JP7656294B2 - Laser Diode - Google Patents

Description

This disclosure relates to laser diodes.

Nitride semiconductors have a direct transition recombination mode, which allows high recombination efficiency and high optical gain to be obtained, making them suitable materials for laser diodes. For example, a technology has been disclosed for oscillating a current-injection type laser diode in the ultraviolet region using a nitride semiconductor (for example, Non-Patent Document 1).

Taking into consideration the technical common sense of laser diodes, it is preferable for a laser diode that emits ultraviolet light to be configured to oscillate at a low oscillation threshold current density. Furthermore, in order to prevent deterioration of element performance due to heat, it is preferable for a laser diode that emits ultraviolet light to have oscillation characteristics at a lower driving power by reducing the element resistance. In order to reduce the element resistance and oscillate the laser diode at a low power, it is considered appropriate to configure the electrodes placed on the mesa structure as close as possible to the side of the mesa structure.

Zhang et al. , Applied Physics Express 12, 124003 (2019)

However, even if such a configuration is applied to a laser diode that emits ultraviolet light, the desired low oscillation threshold current density cannot be obtained. The reason for this is believed to be that near the side surface of the mesa structure, dislocations introduced from the side surface of the mesa structure reach the light-emitting layer directly below the electrode provided on the mesa structure, resulting in the generation of a non-light-emitting region that inhibits optical gain.
SUMMARY OF THE DISCLOSURE In view of the above circumstances, an object of the present disclosure is to provide a laser diode that achieves a lower element resistance.

In order to solve the above-mentioned problems, a laser diode according to an embodiment of the present disclosure includes a nitride semiconductor substrate containing Al, and a semiconductor laminate portion disposed on the nitride semiconductor substrate, the semiconductor laminate portion having: a first conductivity type cladding layer including a nitride semiconductor layer of a first conductivity type disposed on the nitride semiconductor substrate; a light emitting layer formed of a nitride semiconductor including one or more quantum wells disposed on the first conductivity type cladding layer; and a second conductivity type cladding layer including a nitride semiconductor of a second conductivity type disposed on the light emitting layer, at least a portion of the semiconductor laminate portion is a mesa structure formed for optical resonance and emission, and a side surface of the mesa structure that is not perpendicular to the optical resonance direction has a plurality of convex portions in a planar view.
It should be noted that the above summary of the invention does not list all of the features of the present disclosure.

According to one aspect of the present disclosure, a laser diode with a low oscillation threshold current can be provided.

1 is a schematic plan view illustrating a configuration example of a laser diode according to an embodiment of the present disclosure. 1 is a schematic cross-sectional view illustrating a configuration example of a laser diode according to an embodiment of the present disclosure. 4 is a schematic cross-sectional view showing a configuration example of a protrusion of a laser diode according to an embodiment of the present disclosure. FIG. 4 is a schematic cross-sectional view showing a configuration example of a protrusion of a laser diode according to an embodiment of the present disclosure. FIG. 4 is a schematic cross-sectional view showing a configuration example of a protrusion of a laser diode according to an embodiment of the present disclosure. FIG. 10 is a schematic cross-sectional view illustrating another configuration example of a laser diode according to an embodiment of the present disclosure. FIG. 10 is a schematic cross-sectional view illustrating another configuration example of a laser diode according to an embodiment of the present disclosure. FIG. 10 is a schematic cross-sectional view illustrating another configuration example of a laser diode according to an embodiment of the present disclosure. FIG. 10 is a schematic cross-sectional view illustrating another configuration example of a laser diode according to an embodiment of the present disclosure. FIG. 10 is a schematic cross-sectional view illustrating another configuration example of a laser diode according to an embodiment of the present disclosure. FIG.

The laser diode according to the present disclosure will be described below through embodiments, but the following embodiments do not limit the invention according to the claims. Furthermore, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.

For the sake of convenience, hereinafter, "top" refers to the second electrode side depicted in the drawings, and "bottom" refers to the substrate side depicted in the drawings. However, "top" and "bottom" are merely defined for convenience and should not be interpreted in a restrictive manner.

1. Embodiment A laser diode according to an embodiment of the present disclosure will be described.

(1.1) Structure of the ultraviolet laser diode The laser diode according to the present disclosure has the following features:
The semiconductor laminate comprises a nitride semiconductor substrate containing Al, and a semiconductor laminate portion disposed on the nitride semiconductor substrate, the semiconductor laminate portion having: a first conductivity type cladding layer including a nitride semiconductor layer of a first conductivity type disposed on the nitride semiconductor substrate; a light emitting layer formed of a nitride semiconductor including one or more quantum wells disposed on the first conductivity type cladding layer; and a second conductivity type cladding layer including a nitride semiconductor of a second conductivity type disposed on the light emitting layer, at least a portion of the semiconductor laminate portion is a mesa structure formed for optical resonance and emission, and a side of the mesa structure that is not perpendicular to the optical resonance direction has a plurality of convex portions in a planar view.

The reason why the laser diode according to the present disclosure can achieve low element resistance is believed to be that the side surfaces of the mesa structure that are not perpendicular to the optical resonance direction (side surfaces that are not the resonator mirror end faces) have multiple convex portions, which suppresses dislocations introduced from the side surfaces of the mesa structure and reduces non-light-emitting regions that inhibit optical gain compared to a configuration that does not have such convex portions.
Each component of the ultraviolet laser diode according to the present disclosure will be described below.

<Nitride Semiconductor Substrate>
The nitride semiconductor substrate (hereinafter, sometimes referred to as substrate) includes a nitride semiconductor containing Al. The nitride semiconductor containing Al is, for example, AlN. That is, the substrate is preferably an AlN single crystal substrate. The nitride semiconductor containing Al is not limited to AlN, and may be, for example, AlGaN. For example, when the substrate is a nitride semiconductor single crystal substrate such as AlN or AlGaN, the difference in lattice constant with the nitride semiconductor layer formed on the substrate becomes small, and the nitride semiconductor layer is grown in a lattice-matched system, thereby reducing threading dislocations.
The threading dislocation density of the substrate is preferably 5×10 ⁴ cm ⁻² or less. In particular, from the viewpoint of reducing the oscillation threshold current, the threading dislocation density is more preferably 1×10 ³ to 1×10 ⁴ cm ⁻² .

Here, the "comprises" in the expression "comprises a nitride semiconductor" means that the layer mainly contains a nitride semiconductor, but this expression also includes cases where other elements are included. Specifically, this expression also includes cases where the composition of this layer is slightly changed by adding a small amount of an element other than the nitride semiconductor (for example, a few percent or less of elements such as Ga (when Ga is not the main element), In, As, P, or Sb). In expressions regarding the composition of other layers, the wording "comprises" has a similar meaning. Furthermore, the small amount of elements contained is not limited to the above.

The substrate may be doped n-type or p-type with a donor impurity or an acceptor impurity, or may be a mixed crystal of a nitride semiconductor such as AlN and sapphire ( _Al2O3 ), Si, _SiC , _MgO , _Ga2O3 , ZnO, GaN, or InN.

As an example, the substrate preferably has a layer thickness of 100 μm or more and 600 μm or less.
The surface orientation may be c-plane (0001), a-plane (11-20), m-plane (10-10), etc., with a c-plane (0001) substrate being more preferred. Furthermore, the substrate may be formed on a surface tilted at some angle (for example, −4° to 4°, preferably −0.4° to 0.4°) from the normal direction of the c-plane (0001), but is not limited thereto.

<First Conductive Type Cladding Layer>
The first conductive cladding layer is formed on the substrate. Here, for example, the word "on" in the expression "the first conductive cladding layer is formed on the substrate" means that the first conductive cladding layer is formed on one side of the substrate. The above expression also includes the case where another layer is present between the substrate and the first conductive cladding layer. The word "on" has the same meaning in the relationship between other layers. For example, the expression "the second conductive cladding layer is formed on the first conductive waveguide layer" also includes the case where the second conductive cladding layer is formed on the first conductive waveguide layer via an electron block layer. In the description of this embodiment, the "first conductive type" and the "second conductive type" mean semiconductors having different conductive types, and for example, when one is n-type conductive, the other is p-type conductive.

The first conductive cladding layer in the laser diode of this embodiment is a layer of a nitride semiconductor containing Al and Ga. The first conductive cladding layer is formed of, for example, Al _a Ga _(1-a) N (0<a<1). This makes it possible to enhance the crystallinity of the light emitting layer and improve the light emitting efficiency when a material corresponding to the band gap energy of the deep ultraviolet region is formed as the light emitting layer. From the viewpoint of realizing high light emitting efficiency, the nitride semiconductor constituting the first conductive cladding layer is preferably a mixed crystal of AlN and GaN. Moreover, from the viewpoint of growing with complete distortion on the substrate, it is more preferable that the first conductive cladding layer is formed of Al _a Ga _(1-a) N (0.6≦a≦0.8).

The first conductive type cladding layer may be doped with an impurity. Examples of the impurity include Si, Ge, C, and O. Any one of these impurities may be selected as the impurity, or a combination of these impurities may be selected. The concentration of the impurity is not particularly limited, but is preferably 1×10 ¹⁷ cm ⁻³ or more and 1×10 ²⁰ cm ⁻³ or less, and more preferably 5×10 ¹⁸ cm ⁻³ or more and 5×10 ¹⁹ cm ⁻³ or less.

From the viewpoint of lattice relaxation within the first conductivity type cladding layer and from the viewpoint of film resistance, the first conductivity type cladding 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 even more preferably has a layer thickness of 300 nm or more and 500 nm or less.

<Light-emitting layer>
The light emitting layer is provided above the first conductive type cladding layer. The light emitting layer preferably has an emission wavelength of 210 nm or more and 360 nm or less.
The light emitting layer has one or more quantum well layers. In the quantum well layer, a material may be appropriately selected depending on a desired emission wavelength. For the purpose of obtaining an emission wavelength in the ultraviolet region, a material used for the quantum well layer may be, for example, Al _b Ga _1-b N (0≦b≦0.8).

The light-emitting layer has a structure in which the quantum well layer is sandwiched between layers formed of a material different from that of the quantum well layer, either above or below the quantum well layer, or both above and below the quantum well layer. Examples of layers formed of a material different from that of the quantum well layer include an _{AlcGa1 -cN} (0.5≦c<0.7) layer having a thickness of 1 nm to 20 nm.
The light-emitting layer may be partially doped with impurities such as Si and C. The light-emitting layer may have a structure in which no impurities are intentionally doped (an undoped layer).

<Waveguide layer>
From the viewpoint of light confinement, the laser diode of this embodiment may include a first-conductivity-type waveguide layer disposed between the first-conductivity-type cladding layer and the light-emitting layer to confine light in the light-emitting layer, and a second-conductivity-type waveguide layer disposed between the second-conductivity-type cladding layer and the light-emitting layer to confine light in the light-emitting layer.
As a result, the waveguide layers are formed above and below the light emitting layer so as to sandwich the light emitting layer, and have the effect of confining the light emitted from the light emitting layer within the light emitting layer. That is, the waveguide layer is preferably composed of two layers, a first-conductivity-type waveguide layer arranged on the first-conductivity-type cladding layer side of the light emitting layer, and a second-conductivity-type waveguide layer arranged on the second-conductivity-type cladding layer side of the light emitting layer.

From the viewpoint of light confinement, the waveguide layer is preferably a nitride semiconductor containing Al and Ga, which has a band gap with a higher energy than the light emitting layer. The waveguide layer preferably has an Al composition and film thickness that increases the electric field strength distribution of the light present in the device and the overlap of the light emitting layer. In particular, the film thickness ratio of the first conductivity type waveguide layer and the second conductivity type waveguide layer can vary depending on the light confinement to the light emitting layer and the Al composition of the AlGaN in the first conductivity type cladding layer and the second conductivity type cladding layer.

From the viewpoint of carrier confinement in the light emitting layer, when the light emitting layer is Al _b Ga _(1-b) N (0<b<1) and the waveguide layer is Al _d Ga _(1-d) N (0<d<1), it is more preferable that b<d and d≧b+0.05. For example, in the case of a light emitting layer having an emission wavelength of 265 nm, it is preferable that b=0.52 and d is 0.57 or more. Also, from the viewpoint of light confinement and layer resistance, it is preferable that the total film thickness of the first conductivity type waveguide layer and the second conductivity type waveguide layer is 70 nm or more and 150 nm or less.

The Al composition of each of the first conductivity type waveguide layer and the second conductivity type waveguide layer is preferably uniform in the film thickness direction, but is not limited to this. In order to avoid light absorption by the metal (e.g., the second electrode) present above the second conductivity type contact described below, the Al composition of the second conductivity type waveguide layer may be higher than the Al composition of the first conductivity type waveguide layer. For the same purpose, the film thickness of the second conductivity type waveguide layer may be thicker than the film thickness of the first conductivity type waveguide.

When the first conductive type waveguide layer is an n-type conductive semiconductor layer, impurities such as P, As, Sb, and other Group V elements other than N, H, C, O, F, Mg, and Si may be mixed in addition to N in order to obtain the same conductivity type as the first conductive type cladding layer, but this is not limited to the above.

<Second Conductive Type Cladding Layer>
The second-conductivity-type cladding layer is provided on the light-emitting layer. The second-conductivity-type cladding layer has a second-conductivity-type nitride semiconductor layer. The second-conductivity-type cladding layer is formed of, for example, Al _e Ga _1-e N (0<e<1). In addition, when a waveguide layer (second-conductivity-type waveguide layer) is provided on the light-emitting layer, the second-conductivity-type cladding layer is formed in the shape of the waveguide layer (second-conductivity-type waveguide layer). This allows the second-conductivity-type cladding layer to easily lattice-match with the light-emitting layer or the waveguide layer, and makes it possible to suppress threading dislocation density.

The second conductive cladding layer is not particularly limited in its conductive type as long as it has sufficient conductivity to inject carriers (electrons or holes) into the light emitting layer and can increase the overlap between the electric field intensity distribution of the optical mode existing in the device and the light emitting layer (i.e., increase the optical confinement). The second conductive cladding layer may be, for example, p-type AlGaN partially doped with Mg.
When the second conductivity type cladding layer is a p-type conductivity semiconductor layer, impurities such as Group V elements other than N, such as P, As, and Sb, and impurities such as C, H, F, O, Mg, and Si may be mixed in, but the types of impurity elements are not limited to these.

It is more preferable that the second conductivity type cladding layer is entirely or partially free of second conductivity type impurities such as Mg, B, Be, Si, and Li. By not having the second conductivity type impurities in the second conductivity type cladding layer, the oscillation threshold current density of the ultraviolet laser diode can be made lower than that of conventional ultraviolet laser diodes.

From the viewpoint of more efficiently injecting carriers into the light-emitting layer, the second-conductivity-type cladding layer preferably comprises a second-conductivity-type vertical conduction layer which is a compositionally graded layer formed of Al _f Ga _(1-f) N in which the Al composition f decreases in the direction away from the upper surface of the substrate, and a second-conductivity-type horizontal conduction layer containing Al _g Ga _(1-g) N (0<g≦1).
The second conductive type vertical conduction layer and the second conductive type lateral conduction layer will be described below.

(Second Conductive Type Vertical Conductive Layer)
The second-conductivity-type vertical conduction layer is a layer constituting a region of the second-conductivity-type cladding layer on the light-emitting layer side. The second-conductivity-type vertical conduction layer is a layer containing Al _f Ga _1-f N. The second-conductivity-type vertical conduction layer may be a layer in which the Al composition ratio continuously decreases from 1.0 in a direction away from the interface between the light-emitting layer and the second-conductivity-type cladding layer. The second-conductivity-type cladding layer has a gradient in Al composition, thereby serving as a light confinement layer, a carrier injection layer into the light-emitting layer, and an electron blocking layer, and thereby obtaining an effect of lowering the oscillation threshold current density of the laser diode.

The profile (gradient) of the Al composition f in the second conductive type vertical conductive layer may decrease continuously or may decrease intermittently. Here, "intermittently decreasing" means that the second conductive type vertical conductive layer includes a portion in which the Al composition f is the same (constant in the film thickness direction). In other words, the second conductive type vertical conductive layer may include a portion in which the Al composition f does not decrease in the direction away from the substrate, but does not include a portion in which it increases. By including such a composition gradient layer in the second conductive type cladding layer, it plays a role in light confinement, carrier injection into the light emitting layer, and as an electron blocking layer, and is expected to have the effect of lowering the oscillation threshold current density of the ultraviolet laser diode 100.

From the viewpoint of lattice matching, the thickness of the second conductive vertical layer is preferably 500 nm or less. From the viewpoint of light confinement and carrier injection into the light emitting layer, the thickness of the second conductive vertical layer is more preferably 250 nm to 450 nm, and even more preferably 300 nm to 400 nm.

(Second Conductive Type Lateral Conductive Layer)
The second-conductivity-type transverse conduction layer is a layer constituting a region of the second-conductivity-type cladding layer opposite to the light-emitting layer, and is formed on the second-conductivity-type vertical conduction layer. The second-conductivity-type transverse conduction layer is a layer containing Al _g Ga _1-g N (0<g≦1). Here, the Al composition g of the second-conductivity-type transverse conduction layer at the surface facing the second-conductivity-type vertical conduction layer is preferably larger than the minimum value of the Al composition f of the second-conductivity-type vertical conduction layer.
The second conductivity type transverse conduction layer may be intentionally doped with impurities such as H, Mg, Be, Zn, Si, and B for the purpose of controlling the longitudinal resistivity of the second conductivity type transverse conduction layer. The amount of the doped impurity may be, for example, 1×10 ¹⁹ cm ⁻³ to 5×10 ²¹ cm ⁻³ depending on the amount of net electric field induced on the surface and inside of the second conductivity type transverse conduction layer.

The thickness of the second conductive 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 quantum transmission of carriers through the second conductive type lateral conduction layer.

When a second-conductivity-type contact layer (described later) is provided on the second-conductivity-type lateral conduction layer, it is preferable that the Al composition at the interface between the second-conductivity-type lateral conduction layer and the second-conductivity-type contact layer is smaller than the Al composition in the second-conductivity-type contact layer and is completely distorted with respect to the substrate. Such a second-conductivity-type lateral conduction layer can improve the lateral conductivity by inducing holes at the interface as the net internal electric field accumulated on the surface and inside near the surface of the second-conductivity-type lateral conduction layer becomes negative.

The second-conductivity-type vertical conductive layer generates carriers (for example, holes when the second-conductivity-type vertical conductive layer is made of a p-type semiconductor) by the polarization doping effect, and has the effect of efficiently injecting the carriers into the active layer in the light-emitting layer. Therefore, by providing the second-conductivity-type vertical conductive layer between the light-emitting layer and the second-conductivity-type contact layer, the carrier injection efficiency of the laser diode can be increased and the threshold voltage can be reduced.
The second-conductivity-type lateral conduction layer has an effect of spreading in the lateral direction the hole distribution narrowed by the electric field concentrated under the electrode, and this effect allows the second-conductivity-type lateral conduction layer to increase the carrier injection efficiency in the same way as the second-conductivity-type vertical conduction layer.

(Middle class)
Between the second-conductivity-type vertical conduction layer and the second-conductivity-type waveguide layer, an intermediate layer made of Al h Ga (1-h) N (0<h≦1.0) in which the Al composition h increases in the direction away from the upper surface of the substrate may be provided from the viewpoint of improving the conductivity and/or forming the _{second -conductivity-type} lateral conduction layer and the second-conductivity-type contact layer with perfect distortion. The intermediate layer between the second-conductivity-type vertical conduction layer and the second-conductivity-type waveguide layer may be a non-bandgap mixed crystal that does not absorb light of the desired emission wavelength, and further preferably has a thickness of 50 nm or less, and may be undoped.

<Second Conductive Type Contact Layer>
The laser diode of this embodiment may further include a second-conductivity-type contact layer disposed on the second-conductivity-type cladding layer. The nitride semiconductor constituting the second-conductivity-type contact layer is preferably formed of, for example, GaN, AlN, or InN, or a mixed crystal containing these, and more preferably is a nitride semiconductor containing GaN.

The second conductive type contact layer may contain impurities such as P, As, Sb, and other V group elements other than N, C, H, F, O, Mg, Si, and Be. In view of the versatility of the source gas, the impurity contained in the second conductive type contact layer is preferably Mg. In view of reducing the contact resistance, the concentration of Mg is preferably 8×10 ¹⁹ cm ^-3 or more and 5×10 ²¹ cm ^-3 or less, and more preferably 5×10 ²⁰ cm ^-3 or more and 5×10 ²¹ cm ^-3 or less.

The thickness of the second conductive contact layer is preferably 1 nm or more and 20 nm or less. The thinner the second conductive contact layer, the higher the carrier injection efficiency of the laser diode, and the thicker the layer, the lower the carrier injection efficiency.

<First Electrode>
The first electrode is provided on the first conductive clad layer. The first electrode is preferably made of a first conductive material that is electrically connected to the first conductive clad layer. Examples of the first conductive material include an alloy containing any of Mo, Nb, Zr, Ti, Al, W, Ni, Au, V, and In. By using an alloy containing any of V, Ti, Al, Ni, and Au as the first conductive material, the contact resistance can be reduced.

<Second Electrode>
The second electrode is provided on the second conductive clad layer. The second electrode is preferably formed of a second conductive material electrically connected to the second conductive clad layer. Examples of the second conductive material include Ti, Pt, Ni, Pd, Au, Pt, Co, Cu, Ni, Ta, and Ru, or an alloy containing any of them. By using an alloy containing Ti, Pt, Ni, Au, and Pd as the second conductive material, the contact resistance can be reduced.

<Mesa structure and resonator>
The mesa structure is formed to electrically separate the second conductive type layer from the first conductive type layer. The mesa structure is a structure in which a part of the semiconductor laminate is removed. Here, the "semiconductor laminate" refers to at least the first conductive type cladding layer, the light emitting layer, and the second conductive type cladding layer, and also includes the above-mentioned waveguide layers (the first conductive type waveguide layer and the second conductive type waveguide layer), the intermediate layer, and the second conductive type contact layer, if these layers are provided.

The mesa structure desirably has a cavity parallel to the (1-100) crystal orientation of the substrate in plan view, and is configured to produce an edge-emitting laser diode that emits light in the <1-100> direction. This is because, when obtaining the resonator mirror end faces of the laser cavity by various methods such as cleavage or etching, the atomically flat (1-100) plane is the easiest to form the resonator mirror end faces. In other words, it is preferable for the mesa structure to have a cavity in which the resonator mirror end faces are mesa end faces that are parallel to the (1-100) plane in plan view relative to the crystal orientation of the substrate, and to be an edge-emitting laser diode that emits light in the <1-100> direction.

The mesa structure can be formed by etching the nitride semiconductor layer stack using inductively coupled plasma (ICP) etching or the like. Depending on the etching conditions, a large number of dislocations are generated from the side of the mesa structure and propagate toward the inside of the mesa structure. This is because the nitride semiconductor layer stack grows in a pseudomorphic manner, lattice-matched to the substrate, and strain accumulates in the nitride semiconductor stack. The strain is partially released in the device process, including the formation of the mesa structure, but the release of strain is accompanied by the generation of dislocations in the mesa structure. The dislocation lines that are generated typically start from the side of the mesa structure.

If the range in which dislocations occur in the mesa structure reaches the bottom of the second electrode, the oscillation threshold current is significantly deteriorated. For this reason, it is preferable to provide the second electrode inside the mesa structure more than the maximum range in which dislocations occur near the side surface of the mesa structure. This prevents dislocations that occur as the mesa structure is released from reaching the light emitting layer directly below the second electrode, and allows the oscillation threshold current density to reach the best level that the structure is designed to have.
The element resistance of the laser diode is determined by the maximum distance D between the second electrode and the side surface of the mesa structure. From the viewpoint of reducing the element resistance, it is preferable to achieve a smaller D. Therefore, it is required to suppress the maximum range of dislocations generated in the vicinity of the side surface of the mesa structure.

The applicants' intensive study revealed that it is possible to suppress the maximum range of dislocations occurring near the side surface of the mesa structure by defining the shape of the mesa structure in a plan view. The mesa structure is preferably rectangular in plan view having long and short sides, the long sides being parallel to the <1-100> direction, and the convex portion preferably protruding in at least one of the <11-20> direction and the <-1-120> direction in a plan view. In other words, it is preferable that the convex portion is provided on a side surface other than the surface on which the resonator mirror end surface of the mesa structure is formed. By providing the convex portion, it is possible to concentrate the distortion stress of the mesa structure at the tip of the convex portion. Therefore, by intentionally controlling the origin of dislocations introduced into the mesa structure in the stress release process during the formation of the mesa structure, it is possible to efficiently cause dislocation annihilation and suppress the spread of the generated dislocations.

From the viewpoint of achieving sufficient stress concentration at the top of the convex portion and from the viewpoint of efficiently generating dislocation annihilation, the height of the convex portion in a plan view is preferably 1 μm or more and 11 μm or less, and more preferably 1 μm or more and 3.5 μm or less. When the height of the convex portion is within the above-mentioned range, it is possible to suppress an expansion of the maximum range of dislocations due to a decrease in the frequency of dislocation meetings caused by a small unevenness on the side surface of the convex portion resulting in insufficient stress concentration or a large distance from the bottom surface to the top of the convex portion.

In addition, from the viewpoint of achieving sufficient stress concentration at the apex of the convex portion, it is preferable that the minimum angle between the line segments connecting the base and apex of the convex portion in a planar view (hereinafter sometimes referred to as the angle of the apex of the convex portion) is 1 degree or more and 130 degrees or less, and more preferably is greater than 1 degree and 120 degrees or less. When the minimum angle of the apex of the convex portion is within the above-mentioned range, the convex portion is less likely to be damaged, and sufficient stress concentration at the apex of the convex portion can be achieved by suppressing insufficient stress concentration due to minute unevenness on the side surface of the convex portion.

Furthermore, from the viewpoint of suppressing dislocation propagation toward the inside of the mesa, the distance between the tops of the multiple protrusions, which are the starting points for dislocation generation, is preferably 1 μm or more and 10 μm or less, and more preferably 1 μm or more and 6 μm or less. When the distance between the tops of the protrusions is within the above-mentioned range, the protrusions can be easily formed, and the generated dislocations can be efficiently united to suppress dislocation propagation toward the inside of the mesa.

(1.2) Manufacturing Method of Laser Diode The laser diode of this embodiment is manufactured through the steps of forming each layer on a substrate.

(Formation of the Substrate)
The substrate is formed by a general substrate growth method such as a vapor phase growth method, such as a sublimation method or a hydride vapor phase epitaxy (HVPE) method, or a liquid phase growth method.

(Formation of Semiconductor Stack)
Each layer of the semiconductor laminate formed on the substrate can be formed by, for example, molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), metal organic chemical vapor deposition (MOCVD), or the like.
Here, among the layers formed on the substrate, the nitride semiconductor layer can be formed using, for example, an Al source containing trimethylaluminum (TMAl), a Ga source containing trimethylgallium (TMGa) or triethylgallium (TEGa), or the like, or a N source containing ammonia (NH ₃ ).
Laser diodes are manufactured through a process of removing unnecessary portions of each layer of a semiconductor laminate formed on a substrate by etching. The unnecessary portions of each layer of the semiconductor laminate can be removed by, for example, inductively coupled plasma (ICP) etching.

(Formation of electrodes)
The laser diode may be manufactured through a process of forming electrodes. The electrodes, such as the first electrode and the second electrode, may be formed by, for example, resistance heating deposition, electron gun deposition, or sputtering, but are not limited to these methods. The electrodes may be formed as a single layer or may be formed by laminating multiple layers. In addition, the electrodes may be heat-treated in an oxygen, nitrogen, or air atmosphere after the layers are formed.
Finally, the substrate on which the layers have been formed through the above-mentioned steps is divided into individual pieces by dicing, thereby manufacturing the laser diodes.

2. Specific Examples of Laser Diodes Next, the laser diodes of the present embodiment will be described in more detail with reference to the drawings. Note that the detailed configuration of each layer in each of the following specific examples is as described above.

(3.1) First embodiment FIG. 1A is a schematic plan view showing the planar configuration of a laser diode 1 according to a first embodiment, and FIG. 1B is a schematic cross-sectional view showing the cross-sectional configuration of a plane perpendicular to the optical resonance direction of the laser diode 1 shown in FIG. 1A.
In FIG. 1A and FIG. 1B, <1-100>, <11-20>, <0001>, and <-1-120> respectively indicate crystal orientations.

1A, the laser diode 1 of this embodiment includes a substrate 10 containing Al, and a semiconductor laminate portion 20 disposed on the substrate 10. The laser diode 1 also includes a first electrode and a second electrode (not shown).
The semiconductor laminate 20 has a first conductivity type cladding layer 21 including a first conductivity type nitride semiconductor layer, a light emitting layer 22 formed of a nitride semiconductor including one or more quantum wells, and a second conductivity type cladding layer 23 including a second conductivity type nitride semiconductor layer. A part of the first conductivity type cladding layer 21, the light emitting layer 22, and the second conductivity type cladding layer 23 form a mesa structure 201 for optical resonance and emission.

As shown in FIG. 1A, among the side surfaces of the mesa structure 201, the side surface parallel to the optical resonance direction D1 has multiple convex portions in a planar view. The multiple convex portions may be provided on all side surfaces that are not perpendicular to the optical resonance direction D1 as shown in FIG. 1A, or may be provided on one side surface. In addition, the mesa structure 201 may have multiple convex portions over the entire region of the side surface, or may have convex portions only in a partial region.

The multiple protrusions will be described in detail with reference to FIGS. 2A to 2C.
2A to 2C are schematic plan views showing examples of the shapes of multiple protrusions, which show the distance W between the apexes of the protrusions, the height H of the protrusions (the distance from the base of the protrusions to the apex of the protrusions), and the minimum angle φ formed by a line segment connecting the base of the side surface to the apex of the protrusion.
The pattern of the multiple protrusions may be a continuous arrangement of triangular protrusions in a plan view as shown in Fig. 2A, a convex portion having an arc-shaped top in a plan view as shown in Fig. 2B, or a triangular protrusion portion arranged at intervals in a plan view as shown in Fig. 2C. From the viewpoint of achieving sufficient stress concentration at the top of the protrusions, it may be preferable that the top of the protrusions is not flat, as is common to Figs. 2A to 2C.

(3.2) Second embodiment Fig. 3 is a schematic diagram for explaining a laser diode 2 according to a second embodiment. Fig. 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 the laser diode 2 has a semiconductor laminate portion 20A further including a first conductivity type waveguide layer 24 and a second conductivity type waveguide layer 25 .
In such a laser diode 2, the effect of confining light in the light emitting layer 22 is improved, and the emission intensity is improved.

(3.3) Third embodiment Fig. 4 is a schematic diagram for explaining a laser diode 3 according to a third embodiment. Fig. 4 is a schematic cross-sectional view of the laser diode 3.
The laser diode 3 differs from the laser diode 2 according to the second embodiment in that it has a semiconductor laminate portion 20B including a second conductivity type contact layer 26 disposed on a second conductivity type cladding layer 23.
In such a laser diode 3, the efficiency of carrier injection into the light emitting layer 22 is improved, and the light emission efficiency is improved.

The second conductive contact layer 26 may be combined with, for example, the laser diode 2 according to the second embodiment. That is, the laser diode may have the second conductive contact layer 26 disposed on the second conductive cladding layer 23 of the laser diode 2 (not shown).

(3.4) Fourth embodiment Fig. 5 is a schematic diagram for explaining a laser diode 4 according to a fourth embodiment. Fig. 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 has a semiconductor laminate portion 20C including a second conductivity type cladding layer 23 composed of a second conductivity type vertical conduction layer 23A and a second conductivity type horizontal conduction layer 23B, and a second conductivity type contact layer 26.
In such a laser diode 4, the efficiency of carrier injection into the light emitting layer 22 is improved, and the light emission efficiency is improved.

(3.5) Fifth embodiment Fig. 6 is a schematic diagram for explaining a laser diode 5 according to a fifth embodiment. Fig. 6 is a schematic cross-sectional view of the laser diode 5.
The laser diode 5 differs from the laser diode 1 according to the first embodiment in that it has a semiconductor laminate portion 20D including a first conductivity type waveguide layer 24 and a second conductivity type waveguide layer 25, a second conductivity type cladding layer composed of a second conductivity type vertical conductive layer 23A and a second conductivity type lateral conductive layer 23B, and an intermediate layer 27 provided between the second conductivity type vertical conductive layer 23A and the second conductivity type waveguide layer 25.
In the laser diode 5, the carrier conductivity in the intermediate layer 27 is improved, and the second conductive type lateral conduction layer 23B and the second conductive type contact layer 26 are formed with perfect strain, thereby improving the carrier lateral conduction.

(3.6) Sixth embodiment Fig. 7 is a schematic diagram for explaining a laser diode 6 according to a sixth embodiment. Fig. 7 is a schematic cross-sectional view of the laser diode 6.
The laser diode 6 further includes a first conductivity type waveguide layer 24, a second conductivity type waveguide layer 25, and a second conductivity type contact layer 26. The mesa structure of the laser diode 6 is a two-stage structure, and includes a semiconductor laminate portion 20E including a mesa structure 201 including a first conductivity type cladding layer 21 and a mesa structure 203 not including the first conductivity type cladding layer 21, which is different from the laser diode 1 according to the first embodiment.

4. Effects The laser diode of the present disclosure described above has the following effects.
(1) The laser diode has a mesa structure for optical resonance and emission formed in at least a part of a semiconductor laminate, and has a plurality of protrusions on a side surface that is not perpendicular to the optical resonance direction.
This makes it possible to concentrate the strain stress of the mesa structure at the tip of the protrusion, thereby controlling the starting point of dislocations introduced into the mesa structure as a result of the stress release process during the formation of the mesa structure.

(2) In the laser diode, the distance between the tops of the multiple protrusions in a plan view is preferably not less than 1 μm and not more than 10 μm.
This makes it possible to efficiently merge the generated dislocations and suppress the propagation of the dislocations toward the inside of the mesa.
(3) In the laser diode, it is preferable that the height of the protrusions in a plan view is not less than 1 μm and not more than 11 μm.
This reduces the frequency of association of the generated dislocations, thereby suppressing the expansion of the maximum range of dislocations.

(4) In the laser diode, the minimum angle between the line segments connecting the bottom and the top of the convex portion in a plan view is equal to or greater than 1 degree and equal to or less than 120 degrees.
This allows sufficient stress concentration to be achieved at the top of the protrusion.
(5) In the laser diode, the mesa structure has a resonator in which end faces parallel to the (1-100) crystal orientation plane of the nitride semiconductor substrate serve as resonator mirror end faces, and the laser diode is preferably an edge-emitting ultraviolet laser diode that emits light in the <1-100> direction.
This makes it possible to easily form the resonator mirror end faces of the laser resonator into atomically flat (1-100) faces.

(6) In the laser diode, it is preferable that the mesa structure is rectangular having long sides and short sides in a plan view, the long sides are parallel to the <1-100> direction, and the convex portion protrudes in at least one of the <11-20> direction and the <-1-120> direction in the plan view.
This makes it possible to provide a convex portion that can suppress the maximum range of dislocations by concentrating the strain stress of the mesa structure on the side surface other than the surface on which the resonator mirror end face is formed.
(7) In the laser diode, the nitride semiconductor substrate is preferably an AlN single crystal substrate.
This reduces the difference in lattice constant with the nitride semiconductor layer formed above the substrate, and the nitride semiconductor layer can be grown in a lattice-matched system to reduce threading dislocations.

(8) In the laser diode, it is preferable to provide a first conductivity type waveguide layer and a second conductivity type waveguide layer above and below the light emitting layer, which confine light in the light emitting layer.
This improves the effect of confining light in the light-emitting layer, thereby improving the luminous efficiency.

(9) The laser diode preferably further comprises a second-conductivity-type contact layer containing GaN on the second-conductivity-type cladding layer, the second-conductivity-type cladding layer containing Al _f Ga _1-f N (0.1≦f≦1) and having a composition gradient in which the Al composition f decreases with increasing distance from the nitride semiconductor substrate, the second-conductivity-type vertical conduction layer having a film thickness of less than 0.5 μm, and the second-conductivity-type lateral conduction layer containing Al _g Ga _1-g N (0<g≦1).
This allows carriers to be more efficiently injected into the light emitting layer, improving the light emitting efficiency.

(10) The laser diode preferably comprises an intermediate layer disposed between the second conductivity type vertical conducting layer and the second conductivity type waveguide layer and made of Al _h Ga _1-h N (0<h≦1.0).
This allows the second conductive type lateral conduction layer and the second conductive type contact layer to be formed with perfect strain, thereby improving the carrier conductivity.
(11) In the laser diode, the thickness of the second conductivity type vertical conducting layer is preferably 250 nm or more and 450 nm or less.
This improves the effect of confining light in the light emitting layer and also improves the effect of injecting carriers, thereby improving the light emitting efficiency of the laser diode.

(12) In the laser diode, it is preferable that a part or all of the region of the second conductivity type vertical conducting layer, including the interface with the second conductivity type waveguide layer, is a region that is not doped with impurities.
This makes it possible to suppress the diffusion of impurities and efficiently inject carriers into the light emitting layer, thereby improving the oscillation efficiency.

(13) In the laser diode, it is preferable that the first conductivity type cladding layer is made 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 formed with complete strain with respect to the nitride semiconductor substrate.
By forming the first conductivity type cladding layer from Al _a Ga _1-a N (0.6<a≦0.8), the second conductivity type vertical conduction layer and the second conductivity type lateral conduction layer are formed with perfect distortion relative to the substrate, thereby improving the carrier conductivity.
(14) In the laser diode, it is preferable that the Al composition g of the second conductivity type lateral conduction layer at a surface facing the second conductivity type vertical conduction layer is larger than the minimum value of the Al composition f.
This allows the carriers to be diffused in the lateral direction, and the carrier injection efficiency can be improved.

The following describes examples and comparative examples of this disclosure.

Example 1
A 550 μm thick (0001) AlN single crystal substrate was used as the substrate, and annealing was performed on this substrate using a metal organic chemical vapor deposition (MOCVD) apparatus. The annealing was performed in two sets of annealing at 1300° C., with one set being annealing for 5 minutes in an NH ₃ atmosphere and the other being annealing for 5 minutes in an H ₂ atmosphere.

Next, an AlN layer, which is a homoepitaxial layer, was formed on the substrate as a buffer layer. The AlN layer was formed to a thickness of 500 nm in an environment of 1200° C. At this time, the ratio (V/III ratio) of the supply rate of the III group element source gas to the supply rate of the nitrogen source gas was set to 50. The degree of vacuum of the chamber in which the annealing was performed was set to 50 mbar. The growth rate of the AlN layer at this time was 0.5 μm/hr. Trimethylaluminum (TMAl) was used as the Al source. Ammonia (NH ₃ ) was used as the N source.

A first conductive cladding layer was formed on the AlN layer formed as described above. The first conductive cladding layer was an n-type AlGaN layer (Al: 70%, i.e., an Al _0.70 Ga _0.30 N layer) using Si as a dopant impurity. The first conductive cladding layer was formed to a thickness of 350 nm under the conditions of a temperature of 1080° C., a degree of 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 (NH ₃ ) was used as the N raw material. Monosilane (SiH ₄ ) was used as the Si raw material.

Next, an n-type waveguide layer, which is a first conductive type waveguide layer, was formed on the first conductive type cladding layer. The n-type waveguide layer was an n-type AlGaN layer (Al: 63%, i.e., an Al _0.63 Ga _0.37 N layer) using Si as a dopant impurity. The n-type waveguide layer was formed to a thickness of 60 nm under the conditions of a temperature of 1080° C., a degree of 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 (NH ₃ ) was used as the N raw material.

Next, an emission layer was formed on the n-type waveguide layer. The emission layer was formed to have a multiple quantum well structure in which quantum well layers and barrier layers were stacked three times. Here, the quantum well layer was an AlGaN layer (Al: 52%, i.e., an Al _0.52 Ga _0.48 N layer) having a thickness of 3.0 nm. The barrier layer was an AlGaN layer (Al: 63%, i.e., an Al _0.63 Ga _0.37 N layer) having a thickness of 6.0 nm.
The light-emitting layer was formed under conditions of a vacuum degree of 50 mbar and a V/III ratio of 4000. The growth rate of the quantum well layer was 0.18 μm/hr, and the growth rate of the barrier layer was 0.15 μm/hr.

Next, a p-type waveguide layer, which is a second conductive type waveguide layer, was formed on the light emitting layer. The p-type waveguide layer was an AlGaN layer (Al: 63%, i.e., an Al _0.63 Ga _0.37 N layer) containing no dopant. The p-type waveguide layer was formed to a thickness of 60 nm under the conditions of a temperature of 1080° C., a degree of 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. Triethylgallium (TEGa) was used as the Ga raw material.

Next, a second conductive cladding layer was formed on the p-type waveguide layer. The second conductive cladding layer is a layered structure including a second conductive vertical conductive layer and a second conductive horizontal conductive layer, and is a graded layer with a gradient Al composition ratio. The second conductive vertical conductive layer was a p-type AlGaN layer having a thickness of 20 nm and an Al composition distribution in the direction away from the substrate, varying from Al=0.63 to 1.0. The second conductive horizontal conductive layer was a p-type AlGaN layer having a thickness of 320 nm and an Al composition distribution in the direction away from the substrate, varying from Al=1.0 to 0.7. The second conductive cladding layer was formed at a temperature of 1080°C, with a degree of vacuum set to 50 mbar and a V/III ratio of 4000. The growth rate of the second conductive cladding layer at this time was 0.3 to 0.5 μm/hr. In addition, trimethylaluminum (TMAl) was used as the Al source, and triethylgallium (TEGa) was used as the Ga source.

Next, a p-type contact layer, which is a second conductive type contact layer, was formed on the second conductive type cladding layer. Here, the p-type contact layer was formed from an AlGaN layer and a GaN layer. The AlGaN layer was a p-type nitride semiconductor layer with a layer thickness of 30 nm, using Mg as a dopant impurity and with an Al composition distribution in the direction away from the substrate, varying 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 p-type contact layer was formed at a temperature of 950°C, with a degree of vacuum set to 150 mbar and a V/III ratio of 3650. The growth rate of the p-type contact layer was 0.2 μm/hr.

In this way, a semiconductor multilayer was formed on an AlN substrate. When reciprocal lattice mapping measurements were performed on this semiconductor multilayer using XRD, it was found that the semiconductor multilayer had undergone pseudomorphic growth without relaxation up to the p-type contact layer.

The semiconductor laminate formed as described above was annealed in an N ₂ atmosphere at 700° C. for 10 minutes or more to further reduce the resistance of the p-type contact layer.
A resist pattern (photoresist mask for IPC etching) was formed by photolithography on the p-type contact layer to provide multiple convex portions at the mesa boundary in the <1-100> direction. Next, the n-type cladding layer was exposed by performing dry etching with a gas containing _Cl2 and _BCl3 using ICP. The n-type cladding layer was formed in a rectangular region long in the <1-100> direction in plan view. At this time, the ICP conditions were an antenna power of 320 W, a bias power of 30 W, an automatic pressure adjustment (APC: Adaptive Pressure Control valve) of 2 Pa, a process pressure of 600 Pa, and gas flow rates of _Cl2 gas at 20 sccm and _BCl3 gas at 5 sccm.

The mesa structure formed had a length of 600 μm in the <1-100> direction and a length of 40 μm in the <11-20> direction. Here, the length of the mesa structure in the <1-100> direction is the distance between the end faces of the resonator mirror in a plan view, and the length in the <11-20> direction is the distance between the side faces of the mesa structure.

On the p-type contact layer of the mesa structure, Ni and Au were sequentially deposited in a rectangular shape elongated in the <1-100> direction to form a plurality of electrode metal regions as second electrodes. At this time, the width of the second electrode was 5 μm and the length was 600 μm.
The mesa structure was subjected to monochromatic cathode luminescence measurement at the emission wavelength, and the width of the dislocation generation region around the mesa structure, which is a non-emitting region, was confirmed to be 5.6 μm. From the viewpoint of reducing the oscillation threshold of the laser diode, the second electrode was disposed at a position 5.6 μm away from the side surface of the mesa structure so that the second electrode and the dislocation generation region around the mesa structure did not overlap.

In addition, in the region where the n-type cladding layer of the mesa structure was exposed, V, Al, Ni, Ti, and Au were deposited in order in a rectangular shape long in the <1-100> direction to form multiple electrode metal regions as first electrodes. Furthermore, within the electrode metal regions, the substrate was divided into stripes by cleaving multiple times in the <11-20> direction to form individual laser diodes. The final length of the mesa structure in the <1-100> direction was 600 μm.

When the shape of the side surface of the mesa structure in the <1-100> direction (the side surface that is not the resonator mirror end face) in a plan view was measured for the laser diode obtained in this manner, the distance between the apexes of the convex portions was 1.0 μm, the height of the convex portions was 1.0 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 20 degrees. The element resistance of the laser diode was also 5.4 Ω.

Example 2
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and the distance between the apexes of the convex portions was 6 μm, the height of the convex portions was 1 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 20 degrees. The width of the non-light-emitting region around the mesa structure was measured and found to be 10.5 μm. When the second electrode was placed at a position 10.5 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 7 Ω.

Example 3
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and the distance between the apexes of the convex portions was 12 μm, the height of the convex portions was 1 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 20 degrees. The width of the non-light-emitting region around the mesa structure was measured and found to be 11.4 μm. When the second electrode was placed at a position 11.4 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 7.4 Ω.

Example 4
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and the distance between the apexes of the convex portions was 5 μm, the height of the convex portions was 3.5 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 20 degrees. The width of the non-light-emitting region around the mesa structure was measured and found to be 9.3 μm. When the second electrode was placed at a position 9.3 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 6.6 Ω.

Example 5
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and the distance between the apexes of the convex portions was 5 μm, the height of the convex portions was 10 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 20 degrees. The width of the non-light-emitting region around the mesa structure was measured and found to be 11.3 μm. When the second electrode was placed at a position 11.3 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 7.3 Ω.

Example 6
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and the distance between the apexes of the convex portions was 5 μm, the height of the convex portions was 12 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 20 degrees. The width of the non-light-emitting region around the mesa structure was measured and found to be 11.6 μm. When the second electrode was placed at a position 11.6 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 7.4 Ω.

Example 7
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and the distance between the apexes of the convex portions was 5 μm, the height of the convex portions was 1 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 10 degrees. The width of the non-light-emitting region around the mesa structure was measured and found to be 5.4 μm. When the second electrode was placed at a position 5.4 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 5.3 Ω.

Example 8
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and the distance between the apexes of the convex portions was 5 μm, the height of the convex portions was 1 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 60 degrees. The width of the non-light-emitting region around the mesa structure was measured and found to be 7.9 μm. When the second electrode was placed at a position 7.9 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 6.1 Ω.

<Example 9>
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and the distance between the apexes of the convex portions was 5 μm, the height of the convex portions was 1 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 100 degrees. The width of the non-light-emitting region around the mesa structure was measured and found to be 9.5 μm. When the second electrode was placed at a position 9.5 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 6.7 Ω.

Example 10
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and the distance between the apexes of the convex portions was 1 μm, the height of the convex portions was 1 μm, and the minimum angle between the line segments connecting the bottom and top of the convex portions was 130 degrees. The width of the non-light-emitting region around the mesa structure was measured and found to be 11.2 μm. When the second electrode was placed at a position 11.2 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 7.3 Ω.

<Comparative Example 1>
A laser diode was obtained in the same manner as in Example 1, except that the shape of the side surface of the mesa structure was changed by changing the shape of the photoresist mask.
The shape of the side surface (not the resonator mirror end surface) in the <1-100> direction of the mesa structure of the obtained laser diode in plan view was measured, and no protrusions were found. The width of the non-light-emitting region around the mesa structure was measured and found to be 12 μm. When the second electrode was placed at a position 12 μm away from the side surface of the mesa structure so that the second electrode and the dark line region around the mesa structure did not overlap, the element resistance of the laser diode was 7.5 Ω.
The evaluation results of each of the examples and comparative examples are shown in Table 1 below.

As shown in Table 1, the laser diodes of Examples 1 to 10, which have a convex portion on the side surface of the mesa structure of the laser diode, had a lower element resistance than the laser diode of Comparative Example 1, which does not have a convex portion on the side surface of the mesa structure.
Furthermore, the laser diodes of Examples 1 and 2, in which the distance between the tops of the protrusions formed on the side surfaces of the mesa structure is 1 μm or more and 10 μm or less, have an even lower element resistance than the laser diode of Example 3, in which the distance between the tops of the protrusions exceeds 10 μm.

Furthermore, the laser diodes of Examples 7 to 9, in which the minimum angle between the line segments connecting the bottom and top of the convex portion is greater than 1 degree and less than 120 degrees, had an even lower element resistance than the laser diode of Example 10, in which the minimum angle exceeded 120 degrees.
Thus, it was found that in order to reduce the element resistance of the laser diode, it is preferable to provide a convex portion on the side surface of the mesa structure, and it is even more preferable to adjust the shape of the convex portion (distance between apexes, height, and angle of the apex in a planar view).

Although the embodiments of the present disclosure have been described above, the technical scope of the present disclosure is not limited to the technical scope described in the above-mentioned embodiments. Various modifications or improvements can be made to the above-mentioned embodiments, and it is clear from the claims that forms incorporating such modifications or improvements can also be included in the technical scope of the present disclosure.

10 Substrate 20 Semiconductor laminate 21 First conductivity type cladding layer 22 Light emitting layer 23 Second conductivity type cladding layer 23A Second conductivity type vertical conductive layer 23B Second conductivity type lateral conductive layer 24 First conductivity type waveguide layer 25 Second conductivity type waveguide layer 26 Second conductivity type contact layer 27 Intermediate layer 28 Convex portion 31 First electrode 32 Second electrode 201 Mesa structure including first conductivity type cladding layer SS Side surface of mesa structure ES End surface of mesa structure

Claims (14)

a nitride semiconductor substrate containing Al;
a semiconductor laminate portion disposed on the nitride semiconductor substrate;
Equipped with
The semiconductor laminate portion is
a first conductivity type cladding layer including a first conductivity type nitride semiconductor layer disposed on the nitride semiconductor substrate;
a light emitting layer formed of a nitride semiconductor including one or more quantum wells and disposed on the first conductive type cladding layer;
a second conductivity type cladding layer including a second conductivity type nitride semiconductor disposed on the light emitting layer;
having
At least a portion of the semiconductor laminate is a mesa structure including at least the first conductive type cladding layer, the mesa structure being formed for optical resonance and emission;
A laser diode in which the side surfaces of the mesa structure that are not perpendicular to the optical resonance direction have a plurality of protrusions. The distance between the tops of the plurality of protrusions in a plan view is 1 μm or more and 10 μm or less.
2. The laser diode of claim 1. The height of the convex portion in a plan view is 1 μm or more and 11 μm or less.
3. The laser diode according to claim 1 or 2. In a plan view, the minimum angle between the line segments connecting the bottom and the top of the protrusion is 1 degree or more and 120 degrees or less.
4. A laser diode according to claim 1. The laser diode according to any one of claims 1 to 4, wherein the mesa structure has a resonator in which an end face parallel to a (1-100) crystal orientation plane of the nitride semiconductor substrate is a resonator mirror end face, and the laser diode is an edge-emitting ultraviolet laser diode that emits light in a <1-100> direction. The mesa structure has a rectangular shape having long sides and short sides in a plan view, the long sides being parallel to the <1-100> direction,
6. The laser diode according to claim 1, wherein the convex portion protrudes in at least one of a <11-20> direction and a <-1-120> direction in a plan view. 7. The laser diode according to claim 1, wherein the nitride semiconductor substrate is an AlN single crystal substrate. a first conductivity type waveguide layer disposed between the first conductivity type cladding layer and the light emitting layer to confine light to the light emitting layer;
a second conductive type waveguide layer disposed between the second conductive type cladding layer and the light emitting layer to confine light to the light emitting layer;
Equipped with
8. A laser diode according to claim 1. a second conductive type contact layer disposed on the second conductive type cladding layer and formed of a nitride semiconductor containing GaN;
The second conductivity type cladding layer includes a second conductivity type vertical conduction layer having a thickness of less than 0.5 μm, and a second conductivity type lateral conduction layer including Al _{g Ga 1- g} N (0<g≦1), the second conductivity type vertical conduction layer having a composition gradient in which the Al composition e decreases with increasing distance from the nitride semiconductor substrate.
9. The laser diode of claim 8. an intermediate layer disposed between the second conductive type vertical conductive layer and the second conductive type waveguide layer and formed of Al _h Ga _1-h N (0<h≦1.0);
10. The laser diode of claim 9. The thickness of the second conductive type vertical conductive layer is 250 nm or more and 450 nm or less.
11. The laser diode according to claim 9 or 10. a part or all of a region of the second conductive type vertical conduction layer including an interface with the second conductive type waveguide layer is a region that is not doped with impurities;
12. A laser diode according to any one of claims 9 to 11. the first conductive type cladding layer is made of Al _a Ga _1-a N (0.6<a≦0.8);
the second conductive type vertical conduction layer and the second conductive type lateral conduction layer are formed with complete strain with respect to the nitride semiconductor substrate;
13. A laser diode according to any one of claims 9 to 12. the Al composition g of the second conductive type lateral conduction layer at a surface facing the second conductive type vertical conduction layer is greater than a minimum value of the Al composition f of the second conductive type vertical conduction layer;
14. A laser diode according to any one of claims 9 to 13.

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