first n-side cladding layer 22 and the second n-side cladding layer 25 are made of, for example, AlGaN. Either or both of the first n-side cladding layer 22 and the second n-side cladding layer 25 may have the largest bandgap energy in the n-side semiconductor layer 2. The composition and / or n-type impurity concentration of the first n-side cladding layer 22 and the second n-side cladding layer 25 may be the same. The n-side cladding layer may be a single layer, in which case the crack prevention layer may not be provided, or may be provided above or below the n-side cladding layer.

[0027] The first n-side optical guide layer 26 has a smaller band gap energy and a smaller n-type impurity concentration than the first n-side cladding layer 22 and the second n-side cladding layer 25. The first n-side optical guide layer 26 is made of, for example, GaN. The band gap energy of the second n-side optical guide layer 27 is larger than that of the well layer in the active layer 3 and smaller than that of the first n-side optical guide layer 26. Since the second n-side optical guide layer 27 is located closer to the active layer 3 than the first n-side optical guide layer 26, it is preferable for the second n-side optical guide layer 27 to have a smaller n-type impurity concentration than the first n-side optical guide layer 26 in order to reduce optical absorption loss. The second n-side optical guide layer 27 is made of, for example, undoped InGaN.

[0028] The second n-side optical guide layer 27 may be a compositional gradient layer in which the band gap energy decreases as it approaches the active layer 3. When a compositional gradient layer is provided as the n-side optical guide layer, the layer is made up of layers whose composition is changed in steps so that the refractive index increases as it approaches the active layer 3. This continuously forms an optical waveguide barrier in the n-side compositional gradient layer, thereby strengthening optical confinement to the active layer 3. The average value of the compositional gradient layer can be used as a criterion for determining the relative magnitudes of the band gap energy and impurity concentration of the compositional gradient layer with those of other layers. The average value of the compositional gradient layer refers to the sum of the product values ​​of the band gap energy, etc., of each sublayer constituting the compositional gradient layer and the film thickness, divided by the total film thickness. When a compositional gradient layer in the n-side semiconductor layer 2 in which the lattice constant increases as it approaches the active layer 3 is provided, it is preferable to add n-type impurities to that compositional gradient layer. In other words, a compositional gradient layer can be said to consist of multiple sublayers with slightly different compositions. For this reason, even if the rate of compositional change is small, it is difficult to avoid the generation of fixed charges in the compositional gradient layer. By adding n-type impurities, fixed charges can be shielded, thereby reducing the degree of voltage rise caused by the generation of fixed charges.

[0029] Preferably, the hole-blocking layer 28 contains n-type impurities in at least a portion thereof. This allows for more efficient hole blocking. For example, the first hole-blocking layer 281 is made of GaN, and the second hole-blocking layer 282 is made of InGaN.

[0030] (active layer 3) The active layer 3 can be a multilayer structure consisting of nitride semiconductor layers such as GaN and InGaN. The active layer 3 has either a single quantum well structure or a multiple quantum well structure. The multiple quantum well structure is considered to be more likely to yield sufficient gain than the single quantum well structure. When the active layer 3 has a multiple quantum well structure, it has multiple well layers and an intermediate barrier layer sandwiched between the well layers. For example, the active layer 3 includes, in order from the n-side semiconductor layer 2 side, a well layer, an intermediate barrier layer, and another well layer. An n-side barrier layer 31 may be provided between the well layer closest to the n-side semiconductor layer 2 and the n-side semiconductor layer 2. The n-side barrier layer 31 may function as part of the hole block layer 28. The n-side barrier layer 31 may be omitted, and the hole block layer 28 or the n-side optical guide layer (second n-side optical guide layer 27) may function as the n-side barrier layer. Similarly, a p-side barrier layer may be provided between the well layer closest to the p-side semiconductor layer 4 and the p-side semiconductor layer 4. If this p-side barrier layer is not provided or is thin, a part of the p-side semiconductor layer 4 may function as the p-side barrier layer. When a p-side barrier layer is provided in the active layer 3, the thickness of the p-side barrier layer can be, for example, 5 nm or less. In other words, the shortest distance between the p-side semiconductor layer 4 and the well layer in the active layer 3 should be, for example, 5 nm or less. Also, as mentioned above, adding p-type impurities increases the light absorption loss, so it is preferable to form the active layer 3 without adding p-type impurities. Each layer of the active layer 3 is, for example, an undoped layer.

[0031] In the case of a semiconductor laser element with an oscillation wavelength of 530 nm or higher, x Ga 1-x The In composition ratio x of the N well layer varies slightly depending on the layer structure other than the active layer 3, but is, for example, 0.25 or higher. An upper limit for the In composition ratio x of the well layer is, for example, 0.50 or less. In this case, the oscillation wavelength of the semiconductor laser element is considered to be approximately 600 nm or less.

[0032] (p-side semiconductor layer 4) The p-side semiconductor layer 4 can be a multilayer structure made of nitride semiconductors such as GaN, InGaN, or AlGaN. The p-side semiconductor layer 4 may have a p-side cladding layer and a p-side optical guide layer, and may also include other layers. If a transparent conductive film is provided as the p-electrode 6, it can function as a cladding layer, so a cladding layer does not need to be provided in the p-side semiconductor layer 4.

[0033] The p-side semiconductor layer 4 includes one or more p-type semiconductor layers. Examples of the p-type semiconductor layer include a layer made of a nitride semiconductor containing a p-type impurity such as Mg. Since the activation rate of the p-type impurity is lower than that of the n-type impurity such as Si, free carrier absorption loss due to the p-type impurity increases in the p-type semiconductor layer. The greater the absorption loss, the lower the slope efficiency of the semiconductor laser device 100. Generally, the internal loss α i includes the free carrier absorption loss α fc . However, if the internal loss other than the free carrier absorption loss α fc is defined as α int , the threshold mode gain required for laser oscillation is expressed by the following model formula that depends on the free carrier absorption loss. Here, α fc , α i and α m represent the free carrier absorption loss, the average internal loss, and the mirror loss, respectively. For convenience, the mode distribution is not considered and is expressed as an average. Γ is the optical confinement factor in the active region, and g th represents the threshold gain for laser oscillation. Γg th =α fc +α int +α m

[0034] Here, the free carrier absorption loss includes losses outside the active layer 3. For example, in the p-type semiconductor layer, it can be approximately explained by the product of the coefficient σ fc reflecting the p-type impurity concentration n and the free carrier absorption cross-sectional area, and the average leakage light Γ p into the p-type semiconductor layer. That is, even if the impurity concentration of the p-type semiconductor layer is the same, if the leakage light into the p-type semiconductor layer increases, the free carrier absorption loss α fc increases. Similarly, even if the leakage light into the p-type semiconductor layer is the same, if the impurity concentration of the p-type semiconductor layer increases, the free carrier absorption loss α fc increases. Since there is a concern that the driving voltage will increase significantly when the p-type impurity concentration decreases, in order to reduce the free carrier absorption loss α fc , it is particularly effective to reduce the leakage light into the layer with a high p-type impurity concentration. α fc=n × σ fc ×Γ p

[0035] Furthermore, from the above equation, as leakage light into the p-type cladding layer increases, the free carrier absorption loss increases, thus the threshold gain g th It can be understood that g=g increases during laser oscillation. th This is the steady state. In such a steady state, the mode gain depends monotonically on the carrier density, so the carrier density above the laser oscillation threshold current is the threshold carrier density N th It is clamped. The higher the injected carrier density, the more likely it is that localized levels will be shielded, resulting in a larger effective band gap and a tendency for the laser oscillation wavelength to shift to shorter wavelengths. Free carrier absorption loss α fc This reduces the threshold gain g at a lower current. th By reaching the threshold current density j th Threshold carrier density N th Both can be reduced. This reduces the injected carrier density and suppresses the shielding of localized levels, allowing laser oscillation at longer wavelengths. Therefore, from this point of view, it is particularly preferable to reduce free carrier absorption loss in semiconductor laser elements 100 that can emit laser light at long wavelengths such as 530 nm or higher. Furthermore, even in semiconductor laser elements at shorter wavelengths, there is an advantage in that a laser light source with a low threshold current can be obtained.

[0036] Here, we have explained the suppression of localized level shielding, but the same applies to the suppression of the band-filling effect. That is, a short-wavelength shift also occurs due to the band-filling effect, in which the pseudo-Fermi level moves away from the band edge due to current injection, and the effective transition interval widens, but the free carrier absorption loss α fc This can also be suppressed by reducing the threshold carrier density.

[0037] (Part 1 41) The first part 41 is the portion connecting the active layer 3 to the p-type impurity-containing layer in the p-side semiconductor layer 4. The first part 41 does not contain the p-type semiconductor layer. If the p-type impurity concentration and film thickness are such that they do not affect the free carrier absorption loss, a portion of the first part 41 may contain a p-type impurity-containing layer. However, if Mg is doped to make it p-type, 1 × 10 18 / cm 3 A certain degree of p-type impurities is required, and in this case, there is a high possibility that the free carrier absorption loss will increase. Therefore, it is preferable that the first portion 41 is a portion that does not contain a p-type semiconductor layer. It is preferable that the p-type impurity concentration of the first portion 41 is low throughout its entirety to the extent that p-type impurities cannot be detected by analysis such as SIMS. For example, the first portion 41 is formed without intentionally adding p-type impurities throughout its entirety during manufacturing. As described above, the thicker the first portion 41, the more light leakage to the second portion 43 can be reduced, so it is preferable that the thickness of the first portion 41 is 400 nm or more. The upper limit of the thickness of the first portion 41 can be set to the extent that the supply of holes from the second portion 43 is not hindered. Also, as shown in experimental result 3 described later, the thicker the first portion 41, the more electrons overflow, so from this viewpoint, it is preferable that the thickness of the first portion 41 be thin. The thickness of the first portion 41 can be, for example, 660 nm or less. Furthermore, the first portion 41 has a band gap difference with respect to the electron barrier layer 42, which reduces the probability of electron overflow occurring. For this reason, it is preferable that the first portion 41 has a layer with a smaller band gap energy than the electron barrier layer 42 as the layer in contact with the electron barrier layer 42.

[0038] Furthermore, if the first portion 41 is a low-doped portion rather than an undoped portion, it is preferable that the p-type impurity concentration throughout the entire portion be lower than the p-type impurity concentration of the electron barrier layer 42, and even more preferably lower than the p-type impurity concentration of either the electron barrier layer 42 or the second portion 43. Also, the n-type impurity concentration of the first portion 41 is 2 × 10⁻⁶. 18 / cm 3Examples include less than the following. Preferably, the first portion 41 has an n-type impurity concentration that is so low (i.e., at a background level) that n-type impurities are not detected by SIMS analysis. In other words, it is preferable that the first portion 41 is substantially free of n-type impurities.

[0039] (p-side composition gradient layer 411, p-side intermediate layer 412) As shown in Figure 2A, the first part 41 may have a p-side composition gradient layer 411 and a p-side intermediate layer 412. The p-side intermediate layer 412 is provided above the p-side composition gradient layer 411. The p-side intermediate layer 412 may be positioned in contact with the upper surface of the p-side composition gradient layer 411, or in contact with the lower surface of the electron barrier layer 42. The structure of the first part 41 is not limited to having both the p-side composition gradient layer 411 and the p-side intermediate layer 412, but as described above, having the p-side composition gradient layer 411 can enhance optical confinement to the active layer 3, and having the p-side intermediate layer 412 can further increase the thickness of the first part 41. By enhancing optical confinement to the active layer 3 with the p-side composition gradient layer 411, the laser oscillation threshold current density can be reduced. This can suppress shielding of localized levels and suppress the short-wavelength shift of the oscillation wavelength associated with increased current injection, which is advantageous for increasing the oscillation wavelength to a longer wavelength.

[0040] The p-side composition gradient layer 411 is a layer in which the band gap energy increases as it approaches the top. The p-side composition gradient layer 411 has an upper surface and a lower surface, and its band gap energy increases from the lower surface to the upper surface. The band gap energy on the lower surface is smaller than that on the upper surface. In Figure 2A, the p-side composition gradient layer 411 is shown as a slope, but as will be described later, a composition gradient layer can be said to be an aggregate of multiple sublayers with different compositions, so in the p-side composition gradient layer 411, the band gap energy increases in a stepwise manner from the lower surface to the upper surface. An n-side composition gradient layer that is paired with the p-side composition gradient layer 411 may be provided in the n-side semiconductor layer 2. An example of such an n-side composition gradient layer is a layer in which the band gap energy decreases as it approaches the active layer 3. For example, the p-side composition gradient layer 411 and the n-side composition gradient layer may be formed symmetrically with respect to the active layer 3. By providing composition gradient layers on both sides of the active layer 3 in this way, light can be confined to the active layer 3 in a balanced manner from both sides. To improve the photoconfinement effect of the p-side composition gradient layer 411, it is preferable to position the p-side composition gradient layer 411 close to the active layer 3. For this reason, it is preferable to position the p-side composition gradient layer 411 in contact with the active layer 3. Furthermore, it is preferable that the shortest distance between the p-side composition gradient layer 411 and the well layer 32 in the active layer 3 be 5 nm or less.

[0041] The p-side compositionally graded layer 411 functions as, for example, a p-side optical guide layer. The thickness of the p-side compositionally graded layer 411 is greater than the thickness of the well layer 32 and, when there is a p-side barrier layer 34, greater than the thickness of the p-side barrier layer 34. In order to improve the light confinement effect, the thickness of the p-side compositionally graded layer 411 is preferably 200 nm or more. The thickness of the p-side compositionally graded layer 411 can be 500 nm or less, preferably 350 nm or less, and more preferably 300 nm or less. When providing the p-side barrier layer 34, the bandgap energy at the lower end of the p-side compositionally graded layer 411 is preferably smaller than the bandgap energy of the p-side barrier layer 34. The bandgap energy at the upper end of the p-side compositionally graded layer 411 may have a bandgap energy equal to or greater than that of the p-side barrier layer 34. The p-side compositionally graded layer 411 preferably has a structure in which the refractive index monotonically decreases from the active layer 3 side toward the electron barrier layer 42 side and the bandgap energy monotonically increases from the active layer 3 side toward the electron barrier layer 42 side in order to suppress electron overflow while attracting light to the active layer 3.

[0042] As shown in FIG. 5, the p-side compositionally graded layer 411 can be said to be composed of a plurality of sub-layers 411a, 411b, 411c, 411y, 411z having different compositions from each other. FIG. 5 is a partially enlarged view of the p-side compositionally graded layer 411 and its vicinity, and there are a number of sub-layers other than those explicitly shown between the sub-layer 411c and the sub-layer 411y. When the p-side compositionally graded layer 411 is made of InGaN or GaN, the lowermost sub-layer 411a of the p-side compositionally graded layer 411 is composed of In a Ga 1-a N (0 < a < 1), and the uppermost sub-layer 411z of the p-side compositionally graded layer 411 is composed of In z Ga 1-zIt consists of N(0≦z<a). The upper limit value of the In composition ratio a is, for example, 0.25. Considering the suppression of deterioration of crystallinity, the In composition ratio a is preferably 0.1 or less. Also, it is preferable that the lattice constant difference between adjacent sub-layers is small. Thereby, the strain can be reduced. For this purpose, it is preferable that the p-side composition gradient layer 411 changes the composition little by little with a small thickness. Specifically, it is preferable that the In composition ratio of the p-side composition gradient layer 411 decreases for each film thickness of 25 nm or less from the lower surface to the upper surface. That is, it is preferable that the film thicknesses of the respective sub-layers 411a, 411b, 411c, 411y, 411z are 25 nm or less. Furthermore, it is preferable that the film thicknesses of the respective sub-layers 411a, 411b, 411c, 411y, 411z are 20 nm or less. The lower limit value of the film thicknesses of the respective sub-layers 411a, 411b, 411c, 411y, 411z is about 1 atomic layer (about 0.25 nm) for example. Also, the difference in the In composition ratio between adjacent sub-layers (for example, sub-layer 411a and sub-layer 411b) is preferably 0.005 or less. More preferably, it is 0.001 or less. The lower limit value is about 0.00007 for example.

[0043] Such a range is preferably satisfied throughout the entire p-side composition gradient layer 411. That is, it is preferable that all sub-layers are within such a range. For example, in the p-side composition gradient layer 411 with a film thickness of 260 nm, when the lowermost sub-layer 411a is In 0.05 Ga 0.95 N and the uppermost sub-layer 411z is GaN, it is grown under manufacturing conditions in which the composition is gradually changed in 120 steps. The number of times the composition changes in the p-side composition gradient layer 411 is preferably 90 times or more. The composition change rate of the p-side composition gradient layer 411 (that is, the difference in the composition ratio between adjacent sub-layers) may be constant or may vary throughout the entire p-side composition gradient layer 411. The composition change rate of the p-side composition gradient layer 411 is preferably 0.001 or less throughout the entire p-side composition gradient layer 411. When a composition gradient layer is provided on the n-side, the preferable ranges of its composition, composition change rate, and film thickness can be the same as those of the p-side composition gradient layer 411.

[0044] It is preferable that the lower end of the ridge 4a is not located in the p-side composition gradient layer 411. If the lower end of the ridge 4a is located in the p-side composition gradient layer 411, the difference in effective refractive index inside and outside the ridge 4a will vary greatly due to variations in the depth of the ridge 4a. However, if the lower end of the ridge 4a is located in a single-composition layer, the variation in the difference in effective refractive index inside and outside the ridge 4a can be made smaller. For this reason, when a p-side composition gradient layer 411 is provided, it is preferable to provide a single-composition layer for arranging the lower end of the ridge 4a. It is preferable that this single-composition layer has a film thickness greater than the variation in depth when forming the ridge 4a. This allows the lower end of the ridge 4a to be located within the single-composition layer even if the depth of the ridge 4a varies, thereby reducing the variation in the difference in effective refractive index inside and outside the ridge 4a. The film thickness of such a single-composition layer for arranging the lower end of the ridge 4a is preferably thicker than the sub-layers constituting the composition gradient layer, and can be, for example, thicker than 25 nm. The film thickness of the single-composition layer can be, for example, 600 nm or less. A single-composition layer refers to a layer formed without intentionally varying its composition.

[0045] The p-side intermediate layer 412 can be provided as a single-composition layer. Alternatively, the p-side intermediate layer 412 may have a multilayer structure. If the p-side intermediate layer 412 has a multilayer structure, it is preferable that at least the layer on which the lower end of the ridge 4a is located be a single-composition layer. When the p-side intermediate layer 412 has a multilayer structure, as shown in Figure 2B, the p-side intermediate layer 412 can have a first layer 412A and a second layer 412B. The first layer 412A has a band gap energy greater than the average band gap energy of the p-side composition gradient layer 411 and smaller than the band gap energy of the electron barrier layer 42. The second layer 412B has a band gap energy greater than the band gap energy of the first layer 412A and smaller than the band gap energy of the electron barrier layer 42. The first layer 412A and the second layer 412B are undoped. Furthermore, the relationship of refractive indices can be made such that the average refractive index of the p-side composition gradient layer 411 decreases in the order of the average refractive index of the p-side composition gradient layer 411, the refractive index of the first layer 412A, and the refractive index of the second layer 412B. In this specification, the average band gap energy refers to the sum of the product values ​​of the band gap energy of each layer and the film thickness, divided by the total film thickness. In a compositionally graded layer, the bandgap energy of each sublayer constituting it is multiplied by its film thickness, and the sum of these values ​​is divided by the total film thickness to obtain the average bandgap energy of the compositionally graded layer. The same method is used for the average refractive index and average composition ratio.

[0046] By providing the second layer 412B, light leakage into the second portion 43 can be reduced, thereby reducing the free carrier absorption loss occurring in the second portion 43. By providing the second layer 412B, which has a lower refractive index than the first layer 412A, as the p-side intermediate layer 412, the thickness of the p-side intermediate layer 412 required to obtain the same level of light confinement effect can be reduced compared to when the p-side intermediate layer 412 is composed only of the first layer 412A. As described above, a thicker first portion 41 is preferable to reduce light leakage into the second portion 43, but on the other hand, reducing the thickness of the first portion 41 is effective in further reducing the voltage. By providing the p-side composition gradient layer 411 and the second layer 412B, it is possible to reduce light leakage into the second portion 43 while suppressing the voltage rise.

[0047] When the first layer 412A and the second layer 412B are each single-composition layers, it is preferable that the lower end of the ridge 4a is located in either the first layer 412A or the second layer 412B. This reduces the variation in the effective refractive index difference between the inside and outside of the ridge 4a, even if the position of the lower end of the ridge 4a varies during manufacturing, as described above. The lower end of the ridge 4a may be located in the first layer 412A as shown in Figures 2B and 2C, or in the second layer 412B as shown in Figure 2D. The first layer 412A is, for example, a GaN layer. The second layer 412B is, for example, an AlGaN layer. In this case, the Al composition ratio of the second layer 412B can be, for example, 0.01% or more and 10% or less. The film thickness of the second layer 412B can be 1 nm or more and 600 nm or less. When the refractive index of the second layer 412B is smaller than that of the first layer 412A, it is preferable that the film thickness of the second layer 412B be thicker than that of the first layer 412A. This further enhances light confinement to the active layer 3. For example, the film thickness of the second layer 412B is made 50 nm or more thicker than that of the first layer 412A. In this case, since the first layer 412A has less influence on light confinement than the p-side composition gradient layer 411 and the second layer 412B, by making the film thickness of the first layer 412A thinner, it is possible to reduce the amount of light leaking into the second portion 43 while reducing the electrons overflowing from the active layer 3. This makes it possible to improve the slope efficiency of the semiconductor laser element. From this viewpoint, the film thickness of the first layer 412A is preferably 100 nm or less, and more preferably 50 nm or less. Furthermore, the film thickness of the first layer 412A is preferably half or less of the film thickness of the second layer 412B, and more preferably one-quarter or less. The film thickness of the first layer 412A can be 1 nm or more.

[0048] As shown in Figure 2B, the second layer 412B may have a bandgap energy greater than that of the layer in contact with the electron barrier layer 42 in the second portion 43 (the lower p-type semiconductor layer 431 in Figure 2B). By providing a layer with a relatively large bandgap energy in the first portion 41 in this way, it is possible to reduce light leakage into the second portion 43, which has a greater absorption loss than the first portion 41. In this case, it is preferable to use a material that functions as a cladding layer for the p electrode 6. This eliminates the need to provide a p-type cladding layer in the second portion 43, thus reducing the voltage when bias is applied. These points will be described in detail below.

[0049] First, when the p-electrode 6 functions as a cladding layer, a translucent material is used for the p-electrode 6, but even with a translucent material, absorption loss can occur. Therefore, if there is a lot of light leakage to the p-electrode 6 and it is desired to reduce it, a layer that functions as a p-type cladding layer, for example, a p-type layer containing Al, is provided in the second part 43. To function as a p-type cladding layer, it is preferable that the Al composition ratio is relatively large, but on the other hand, the more the Al composition ratio is increased, the higher the activation energy required to activate the p-type impurities in the layer. If the p-type conversion of the second part 43 is insufficient, the series resistance increases and the voltage when bias is applied increases, so when a p-type cladding layer is provided in the second part 43, the acceptor concentration is increased, for example by increasing the amount of p-type impurities added. However, as mentioned above, if the amount of p-type impurities added increases, the light absorption loss increases and the light output decreases. In a structure where an undoped AlGaN layer is provided between the active layer 3 and the electron barrier layer 42, as shown in Figure 2B, this AlGaN layer does not need to be p-type, so the voltage does not increase easily even if the Al composition ratio is increased. Furthermore, because it is an undoped AlGaN layer, the light absorption loss is less likely to increase. Generally, undoped layers are high-resistance layers, and their inclusion tends to increase the voltage. However, providing an undoped AlGaN layer in the first section 41 differs from this general tendency. This is thought to be because, in the undoped AlGaN layer placed between the active layer 3 and the electron barrier layer 42, when a bias is applied, donors become dominant, and acceptors with activation energies greater than those of the donors function as minority carriers. Therefore, an AlGaN layer with a relatively large Al composition ratio, such as the second layer 412B, can be provided in the first section 41. In other words, a layer with a large bandgap energy can be provided in the first section 41. By providing such a layer, light leakage to the second section 43 can be reduced, eliminating the need for a p-type cladding layer in the second section 43. That is, the Al composition ratio of the second section 43 can be reduced. This allows for a reduction in the series resistance of the second section 43, making it possible to reduce the voltage of the semiconductor laser element 100.

[0050] Figures 3A to 3C schematically show an example of the relationship between the band gap energies of the uppermost layer of the first part 41, the electron barrier layer 42, and the bottommost layer of the second part 43. The uppermost layer of the first part 41 is in contact with the lower surface of the electron barrier layer 42, and the bottommost layer of the second part 43 is in contact with the upper surface of the electron barrier layer 42. In Figure 3A, the band gap energy of the bottommost layer is smaller than that of the uppermost layer. In Figure 3B, the band gap energy of the bottommost layer is equal to that of the uppermost layer. In Figure 3C, the band gap energy of the bottommost layer is larger than that of the uppermost layer. For the reasons described above, as shown in Figure 3A, it is preferable that the band gap energy of the bottommost layer of the second part 43 (e.g., the lower p-type semiconductor layer 431) is smaller than that of the uppermost layer of the first part 41 (e.g., the second layer 412B). This makes it possible to reduce light leakage into the second part 43 and is also suitable for a configuration in which a material that functions as a cladding layer is used for the p electrode 6. In this configuration, the voltage applied when bias is applied to the semiconductor laser element 100 can be reduced. Regarding the relationship between the magnitudes of the bandgap energies, if the uppermost and / or lowermost layers are layers with non-constant bandgap energies, such as superlattice layers or compositionally graded layers, the relationship between the magnitudes can be compared using their average bandgap energies. In the case of a superlattice layer, the bandgap energy of each sublayer constituting the superlattice layer is multiplied by its film thickness, and the sum of these values ​​is divided by the total film thickness of the superlattice layer to obtain the average bandgap energy of the superlattice layer. When the uppermost and lowermost layers are AlGaN layers, this relationship between the magnitudes of the bandgap energies can be rephrased as the relationship between the Al composition ratios.

[0051] The bottom layer can be an AlGaN layer with an Al composition ratio of 4% or less. The bottom layer may also be a layer with a substantially zero Al composition ratio, i.e., a GaN layer. The bottom layer may be a p-type semiconductor layer containing p-type impurities such as Mg. The bottom layer may also be a quaternary layer such as AlInGaN. For low voltage, the average Al composition ratio of the second part 43, including the bottom layer, is preferably 4% or less. The top layer provided in the first part 41 preferably contains 0.01% or more Al in part or all of it. More preferably, the average Al composition ratio of the top layer is greater than 4%. The top layer may also have a band gap energy greater than any of the band gap energies of each layer (which may be a single layer) constituting the second part 43. The top layer may be a superlattice layer or a composition gradient layer containing AlGaN or AlInGaN. One or more layers connecting the top layer to the active layer 3 may each have a band gap energy smaller than the band gap energy of the top layer. Such one or more layers include, for example, the first layer 412A and the p-side composition gradient layer 411 shown in Figure 2B, but these layers do not have to be these.

[0052] The semiconductor laser element 100 may have the following configuration: It has an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4, arranged in order upwards, each made of a nitride semiconductor, and the p-side semiconductor layer 4 has an upwardly projecting ridge 4a. The p-side semiconductor layer 4 is arranged in contact with the upper surface of the active layer 3 and has one or more semiconductor layers and is undoped, and has an electron barrier layer 42 arranged in contact with the upper surface of the first portion 41 and having a larger band gap energy than the first portion 41 and containing p-type impurities, and a second portion 43 arranged in contact with the upper surface of the electron barrier layer 42 and having one or more p-type semiconductor layers containing p-type impurities. The first portion 41 has an uppermost layer in contact with the lower surface of the electron barrier layer 42, and the second portion 43 has a lowermost layer in contact with the upper surface of the electron barrier layer 42, and the band gap energy of the lowermost layer is smaller than the band gap energy of the uppermost layer. The lower end of the ridge 4a is located in the first portion 41.

[0053] (Electron barrier layer 42) The electron barrier layer 42 contains p-type impurities such as Mg. The band gap energy of the electron barrier layer 42 is greater than the band gap energy of the first portion 41. If the first portion 41 has a multilayer structure as described above, the electron barrier layer 42 is a layer with a larger band gap energy than any of the layers constituting the first portion 41. By having such a large band gap energy, the electron barrier layer 42 can function as a barrier against electrons overflowing from the active layer 3. Preferably, the band gap energy difference between the electron barrier layer 42 and the uppermost layer of the first portion 41 is 0.1 eV or more. These band gap energy differences can be, for example, 1 eV or less. The electron barrier layer 42 is, for example, the layer with the highest band gap energy among the p-side semiconductor layers 4. The electron barrier layer 42 may be a layer with a smaller film thickness than the p-side composition gradient layer 411. The electron barrier layer 42 may have a multilayer structure. In this case, the electron barrier layer 42 has a layer with a larger band gap energy than any of the layers constituting the first portion 41. For example, as shown in Figure 2A, the first electron barrier layer 42A and the second electron barrier layer 42B may be present. Note that if the first portion 41 or the electron barrier layer 42 has a superlattice layer, the relationship between them is compared using the average band gap energy of the superlattice layer, rather than the band gap energies of each layer constituting the superlattice layer. The electron barrier layer 42 is made of, for example, AlGaN. When the electron barrier layer 42 is AlGaN, its Al composition ratio may be 8-30%. The thickness of the electron barrier layer 42 can be, for example, 5 nm or more, and 100 nm or less.

[0054] As shown in Figures 2C and 2D, it is preferable that the shortest distance from the bottom surface of the ridge 4a to the electron barrier layer 42 is greater than the shortest distance from the top surface of the ridge 4a to the electron barrier layer 42. This arrangement allows the peak of light intensity to be moved away from the portion containing p-type impurities, such as the electron barrier layer 42, and also shortens the distance between the lower end of the ridge 4a and the active layer 3. The shortest distance from the bottom surface of the ridge 4a to the electron barrier layer 42 refers to the shortest distance from the imaginary straight line connecting the lower ends of the ridge 4a to the lower surface of the electron barrier layer 42 in a cross-sectional view as shown in Figure 1. In other words, the electron barrier layer 42 is located towards the upper part of the ridge 4a. Furthermore, if the shortest distance from the bottom surface of the ridge 4a to the active layer 3 is, for example, around 436 nm, kinks may occur in the IL characteristic curve. For this reason, it is preferable that the shortest distance from the bottom surface of the ridge 4a to the active layer 3 be 430 nm or less. This enhances horizontal lateral light confinement.

[0055] (Second part 43) The second part 43 has one or more p-type semiconductor layers containing p-type impurities. The concentration of p-type impurities in the p-type semiconductor layers of the second part 43 is, for example, 1 × 10⁻⁶ 18 / cm 3 It can be set to the above 1 × 10 22 / cm 3 The following is possible. As described above, the driving voltage can be reduced by making the thickness of the second portion 43 thinner, so the thickness of the second portion 43 is preferably 260 nm or less. The thickness of the second portion 43 can be 10 nm or more. The second portion 43 may contain an undoped layer, but if there is an undoped layer in the second portion 43 the resistance will be higher than that of the second portion 43, so it is preferable to contain p-type impurities throughout the second portion 43. In the case of a superlattice layer, its average p-type impurity concentration can be considered as the p-type impurity concentration of the superlattice layer, so if the second portion 43 has a superlattice layer, the superlattice layer may have a laminated structure of an undoped layer and a p-type impurity-containing layer.

[0056] The second part 43, as shown in Figure 2A, may have an upper p-type semiconductor layer 432 and a lower p-type semiconductor layer 431. The upper p-type semiconductor layer 432 constitutes the upper surface of the ridge 4a. That is, the upper p-type semiconductor layer 432 is the uppermost layer of the second part 43 and the uppermost layer of the ridge 4a. The upper p-type semiconductor layer 432 functions as a p-side contact layer. The lower p-type semiconductor layer 431 is located between the upper p-type semiconductor layer 432 and the electron barrier layer 42 and has a larger bandgap energy than the upper p-type semiconductor layer 432.

[0057] The lower p-type semiconductor layer 431 is made of, for example, AlGaN. The upper p-type semiconductor layer 432 is made of, for example, GaN. The lower p-type semiconductor layer 431 preferably has a band gap energy between the electron barrier layer 42 and the upper p-type semiconductor layer 432. Since AlGaN containing p-type impurities tends to have higher resistance than GaN containing p-type impurities, the upper p-type semiconductor layer 432 is preferably a GaN layer doped with p-type impurities. By providing a lower p-type semiconductor layer made of AlGaN below the upper p-type semiconductor layer 432, photoconfinement to the active layer 3 can be strengthened compared to the case where the second portion 43 is formed only of a GaN layer. Furthermore, by making the Al composition ratio of the lower p-type semiconductor layer 431 smaller than that of the electron barrier layer 42, it is possible to make the lower p-type semiconductor layer 431 have lower resistance than the electron barrier layer 42. The lower p-type semiconductor layer 431 may also function as a p-side cladding layer. The lower p-type semiconductor layer 431 may also be a p-type GaN layer, which can further reduce the resistance of the second portion 43. In this case, it is preferable to form the p-electrode with a material that functions as a cladding layer, such as ITO.

[0058] The film thickness of the upper p-type semiconductor layer 432 can be, for example, 5 to 30 nm. The film thickness of the lower p-type semiconductor layer 431 can be, for example, 1 to 260 nm. The film thickness of the lower p-type semiconductor layer 431 can be thinner than the film thickness of the p-side intermediate layer 412, and even thinner than the film thickness of the second layer 412B. Both the lower p-type semiconductor layer 431 and the second layer 412B may be AlGaN layers, and their Al composition ratios may be the same. The lower p-type semiconductor layer 431 is, for example, thicker than the electron barrier layer 42. Therefore, in order to reduce free carrier absorption loss, it is preferable that the p-type impurity concentration of the lower p-type semiconductor layer 431 is lower than the p-type impurity concentration of the electron barrier layer 42.

[0059] (Insulating film 5, n electrode 8, p electrode 6, p-side pad electrode 7) The insulating film 5 can be formed from a single-layer or multilayer film of an oxide or nitride such as Si, Al, Zr, Ti, Nb, or Ta. The n-electrode 8 is provided over almost the entire lower surface of the n-type substrate 1, for example. The p-electrode 6 is provided on the upper surface of the ridge 4a. If the width of the p-electrode 6 is narrow, a p-side pad electrode 7 wider than the p-electrode 6 can be provided on top of the p-electrode 6, and a wire or the like can be connected to the p-side pad electrode 7. The material of each electrode can be a single-layer or multilayer film of a conductive oxide containing at least one selected from metals or alloys such as Ni, Rh, Cr, Au, W, Pt, Ti, Al, Zn, In, and Sn. Examples of conductive oxides include ITO (Indium Tin Oxide), IZO (Indium Zinc Oxide), and GZO (Gallium-doped Zinc Oxide). The thickness of the electrode can usually be any thickness that allows it to function as an electrode of a semiconductor device. For example, it can be about 0.1 μm to 2 μm.

[0060] The p electrode 6 is preferably a transparent conductive film having a refractive index smaller than that of the active layer 3. This allows it to function as a cladding layer. Furthermore, the p electrode 6 is preferably a transparent conductive film having a refractive index smaller than that of the second portion 43. This allows for a greater light confinement effect. Furthermore, when a p-side cladding layer is provided in the second portion 43, for example, an AlGaN layer with a relatively high Al composition ratio and p-type impurities is provided as the p-side cladding layer. However, the higher the Al composition ratio, the higher the resistance tends to be. If the p-electrode 6 functions as a cladding layer, it is not necessary to provide a p-side cladding layer in the second portion 43, or if a p-side cladding layer is provided, its Al composition ratio can be lowered. As a result, the resistance can be reduced, and the driving voltage of the semiconductor laser element 100 can be reduced. An example of a p-electrode 6 that functions as a cladding layer is a p-electrode 6 made of ITO.

[0061] (Manufacturing method) The manufacturing method of the semiconductor laser element 100 according to the embodiment may include steps S101 to S106 shown in the flowchart of Figure 6A. In step S101, an n-side semiconductor layer 2 is formed on the substrate 1. In step S102, an active layer 3 is formed on the n-side semiconductor layer 2. In step S103, a first portion 41 having one or more semiconductor layers is formed on the upper surface of the active layer 3 by undoping. In step S104, an electron barrier layer 42 having a band gap energy greater than the band gap energy of the first portion 41 is formed on the upper surface of the first portion 41 by doping with p-type impurities. In step S105, a second portion 43 having one or more p-type semiconductor layers formed by doping with p-type impurities is formed on the upper surface of the electron barrier layer 42. In step S106, a ridge 4a protruding upward is formed by removing a part of the p-side semiconductor layer 4 including the first portion 41, the electron barrier layer 42, and the second portion 43. These steps enable the creation of a semiconductor laser element 100 having an n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4 arranged in that order upwards, with an upwardly projecting ridge 4a provided on the p-side semiconductor layer 4. The effects and preferred configurations of the layers obtained by each step are as described above. For example, as shown in Figure 6B, the step S103 for forming the first portion 41 may include a step S103A for forming a p-side composition gradient layer 411 with increasing bandgap energy towards the top using an undoped method, and a step S103B for forming a p-side intermediate layer 412 above the p-side composition gradient layer 411 using an undoped method. In this case, in the step S106 for forming the ridge 4a, a portion of the p-side semiconductor layer 4 can be removed so that the lower end of the ridge 4a is located on the p-side intermediate layer 412. Furthermore, in the step S105 for forming the second portion 43, a second portion 43 having a thickness thinner than the first portion 41 can be formed. In this case, in step S106, which forms the ridge 4a, a portion of the p-side semiconductor layer 4 can be removed so that the lower end of the ridge 4a is located in the first portion 41. Also, in step S105, which forms the second portion 43, a layer having a band gap energy smaller than that of the uppermost layer of the first portion 41 may be formed as the bottom layer of the second portion 43.

[0062] (Example 1) As Example 1, a semiconductor laser element having a p-side semiconductor layer 4 as shown in Figures 2A and 4 was fabricated. An MOCVD apparatus was used to fabricate the epitaxial wafer that would become the semiconductor laser element. In addition, trimethylgallium (TMG), triethylgallium (TEG), trimethylaluminum (TMA), trimethylindium (TMI), ammonia (NH3), silane gas, and bis(cyclopentadienyl)magnesium (Cp2Mg) were used as raw materials as appropriate.

[0063] An n-side semiconductor layer 2, an active layer 3, and a p-side semiconductor layer 4 were grown on an n-type GaN substrate (substrate 1) with the +c plane facing upwards. First, as the n-side semiconductor layer 2, a 1.0 μm thick layer of Al with Si doped into it is used. 0.018 Ga 0.982 N layer (underlayer 21) and Al layer with Si doped, 250 nm thick 0.08 Ga 0.92 N layer (first n-side cladding layer 22) and In layer with Si doped, 150 nm thick 0.04 Ga 0.96 The structure consists of an N layer (crack prevention layer 23), a GaN layer with a thickness of 10 nm and Si doped (intermediate layer 24), and an Al layer with a thickness of 650 nm and Si doped. 0.08 Ga 0.92 The structure consists of an N layer (second n-side cladding layer 25), a Si-doped GaN layer with a thickness of 200 nm (first n-side optical guide layer 26), and an undoped In layer with a thickness of 260 nm. 0.03 Ga 0.97 The structure consists of an N layer (second n-side optical guide layer 27), a GaN layer with a Si doping thickness of 1.2 nm (first hole block layer 281), and an In layer with a Si doping thickness of 4 nm. 0.05 Ga 0.95 The N layer (second hole block layer 282) and the others were grown in this order. Next, a Si-doped GaN layer (n-side barrier layer 31) and an undoped In 0.25 Ga 0.75 The N layer (well layer 32), the undoped GaN layer (intermediate barrier layer), and the undoped In 0.25 Ga 0.75An active layer 3 was grown containing an N layer (well layer 32) and an undoped GaN layer (p-side barrier layer 34) in that order. Next, as the p-side semiconductor layer 4, there is an undoped composition gradient layer with a thickness of 260 nm (p-side composition gradient layer 411), an undoped GaN layer with a thickness of 200 nm (p-side intermediate layer 412), and an Al layer with Mg doped and a thickness of 3.9 nm. 0.10 Ga 0.90 N layer (first electron barrier layer 42A) and Mg-doped Al with a film thickness of 7 nm 0.16 Ga 0.84 N layer (second electron barrier layer 42B) and Mg-doped Al with a film thickness of 300 nm 0.04 Ga 0.96 An N layer (lower p-type semiconductor layer 431) and a 15 nm thick GaN layer with Mg doping (upper p-type semiconductor layer 432) were grown in this order. The p-side composition gradient layer 411 has its growth starting point in In 0.05 Ga 0.95 With N as the starting element and GaN as the end of the growth cycle, the In composition was substantially monotonically decreased in 120 steps so that the compositional gradient was nearly linear.

[0064] Then, the epitaxial wafer on which the above layers were formed was removed from the MOCVD apparatus, and a ridge 4a, insulating film 5, p electrode 6, p-side pad electrode 7, and n electrode 8 were formed. A reflective film was then formed on the light-emitting end face and the light-reflecting end face, respectively, and the wafer was separated to obtain a semiconductor laser element 100. The depth of ridge 4a was set to approximately 340 nm. That is, ridge 4a was formed so that its lower end was located in the first layer 412A. In addition, an ITO film with a thickness of 200 nm was formed as the p electrode 6. The peak wavelength of the laser light emitted by the semiconductor laser element 100 according to Example 1 was approximately 530 nm.

[0065] (Example 2) As Example 2, a semiconductor laser element having the p-side semiconductor layer 4 shown in Figure 2B was fabricated. In other words, the p-side intermediate layer 412 is made of Al, which has an undoped film thickness of 100 nm, in addition to the first layer 412A. 0.05 Ga 0.95A semiconductor laser element similar to that in Example 1 was fabricated, except that an N layer (second layer 412B) was also formed and the film thickness of the lower p-type semiconductor layer 431 was set to 200 nm.

[0066] (Example 3) As Example 3, a semiconductor laser element having the p-side semiconductor layer 4 shown in Figure 2C was fabricated. In other words, a semiconductor laser element similar to that of Example 2 was fabricated, except that the film thickness of the second layer 412B was set to 200 nm and the film thickness of the lower p-type semiconductor layer 431 was set to 100 nm.

[0067] (Example 4) As Example 4, a semiconductor laser device similar to that in Example 3 was fabricated, except that the film thickness of the first layer 412A was set to 100 nm.

[0068] (Example 5) As Example 5, a semiconductor laser device similar to that in Example 3 was fabricated, except that the film thickness of the first layer 412A was set to 50 nm.

[0069] (Example 6) As Example 6, a semiconductor laser element having the p-side semiconductor layer 4 shown in Figure 2D was fabricated. In Example 6, a semiconductor laser element similar to that in Example 3 was fabricated, except that the depth of the ridge 4a was set to 270 nm. That is, in the semiconductor laser elements of Examples 1 to 5, the lower end of the ridge 4a was formed so that it was located in the first layer 412A, while in the semiconductor laser element of Example 6, the lower end of the ridge 4a was formed so that it was located in the second layer 412B.

[0070] (Comparative Examples 1-4) As Comparative Example 1, a semiconductor laser element similar to that in Example 1 was fabricated, except that the depth of the ridge 4a was 270 nm. That is, the semiconductor laser element of Comparative Example 1 was formed so that the lower end of the ridge 4a was located on the lower p-type semiconductor layer 431. As Comparative Examples 2 and 3, semiconductor laser elements similar to those in Comparative Example 1 were fabricated, except that the film thickness of the p-side intermediate layer 412 was different. The film thickness of the p-side intermediate layer 412 in Comparative Example 2 was 300 nm, and the film thickness of the p-side intermediate layer 412 in Comparative Example 3 was 400 nm. As Comparative Example 4, a semiconductor laser element similar to that in Comparative Example 1 was fabricated, except that the p-side intermediate layer 412 was not provided. That is, in all of the semiconductor laser elements of Comparative Examples 1 to 4, the lower end of the ridge 4a was located on the lower p-type semiconductor layer 431, and the shortest distance from the active layer 3 to the electron barrier layer 42 was formed so that it increased in the order of Comparative Example 4, Comparative Example 1, Comparative Example 2, and Comparative Example 3. The peak wavelength of the laser light emitted by the semiconductor laser elements of Comparative Examples 1 to 4 was approximately 525 nm.

[0071] (Experimental result 1) First, Figure 8 shows the IL characteristics of the semiconductor laser elements of Comparative Examples 1 to 4. In the graph in Figure 8, the horizontal axis represents current, and the vertical axis represents optical output. In Figure 8, the thin solid line represents Comparative Example 1, the thin dashed line represents Comparative Example 2, the thick dashed line represents Comparative Example 3, and the thick solid line represents Comparative Example 4. As shown in Figure 8, in Comparative Examples 1 to 4, where the lower end of the ridge 4a is located above the electron barrier layer 42, the IL characteristics became more unstable as the shortest distance from the active layer 3 to the electron barrier layer 42 increased. Comparative Examples 1 and 2 showed improved optical output and slope efficiency compared to Comparative Example 4, but kink occurred in the IL characteristic graph, causing a bend in part. In the semiconductor laser elements of Comparative Examples 1 and 2, the distance from the active layer 3 to the lower end of the ridge 4a also increased due to the provision of the p-side intermediate layer 412, resulting in a decrease in the effective refractive index difference between the inside and outside of the ridge 4a compared to Comparative Example 4. This is thought to have caused the horizontal transverse mode to become unstable in the semiconductor laser elements of Comparative Examples 1 and 2, resulting in kink. Furthermore, the semiconductor laser element of Comparative Example 3, in which the p-side intermediate layer 412 was made even thicker, not only experienced kinking but also exhibited lower optical output than the semiconductor laser element of Comparative Example 4. Thus, when the lower end of the ridge 4a is located above the electron barrier layer 42, even when attempting to improve efficiency by providing the p-side intermediate layer 412, kinking occurs and the IL characteristics become unstable.

[0072] (Experimental result 2) Figure 9A shows the IL characteristics of the semiconductor laser elements of Examples 1 to 3, and Figure 9B shows the IV characteristics. In the graph of Figure 9A, the horizontal axis represents current and the vertical axis represents optical output. In the graph of Figure 9B, the horizontal axis represents current and the vertical axis represents voltage. In Figures 9A and 9B, the thin solid line represents Example 1, the thick solid line represents Example 2, and the dashed line represents Example 3. First, Examples 1 to 3 and Comparative Examples 1 to 3 are compared using Figures 8 and 9A. The shortest distance from the active layer 3 to the electron barrier layer 42 is the same for Example 1 and Comparative Example 1, the same for Example 2 and Comparative Example 2, and the same for Example 3 and Comparative Example 3. As can be seen from Figures 8 and 9A, in Comparative Examples 1 to 3, the IL characteristics became more unstable as the distance from the active layer 3 to the electron barrier layer 42 increased, but in Examples 1 to 3, the IL characteristics were almost the same regardless of the distance from the active layer 3 to the electron barrier layer 42. This is thought to be because the formation of a deeper ridge 4a in the semiconductor laser elements 100 of Examples 1 to 3 strengthened lateral optical confinement, stabilizing the horizontal transverse modes. It should be noted that the higher optical output in Comparative Examples 1 to 3 compared to Examples 1 to 3 is due to the shorter oscillation wavelength of Comparative Examples 1 to 3, and cannot be attributed to the difference in ridge 4a depth.

[0073] Furthermore, the film thickness of the lower p-type semiconductor layer 431 decreases in the order of Example 1, Example 2, and Example 3. As shown in Figure 9B, it was confirmed that thinning the lower p-type semiconductor layer 431 reduces the voltage required to drive the semiconductor laser element 100. This is thought to be because the lower p-type semiconductor layer 431 is an AlGaN layer, which has a relatively high series resistance. The second layer 412B is also an AlGaN layer. While providing such an undoped AlGaN layer in the p-side semiconductor layer 4 may increase electrical resistance and reduce the probability of hole injection into the active layer 3, this effect was not observed in the results shown in Figures 9A and 9B. This is thought to be because the band of the second layer 412B bends when voltage is applied, thereby promoting hole injection. In addition, it is thought that when voltage is applied, electrons overflowing from the active layer 3 fill the first portion 41.

[0074] (Experimental result 3) Figure 10A shows the IL characteristics of the semiconductor laser elements of Examples 3 to 5, and Figure 10B shows the IV characteristics. In the graph of Figure 10A, the horizontal axis represents current and the vertical axis represents optical output. In the graph of Figure 10B, the horizontal axis represents current and the vertical axis represents voltage. In Figures 10A and 10B, the dashed line represents Example 3, the solid line represents Example 4, and the dashed line represents Example 5. The film thickness of the first layer 412A decreases in the order of Example 3, Example 4, and Example 5, and as shown in Figure 10A, it was confirmed that the slope efficiency improves by reducing the film thickness of the first layer 412A. As described above, by moving the electron barrier layer 42 and the second portion 43 containing p-type impurities away from the active layer 3, the optical leakage Γp to the second portion 43, which has a large free carrier absorption loss, can be reduced. However, on the other hand, the further the electron barrier layer 42 is moved away from the active layer 3, the more electrons overflow from the active layer 3. Therefore, by reducing the film thickness of the first layer 412A, which has less influence on light confinement than the p-side composition gradient layer 411 and the second layer 412B, it is possible to suppress the increase in overflowing electrons while also reducing light leakage into the second portion 43, thereby improving the slope efficiency. Furthermore, as shown in Figure 10B, it was confirmed that the change in the film thickness of the first layer 412A has almost no effect on the driving voltage.

[0075] (Experimental result 4) Figure 11A shows the IL characteristics of the semiconductor laser elements of Examples 3 and 6, and Figure 11B shows the IV characteristics. In the graph of Figure 11A, the horizontal axis represents current and the vertical axis represents optical output. In the graph of Figure 11B, the horizontal axis represents current and the vertical axis represents voltage. In Figures 11A and 11B, the dashed line represents Example 3 and the solid line represents Example 6. The depth of the ridge 4a is shallower in Example 6 than in Example 3, but as shown in Figures 11A and 11B, both showed similar characteristics. [Explanation of Symbols]

[0076] 100 semiconductor laser elements 1 circuit board 2 n-side semiconductor layer 21 Base layer 22 First n-side cladding layer 23. Crack prevention layer 24 Middle Class 25 Second n-side cladding layer 26. First n-side optical guide layer 27. Second n-side optical guide layer 28 Hole Block Layer 281 First Hole Block Layer 282 Second Hole Block Layer 3 Active layer 31 n-side barrier layer 32 Well layer 34 p-side barrier layer 4 p-side semiconductor layer 41 Part 1 411 p-side compositionally graded layer 412 p side middle layer 412A 1st layer 412B 2nd layer 42 Electron barrier layer 42A First electron barrier layer 42B Second electron barrier layer 43 Part 2 431 Lower p-type semiconductor layer 432 Upper p-type semiconductor layer 4a Ridge 5. Insulating film 6p electrode 7 p-side pad electrode 8n electrode 411a, 411b, 411c, 411y, 411z sublayers

Claims

1. A semiconductor laser element having, in order from top to bottom, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each made of a nitride semiconductor, and having an upwardly protruding ridge on the p-side semiconductor layer, The p-side semiconductor layer is Distended in contact with the upper surface of the active layer, having one or more semiconductor layers, and comprising a first portion which is undoped, An electron barrier layer is provided which is positioned in contact with the upper surface of the first portion, has a larger band gap energy than the first portion, and contains p-type impurities. A second portion is disposed in contact with the upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities, The first part is, As you move upwards, the band gap energy increases, and the p-side composition gradient layer is undoped, A semiconductor laser element having a p-side intermediate layer which is undoped and positioned above the p-side composition gradient layer, wherein the lower end of the ridge is located in the p-side intermediate layer.

2. The second part is, The upper p-type semiconductor layer that constitutes the upper surface of the ridge, The semiconductor laser element according to claim 1, comprising a lower p-type semiconductor layer disposed between the upper p-type semiconductor layer and the electron barrier layer, and having a bandgap energy greater than that of the upper p-type semiconductor layer.

3. The first part is the p-side intermediate layer, A first layer having a band gap energy greater than the average band gap energy of the p-side composition gradient layer and smaller than the band gap energy of the electron barrier layer, and being undoped, A semiconductor laser element according to claim 1 or 2, comprising: a second layer having a band gap energy greater than the band gap energy of the first layer and less than the band gap energy of the electron barrier layer, and being undoped.

4. The semiconductor laser element according to claim 3, wherein the first layer and the second layer are each layers of a single composition, and the lower end of the ridge is located in the first layer or the second layer.

5. The semiconductor laser element according to claim 3 or 4, wherein the first layer is a GaN layer.

6. The semiconductor laser element according to any one of claims 3 to 5, wherein the second layer is an AlGaN layer.

7. The aforementioned p-side composition gradient layer consists of a plurality of sub-layers with different compositions. The lowest sublayer of the aforementioned p-side composition gradient layer is In ａ Ga １－ａ N consists of (0 < a < 1), The uppermost sublayer of the p-side composition gradient layer is In ｚ Ga １－ｚ A semiconductor laser element according to any one of claims 1 to 6, wherein N (0 ≤ z < a).

8. A semiconductor laser element having, in order from top to bottom, an n-side semiconductor layer, an active layer, and a p-side semiconductor layer, each made of a nitride semiconductor, and having an upwardly protruding ridge on the p-side semiconductor layer, The p-side semiconductor layer is Distended in contact with the upper surface of the active layer, having one or more semiconductor layers, and comprising a first portion which is undoped, An electron barrier layer is provided which is positioned in contact with the upper surface of the first portion, has a larger band gap energy than the first portion, and contains p-type impurities. A second portion is disposed in contact with the upper surface of the electron barrier layer and has one or more p-type semiconductor layers containing p-type impurities, The thickness of the second part is thinner than the thickness of the first part. The lower end of the ridge is a semiconductor laser element located in the first portion.

9. The semiconductor laser element according to any one of claims 1 to 8, wherein the thickness of the first portion is 400 nm or more.

10. The semiconductor laser element according to any one of claims 1 to 9, wherein the shortest distance from the bottom surface of the ridge to the electron barrier layer is greater than the shortest distance from the top surface of the ridge to the electron barrier layer.

11. The ridge has a p electrode provided on its upper surface, The semiconductor laser element according to any one of claims 1 to 10, wherein the p electrode is a transparent conductive film having a refractive index smaller than that of the second portion.

12. The semiconductor laser element according to any one of claims 1 to 11, wherein the semiconductor laser element is capable of emitting laser light with a wavelength of 530 nm or more.

13. The first portion has an uppermost layer in contact with the lower surface of the electron barrier layer, The second portion has a bottom layer in contact with the upper surface of the electron barrier layer, The semiconductor laser element according to any one of claims 1 to 12, wherein the band gap energy of the lowest layer is smaller than the band gap energy of the uppermost layer.

14. A process of forming an n-side semiconductor layer on a substrate, The process of forming an active layer on the n-side semiconductor layer, A step of forming a first portion having one or more semiconductor layers on the upper surface of the active layer by undoping, The process involves forming an electron barrier layer having a band gap energy greater than the band gap energy of the first portion on the upper surface of the first portion by doping it with p-type impurities. A step of forming a second portion having one or more p-type semiconductor layers formed by doping p-type impurities on the upper surface of the electron barrier layer, The process includes a step of forming an upwardly protruding ridge by removing a portion of the p-side semiconductor layer, which includes the first portion, the electron barrier layer, and the second portion. The process of forming the first portion by undoping is as follows: A process of forming a p-side composition gradient layer in which the band gap energy increases as you move upwards using undoping, A method for manufacturing a semiconductor laser element, comprising the steps of forming a p-side intermediate layer by undoping above the p-side composition gradient layer, wherein in the step of forming the ridge, a portion of the p-side semiconductor layer is removed such that the lower end of the ridge is located in the p-side intermediate layer.

15. A process of forming an n-side semiconductor layer on a substrate, The process of forming an active layer on the n-side semiconductor layer, A step of forming a first portion having one or more semiconductor layers on the upper surface of the active layer by undoping, The process involves forming an electron barrier layer having a band gap energy greater than the band gap energy of the first portion on the upper surface of the first portion by doping it with p-type impurities. A step of forming a second portion having one or more p-type semiconductor layers formed by doping p-type impurities on the upper surface of the electron barrier layer, The process includes a step of forming an upwardly protruding ridge by removing a portion of the p-side semiconductor layer, which includes the first portion, the electron barrier layer, and the second portion. In the process of forming the second portion, the second portion having a thickness thinner than the thickness of the first portion is formed. A method for manufacturing a semiconductor laser element, comprising the step of forming the ridge, wherein a portion of the p-side semiconductor layer is removed such that the lower end of the ridge is located in the first portion.

Images