Laser diode

Abstract

The present invention provides a laser diode having a low oscillation threshold current and drive voltage. This laser diode comprises a nitride semiconductor substrate containing Al, and a semiconductor lamination part disposed on the nitride semiconductor substrate. The semiconductor lamination part has a first conductivity type cladding layer that is disposed on the nitride semiconductor substrate and includes a first conductivity type nitride semiconductor layer, a nitride semiconductor structure light emitting layer that is formed on the first conductivity type cladding layer and that includes one or more quantum wells, and a second conductivity type cladding layer that is disposed on the light emitting layer and includes a second conductivity type nitride semiconductor layer. At least some of the semiconductor lamination part is a mesa structure for optical resonance and emission, and the side surfaces of the mesa structure are inclined planes that are inclined outward toward the first conductivity type cladding layer from the top surface of the mesa structure.

Description

Technical Field

[0001] The present disclosure relates to laser diodes.

Background Art

[0002] Conventionally, nitride semiconductors have been used as materials for forming light-emitting diodes (LEDs) and laser diodes (LDs). Since nitride semiconductors have a recombination form of direct transition, they are suitable as materials for LEDs and LDs in that high recombination efficiency and high optical gain can be obtained. As an example of a laser diode using such a nitride semiconductor, a technique for oscillating a current-injection type laser diode in the ultraviolet region has been disclosed (for example, Non-Patent Document 1).

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The above-described ultraviolet laser diode has low-temperature continuous oscillation, and room-temperature continuous oscillation is required for application to actual applications. For this room-temperature continuous oscillation, reduction of the oscillation threshold current and reduction of the driving voltage are required. An object of the present disclosure is to provide a laser diode having a low oscillation threshold current and a low driving voltage.

Means for Solving the Problems

[0005] In order to solve the above problems, a laser diode according to one aspect of the present disclosure includes a nitride semiconductor substrate containing Al, and a semiconductor laminate disposed on the nitride semiconductor substrate. The semiconductor laminate is disposed on the nitride semiconductor substrate and includes a first-conductivity-type clad layer containing a first-conductivity-type nitride semiconductor layer, a light-emitting layer of a nitride semiconductor structure formed on the first-conductivity-type clad layer and including one or more quantum wells, and a second-conductivity-type clad layer disposed on the light-emitting layer and containing a second-conductivity-type nitride semiconductor layer. At least a part of the semiconductor laminate is a mesa structure for optical resonance and emission, and a side surface of the mesa structure is an inclined surface that inclines outward from the upper surface of the mesa structure toward the first-conductivity-type clad layer. Note that the above summary of the invention does not list all the features of the invention according to the present disclosure.

Effect of the Invention

[0006] According to the present disclosure, a laser diode with a low oscillation threshold current and a low driving voltage can be provided.

Brief Description of the Drawings

[0007] [Figure 1] It is a schematic plan view showing a configuration example of a laser diode according to a first embodiment of the present disclosure. [Figure 2] It is a schematic cross-sectional view showing a configuration example of a laser diode according to a first embodiment of the present disclosure. [Figure 3] It is a schematic cross-sectional view showing another configuration example of a laser diode according to a second embodiment of the present disclosure. [Figure 4] It is a schematic cross-sectional view showing another configuration example of a laser diode according to a third embodiment of the present disclosure. [Figure 5] It is a schematic cross-sectional view showing another configuration example of a laser diode according to a fourth embodiment of the present disclosure.

Mode for Carrying Out the Invention

[0008] The laser diodes relating to this disclosure will be described below through embodiments, but these embodiments are not intended to limit the invention as claimed. Furthermore, not all combinations of features described in the embodiments are necessarily essential to the solution of the invention. Furthermore, in the following explanation, "up" and "down" do not necessarily mean the vertical direction relative to the ground. In other words, the directions of "up" and "down" are not limited to the direction of gravity. "Up" and "down" are merely convenient expressions to specify the relative positional relationship on a surface, film, substrate, etc., and do not limit the technical concept of this disclosure. For example, it goes without saying that if the paper is rotated 180 degrees, "up" becomes "down" and "down" becomes "up".

[0009] 1. Embodiment A laser diode according to an embodiment of this disclosure will be described. (1.1) Laser Diode Configuration The laser diode according to this embodiment comprises an Al-containing nitride semiconductor substrate and a semiconductor laminate disposed on the nitride semiconductor substrate. The semiconductor laminate is disposed on the nitride semiconductor substrate and includes a first conductivity type cladding layer containing a first conductivity type nitride semiconductor layer, an emissive layer formed on the first conductivity type cladding layer and having a nitride semiconductor structure containing one or more quantum wells, and a second conductivity type cladding layer disposed on the emissive layer and containing a second conductivity type nitride semiconductor layer. At least a portion of the semiconductor laminate is a mesa structure for optical resonance and emission, and the side surface of the mesa structure is an inclined surface that slopes outward from the top surface of the mesa structure toward the first conductivity type cladding layer. Here, "side surface" refers to a side surface of the mesa structure other than the resonant mirror end surface (resonant mirror end surface ES shown in Figure 1, details will be described later) from which the laser light is emitted (side surface SS shown in Figure 1). The following provides a detailed explanation of each layer of the laser diode.

[0010] <Nitride semiconductor substrates> The nitride semiconductor substrate (hereinafter sometimes referred to as "substrate") used in the manufacture of the laser diode according to this embodiment contains an Al-containing nitride semiconductor. The Al-containing nitride semiconductor is, for example, AlN. That is, the substrate is preferably an AlN single crystal substrate. Furthermore, the Al-containing nitride semiconductor is not limited to AlN, and may be, for example, AlGaN. For example, when the substrate is an AlN, AlGaN, or other nitride semiconductor single crystal substrate, the lattice constant difference with the nitride semiconductor layer formed on the upper side of the substrate becomes smaller, and the number of threading dislocations can be reduced by growing the nitride semiconductor layer in a lattice-matched system. The through-dislocation density of the substrate is 5 × 10⁻⁶ 4 cm -2 The following is preferable. In particular, from the viewpoint of reducing the oscillation threshold current, the through-dislocation density is 1 × 10⁻⁶. 3 The above 1 x 10 4 cm -2 The following is more preferable:

[0011] Here, the phrase "contains nitride semiconductors" means that nitride semiconductors are primarily contained within the layer, but it also includes cases where other elements are present. Specifically, this expression includes cases where minor changes are made to the composition of this layer, such as adding small amounts of elements other than nitride semiconductors (for example, a few percent or less of elements such as Ga (if Ga is not the main element), In, As, P, or Sb). The word "contains" has the same meaning when describing the composition of other layers. Furthermore, the above does not apply to the small amounts of elements that may be included.

[0012] Furthermore, the substrate may be doped to an n-type or p-type with donor or acceptor impurities. The substrate may also be a mixed crystal of a nitride semiconductor such as AlN and sapphire (Al2O3), Si, SiC, MgO, Ga2O3, ZnO, GaN, or InN. The substrate preferably has a layer thickness of 100 μm to 600 μm, for example. Furthermore, while the surface orientations include the c-plane (0001), a-plane (11-20), and m-plane (10-10), the c-plane (0001) substrate is more preferred. Moreover, it can be formed on a surface inclined at some angle (for example, -4° to 4°, preferably -0.4° to 0.4°) from the c-plane (0001) normal direction, but is not limited to this.

[0013] <buffer layer> The buffer layer is formed between the substrate and the first conductivity type cladding layer, and preferably covers the entire surface of the substrate. By providing a buffer layer, a nitride semiconductor layer with small differences in lattice constants and thermal expansion coefficients and few defects is formed on the buffer layer. Furthermore, by providing a buffer layer, the first conductivity type cladding layer can be grown under compressive stress, and the occurrence of cracks in the first conductivity type cladding layer can be suppressed. For this reason, even when the substrate is made of a nitride semiconductor such as AlN or AlGaN, a nitride semiconductor layer with few defects can be formed above the buffer layer. The buffer layer is formed from a nitride semiconductor such as AlN or AlGaN. The buffer layer may also contain impurities such as C, Si, Fe, and Mg.

[0014] The buffer layer has a thickness of, for example, several micrometers. Specifically, the buffer layer is preferably thicker than 10 nm and thinner than 10 μm. When the buffer layer is thicker than 10 nm, the crystallinity of nitride semiconductors such as AlN is increased. When the buffer layer is thinner than 10 μm, cracks are less likely to occur in the buffer layer formed by crystal growth across the entire wafer surface. Furthermore, it is more preferable that the buffer layer is thicker than 50 nm and thinner than 5 μm. When the buffer layer is thicker than 50 nm, a highly crystallinity layer can be formed. Furthermore, when the buffer layer is thinner than 5 μm, cracks in the buffer layer are even less likely to occur.

[0015] <First conductive cladding layer> The first conductivity type clad layer is formed on the substrate. Here, for example, in the expression "the first conductivity type clad layer is formed on the substrate", the word "on" means that the first conductivity type clad layer is formed on one surface of the substrate. Also, the above expression includes the case where another layer further exists between the substrate and the first conductivity type clad layer. In the relationship between other layers, the word "above" has the same meaning. For example, when the second conductivity type clad layer is formed via an electron blocking layer on the second waveguide layer described later, it is also included in the expression "the second conductivity type clad layer is formed on the first waveguide layer". In the description of this embodiment, "the first conductivity type" and "the second conductivity type" each mean a semiconductor indicating a different conductivity type. For example, when one is n-type conductivity, the other is p-type conductivity. There may be cases where the description is made on the premise that the first conductivity type is n-type and the second conductivity type is p-type without special explanation, but it is not limited to this.

[0016] The first conductivity type clad layer is a layer of a nitride semiconductor containing Al and Ga. The first conductivity type clad layer is, for example, Al a Ga 1-a formed by N (0 < a < 1). Thereby, when forming a material corresponding to the bandgap energy in the deep ultraviolet region as the light-emitting layer, it becomes possible to improve the crystallinity of the light-emitting layer and the light-emitting efficiency. From the viewpoint of realizing high light-emitting efficiency, the nitride semiconductor constituting the first conductivity type clad layer is preferably a mixed crystal of AlN and GaN. Also, from the viewpoint of growing with complete strain with respect to the substrate, the first conductivity type clad layer is more preferably formed by Al a Ga N (0.6 < a ≤ 0.8). N(0.6 < a ≤ 0.8).

[0017] The first conductivity type clad layer may be a gradient layer in which the Al composition increases as it moves away from the substrate for the purpose of controlling the longitudinal conductivity. In this case, the above limitation on the Al composition can be the Al composition averaged by the film thickness of the first conductivity type clad layer with respect to the position in the film thickness direction within the first conductivity type clad layer.

[0018] When the first conductive type clad layer is an n-type conductive semiconductor layer, it may contain impurities such as group V elements other than N such as In, P, As, Sb, and impurities such as C, H, F, O, Mg, Si, etc., but the types of impurity elements are not limited to this. From the viewpoints of reducing electrical resistance and the availability of raw materials, the impurity contained in the first conductive type clad layer is preferably Si, and the impurity concentration is 5×10 18 cm -3 or more and 5×10 19 cm -3 or less. From the viewpoints of lattice relaxation within the first conductive type clad layer and film resistance, the first conductive type clad layer preferably has a layer thickness of 200 nm or more and 800 nm or less, more preferably has a layer thickness of 300 nm or more and 750 nm or less, and still more preferably has a layer thickness of 300 nm or more and 500 nm or less.

[0019] <Light-emitting layer> The light-emitting layer is a layer of a nitride semiconductor containing Al and Ga. The nitride semiconductor contained in the light-emitting layer is preferably, for example, a mixed crystal of AlN and GaN from the viewpoint of realizing high luminous efficiency. For example, Al b Ga 1-b N (0 < b < 1). In addition to N, the light-emitting layer may contain impurities such as group V elements other than N such as P, As, Sb, and impurities such as C, H, F, O, Mg, Si, etc., but it is not limited to this.

[0020] Also, the light-emitting layer can have either a multiple quantum well structure or a single quantum well structure. Depending on the longitudinal conductivity of the first conductive type clad layer and the second conductive type clad layer, the number of quantum well structures is preferably any one of 1 to 3. Also, for the purpose of reducing the influence of crystal defects in the light-emitting layer, etc., elements such as Si, Sb, P, etc. may be contained in part or all of the light-emitting layer at 1×10 15 cm -3 or more.

[0021] <Waveguide layer> From the perspective of light confinement, the laser diode of this embodiment may further include a waveguide layer formed above and below the light-emitting layer so as to sandwich the light-emitting layer and having an effect of confining the light emitted from the light-emitting layer within the light-emitting layer. The waveguide layer is preferably composed of two layers, a first waveguide layer disposed on the side of the first conductive type clad layer with respect to the light-emitting layer and a second waveguide layer disposed on the side of the second conductive type clad layer with respect to the light-emitting layer.

[0022] From the perspective of light confinement, the waveguide layer is preferably a nitride semiconductor containing Al and Ga having a bandgap higher in energy than the light-emitting layer. The waveguide layer preferably has an Al composition and a film thickness that increase the overlap of the electric field intensity distribution of the light standing in the device and the light-emitting layer. From the perspective of carrier confinement in the light-emitting layer, the light-emitting layer is made of Al b Ga 1-b N (0 < b < 1), and when the waveguide layer is made of Al c Ga 1-c N (0 < c < 1), it is preferable that b < c and c ≧ b + 0.05. For example, when taking a light-emitting layer with an emission wavelength of 265 nm as an example, b is 0.52, and it is preferable that c is 0.57 or more. Also, from the perspectives of light confinement and sheet resistance, the total film thickness of the first waveguide layer and the second waveguide layer is preferably 70 nm or more and 150 nm or less.

[0023] Each of the Al compositions of the first waveguide layer and the second waveguide layer is preferably uniform in the film thickness direction, but this is not the limit. In order to avoid light absorption into the metal (for example, the second electrode) existing above the second conductive type clad layer described later, the Al composition of the second waveguide layer may be higher than the Al composition of the first waveguide layer. For the same purpose, the film thickness of the second waveguide layer may be thicker than the film thickness of the first waveguide. The first waveguide layer may contain impurities such as P, As, Sb and other group V elements other than N, and H, C, O, F, Mg, Si and other impurities for the purpose of obtaining the same conductivity type as the first conductive type clad layer, but the types of impurity elements are not limited to this.

[0024] <Second Conductive Type Clad Layer> The second-conductivity-type clad layer is a nitride semiconductor layer containing Al and Ga having conductivity of the second conductivity type, formed on the light-emitting layer. The second-conductivity-type clad layer is, for example, Al d Ga 1-d formed by N (0 < d ≦ 1), and preferably formed by Al d Ga 1-d N (0.1 ≦ d < 1). Further, when a waveguide layer (second waveguide layer) is provided on the light-emitting layer, the second-conductivity-type clad layer is formed on the waveguide layer (second waveguide layer). Thereby, the second-conductivity-type clad layer is easily lattice-matched to the light-emitting layer or the waveguide layer, and it is possible to suppress the threading dislocation density.

[0025] The second-conductivity-type clad layer has conductivity sufficient to inject carriers (electrons or holes) into the light-emitting layer, and if it is possible to increase the overlap between the electric field strength distribution of the standing optical mode in the device and the light-emitting layer (that is, increase the optical confinement), the conductivity type is not particularly limited. The second-conductivity-type clad layer may be, for example, p-type AlGaN doped with Mg as an impurity. Further, the second-conductivity-type clad layer may contain impurities such as group V elements other than N such as P, As, Sb, and impurities such as C, H, F, O, Mg, Si, etc., but the types of impurity elements are not limited to this.

[0026] From the viewpoint of injecting carriers into the light-emitting layer more efficiently, the second-conductivity-type clad layer has a decreasing Al composition e as it moves away from the substrate, that is, the Al composition e is inclined so as to decrease in the direction away from the upper surface of the substrate, and is an Al e Ga 1-e compositionally graded layer (second-conductivity-type vertical conductive layer) formed by N (0.1 ≦ e ≦ 1), and preferably includes a second-conductivity-type lateral conductive layer containing Al f Ga 1-f N (0 < f ≦ 1). Hereinafter, the second-conductivity-type vertical conductive layer and the second-conductivity-type lateral conductive layer will be described.

[0027] (Second-Conductivity-Type Vertical Conductive Layer) The second conductivity type longitudinal conduction layer is a layer that constitutes the region of the second conductivity type cladding layer that is on the light-emitting layer side. The second conductivity type longitudinal conductive layer is Al e Ga 1-e This is a layer containing N (0.1 ≤ e ≤ 1). The profile (slope) of Al composition e in the second conductivity type longitudinal conduction layer may decrease continuously or intermittently. Here, "intermittently decreasing" means that a portion of the film in the second conductivity type longitudinal conduction layer contains a part where the Al composition e is the same (constant in the film thickness direction). In other words, the second conductivity type longitudinal conduction layer may contain a portion where the Al composition e does not decrease in the direction away from the substrate, but it does not contain a portion where it increases.

[0028] The film thickness of the second conductivity type longitudinal conduction layer is preferably 500 nm or less from the viewpoint of lattice matching. Furthermore, from the viewpoint of light confinement to the light-emitting layer and carrier injection, the film thickness of the second conductivity type longitudinal conduction layer is more preferably 250 nm to 450 nm, and even more preferably 300 nm to 400 nm.

[0029] In the second conductivity type longitudinal conduction layer, it is preferable that the region of the second conductivity type longitudinal conduction layer close to the light-emitting layer is not doped (not intentionally mixed with) impurities such as H, Mg, Be, Zn, Si, and B, in order to suppress the diffusion of impurities. Here, "undoped" means that the aforementioned impurities are not intentionally supplied as elements during the process of forming the target layer, but the elements originating from the raw materials and manufacturing equipment are, for example, 1 × 10⁻⁶ 16 cm -3 This does not apply if the contamination is within the following ranges. Furthermore, the undoped region of the second conductivity type longitudinal conduction layer includes at least the boundary with the light-emitting layer (or the second waveguide layer if a second waveguide layer is provided), but its size is not limited. For example, the entire region of the second conductivity type longitudinal conduction layer may be undoped. Alternatively, 50% of the region of the second conductivity type longitudinal conduction layer closest to the light-emitting layer may be undoped. Alternatively, approximately 10% of the region of the second conductivity type longitudinal conduction layer closest to the light-emitting layer may be undoped.

[0030] (p-type lateral conduction layer) The p-type lateral conduction layer is a layer that constitutes a region on the opposite side of the light-emitting layer in the p-type cladding layer, and is formed on the p-type vertical conduction layer. The p-type lateral conduction layer is Al f Ga 1-f N (0 < f ≤ 1) containing layer. Here, the Al composition f on the surface of the p-type lateral conduction layer facing the p-type vertical conduction layer is preferably larger than the minimum value of the Al composition e of the p-type vertical conduction layer.

[0031] For the purpose of controlling the longitudinal resistivity of the p-type lateral conduction layer, impurities such as H, Mg, Be, Zn, Si, B, etc. may be intentionally incorporated. The amount of the incorporated impurities may be, for example, 1×10 19 cm -3 or more and 5×10 21 cm -3 or less.

[0032] The film thickness of the p-type lateral conduction layer is preferably 20 nm or less, more preferably 10 nm or less, and even more preferably 5 nm or less from the viewpoint of facilitating the quantum tunneling of carriers through the p-type lateral conduction layer.

[0033] When a p-type contact layer described later is provided on the p-type lateral conduction layer, the Al composition at the interface of the p-type lateral conduction layer with the p-type contact layer is preferably smaller than the Al composition in the p-type contact layer and is completely strained with respect to the substrate. Such a p-type lateral conduction layer can improve the lateral conductivity by making the net internal electric field accumulated on the surface and near the surface of the p-type lateral conduction layer negative, and inducing carriers at the interface.

[0034] Thus, the second conductivity type longitudinal conduction layer generates carriers (for example, holes if the second conductivity type longitudinal conduction layer is made of a p-type semiconductor) through a polarization doping effect, and efficiently injects these carriers into the active layer within the light-emitting layer. For this reason, by providing the second conductivity type longitudinal conduction layer on the light-emitting layer, the carrier injection efficiency of the laser diode can be increased and the threshold voltage can be reduced.

[0035] Furthermore, the second conductivity type transverse conduction layer has the effect of broadening the carrier distribution, which is narrowed by the electric field concentrated below the electrode, in the transverse direction (within the plane of the second conductivity type transverse conduction layer). Due to this effect, the second conductivity type transverse conduction layer can increase the carrier injection efficiency into the light-emitting layer, similar to the second conductivity type longitudinal conduction layer.

[0036] <Second conductive contact layer> The semiconductor laminate of the laser diode in this embodiment may further include a second conductivity type contact layer disposed on a second conductivity type cladding layer. The nitride semiconductor constituting the second conductivity type contact layer is preferably formed from, for example, GaN, AlN, or InN, and mixed crystals containing them, and more preferably from a nitride semiconductor containing GaN.

[0037] In the case of a p-type contact layer, the second conductive contact layer may contain impurities such as P, As, Sb, other group V elements besides N, C, H, F, O, Mg, Si, Be, etc. Due to the versatility of the source gas, the impurity contained in the p-type contact layer is preferably Mg. From the viewpoint of reducing contact resistance, the concentration of Mg is 8 × 10⁻⁶. 19 cm -3 The above 5 x 10 21 cm -3 The following is preferable: 5 × 10 20 cm -3 The above 5 x 10 21 cm -3 The following is more preferable:

[0038] Furthermore, the thickness of the second conductive contact layer is preferably between 1 nm and 20 nm. The thinner the second conductive contact layer, the better the carrier injection efficiency of the laser diode; conversely, the thicker the layer, the worse the carrier injection efficiency.

[0039] <Electronic Block Layer> The semiconductor laminate of the laser diode in this embodiment may further have an electron blocking layer above the light-emitting layer, the electron blocking layer having a band gap larger than that of the second waveguide layer. The electron blocking layer can be provided, for example, inside the second waveguide layer, between the second waveguide layer and the light-emitting layer, or between the second waveguide layer and the second conductivity type longitudinal conduction layer. The thickness of the electron blocking layer is preferably 30 nm or less, more preferably 20 nm or less, and even more preferably 15 nm or less, so that carriers (holes) can easily quantum penetrate the electron blocking layer.

[0040] <Mesa Structures and Resonators> The mesa structure is formed to electrically isolate the second conductivity type layer from the first conductivity type layer. The mesa structure is a structure in which a portion of the semiconductor stack is removed. Here, "semiconductor stack" refers to at least the first conductivity type cladding layer, the light-emitting layer, and the second conductivity type cladding layer, and also includes the waveguide layer (first waveguide layer and second waveguide layer) and the second conductivity type contact layer if they are provided.

[0041] From the viewpoint of laser current constriction and optical amplification due to reflection at the end face, the mesa structure is preferably rectangular in shape with a long side and a short side in a plan view, and the long side extends in the <1-100> direction. Here, the side surface of the mesa structure is formed on the long side of the mesa structure in a plan view, and the resonant mirror end face of the mesa structure is formed on the short side of the mesa structure in a plan view. This is because, when obtaining the resonant mirror end face of a laser resonator by various methods such as cleavage or etching, a stable surface and an atomically flat (1-100) surface can be formed most easily. In other words, the mesa structure has a resonant mirror end face of a resonator parallel to the crystal plane (1-100) of the nitride semiconductor substrate. As a result, the laser diode of this embodiment is an end-face emitting laser diode in which the mesa structure emits light in the <1-100> direction.

[0042] A mesa structure can be formed by etching the semiconductor stack with inductively coupled plasma (ICP) or the like. In this process, the sides of the mesa structure can have various structures, from perpendicular to the surface of the first conductivity type cladding layer. From a manufacturing perspective, it is generally preferable for the sides of the mesa structure to be formed perpendicularly; however, when a mesa structure with perpendicular sides is formed, a damage layer is formed on the side. If optical modes interfere with this damage layer, optical loss occurs, leading to a deterioration of the oscillation threshold current. Therefore, from the viewpoint of suppressing optical loss, it is preferable that the sides of the mesa structure are inclined. Similarly, it is preferable that the sides of the mesa structure are inclined to be convex outward or inward. In other words, this application does not include structures with a reverse taper. Also, from the viewpoint of the number of manufacturing steps, it is preferable that the sides of the mesa structure are inclined to be convex outward (i.e., convex upward).

[0043] The slope of the mesa structure begins at the second conductivity type cladding layer (or the second conductivity type contact layer if one is present). The starting point of the sloped surface is defined as the point where the angle of the top surface of the mesa structure changes when viewed from the crystal plane (1-100) of the nitride semiconductor substrate. The side surface of the mesa structure slopes down to the first conductivity type cladding layer and connects to a flat surface. The point where the angle changes upon connection to this flat surface is defined as the ending point of the sloped surface. If the starting point of the sloped surface is above the ending point in the stacking direction on the substrate, the side surface of the mesa structure is defined as the sloped surface. In this case, if the starting point of the sloped surface and the top surface of the mesa structure do not perfectly coincide, the starting point of the sloped surface is defined as the point where the angle changes (for example, the point where the angle of the side surface of the mesa structure changes). If the sloped surface of the side surface of the mesa structure is above the line connecting the starting point and the ending point of the sloped surface over its entire range, the sloped surface is defined as an "outwardly convex sloped surface". Furthermore, if the inclined surface of the side of a mesa structure is below the aforementioned straight line over its entire range, the inclined surface is considered to be an "inwardly convex inclined surface." If a portion of the inclined surface of the side of a mesa structure is above the aforementioned straight line, and another portion of the inclined surface is below the aforementioned straight line, the side of the mesa structure is considered to be an inclined surface.

[0044] When the angle between the first straight line connecting the start and end points of the inclined surface and the surface of the first conductive cladding layer is defined as θa (0° < θa < 90°) (see Figure 3 described later), it is preferable that θa is 6° or more and 30° or less from the viewpoint of oscillation threshold and driving voltage. More preferably, θa is 11° or more and 25° or less. Furthermore, the angle between the third straight line, which connects the point where a second straight line parallel to the first straight line connecting the start and end points of the inclined surface touches the convex inclined surface and the start point of the inclined surface, and the first straight line described above is defined as θb (0° < θb < 90°) (see Figure 3 described later). In this case, it is preferable that θb is 0.1° or more and 5.0° or less from the viewpoint of oscillation threshold and driving voltage.

[0045] As described below, the first electrode and the second electrode are formed on the first and second conductivity type cladding layers, respectively, with an inclined surface in between. In this case, the length of the inclined surface is preferably 0.7 μm to 4.5 μm when viewed from above perpendicular to the substrate, from the viewpoint of driving voltage and oscillation threshold. More preferably, it is 0.9 μm to 2.2 μm. These inclined surface shapes can further reduce optical loss and reduce the oscillation threshold current. In addition, the resistance between the first and second electrodes can be reduced, making it possible to reduce the driving voltage.

[0046] <Electrode> A laser diode can oscillate by injecting current through a second electrode placed on a second conductivity type cladding layer and a first electrode placed on a first conductivity type cladding layer. In this case, the first electrode is formed to be in electrical contact with the first conductivity type cladding layer, and the second electrode is formed to be in electrical contact with the second conductivity type cladding layer. The first electrode can be placed, for example, on the back side of the substrate. Alternatively, the first electrode can be placed on the first conductivity type cladding layer that is exposed by removing the layers above the first conductivity type cladding layer of the semiconductor stack (for example, one or more layers near the second conductivity type contact layer) by, for example, chemical etching or dry etching. In other words, the first electrode is placed on a region of the first conductivity type cladding layer that does not form a mesa structure.

[0047] When the first conductive cladding layer is an n-type cladding layer, the first electrode is formed from metals such as Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zr, mixed crystals thereof, or conductive oxides such as ITO or Ga2O3. When the first conductive cladding layer is a p-type cladding layer, the first electrode is formed from metals such as Ni, Au, Pt, Ag, Rh, Pd, Pt, Cu, Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Ir, Zr, mixed crystals thereof, or conductive oxides such as ITO or Ga2O3.

[0048] When the second conductive cladding layer is an n-type cladding layer, the second electrode is formed from metals such as Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zr, mixed crystals thereof, or conductive oxides such as ITO or Ga2O3. When the second conductive cladding layer is a p-type cladding layer, the second electrode is formed from metals such as Ni, Au, Pt, Ag, Rh, Pd, Pt, Cu, Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Ir, Zr, mixed crystals thereof, or conductive oxides such as ITO or Ga2O3.

[0049] The placement regions and shapes of the first and second electrodes are not limited as long as electrical contact is obtained with the first conductivity type cladding layer and the second conductivity type cladding layer (or the second conductivity type contact layer if a second conductivity type contact layer is provided). From the viewpoint of driving voltage and manufacturing dimensional accuracy, the distance D1 from the second electrode to the side edge of the mesa structure (edge ​​L in Figure 1, described later) is preferably 1.0 μm or more and 4.5 μm or less when viewed from above in a direction perpendicular to the substrate. Here, in this embodiment, "side edge of the mesa structure" refers to the side edge of the top surface of the mesa structure, for example, the edge on the top surface of the second conductivity type cladding layer (or the edge on the top surface of the second conductivity type contact layer if a second conductivity type contact layer is provided). Furthermore, from the viewpoint of driving voltage, the distance between the first electrode and the second electrode formed on the first conductivity type cladding layer is preferably 3.5 μm or more and 10.0 μm or less when viewed from above. Here, distance means the shortest distance connecting the relevant points. Similarly, when the first electrodes are positioned on both sides of a mesa structure, distance refers to the shortest distance.

[0050] (1.2) Method for manufacturing ultraviolet laser diodes The laser diode of this embodiment is manufactured through a process of forming each layer on a substrate.

[0051] (Formation of substrate) The substrate is formed by general substrate growth methods such as sublimation, hydride vapor phase epitaxy (HVPE), and liquid phase epitaxy.

[0052] (Formation of semiconductor stacked layers) Each layer of the semiconductor stack formed on the substrate can be formed by, for example, molecular beam epitaxy (MBE), hydride vapor deposition (HVPE), or metal-organic chemical vapor deposition (MOCVD). Here, among the layers formed on the substrate, the nitride semiconductor layer can be formed using, for example, an Al raw material containing trimethylaluminum (TMAl), a Ga raw material containing trimethylgallium (TMGa) or triethylgallium (TEGa), or an N raw material containing ammonia (NH3).

[0053] A first conductivity type cladding layer containing a first conductivity type nitride semiconductor is formed on a buffer layer formed on a substrate. Next, an emissive layer is formed on the first conductivity type cladding layer using a nitride semiconductor (such as AlGaN) containing one or more quantum wells, and a second conductivity type cladding layer is formed on the emissive layer. If necessary, waveguide layers made of a nitride semiconductor such as AlGaN may be formed above and below the light-emitting layer. Alternatively, an intermediate layer made of a nitride semiconductor such as AlGaN may be formed between the second conductivity type cladding layer and the waveguide, or a second conductivity type contact layer made of a nitride semiconductor containing GaN or the like may be provided on the second conductivity type cladding layer.

[0054] Mesa structures are manufactured through a process (mesa structure formation process) in which unwanted portions of each layer of a semiconductor stack formed on a substrate are removed by etching. The removal of unwanted portions of each layer of the semiconductor stack can be performed, for example, by inductively coupled plasma (ICP) etching. During this process, the unwanted portions of each layer of the semiconductor stack are removed by etching in such a way that the sides of the mesa structure become inclined surfaces. For example, by using OFPR resist manufactured by Tokyo Ohka Kogyo and processing at a bake temperature of 150°C to 250°C, resist burning can be avoided, and long-term etching resistance can be obtained. This makes it possible to form inclined surfaces on the sides of the mesa structure. In the mesa structure formation process, unnecessary portions of each layer of the conductive laminate are removed by etching, exposing a portion of the first conductive cladding layer.

[0055] (Formation of electrodes) Furthermore, laser diodes can be manufactured through a process of forming electrodes. Electrodes such as the first and second electrodes are formed by methods such as resistance heating deposition, electron gun deposition, or sputtering, but are not limited to these methods. Each electrode may be formed as a single layer or as multiple layers stacked together. In addition, each electrode may be heat-treated in an oxygen, nitrogen, or air atmosphere after layer formation. Specifically, a first electrode is formed on the surface of the first conductivity type cladding layer. The second electrode is formed on the uppermost layer of the mesa structure (for example, the second conductivity type cladding layer) where a portion of the semiconductor stack is formed.

[0056] The formed electrodes can be alloyed by heating with an infrared lamp using a rapid thermal annealing (RTA) device, or by laser annealing using laser pulses, thereby achieving contact with a nitride semiconductor layer. In this case, the alloying method is not particularly limited, as long as sufficient contact with the nitride semiconductor layer is achieved and dislocations are not introduced into the mesa structure. Finally, the substrate, with each layer formed through the process described above, is divided into individual pieces by dicing to manufacture the laser diode.

[0057] 2. Measurement methods for the physical properties of laser diodes. The physical properties of the laser diode described above can be measured as follows.

[0058] (Measurement of impurity and doping concentrations) The concentrations of dopants and impurities in each layer of the substrate and semiconductor stack that constitute a laser diode can be measured by secondary ion mass spectrometry (SIMS). When measuring the concentration of dopants and impurities in each layer of a semiconductor stack using SIMS after the device has been fabricated, the measurement can be performed with the electrodes removed by chemical etching or physical polishing. Alternatively, the concentration of dopants and impurities in each layer of the semiconductor stack can also be measured by sputtering from the substrate side where electrodes are not formed. Specifically, SIMS measurements will be performed using measurement conditions provided by Evans Analytical Group (EAG). A cesium (Cs) ion beam with an energy of 14.5 keV will be used to sputter the sample during measurement.

[0059] (Method for measuring layer thickness) The thickness of each layer constituting a laser diode can be measured by cutting a predetermined cross-section perpendicular to the substrate, observing this cross-section with a transmission electron microscope (TEM), and using the TEM's length-measuring function. The measurement method involves first observing a cross-section perpendicular to the main surface of the laser diode substrate using a TEM. Specifically, for example, the observation width is defined as a range of 2 μm or more in the direction parallel to the main surface of the substrate within the TEM image showing the cross-section perpendicular to the main surface of the laser diode substrate. Within this observation width, contrast is observed at the interface between two layers with different compositions; therefore, the thickness up to this interface is observed in a continuous observation area with a width of 200 nm. The average thickness of each layer within this 200 nm observation area is then calculated from five arbitrarily selected locations within the aforementioned observation width of 2 μm or more, thereby obtaining the thickness of each layer.

[0060] (Method for measuring relaxation) The presence or absence of relaxation can be determined by the lattice constant calculated from the lattice diffraction pattern obtained by TEM. Strain can be represented by how much the lattice constant in the in-plane direction along the a-axis of each layer of the laser diode has changed from the original lattice constant. If the lattice constant is the same as that of the substrate, it is strain; if there is a difference, it can be said to be relaxation. Here, relaxation is defined as a relaxation of 20% or more calculated by the analysis software. The difference in lattice constants can be measured by automatically mapping the lattice diffraction pattern at each spot and displaying it as a mapping using analysis software. Specifically, the measurement can be performed using analysis software (ASTER from NanoMEGAS).

[0061] (Method for measuring atomic concentration in each layer) One method for measuring the atomic concentration in each layer constituting a laser diode is reciprocal space mapping (RSM) using X-ray diffraction (XRD). Specifically, by analyzing the reciprocal space mapping data near the diffraction peak obtained using an asymmetric plane as the diffraction plane, the lattice relaxation rate and Al composition relative to the substrate can be obtained. Examples of diffraction planes include the (10-15) plane and the (20-24) plane. Furthermore, layers and regions where sufficient reflectance cannot be obtained by XRD, such as the emissive layer, gradient layers, and hillocks formed in each layer, can be measured by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and electron energy-loss spectroscopy (EELS).

[0062] EELS analyzes the composition of a sample by measuring the energy lost when an electron beam passes through the sample. Specifically, for example, in thinned samples used for TEM observation, the energy loss spectrum of the transmitted electron beam intensity is measured and analyzed. By utilizing the fact that the peak position that appears around 20 eV of energy loss changes according to the composition of each layer, the composition can be determined from the peak position. Similar to the layer thickness calculation method using TEM observation described above, the average value of the Al composition in an observation width of 200 nm is calculated from five locations arbitrarily selected from the observation area of ​​2 μm or more, thereby obtaining the Al composition of each layer.

[0063] In EDX, characteristic X-rays generated by the electron beam in the thinned sample used for TEM observation as described above are measured and analyzed. Similar to the layer thickness calculation method using TEM observation described above, the average value of the Al composition at an observation width of 200 nm is calculated from five locations arbitrarily selected from the observation area of ​​2 μm or more, thereby obtaining the Al composition of each layer.

[0064] XPS allows for depth-direction evaluation by performing XPS measurements while sputter etching using an ion beam. While Ar+ is commonly used as the ion beam, other ion species, such as Ar cluster ions, can be used as long as they can be irradiated by the etching ion gun mounted on the XPS apparatus. The depth-direction distribution of the Al composition in each layer is obtained by measuring and analyzing the XPS peak intensities of Al, Ga, and N. Alternatively, instead of sputter etching, a laser diode can be obliquely polished so that a cross-section perpendicular to the main surface of the substrate is magnified and exposed, and the exposed cross-section can be measured with XPS.

[0065] The composition of each layer can be measured not only using XPS but also using Auger electron spectroscopy (AES). In this case, the composition can be measured by performing Auger electron spectroscopy on a cross-section exposed by sputter etching or oblique polishing. Furthermore, the composition of each layer can also be measured by SEM-EDX measurement on a cross-section exposed by oblique polishing.

[0066] (Application fields of laser diodes) The laser diodes relating to this disclosure are applicable to devices in fields such as medicine and life sciences, environmental science, industry, consumer electronics, agriculture, and other fields. The laser diodes are applicable to synthesis and decomposition equipment for pharmaceuticals or chemical substances, sterilization equipment for liquids, gases, and solids (containers, food, medical devices, etc.), cleaning equipment for semiconductors, surface modification equipment for films, glass, metals, etc., exposure equipment for the manufacture of semiconductors, FPDs (Flat Panel Displays), PCBs (Printed Wiring Boards), and other electronic products, printing and coating equipment, bonding and sealing equipment, transfer and molding equipment for films, patterns, mockups, etc., and measurement and inspection equipment for banknotes, scratches, blood, chemical substances, etc.

[0067] Examples of liquid sterilization equipment include, but are not limited to, automatic ice makers, ice trays and storage containers in refrigerators, water tanks for ice makers, freezers, ice makers, humidifiers, dehumidifiers, cold water tanks, hot water tanks and flow piping for water dispensers, stationary water purifiers, portable water purifiers, water dispensers, hot water heaters, wastewater treatment equipment, garbage disposals, toilet drain traps, washing machines, dialysis water sterilization modules, peritoneal dialysis connector sterilizers, and disaster relief water storage systems.

[0068] Examples of gaseous sterilization devices include, but are not limited to, air purifiers, air conditioners, ceiling fans, vacuum cleaners for floors or bedding, futon dryers, shoe dryers, washing machines, clothes dryers, indoor germicidal lamps, storage room ventilation systems, shoe boxes, chests of drawers, etc. Examples of solid sterilization equipment (including surface sterilization equipment) include, but are not limited to, vacuum packers, belt conveyors, hand tool sterilization equipment for medical, dental, barber, and beauty salon use, toothbrushes, toothbrush holders, chopstick cases, cosmetic pouches, drain covers, toilet bidet devices, and toilet lids.

[0069] 3. Specific Examples of Laser Diodes The laser diode of this embodiment will be described in more detail below with reference to Figures 1 and 5. The detailed configuration of each layer in each of the following embodiments is as described above.

[0070] (3.1) First Embodiment Figures 1 and 2 are schematic diagrams illustrating the laser diode 1 according to the first embodiment. Figure 1 is a plan view schematic of the laser diode 1, and Figure 2 is a cross-sectional view schematic of the laser diode 1. In Figure 1, <1-100><11-20> <0001> These indicate the crystal orientation.

[0071] As shown in Figure 2, the laser diode 1 comprises a substrate 11, a semiconductor laminate 10 disposed on the substrate 11, a first electrode 21, and a second electrode 22. The semiconductor laminate 10 has a first conductivity type cladding layer 101, an emitting layer 102, and a second conductivity type cladding layer 103. A portion of the semiconductor laminate 10 is a mesa structure 20. The resonant mirror end face ES of the mesa structure 20 (see Figure 2) is a resonator structure for optical resonance and emission, and laser light is emitted in a direction perpendicular to the resonant mirror end face ES (direction of the arrow in Figure 1). The mesa structure 20 has an inclined surface 201 on its side surface SS. The inclined surface 201 has a starting point 202 which is the upper end of the second conductivity type cladding layer 103, and an ending point 203 which is the intersection of the inclined surface 201 and the first conductivity type cladding layer 101 (see Figure 1).

[0072] (3.2) Second Embodiment Figure 3 is a schematic diagram illustrating the laser diode 2 according to the second embodiment. Figure 3 is a schematic cross-sectional view of the laser diode 2. The laser diode 2 differs from the laser diode 1 according to the first embodiment in that it includes a semiconductor laminate 10A having a mesa structure 20A whose side surface is an outwardly convex inclined surface 201A. A portion of the semiconductor laminate 10A is the mesa structure 20A. Such a laser diode 2 can reduce optical loss at the side surface SS of the mesa structure 20A (see Figure 1), and the oscillation threshold current of the laser diode 2 is improved.

[0073] (3.3) Third Embodiment Figure 4 is a schematic diagram illustrating the laser diode 3 according to the third embodiment. Figure 4 is a schematic cross-sectional view of the laser diode 3. The laser diode 3 differs from the laser diode 1 according to the first embodiment in that it further comprises a semiconductor laminate 10B having a first waveguide layer 12 and a second waveguide layer 13. The first waveguide layer 12 is disposed between a first conductive cladding layer 101 and an emitting layer 102, and the second waveguide layer 13 is disposed between a second conductive cladding layer 103 and an emitting layer 102. A part of the semiconductor laminate 10B is a mesa structure 20B. The side surface of the mesa structure 20B is an inclined surface 201B which is a flat inclined surface. Such a laser diode 3 improves the light confinement effect on the light-emitting layer 102 and improves the oscillation threshold current of the laser diode 3.

[0074] (3.4) Fourth Embodiment Figure 5 is a schematic diagram illustrating the laser diode 4 according to the fourth embodiment. Figure 5 is a schematic cross-sectional view of the laser diode 4. The laser diode 4 differs from the laser diode 1 according to the first embodiment in that it includes a semiconductor laminate 10C comprising a second conductivity type cladding layer 103C composed of a second conductivity type longitudinal conductive layer 103A and a second conductivity type transverse conductive layer 103B, and a second conductivity type contact layer 14. A portion of the semiconductor laminate 10C is a mesa structure 20C. The side surface of the mesa structure 20C is an inclined surface 201C, which is a flat inclined surface. Such a laser diode 4 improves the carrier injection efficiency into the light-emitting layer 102 and improves the oscillation threshold current of the laser diode 4.

[0075] 4. Effects The laser diode described above has the following effects. (1) The laser diode of the present disclosure comprises an Al-containing nitride semiconductor substrate and a semiconductor laminate disposed on the nitride semiconductor substrate, the semiconductor laminate having a first conductivity type cladding layer disposed on the nitride semiconductor substrate and including a first conductivity type nitride semiconductor layer, an emissive layer formed on the first conductivity type cladding layer and having a nitride semiconductor structure including one or more quantum wells, and a second conductivity type cladding layer disposed on the emissive layer and including a second conductivity type nitride semiconductor layer, at least a portion of the semiconductor laminate is a mesa structure for optical resonance and emission, and the side surface of the mesa structure is an inclined surface that slopes outward from the top surface of the mesa structure toward the first conductivity type cladding layer. This makes it possible to provide laser diodes with low oscillation threshold current and drive voltage.

[0076] (2) In the laser diode of the present disclosure, the inclined surface is preferably inclined to be convex outward or inward. This makes it possible to suppress optical loss and reduce the oscillation threshold current density.

[0077] (3) In the laser diode of the present disclosure, the inclined surface is preferably convex upward. This makes it easier to form inclined surfaces from the perspective of the number of manufacturing steps.

[0078] (4) In the laser diode of this disclosure, the length of the inclined surface is preferably 0.7 μm or more and 4.5 μm or less when viewed from above. This makes it possible to reduce optical loss, lower the oscillation threshold current, and reduce the resistance between the first and second electrodes, thereby providing a laser diode with a low drive voltage.

[0079] (5) In the laser diode of the present disclosure, it is preferable that the angle θa between the straight line connecting the start and end points of the inclined surface and the surface of the first conductivity type cladding layer is 6° or more and 30° or less. This makes it possible to provide laser diodes with low oscillation threshold current and drive voltage.

[0080] (6) In the laser diode of the present disclosure, it is preferable that the angle θb between the straight line connecting the starting point and the inflection point of the inclined surface and the straight line connecting the starting point and the ending point of the inclined surface is 0.1° or more and 5.0° or less. This makes it possible to provide laser diodes with low oscillation threshold current and drive voltage.

[0081] (7) In the laser diode of the present disclosure, the distance from the second electrode formed on the mesa structure to the side edge of the mesa structure is preferably 1.0 μm or more and 4.5 μm or less when viewed from above. This improves the accuracy of manufacturing dimensions during laser diode production and makes it possible to provide laser diodes with lower drive voltages.

[0082] (8) In the laser diode of the present disclosure, the distance between the first electrode formed on the first conductivity type cladding layer and the second electrode formed on the mesa structure is preferably 3.5 μm or more and 10.0 μm or less when viewed from above. This makes it possible to provide laser diodes with low drive voltages.

[0083] (9) In the laser diode of the present disclosure, the nitride semiconductor substrate is preferably an AlN single crystal substrate. This reduces the lattice constant difference between the substrate and the nitride semiconductor layer formed on top of the substrate. By growing the nitride semiconductor layer in a lattice-matched system, the number of threading dislocations can be reduced, and a highly stable nitride semiconductor layer can be formed.

[0084] (10) In the laser diode of the present disclosure, the semiconductor laminate preferably comprises a first waveguide layer disposed between a first conductivity type cladding layer and an emitting layer, and a second waveguide layer disposed between a second conductivity type cladding layer and an emitting layer. This improves the effect of confining light in the light-emitting layer and reduces the oscillation threshold current.

[0085] (11) In the laser diode of the present disclosure, a second-conductivity-type contact layer formed of a nitride semiconductor containing GaN and disposed on the second-conductivity-type clad layer is provided. The second-conductivity-type clad layer contains Al e Ga 1-e N (0.1 ≦ e ≦ 1), has a composition gradient in which the Al composition e decreases as it moves away from the nitride semiconductor substrate, and has a second-conductivity-type vertical conduction layer with a film thickness of less than 0.5 μm, and Al f Ga 1-f N (0 < f ≦ 1), and preferably has a second-conductivity-type lateral conduction layer. Thereby, carriers can be injected into the light-emitting layer more efficiently, and the oscillation threshold current can be reduced.

[0086] (12) In the laser diode of the present disclosure, the film thickness of the second-conductivity-type vertical conduction layer is preferably 250 nm or more and 450 nm or less. Thereby, the light confinement effect on the light-emitting layer is improved, the carrier injection effect is improved, and the oscillation threshold current of the laser diode is reduced.

[0087] (13) In the laser diode of the present disclosure, the first-conductivity-type clad layer is formed of Al a Ga 1-a N (0.6 < a ≦ 0.8), and the second-conductivity-type vertical conduction layer and the second-conductivity-type lateral conduction layer are preferably formed with complete strain with respect to the nitride semiconductor substrate. Thereby, the conductivity of carriers can be improved.

[0088] (14) In the laser diode of the present disclosure, the mesa structure is rectangular having a long side and a short side in plan view, and the long side is preferably parallel to the <1-100> direction. Thereby, the resonant mirror end face of the laser resonator is formed on an atomically flat (1-100) plane, and the resonant mirror end face can be easily formed.

Example

[0089] The laser diodes of this disclosure will be described below with reference to examples and comparative examples. However, the laser diodes of this disclosure are not limited to these examples.

[0090] <Example 1> A 550 μm thick (0001) plane AlN single crystal substrate was used as the substrate. Next, an AlN buffer layer was formed on the substrate. The AlN layer was formed to a thickness of 500 nm under conditions of 1200°C. At this time, the ratio of the supply rate of Group III element raw material gas to the supply rate of nitrogen raw material gas (V / III ratio) was set to 50. The growth rate of the AlN layer at this time was 0.5 μm / hr. Trimethylaluminum (TMAl) was used as the Al raw material, and ammonia (NH3) was used as the N raw material.

[0091] A first conductivity type cladding layer was formed on this substrate. The first conductivity type cladding layer is an n-type AlGaN layer (Al: 75%, i.e., Al) using Si as a dopant impurity. 0.75 Ga 0.25 The N layer was used. The first conductive cladding layer was formed to a thickness of 400 nm at a temperature of 1080°C, with a vacuum of 50 mbar and a V / III ratio of 4000. The growth rate of the first conductive cladding layer at this time was 0.4 μm / hr. Trimethylaluminum (TMAl) was used as the Al raw material. Triethylgallium (TEGa) was used as the Ga raw material. Ammonia (NH3) was used as the N raw material. Monosilane (SiH4) was used as the Si raw material.

[0092] Next, an n-type waveguide layer, which is the first waveguide layer, was formed on the first conductive cladding layer. The n-type waveguide layer is an AlGaN layer that does not contain dopants (Al: 63%, i.e., Al 0.63 Ga 0.37The n-type waveguide layer was formed to a thickness of 40 nm at a temperature of 1080°C, with a vacuum of 50 mbar and a V / III ratio of 4000. The growth rate of the n-type waveguide layer at this time was 0.35 μm / hr. Trimethylaluminum (TMAl) was used as the Al raw material. Triethylgallium (TEGa) was used as the Ga raw material. Ammonia (NH3) was used as the N raw material.

[0093] Next, an emissive layer was formed on the first waveguide layer. The emissive layer was formed by depositing a film to have a multiple quantum well structure in which a quantum well layer and a barrier layer are stacked in two periods. Here, the quantum well layer is an AlGaN layer with a thickness of 4.5 nm (Al: 52%, i.e., Al 0.52 Ga 0.48 The N layer was made of AlGaN (Al: 63%, i.e., Al 0.63 Ga 0.37 (N layers) The luminescent layer was formed under conditions of a vacuum of 50 mbar and a V / III ratio of 4000. The growth rate of the quantum well layer at this time was 0.18 μm / hr. The growth rate of the barrier layer was 0.15 μm / hr.

[0094] Next, a second waveguide layer, a p-type waveguide layer, was formed on the light-emitting layer. The p-type waveguide layer was an AlGaN layer that did not contain dopants (Al: 63%, i.e., Al 0.63 Ga 0.37 The N-type waveguide layer was formed to a thickness of 70 nm at a temperature of 1080°C, with a vacuum of 50 mbar and a V / III ratio of 4000. The growth rate of the p-type waveguide layer at this time was 0.35 μm / hr. Trimethylaluminum (TMAl) was used as the Al raw material, and triethylgallium (TEGa) was used as the Ga raw material.

[0095] Next, a second conductivity type cladding layer was formed on the p-type waveguide layer. The second conductivity type cladding layer is a laminated structure comprising a second conductivity type longitudinal conductive layer and a second conductivity type transverse conductive layer, and is a graded layer with a gradient Al composition ratio. The second conductivity type longitudinal conductive layer is an AlGaN layer with a thickness of 330 nm, where the Al composition is distributed in the direction away from the substrate, changing from Al=1.0 to 0.7. The second conductivity type cladding layer was formed at a temperature of 1080°C, with a vacuum of 50 mbar and a V / III ratio of 4000. The growth rate of the second conductivity type cladding layer at this time was 0.3 to 0.5 μm / hr. Trimethylaluminum (TMAl) was used as the Al raw material, and triethylgallium (TEGa) was used as the Ga raw material. The second conductivity type transverse conductive layer is an AlGaN layer with Al=0.8 and a thickness of 5 nm. At this time, the average Al composition d' in the second conductivity type cladding layer was 0.85, which was greater than the average Al composition a' in the first conductivity type cladding layer.

[0096] Next, a p-type contact layer, which is a second conductivity type contact layer, was formed on the second conductivity type cladding layer. Here, the p-type contact layer was formed from an AlGaN layer and a GaN layer. The AlGaN layer was a 30 nm thick p-type nitride semiconductor layer in which Mg was used as a dopant impurity, and the Al composition was distributed in the direction away from the substrate, changing from Al=0.7 to 0.4. The GaN layer was formed from GaN (i.e., Al:0%) with a thickness of 10 nm. The second conductive contact layer was formed at a temperature of 950°C, with a vacuum of 150 mbar and a V / III ratio of 3650. The growth rate of the second conductive contact layer at this time was 0.2 μm / hr.

[0097] As described above, a semiconductor laminate was formed on the AlN substrate. Reciprocal lattice mapping measurements were performed on this semiconductor laminate using XRD, and it was found that the semiconductor laminate underwent pseudomorphic growth without relaxation up to the second conductivity type contact layer.

[0098] The second conductivity type contact layer was further reduced in resistance by annealing the semiconductor laminate formed as described above at 700°C for more than 10 minutes in an N2 atmosphere. A mesa structure was formed by dry etching with a Cl2-containing gas using ICP, exposing the first conductivity type cladding layer. At this time, unnecessary portions of each layer of the semiconductor stack were removed by etching so that the sides of the mesa structure were inclined surfaces. The mesa structure was formed with the convex direction of the inclined surface facing outwards, and the length of the inclined surface when viewed from above was 4.6 μm. Furthermore, the inclined surface was formed such that the angle θa between the line connecting the start and end points of the inclined surface and the surface of the first conductive cladding layer was 5.0°, and the angle θb between the line connecting the start point and the inflection point and the line connecting the start point and the end point of the inclined surface was 0.3°.

[0099] The formed mesa structure had a length of 700 μm in the <1-100> direction and a length of 40 μm in the <11-20> direction on the upper surface of the mesa structure. Here, the length of the mesa structure in the <1-100> direction is the distance between the end faces of the resonator mirrors in a plan view, and the length in the <11-20> direction is the shortest distance between the sides of the mesa structure. In other words, the inclined surface of the mesa structure was formed to slope outward from the long side end of the upper surface of the mesa structure toward the first conductive cladding layer.

[0100] Multiple electrode metal regions were formed on the second conductive contact layer in the mesa structure by sequentially depositing Ni and Au in a long rectangular shape in the <1-100> direction to create a p-type second electrode. The width of the second electrode was 5 μm and the length was 700 μm. The second electrode was formed at a position where the distance from the long side end of the mesa structure was 3.5 μm when viewed from above. Furthermore, in the region where the n-type cladding layer of the mesa structure was exposed, V, Al, Ni, Ti, and Au were sequentially deposited in a long rectangular shape in the <1-100> direction to form multiple electrode metal regions, which served as the first n-type electrode. The first electrode was formed at a position where the distance between it and the second electrode was 9.1 μm when viewed from above. Finally, the first and second electrodes were annealed at 750°C for 60 seconds under a nitrogen atmosphere using an RTA (Real-Time Analysis) device.

[0101] Furthermore, by performing multiple cleavage operations parallel to the <11-20> direction within the electrode metal region, the substrate was divided into stripes, forming individual laser diodes. The length of the mesa structure in the <1-100> direction after division was 600 μm. When the laser diode obtained in this manner was subjected to current injection and current-endface emission intensity measurement, the threshold voltage was found to be 9.3V and the oscillation threshold current density was 10.0kA / cm². 2 That was the case.

[0102] <Example 2> The laser diode of Example 2 was formed in the same manner as in Example 1, except that a mesa structure was formed such that the length of the inclined surface in a top view was 2.3 μm and the angle θa was 10.0°, and the first electrode was formed at a position where the distance from the second electrode was 6.8 μm in a top view.

[0103] <Example 3> The laser diode of Example 3 was formed in the same manner as in Example 1, except that a mesa structure was formed such that the length of the inclined surface in a top view was 1.5 μm and the angle θa was 15.0°, and the first electrode was formed at a position where the distance from the second electrode was 6.0 μm in a top view.

[0104] <Example 4> The laser diode of Example 4 was formed in the same manner as in Example 1, except that a mesa structure was formed such that the length of the inclined surface in a top view was 0.8 μm and the angle θa was 26.0°, and the first electrode was formed at a position where the distance from the second electrode was 5.3 μm in a top view.

[0105] <Example 5> The laser diode of Example 5 was formed in the same manner as in Example 1, except that a mesa structure was formed such that the length of the inclined surface in a top view was 0.6 μm and the angle θa was 33.0°, and the first electrode was formed at a position where the distance from the second electrode was 5.1 μm in a top view.

[0106] <Example 6> The laser diode of Example 6 was formed in the same manner as in Example 1, except that a mesa structure was formed such that the length of the inclined surface in a top view was 1.5 μm, the angle θa was 15.0°, and the angle θb was 0.1°, and the first electrode was formed at a position where the distance between it and the second electrode was 6.0 μm in a top view.

[0107] <Example 7> The laser diode of Example 7 was formed in the same manner as in Example 6, except that a mesa structure was formed so that the angle θb was 4.0°.

[0108] <Example 8> The laser diode of Example 8 was formed in the same manner as in Example 6, except that a mesa structure was formed so that the angle θb was 6.0°.

[0109] <Example 9> The laser diode of Example 9 was formed in the same manner as in Example 6, except that a mesa structure was formed such that the angle θb was 0.3°, a second electrode was formed at a position where the distance from the long side end of the mesa structure was 1.2 μm when viewed from above, and a first electrode was formed at a position where the distance between it and the second electrode was 3.7 μm when viewed from above.

[0110] <Example 10> The laser diode of Example 10 was formed in the same manner as in Example 9, except that a second electrode was formed at a position where the distance from the long side end of the mesa structure was 2.2 μm when viewed from above, and a first electrode was formed at a position where the distance between it and the second electrode was 4.7 μm when viewed from above.

[0111] <Example 11> The laser diode of Example 11 was formed in the same manner as in Example 9, except that a second electrode was formed at a position where the distance from the long side end of the mesa structure was 4.4 μm when viewed from above, and a first electrode was formed at a position where the distance between it and the second electrode was 6.9 μm when viewed from above.

[0112] <Example 12> The laser diode of Example 12 was formed in the same manner as in Example 9, except that a second electrode was formed at a position where the distance from the long side end of the mesa structure was 5.5 μm when viewed from above, and a first electrode was formed at a position where the distance between it and the second electrode was 8.0 μm when viewed from above.

[0113] <Comparative Example 1> The laser diode of Comparative Example 1 was formed in the same manner as in Example 1, except that the side surface of the mesa structure was made vertical instead of sloped, and the first electrode was formed at a position where the distance between it and the second electrode was 4.5 μm when viewed from above.

[0114] <Comparative Example 2> A laser diode for Comparative Example 2 was formed in the same manner as in Example 1, except that the sides of the mesa structure were not sloped but formed vertical surfaces, a second electrode was formed at a position where the distance from the long side end of the mesa structure was 10.0 μm when viewed from above, and a first electrode was formed at a position where the distance between it and the second electrode was 4.5 μm when viewed from above.

[0115] [Current-Endface Emission Intensity Measurement] For the laser diodes of each of the above-described examples and comparative examples, current-endface emission intensity measurements were performed by current injection to measure the threshold voltage and oscillation threshold current density. Table 1 below shows the evaluation results for each example and comparative example.

[0116] [Table 1]

[0117] As shown in Table 1, the laser diodes of each embodiment, which have a mesa structure with a sloping surface on the side, show a reduction in both threshold voltage and oscillation threshold current density compared to the laser diodes of each comparative example, which have a mesa structure without a sloping surface on the side.

[0118] Furthermore, the laser diodes of Examples 2-12, in which the length of the inclined surface in a top view was 4.5 μm or less and the angle θa of the inclined surface was 6° or more, showed a further reduction in threshold voltage and oscillation threshold current density compared to the laser diode of Example 1. In addition, the laser diodes of Examples 2-4 and 6-12, in which the length of the inclined surface in a top view was 0.7 μm or more and the angle θa of the inclined surface was 30° or less, showed a further reduction in oscillation threshold current density. Furthermore, the laser diodes of Examples 2-4, 6, 7 and 9-12, in which the angle θb of the inclined surface was 0.1° or more and 5.0° or less, showed a further reduction in oscillation threshold current density. Furthermore, the laser diodes of Examples 2-4, 6, 7, and 9-11, in which the distance from the second electrode to the long-side end of the mesa structure was 4.5 μm or less when viewed from above, exhibited an even further reduction in oscillation threshold current density.

[0119] While embodiments of the present disclosure have been described above, these embodiments are merely illustrative examples of apparatus and methods for realizing the technical concept of the present disclosure, and the technical concept of the present disclosure does not specify the material, shape, structure, arrangement, etc., of the components. The technical concept of the present disclosure can be modified in various ways within the technical scope defined by the claims described in the claims. [Explanation of symbols]

[0120] 1,2,3,4 Laser Diodes 10 Semiconductor stacked section 10A, 10B, 10C Semiconductor stacked section 101 First conductive cladding layer 102 Emitting layer 103,103C Second conductive cladding layer 103A Second Conductive Type Longitudinal Layer 103B Second Conductive Type Transverse Conductive Layer 11 circuit boards 12. First Waveguide Layer 13. Second Waveguide Layer 14. Second conductive contact layer 20, 20A, 20B Mesa structure 201,201A Slope 202 Starting point 203 Final stop 21 1st electrode 22 2nd electrode

Claims

1. Al-containing nitride semiconductor substrate, A semiconductor laminate disposed on the nitride semiconductor substrate, Equipped with, The aforementioned semiconductor stacked portion is Disposed on the nitride semiconductor substrate, a first conductivity type cladding layer including a first conductivity type nitride semiconductor layer, A light-emitting layer of a nitride semiconductor structure, formed on the first conductive cladding layer and containing one or more quantum wells, Disposed on the light-emitting layer is a second conductivity type cladding layer including a second conductivity type nitride semiconductor layer, It has, At least a portion of the semiconductor stack is a mesa structure for optical resonance and emission. The mesa structure is rectangular in shape, having a long side and a short side, with the long side extending in the <1-100> direction. The side surface of the mesa structure is an inclined surface that is convex outward from the upper surface of the mesa structure toward the first conductive cladding layer. The mesa structure is a laser diode in which, when the starting point of the inclined surface is the point on the upper surface of the second conductive cladding layer where the angle changes from the upper surface, the inclined surface extends to the first conductive cladding layer, and the ending point of the inclined surface is the point where the angle changes upon connecting to the flat surface of the first conductive cladding layer, θa is the low-angle side of the angle that the first straight line connecting the starting point and the ending point of the inclined surface makes with the surface of the first conductive cladding layer, and θa is 6° or more and 30° or less.

2. The laser diode according to claim 1, wherein the length of the inclined surface from the upper surface to the lower surface of the mesa structure is 0.7 μm or more and 4.5 μm or less when viewed from above.

3. The laser diode according to claim 1, wherein the angle θb made by a third line, which connects the point of contact where a second line parallel to a first line connecting the start and end points of the inclined surface touches the inclined surface and the start point of the inclined surface, with the first line is 0.1° or more and 5.0° or less.

4. The laser diode according to claim 1, wherein the distance from the second electrode formed on the mesa structure to the side end of the mesa structure is 1.0 μm or more and 4.5 μm or less when viewed from above.

5. The laser diode according to claim 1, wherein the distance between the first electrode formed on the first conductive cladding layer and the second electrode formed on the mesa structure is 3.5 μm or more and 10.0 μm or less when viewed from above.

6. The laser diode according to claim 1, wherein the nitride semiconductor substrate is an AlN single crystal substrate.

7. The aforementioned semiconductor stacked portion is A first waveguide layer disposed between the first conductive cladding layer and the light-emitting layer, The laser diode according to claim 1, further comprising a second waveguide layer disposed between the second conductive cladding layer and the light-emitting layer.

8. The second conductive contact layer is disposed on the second conductive cladding layer and is made of a nitride semiconductor containing GaN, The second conductive cladding layer is Al e Ga 1-e A second conductivity type longitudinal conductive layer containing N (0.1 ≤ e ≤ 1), having a compositional gradient in which the Al composition e decreases as it moves away from the nitride semiconductor substrate, and having a film thickness of less than 0.5 μm, and Al f Ga 1-f A laser diode according to claim 1, comprising a second conductivity type transverse conduction layer containing N (0 < f ≤ 1).

9. The laser diode according to claim 8, wherein the thickness of the second conductive longitudinal conductive layer is 250 nm or more and 450 nm or less.

10. The first conductive cladding layer is Al a Ga 1-a It is formed by N (0.6 < a ≤ 0.8), The laser diode according to claim 8, wherein the second conductivity type longitudinal conduction layer and the second conductivity type transverse conduction layer are formed with perfect strain relative to the nitride semiconductor substrate.

11. The laser diode according to claim 1, wherein the length of the inclined surface from the upper surface to the lower surface of the mesa structure is 0.9 μm or more and 2.2 μm or less when viewed from above.

12. The laser diode according to claim 1, wherein the angle θa between the first straight line connecting the start and end points of the inclined surface and the surface of the first conductive cladding layer is 11° or more and 25° or less.

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