JP7614607B2 - Laser diode manufacturing method, laser diode - Google Patents

Description

The present invention relates to a method for manufacturing a laser diode and a laser diode.

Conventionally, nitride semiconductors have been used as materials for forming laser diodes. Nitride semiconductors have a direct transition recombination mode, and therefore are suitable as materials for laser diodes in that they can provide high recombination efficiency and high optical gain. As an example of a laser diode using such a nitride semiconductor, a technology for oscillating a current-injection type laser diode in the ultraviolet region has been disclosed (for example, Non-Patent Document 1).

K. Sato et al. , Applied Physics Express 13, 031004 (2020)

The above-mentioned ultraviolet laser diode is a pulse-driven type, and continuous oscillation is required for practical applications, which requires reduction of the oscillation threshold current and threshold voltage.
An object of the present disclosure is to provide a manufacturing method for a laser diode and a laser diode capable of reducing the oscillation threshold current and threshold voltage.

In order to solve the above-mentioned problems, a method for manufacturing a laser diode according to an embodiment of the present disclosure includes forming a nitride semiconductor substrate having a surface region on which Al-containing nitride semiconductor substrate is provided with a plurality of Al-containing nitride semiconductor layers each having a diameter of 300 nm or more and 1000 nm or less and a density of 10 ² particles/cm ² or more and 10 ⁵ particles/cm 2 or more. forming a first conductivity type cladding layer on the ^nitride semiconductor substrate, the first conductivity type cladding layer including a nitride semiconductor having one or more quantum wells; forming a second conductivity type cladding layer on the light-emitting layer, the second conductivity type cladding layer including a nitride semiconductor having a second conductivity type and having the hillocks formed above the convex portion and exposed on its surface; etching a region where the hillocks are formed in a semiconductor laminate including the first conductivity type cladding layer, the light-emitting layer and the second conductivity type cladding layer, exposing the first conductivity type cladding layer to form a mesa structure having a resonator; forming a first electrode in the region where the hillocks are exposed on the exposed surface of the first conductivity type cladding layer; and forming a second electrode on the second conductivity type cladding layer.

In order to solve the above-mentioned problems, a method for manufacturing a laser diode according to another aspect of the present disclosure includes exposing a partial region of a surface of a nitride semiconductor substrate containing Al, forming a resist mask covering the remaining region of the surface, and etching the partial region of the surface of the nitride semiconductor substrate by an etching method to form a laser diode having a diameter of 300 nm or more and 1000 nm or less and a density of ¹⁰² particles/ ^cm2 to ¹⁰⁵ particles/cm2 in the partial region. forming a first conductivity type cladding layer on the nitride semiconductor substrate, the first conductivity type cladding layer including a nitride semiconductor having one ^or more quantum wells, forming a second conductivity type cladding layer on the light-emitting layer, the second conductivity type cladding layer including a nitride semiconductor having a second conductivity type and having the hillocks formed above the convex portions and exposed on the surface; etching a region where the hillocks are formed in a semiconductor laminate including the first conductivity type cladding layer, the light-emitting layer and the second conductivity type cladding layer, exposing the first conductivity type cladding layer to form a mesa structure having a resonator, forming a first electrode in the region where the hillocks are exposed on the exposed surface of the first conductivity type cladding layer, and forming a second electrode on the second conductivity type cladding layer.

In order to solve the above-mentioned problems, a laser diode according to one aspect 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 disposed on the nitride semiconductor substrate and containing a nitride semiconductor of a first conductivity type, a light emitting layer disposed on the first conductivity type cladding layer and formed of a nitride semiconductor including one or more quantum wells, and a second conductivity type cladding layer disposed on the light emitting layer and containing a nitride semiconductor of a second conductivity type, wherein a part of the first conductivity type cladding layer, the light emitting layer, and the second conductivity type cladding layer form a mesa structure having a resonator, a first electrode is disposed on the first conductivity type cladding layer in a region not forming the mesa structure, one or more hillocks are present on a surface of the first conductivity type cladding layer facing the first electrode, and one or more hillocks are present on the surface of the second conductivity type cladding layer facing the second electrode, a second electrode is disposed on the second conductivity type cladding layer, and no hillocks are present on a surface of the second conductivity type cladding layer facing the second electrode.
It should be noted that the above summary of the invention does not list all of the features of the present disclosure.

The present disclosure provides a method for manufacturing a laser diode and a laser diode that can reduce the oscillation threshold current and threshold voltage.

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. FIG. 11 is a cross-sectional view illustrating another configuration example of a laser diode according to an embodiment of the present disclosure. FIG. 11 is a cross-sectional view illustrating another configuration example of a laser diode according to an embodiment of the present disclosure. FIG. 11 is a cross-sectional view illustrating another configuration example of a laser diode according to an embodiment of the present disclosure. FIG. 11 is a cross-sectional view illustrating another configuration example of a laser diode according to an embodiment of the present disclosure. 10 is a schematic plan 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 and the method for manufacturing the laser diode according to the present disclosure will be described below through the embodiments, but the following embodiments do not limit the invention according to the claims. Also, not all of the combinations of features described in the embodiments are necessarily essential to the solution of the invention.
In the following description, the positive direction of the Z axis may be referred to as "up" and the negative direction of the Z axis may be referred to as "down". "Up" and "down" do not necessarily mean the vertical direction with respect 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 for specifying the relative positional relationship of a surface, a film, a substrate, etc., and do not limit the technical idea of the present invention. For example, if the paper surface is rotated 180 degrees, "up" will of course become "down" and "down" will become "up".

1. Embodiments A laser diode and a method for manufacturing a laser diode according to an embodiment of the present disclosure will be described.
(1.1) Configuration of the laser diode The laser diode according to this embodiment includes a nitride semiconductor substrate containing Al and a semiconductor laminate disposed on the nitride semiconductor substrate. The semiconductor laminate includes a first conductive type cladding layer disposed on the nitride semiconductor substrate and containing a first conductive type nitride semiconductor, a light emitting layer disposed on the first conductive type cladding layer and formed of a nitride semiconductor containing one or more quantum wells, and a second conductive type cladding layer disposed on the light emitting layer and containing a second conductive type nitride semiconductor. A part of the first conductive type cladding layer, the light emitting layer, and the second conductive type cladding layer of the semiconductor laminate form a mesa structure having a resonator. A first electrode is disposed on the first conductive type cladding layer in an area not forming a mesa structure, and one to 30 hillocks are present on the surface of the first conductive type cladding layer facing the first electrode. A second electrode is disposed on the second conductive type cladding layer, and no hillocks are present on the surface of the second conductive type cladding layer facing the second electrode.

(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 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 strain on the substrate, the first conductive cladding layer is more preferably formed of Al _a Ga _(1-a) N (0.6<a≦0.8).

The first conductivity type cladding layer may be a gradient layer in which the Al composition increases with increasing distance from the substrate, for example, to control the longitudinal conductivity. In this case, the above-mentioned limitation on the Al composition may be the Al composition averaged over the thickness of the first conductivity type cladding layer at positions in the thickness direction within the first conductivity type cladding layer.

When the first conductivity type cladding 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, etc., C, H, F, O, Mg, Si, etc., but the types of impurity elements are not limited to these. From the viewpoint of reducing electrical resistance and the ease of obtaining raw materials, the impurity contained in the first conductivity type cladding layer is preferably Si, and the impurity concentration is preferably 5×10 ¹⁸ cm ^-3 or more and 5×10 ¹⁹ cm ^-3 or more.

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 a layer of a nitride semiconductor containing Al and Ga. From the viewpoint of realizing high light emitting efficiency, the nitride semiconductor contained in the light emitting layer is preferably a mixed crystal of, for example, AlN or GaN, and is formed, for example, of Al _b Ga _(1-b) N (0<b<1). In addition to N, the light emitting layer may contain impurities such as P, As, Sb, and other group V elements other than N, and C, H, F, O, Mg, and Si, but is not limited thereto.

The light emitting layer may have either a multiple quantum well structure or a single quantum well structure. The number of quantum well structures is preferably one to three, depending on the longitudinal conductivity of the first and second conductive cladding layers.
Furthermore, for the purpose of reducing the influence of crystal defects in the light-emitting layer, a part or the whole of the light-emitting layer may contain elements such as Si, Sb, P at 1×10 ¹⁵ cm ⁻³ or more.

(Waveguide layer)
From the viewpoint of light confinement, the laser diode of this embodiment may include waveguide layers formed above and below the light emitting layer so as to sandwich the light emitting layer and having the effect of confining light emitted from the light emitting layer within the light emitting layer. The waveguide layers are 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.
That is, the laser diode of this embodiment may include, for example, 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.

From the viewpoint of light confinement, the waveguide layer is preferably a nitride semiconductor containing Al and Ga having a band gap with a higher energy than the light emitting layer. The waveguide layer preferably has an Al composition and a film thickness that increase the electric field intensity distribution of light present in the device and the overlap of the light emitting 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 _c Ga _(1-c) N (0<c<1), it is more preferable that b<c and c≧b+0.05. For example, in the case of a light emitting layer having an emission wavelength of 265 nm, it is preferable that c=0.52 and d is 0.57 or more. Also, from the viewpoint of light confinement and layer resistance, the film thickness of the waveguide layer is preferably 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 cladding layer described later, 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 a nitride semiconductor layer formed on the light-emitting layer and containing Al and Ga, and having second-conductivity-type conductivity. The second-conductivity-type cladding layer is formed of, for example, Al _d Ga _(1-d) N (0<d<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 on 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 conductivity type cladding layer is not particularly limited 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 strength distribution of the optical mode existing in the device and the light emitting layer (i.e., increase the optical confinement). The second conductivity type cladding layer may be, for example, p-type AlGaN doped with Mg.
Furthermore, 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.

From the viewpoint of more efficiently injecting carriers into the light-emitting layer, the second-conductivity-type cladding layer preferably comprises a compositionally graded layer (second-conductivity-type vertical conduction layer) formed of Al _e Ga _(1-e) N (0.1≦e≦1) in which the Al composition e decreases with increasing distance from the substrate, i.e., the Al composition e decreases in the direction away from the upper surface of the substrate, and a second-conductivity-type lateral conduction layer containing Al _f Ga _(1-f) N (0<f≦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 conductive type vertical conductive layer is a layer that constitutes a region of the second conductive type cladding layer on the light emitting layer side.
The second-conductivity-type vertical conduction layer is a layer containing Al _e Ga _(1-e) N. The profile (slope) of the Al composition e in the second-conductivity-type vertical conduction layer may decrease continuously or may decrease intermittently. Here, "intermittently decreasing" means that the second-conductivity-type vertical conduction layer includes a portion in which the Al composition f is the same (constant in the film thickness direction). In other words, the second-conductivity-type vertical conduction 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.

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 or more and 450 nm or less, and even more preferably 300 nm or more and 400 nm or less.

For the purpose of suppressing the diffusion of impurities, it is preferable that the second conductive type vertical conduction layer is not doped (intentionally mixed) with impurities such as H, Mg, Be, Zn, Si, B, etc. in the region close to the light emitting layer of the second conductive type vertical conduction layer, that is, is in an undoped state. Here, "undoped" means that the above-mentioned impurities are not intentionally supplied as elements in the process of forming the target layer, but this does not apply when elements derived from raw materials and manufacturing equipment are mixed in the range of, for example, 1×10 ¹⁶ cm ^-3 or less. In addition, the undoped region of the second conductive type vertical conduction layer includes at least the boundary with the light emitting layer (the second conductive type waveguide layer in the case where a second conductive type waveguide layer is provided), but the size is not limited. For example, the entire region of the second conductive type vertical conduction layer may be in an undoped state. In addition, as another example, 50% of the region of the second conductive type vertical conduction layer close to the light emitting layer may be in an undoped state. As another example, about 10% of the second conductivity type vertical conductive layer close to the light emitting layer may be in an undoped state.

(Second Conductive Type Lateral Conductive Layer)
The second-conductivity-type lateral conduction layer is a layer that constitutes 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 vertical conduction layer is a layer containing Al _f Ga _(1-f) N (0<f≦1). Here, the Al composition f of the second-conductivity-type horizontal conduction layer at a surface facing the second-conductivity-type vertical conduction layer is preferably larger than the minimum value of the Al composition e 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 guiding carriers to 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.

In this way, the second conductive type vertical conductive layer generates carriers (for example, holes when the second conductive 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 conductive type vertical conductive layer on the light emitting layer, the carrier injection efficiency of the laser diode can be increased and the threshold voltage can be reduced.
In addition, the second-conductivity-type lateral conduction layer has the effect of spreading the carrier distribution, which is narrowed by the electric field concentrated under the electrode, in the lateral direction (within the plane of the second-conductivity-type lateral conduction layer), which allows the second-conductivity-type lateral conduction layer to increase the carrier injection efficiency into the light-emitting layer, similar to the second-conductivity-type vertical conduction layer.

(Middle class)
Between the second conductivity type cladding layer and the waveguide, i.e., between the second conductivity type vertical conduction layer and the second conductivity type waveguide layer, an intermediate layer made of Al g Ga (1-g) N _(0<g≦1.0) in which the Al _composition g increases in the direction away from the upper surface of the substrate may be provided from the viewpoint of improving the carrier conductivity and/or for forming the second conductivity type lateral conduction layer and the second conductivity type contact layer with perfect distortion. The intermediate 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 an undoped layer containing no impurities.

(Second Conductive Type Contact Layer)
The semiconductor laminate of 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.

(Electron Blocking Layer)
The semiconductor laminate of the laser diode of this embodiment may further include an electron blocking layer above the light emitting layer, the electron blocking layer having a band gap larger than that of the second conductive type waveguide layer. The electron blocking layer may be provided, for example, inside the second conductive type waveguide layer, between the second conductive type waveguide layer and the light emitting layer, or between the second conductive type waveguide layer and the second conductive type vertical conductive layer.
The thickness of the electron blocking layer is preferably 30 nm or less, more preferably 20 nm or less, and further preferably 15 nm or less, so that carriers (holes) can easily quantum-transport through the electron blocking layer.

(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.

From the viewpoint of current confinement of the laser and amplification by reflection at the end facets, it is preferable that the mesa structure has a rectangular shape having long and short sides in a plan view, with the long sides extending in the <1-100> direction. This is because, when obtaining the end faces of the laser resonator (for example, the resonator mirror end faces) by various methods such as cleavage and etching, the atomically flat (1-100) plane can be most easily formed.
In other words, the mesa structure has a resonator in which the mesa end faces parallel to the (1-100) crystal plane of the nitride semiconductor substrate serve as resonator mirror end faces. As a result, the laser diode of this embodiment is an edge-emitting laser diode in which the mesa structure emits light in the <1-100> direction.

The distance between the side surfaces of the mesa structure is also correlated with the threshold voltage and the oscillation threshold current. For this reason, in a rectangular mesa structure, it is preferable that the ratio of the distance between the side surfaces of the mesa structure in a plan view to the distance between the end faces of the resonator is 1:5 or more and 1:500 or less. In a plan view, the distance between the side surfaces of the mesa structure corresponds to the length of the end face of the resonator, and the distance between the end faces of the resonator mirror corresponds to the length of the side surface of the mesa structure. In other words, it is preferable that the ratio of the length of the end face of the resonator in a plan view to the length of the side surface of the mesa structure is 1:5 or more and 1:500 or less.

The mesa structure can be formed by etching the semiconductor layer stack using inductively coupled plasma (ICP) or similar.

(electrode)
The laser diode can oscillate by injecting a current through a second electrode disposed on the second conductive type cladding layer and a first electrode disposed on the first conductive type cladding layer, where the first electrode is formed so as to be in electrical contact with the first conductive type cladding layer, and the second electrode is formed so as to be in electrical contact with the second conductive type cladding layer.
The first electrode can be disposed, for example, on the back side of the substrate. The first electrode is disposed on the first conductive cladding layer exposed by removing layers above the first conductive cladding layer of the semiconductor laminate by, for example, chemical etching or dry etching. In other words, the first electrode is disposed on a region of the first conductive cladding layer that does not form a mesa structure.

When the first conductive type cladding layer is an n-type cladding layer, the first electrode is formed of a metal such as Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zr, or a mixed crystal thereof, or _a conductive oxide such as ITO or _Ga2O3 .
When the first conductive type cladding layer is a p-type cladding layer, the first electrode is formed of a metal such as Ni, Au, Pt, Ag, Rh, Pd, Pt, Cu, Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Ir, Zr, or a mixed crystal thereof, or a conductive oxide such as _ITO or _Ga2O3 .

When the second conductive type cladding layer is an n-type cladding layer, the second electrode is formed of a metal such as Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zr, or a mixed crystal thereof, or _a conductive oxide such as ITO or _Ga2O3 .
When the second conductive type cladding layer is a p-type cladding layer, the second electrode is formed of a metal such as Ni, Au, Pt, Ag, Rh, Pd, Pt, Cu, Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Ir, Zr, or a mixed crystal thereof, or a conductive oxide such as _ITO or _Ga2O3 .

The arrangement area and shape of the first electrode and the second electrode are not limited as long as electrical contact is obtained with the first conductivity type cladding layer and the second conductivity type cladding layer (the second conductivity type contact layer if a second conductivity type contact layer is provided). Since carriers are captured and preferentially consumed in areas with many dislocations, resulting in worsening internal loss, it is preferable that the second electrode is arranged in an area 5 μm or more away from the side of the mesa structure on the second conductivity type cladding layer in a plan view.

(Buffer layer)
The buffer layer is formed between the substrate and the first conductive type cladding layer, and is preferably formed on the entire surface of the substrate. By providing the buffer layer, a nitride semiconductor layer having a small lattice constant difference and a small thermal expansion coefficient difference and having few defects is formed on the buffer layer. Furthermore, by providing the buffer layer, the first conductive type cladding layer can be grown under compressive stress, and the occurrence of cracks in the first conductive type cladding layer can be suppressed. Therefore, even if the substrate is formed of a nitride semiconductor such as AlN or AlGaN, a nitride semiconductor layer having few defects can be formed above the buffer layer.

The buffer layer is formed of a nitride semiconductor such as AlN or AlGaN. The buffer layer may also contain impurities such as C, Si, Fe, and Mg.

The buffer layer has a thickness of, for example, several micrometers. Specifically, the thickness of 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 the nitride semiconductor such as AlN is high. When the buffer layer is thinner than 10 μm, cracks are less likely to occur in the buffer layer formed by crystal growth over the entire surface of the wafer. 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 layer with high crystallinity can be formed. When the buffer layer is thinner than 5 μm, cracks are less likely to occur in the buffer layer.

(Hillock)
Hillocks are formed on the surface of the first conductivity type cladding layer. The hillocks are made of a nitride semiconductor containing Al and Ga. The hillocks have a protruding shape on the surface of the first conductivity type cladding layer. The hillocks are formed on the surface of a region on the first conductivity type cladding layer where a mesa structure is not formed. More specifically, the hillocks are formed on the surface of the first conductivity type cladding layer facing the first electrode.
As will be described in detail later, hillocks are formed with the growth of the semiconductor laminate, starting from protrusions or irregularities formed on the substrate surface. In other words, hillocks are formed extending from the substrate surface in the thickness direction of the first conductivity type cladding layer, and protrude above the first conductivity type cladding layer.

The hillocks have an n-sided pyramid shape (n is an integer of 3 or more) whose diameter expands from the surface of the substrate upward. The hillocks form an n-sided shape in plan view because the faces that are easy to grow are preferentially formed. More specifically, the hillocks tend to form a shape having three or six sides in plan view. That is, the hillocks tend to form a triangular or hexagonal pyramid shape. The shape (top surface shape) of the hillock in plan view is not limited to a shape having three or six sides, and it can be formed into a polygonal shape having three or more sides. The hillocks may be a concave polygon shape in plan view, that is, a concave polygonal pyramid. The hillocks are formed while expanding their diameter with the growth of the semiconductor laminate starting from the substrate surface. That is, the hillocks are n-sided pyramid shapes with the substrate surface side as the apex. For this reason, the bottom side region of the n-sided pyramid hillock protrudes from the surface of the first conductive type cladding layer, and the remaining part is formed within the first conductive type cladding layer. In this case, the top of the hillock is not limited to a point, but may be a surface with a smaller area than the bottom of the hillock.

When forming a semiconductor laminate, for example, when a thin film containing AlGaN is grown by metal organic vapor deposition (MOVPE), Ga atoms preferentially aggregate at the ends of protrusions and irregularities that are the starting points of hillocks due to the difference in thermal diffusion length compared to Al. For this reason, the Ga content of the hillocks is higher than the Ga content of the regions other than the hillocks. In other words, in a region at the same height as the surface of the substrate, i.e., in a plane parallel to the surface of the substrate (parallel plane), the Ga content "x" of the hillocks and the Ga content "y" of the regions other than the hillocks have a relationship of "x>y". For example, the Ga content of the hillocks in the first conductive cladding layer is higher than the Ga content of the regions other than the hillocks in the first conductive cladding layer. Here, the Ga concentration (content) of the hillocks is preferably 5% or more higher than the Ga concentration of the regions other than the hillocks, and more preferably 5% to 10% higher.

In addition, as described above, the Ga content of the hillocks is greater than the Ga content of the regions other than the hillocks in the parallel plane of the first conductive type cladding layer, so the electrical resistance of the hillocks is lower than the electrical resistance of the regions other than the hillocks in the parallel plane. Therefore, in the laser diode according to this embodiment, the presence of hillocks on the surface of the first cladding layer facing the first electrode can reduce the threshold voltage.

From the viewpoint of reducing the threshold voltage, it is preferable that the first conductive type cladding layer has 1 to 30 hillocks on the surface facing the first electrode, and the density of the hillocks on the surface facing the first electrode of the first conductive type cladding layer is preferably 10 ^{/ cm2} to ¹⁰ / ^cm2 .

In addition, from the viewpoint of reducing the oscillation threshold current, hillocks are not formed in the region in the semiconductor laminate where the mesa structure is formed. Specifically, if hillocks are present in the region located below the second electrode in the mesa structure, carriers are preferentially consumed in the hillock portion, and the oscillation threshold current deteriorates. Therefore, since there are no hillocks in the region located below the second electrode in the semiconductor laminate (the surface facing the second electrode), the injected current can be efficiently coupled in the light emitting layer, and the oscillation threshold current can be reduced.
In this way, by configuring the surface of the first conductivity type cladding layer facing the first electrode, i.e., the region overlapping with the first electrode in a planar view, to have hillocks, and the surface of the second conductivity type cladding layer facing the second electrode, i.e., the region overlapping with the second electrode in a planar view, to have no hillocks, it is possible to realize a laser diode that can reduce the oscillation threshold current and threshold voltage.

Furthermore, a hillock periphery is formed along the outer periphery of the hillock on the surface of the first conductivity type cladding layer. That is, a hillock structure consisting of the hillock and the hillock periphery is formed on the first conductivity type cladding layer. Furthermore, the first electrode is formed on the first conductivity type cladding layer so as to cover the hillock structure consisting of the hillock and the hillock periphery.
As described above, a hillock is a protruding structure formed on the first-conductivity-type cladding layer as a result of thin film growth on the substrate, i.e., growth of the semiconductor laminate. In contrast, a hillock periphery is a protruding structure formed on the first-conductivity-type cladding layer when the semiconductor laminate is removed to expose the first-conductivity-type cladding layer. In other words, the hillock structure on the first-conductivity-type cladding layer is composed of two types of components that are formed by different processes.

Here, we will explain the shape and size (maximum diameter, height) of the hillock structure that is composed of the hillock and the hillock periphery.

The hillock structure has a polygonal shape in plan view. More specifically, the hillock structure has a shape having three or more sides in plan view. The shape of the hillock structure in plan view (top surface shape of the hillock structure) is formed based on the top surface shape of the hillock. As described above, the hillock is an n-sided pyramid (n is an integer equal to or greater than 3) and has an n-sided shape in plan view. Therefore, the hillock structure has an n-sided shape similar to the hillock in plan view. For example, if the hillock has a three-sided shape in plan view, the hillock outer periphery also has a three-sided shape in plan view. Also, for example, if the hillock has a six-sided shape in plan view, the hillock outer periphery also has a six-sided shape in plan view.

From the viewpoint of reducing electrical resistance and increasing process yield, the maximum diameter of the hillock structure, i.e., the maximum diameter of the hillock outer periphery including the hillock, is preferably 1 μm to 30 μm. Here, the diameter refers to the maximum diameter of the hillock structure in a plan view.
From the viewpoint of adhesion between the first conductive type cladding layer and the first electrode, the height of the hillock structure is preferably 10 nm to 1000 nm based on the surface of the first conductive type cladding layer facing the first electrode. Note that since the height of the hillock and the height of the hillock outer periphery are the same, the height of the hillock structure indicates the height of the hillock.

(1.2) Method for manufacturing ultraviolet light-emitting device The laser diode of this embodiment is manufactured through a process 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 protrusions on substrate)
Furthermore, convex portions that become the starting points of hillocks are formed in a partial region of the surface of the formed substrate. The convex portions may be formed, for example, with a diameter of 300 nm or more and 1000 nm or less and with a density of 10 ² pieces/cm ² or more and 10 ⁵ pieces/cm ² or less. By setting the density of the convex portions within the above-mentioned range, the density of hillocks formed starting from the convex portions can also be controlled within the above-mentioned range. The convex portions may have any shape that protrudes from the substrate surface, and may be spherical, hemispherical, or polyhedral, or may be mountain-shaped protrusions.

The convex portion on the substrate surface may be composed of, for example, microparticles. The material of the microparticles is not limited as long as the diameter is 300 nm or more and 1000 nm or less, but for example, alumina (Al ₂ O ₃ ) particles can be used. The microparticles may be implanted in a partial region of the substrate surface at a density of 10 ² particles/cm ² or more and 10 ⁵ particles/cm ² or less, and the device used for implantation is not particularly limited. For example, a micro air blaster device can be used to implant (spray) the microparticles in the above-mentioned range of density on the substrate surface to form convex portions on the substrate surface. In this case, it is preferable to protect the remaining region of the substrate, excluding the partial region of the surface, by a mask such as SiO ₂ .

The convex portions on the surface of the substrate may be formed by patterning. More specifically, the convex portions on the surface of the substrate may be formed by etching a partial area on the surface of the substrate by an etching method.
When forming a convex portion by patterning, for example, a part of the surface of the substrate is exposed, and a resist mask is formed to cover the remaining area. Then, a part of the surface of the substrate is etched by an etching method to form a convex portion having a diameter of 300 nm to 1000 nm and a density of 10 ² pieces/cm ² to 10 ⁵ pieces/cm ^2. The device used for etching the substrate surface is not particularly limited as long as it can form convex portions having a diameter and density within the above range, but for example, an inductively coupled plasma device (ICP) can be used. In addition, the material of the resist mask covering the remaining area of the substrate surface, i.e., the area where the convex portion is not formed, is not limited, but for example, a photoresist, SiO _2, Ni, or other mask can be used.

As described above, the convex portions on the substrate surface are the starting points for hillocks, and hillocks are formed on the convex portions. Therefore, by forming convex portions in a partial region of the substrate surface that corresponds to the region in which the first electrode is formed in the laser diode, hillocks can be formed on the surface of the laser diode that faces the first electrode on the first conductivity type cladding layer. Furthermore, by making the density of the convex portions uniform in the partial region of the substrate surface, the density of hillocks on the surface that faces the first electrode on the first conductivity type cladding layer, i.e., the number of hillocks, can be controlled with higher precision.

(Formation of Semiconductor Stack)
Each layer of the semiconductor laminate formed on the substrate can be formed by, for example, a molecular beam epitaxy (MBE) method, a hydride vapor phase epitaxy (HVPE) method, or a metal organic chemical vapor deposition (MOCVD) method.

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 ).

Hillocks are formed starting from protrusions formed on the substrate surface as a thin film containing a nitride semiconductor is grown from the above-mentioned raw materials.
Specifically, a first-conductivity-type cladding layer including a first-conductivity-type nitride semiconductor is first formed on a substrate. At this time, as the first-conductivity-type cladding layer grows, hillocks also grow starting from above the convex portions on the substrate surface. The hillocks are made of the same nitride semiconductor as the first-conductivity-type cladding layer. If the first-conductivity-type cladding layer is a nitride semiconductor (AlGaN) including Al and GaN, the hillocks are also made of AlGaN. When the thin film is formed, if the convex portions on the substrate surface are covered with the thin film, the covered portions of the thin film protrude from other regions. Therefore, as the first-conductivity-type cladding layer grows, the hillocks extending in the thickness direction of the first-conductivity-type cladding layer protrude above the first-conductivity-type cladding layer.
In this manner, a first conductivity type cladding layer is formed on the substrate, the cladding layer including a first conductivity type nitride semiconductor and having hillocks formed above the convex portions on the substrate surface and exposed on the surface.
When the first conductivity type cladding layer is formed, the hillock periphery is not formed on the first conductivity type cladding layer, and the hillock is exposed in a protruding shape on the surface of the first conductivity type cladding layer.

Next, a light emitting layer is formed on the first conductive cladding layer using a nitride semiconductor (such as AlGaN) including one or more quantum wells, and a second conductive cladding layer is formed on the light emitting layer, the second conductive cladding layer including a nitride semiconductor (such as AlGaN) and having hillocks formed above the convex portions of the substrate surface and exposed on the surface. This forms a semiconductor laminate. During the formation of the light emitting layer and the second conductive cladding layer, hillocks extending in the thickness direction are formed as each layer grows, similar to the formation of the first conductive cladding layer.
In the semiconductor laminate after film formation, hillocks start from convex portions on the substrate surface and extend in the thickness direction of the semiconductor laminate as the thin film grows, and are exposed in a protruding shape on the surface of the top layer (e.g., the second conductivity type cladding layer) of the semiconductor laminate.

If necessary, a buffer layer made of a nitride semiconductor such as AlN or AlGaN may be formed between the substrate and the first conductive cladding layer, and a waveguide layer made of a nitride semiconductor such as AlGaN may be formed above and below the light emitting layer. In addition, an intermediate layer made of a nitride semiconductor such as AlGaN may be formed between the second conductive cladding layer and the waveguide, a nitride semiconductor second conductive contact layer containing GaN or the like may be provided on the second conductive cladding layer, or an electron blocking layer may be formed above the light emitting layer. When a second conductive contact layer is provided, hillocks are formed protruding above the second conductive contact layer after deposition.

Hillocks grow while maintaining their shape in a plan view, that is, their top surface shape, which is an n-sided shape (n is an integer of 3 or more), so as the thickness of the laminated films (first conductivity type cladding layer, light-emitting layer, second conductivity type cladding layer, etc.) formed on the substrate increases, the size of the top surface shape, that is, the maximum diameter, increases. For this reason, hillocks grow in the shape of an n-sided pyramid whose diameter increases upward from the convex portion on the substrate surface, as described above.

Due to the formation mechanism described above, the size (maximum diameter) of the top surface shape of the hillock changes depending on the grown film thickness of the semiconductor laminate and the growth conditions of each layer (thin film). Specifically, as described above, the size of the top surface shape of the hillock changes depending on the film thickness of each layer on the substrate. For this reason, for example, the top surface shape of a hillock exposed on the second conductivity type cladding layer, the topmost layer of the semiconductor laminate, has a larger maximum diameter than the top surface shape of a hillock exposed on the first conductivity type cladding layer at the time the first conductivity type cladding layer is formed.

For example, when a thin film is grown by metal organic vapor deposition (MOVPE), Ga atoms preferentially aggregate at the end of the starting point on the substrate surface due to the difference in thermal diffusion length. Here, from the viewpoint of controlling the thermal diffusion length, it is preferable that the V/III ratio, which is the ratio of the group V and group III raw materials, is 500 to 10,000. This allows the maximum diameter of the hillocks in the top layer of the semiconductor laminate to be 1 μm to 30 μm. In addition, it is preferable that the growth rate of the thin film is 0.1 μm/h to 3.0 μm/h. This allows the height of the hillocks (height of the protruding shape) based on the surface of the top layer of the semiconductor laminate to be 10 nm to 1000 nm.

During film formation, the size of the hillocks protruding from the top layer of the semiconductor laminate is preferably such that the diameter (maximum diameter) of the top surface shape is 1 μm or more and 30 μm or less, and the height based on the surface of the top layer is 10 nm or more and 1000 nm or less. Therefore, the convex portion formed on the substrate surface may be configured to form hillocks with maximum diameter and height within the above-mentioned ranges on the top layer of the semiconductor laminate. Furthermore, the starting point of the hillock is not limited to the convex portion; for example, the hillock may be formed on a concave portion formed on the substrate surface. The structure (protrusion or uneven shape) that serves as the starting point of the hillock on the substrate surface is also referred to as the "starting point portion."

Furthermore, the density of hillocks changes based on the density of the convex portions formed on the substrate. That is, as described above, when the convex portions are formed on the substrate at a density of ¹⁰² /cm2 or ^more and ¹⁰⁵ / ^cm2 , hillocks are formed at a density of ¹⁰² /cm2 or ^more and ¹⁰⁵ / ^cm2 in the region of the semiconductor laminate overlapping the partial region of the substrate surface where the convex portions are formed in a plan view. That is, the number of hillocks in the region of the semiconductor laminate overlapping the partial region of the substrate surface where the convex portions are formed in a plan view changes depending on the number of convex portions in the partial region of the substrate surface.

Laser diodes are manufactured through a process (mesa structure formation process) in which unnecessary portions of each layer of the semiconductor laminate formed on the substrate are removed by etching. In the mesa structure formation process, the area where the hillocks are formed is removed by etching to form a mesa structure having a resonator. The unnecessary portions of each layer of the semiconductor laminate can be removed by, for example, inductively coupled plasma (ICP) etching.

In the mesa structure formation process, unnecessary parts of each layer of the conductor laminate are removed by etching, thereby exposing a part of the first conductive clad layer. In addition, in the mesa structure formation process, the semiconductor laminate is physically etched, so that the etching is performed while maintaining the shape of the surface of the top layer of the semiconductor laminate. Therefore, etching is performed up to the first conductive clad layer while maintaining the protruding shape of the hillock on the top layer (second conductive clad layer) of the semiconductor laminate, that is, the top surface shape (shape in plan view) and size (height, maximum diameter) of the hillock protruding on the top layer. As a result, a hillock periphery is formed so as to surround the outer peripheral surface of the hillock protruding on the first conductive clad layer. The hillock periphery is formed as a history of the shape and size of the hillock protruding on the top layer of the semiconductor laminate, so it has the same shape (appearance) as the hillock of the p-type contact layer. If multiple hillocks are formed on the top layer of the semiconductor laminate, in the mesa structure formation process, hillock peripheries are formed around each hillock on the exposed first conductivity type cladding layer.

In this way, a hillock structure is formed on the first conductive cladding layer, which is composed of the hillock formed during film formation and the hillock periphery formed as a history by etching. As described above, after the mesa structure formation process, the hillock covered by the hillock periphery, i.e., the hillock structure, is observed as the appearance of the hillock.

(Formation of electrodes)
The laser diode may also be manufactured through a process of forming electrodes. The electrodes, such as the first electrode and the second electrode, may be formed by various methods of depositing metal by electron beam deposition (EB) 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 may be formed by laminating multiple layers. Each electrode may also 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 to manufacture the laser diodes.

Specifically, the first electrode is formed on the surface of the first conductive cladding layer where the hillocks are exposed, that is, on the surface where the hillock structure is formed. At this time, 1 to 30 hillocks are present on the surface of the first conductive cladding layer facing the first electrode. Also, hillocks are present on the surface facing the first electrode at a density of 10 ² /cm ² to 10 ⁵ /cm ^2. This is because the convex parts are formed at this density in a part of the substrate surface (a region assumed to overlap with the first electrode in a plan view) when the convex parts are formed. In this way, the region on the substrate surface where the convex parts are formed overlaps with the region on the first conductive cladding layer where the first electrode is formed in a plan view. This allows hillocks to be formed on the surface of the first conductive cladding layer facing the first electrode, and the threshold voltage of the laser diode can be reduced.

The second electrode is formed on the top layer (e.g., the second conductive cladding layer) of a mesa structure that is a part of the semiconductor laminate. When forming the convex portion on the substrate surface, the region (remaining region) that is assumed to overlap with the region of the second electrode on the second conductive cladding layer in a plan view is protected by, for example, a mask, and the convex portion is not formed. This makes it possible to reliably prevent hillocks from being formed on the surface of the second conductive cladding layer that faces the second electrode.
By forming the second electrode on a mesa structure that is free of hillocks, i.e., by having no hillocks on the surface of the mesa structure that faces the second electrode, it is possible to reduce the oscillation threshold current in the laser diode.
In this manner, according to the method for manufacturing a laser diode according to the present embodiment, it is possible to reduce the oscillation threshold current and the threshold voltage.

2. Method for Measuring the Physical Properties of the Ultraviolet Light Emitting Device The physical properties of the above-mentioned laser diode can be measured as follows.

(Measurement of impurity and doping concentrations)
The concentrations of dopants and impurities contained in the substrate and each layer of the semiconductor laminate constituting the laser diode can be measured by secondary ion mass spectrometry (SIMS).
When the concentration of dopants or impurities contained in each layer of the semiconductor laminate is measured by SIMS after processing into a device, the measurement can be performed in a state where the electrodes are removed by chemical etching or physical polishing. The concentration of dopants or impurities contained in each layer of the semiconductor laminate can also be measured by sputtering from the substrate side where no electrodes are formed.
Specifically, the SIMS measurement is performed under the measurement conditions provided by Evans Analytical Group (EAG), Inc. A cesium (Cs) ion beam having an energy of 14.5 keV is used for sputtering the sample during the measurement.

(Method of measuring layer thickness)
The thickness of each layer constituting the laser diode can be measured by cutting out a predetermined cross section perpendicular to the substrate, observing this cross section with a transmission electron microscope (TEM), and using the length measurement function of the TEM. As a measurement method, first, a cross section perpendicular to the main surface of the substrate of the laser diode is observed using a TEM. Specifically, for example, the observation width is set to a range of 2 μm or more in a direction parallel to the main surface of the substrate in a TEM image showing a cross section perpendicular to the main surface of the substrate of the laser diode. In the range of this observation width, contrast is observed at the interface between two layers with different compositions, so the thickness to this interface is observed in a continuous observation region with a width of 200 nm. The average value of the thickness of each layer included in this 200 nm wide observation region can be calculated from five points arbitrarily extracted from the above-mentioned observation width of 2 μm or more to obtain the layer thickness of each layer.

(Method of measuring atomic concentration of each layer)
A method for measuring the atomic concentration of each layer constituting a laser diode includes reciprocal space mapping (RSM) using an X-ray diffraction (XRD) method. Specifically, the lattice relaxation rate and Al composition relative to the underlayer can be obtained by analyzing reciprocal lattice mapping data in the vicinity of a diffraction peak obtained with an asymmetric plane as a diffraction plane. Examples of the diffraction plane include the (10-15) plane and the (20-24) plane.
Furthermore, layers or regions in which sufficient reflection intensity cannot be obtained by XRD, such as the light-emitting layer, the inclined layer, 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).

In EELS, the composition of a sample is analyzed by measuring the energy lost when an electron beam passes through the sample. Specifically, for example, the energy loss spectrum of the intensity of the transmitted electron beam is measured and analyzed for a thin sliced sample used in TEM observation. The position of the peak that appears near the energy loss of 20 eV changes depending on the composition of each layer, and the composition can be determined from the peak position.
In the same manner as in the method of calculating layer thickness by TEM observation described above, the average value of the Al composition in an observation width of 200 nm is calculated from five points arbitrarily selected from an observation region of 2 μm or more, thereby obtaining the Al composition of each layer.

In EDX, characteristic X-rays generated by an electron beam in a thin-sectioned sample used in the above-mentioned TEM observation are measured and analyzed. In the same manner as the method for calculating layer thickness using TEM observation described above, the Al composition of each layer is obtained by calculating the average value of the Al composition in an observation width of 200 nm from five points arbitrarily selected from an observation area of 2 μm or more.

In XPS, depth direction evaluation is possible by performing XPS measurement while performing sputter etching using an ion beam. Although Ar+ is generally used for the ion beam, other ion species such as Ar cluster ions can be used as long as they can be irradiated with the etching ion gun mounted on the XPS device. The XPS peak intensities of Al, Ga, and N are measured and analyzed to obtain the depth direction distribution of the Al composition of each layer. Instead of sputter etching, the laser diode can be polished at an angle so that a cross section perpendicular to the main surface of the substrate is enlarged and exposed, and the exposed cross section can be measured by XPS.

The composition of each layer can be measured not only by XPS but also by 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 angled polishing. The composition of each layer can also be measured by SEM-EDX measurement of a cross section exposed by angled polishing.

(Measurement of the size and top surface shape of the hillock structure)
The size (maximum diameter and height) and the top surface shape of the hillock structure including the hillock formed in the laser diode are measured after the mesa structure is formed and before the first electrode is formed during the manufacture of the laser diode. Alternatively, the measurements may be performed after removing the first electrode from the laser diode.
Specifically, the maximum diameter and height of the hillock structure can be measured, for example, by observing the exposed first conductivity type cladding layer after the formation of the mesa structure with a white light interferometer laser microscope.
When a plurality of hillock structures including hillocks are present at an arbitrary measurement point, the median of the maximum diameter and the median of the height of the plurality of hillock structures at the measurement point are calculated and these are set as the maximum diameter and height of the hillock structure, respectively. Furthermore, the top surface shape of the hillock structure can be measured, for example, by observing the first conductive type cladding layer exposed after the formation of the mesa structure with a white light interferometer laser microscope.

(Measurement of the number of hillock structures)
When hillocks are formed, a hillock periphery is formed in the laser diode structure. Therefore, the number and density of hillocks are the number of hillock structures. The number of hillock structures is measured for hillock structures covered by the first electrode, i.e., hillock structures appearing on the first electrode, after the laser diode is fabricated. The number of hillock structures can be measured by observing the first electrode with a white light interferometer laser microscope. Alternatively, the number of hillock structures may be measured after removing the first electrode in the laser diode.
Regarding the number of hillock structures, when half or more of the surface area of a hillock structure overlaps with the measurement point, the number of hillocks at the measurement point is counted as 1. This is because, in a hillock structure including a hillock and a hillock periphery, when half or more of the surface area overlaps with the measurement point, the hillock included in the hillock structure is considered to overlap with the measurement point. As an example, the expression "on the first conductive type cladding layer, there are p hillocks (p is an integer of 1 or more) on the surface facing the first electrode" means that the number of hillock structures whose surface area overlaps with half or more of the region where the first electrode is formed is p.

Furthermore, the number of hillock structures that overlap less than half of the surface area with the measurement point is not counted. In other words, even if there are multiple hillock structures that overlap less than half of the surface area with the measurement point, the number of hillocks for the multiple hillock structures is not counted.
As an example, the expression "there are no hillocks on the surface of the second conductive type cladding layer facing the second electrode" means that the number of hillock structures that overlap with the second electrode region by half or more of their surface area is zero. Hillocks originating from convex portions are not formed in the region overlapping with the second electrode, but hillocks may occur unintentionally. Even in this case, if the surface area of the hillocks that do occur that overlap with the second electrode region is less than half, it can be said that there are "no hillocks."
(Calculation of density of hillock structure)
The density of the hillocks can be calculated by dividing the area of the region where the density is measured by the number of hillocks, for example, by the area of the region where the first electrode is formed (e.g., the bottom area of the first electrode) by the number of hillocks in the region (the number of hillock structures whose surface area overlaps with the region by half or more).

(Measurement of Hillock Cross-Sectional Shape)
The cross-sectional shape of the hillock extending in the thickness direction of the first conductive cladding layer starting from the substrate surface can be measured by cutting out a predetermined cross section perpendicular to the substrate surface, observing this cross section with a transmission electron microscope (TEM), and using the length measurement function of the TEM. As a measurement method, first, a cross section perpendicular to the main surface of the substrate of the laser diode is observed using a TEM. For example, when the top surface shape of the hillock structure including the hillock is an s-gon (s is an even number of 4 or more) in a plan view, the shape of the hillock perpendicular to the substrate can be inferred by measuring the shape of the hillock in a TEM image showing a cross section perpendicular to the diagonal. When the top surface shape of the hillock structure is a t-gon (t is an odd number of 5 or more) in a plan view, the shape of the hillock perpendicular to the substrate can be inferred by measuring the shape of the hillock in a TEM image showing a cross section perpendicular to at least two diagonals. Furthermore, when the top surface shape of the hillock structure is triangular in plan view, it is possible to infer the shape of the hillock in the direction perpendicular to the substrate by measuring the shape in a TEM image showing a cross section taken along a perpendicular line dropped from one vertex in plan view.

(Applications of ultraviolet light emitting elements)
The laser diode according to the present disclosure can be applied to devices in, for example, the medical and life science fields, the environmental field, the industrial and manufacturing fields, the household appliances field, the agricultural field, and other fields. The laser diode can be applied to a drug or chemical synthesis/decomposition device, a liquid/gas/solid (container, food, medical equipment, etc.) sterilization device, a semiconductor cleaning device, etc., a surface modification device for film, glass, metal, etc., an exposure device for manufacturing semiconductors, FPD (Flat Panel Display), PCB (Printed Wiring Board), and other electronic products, a printing/coating device, an adhesive/sealing device, a transfer/molding device for film, pattern, mockup, etc., and a measurement/inspection device for banknotes, wounds, blood, chemical substances, etc.

Examples of liquid sterilization devices include, but are not limited to, automatic ice makers, ice trays and ice storage containers in refrigerators, water tanks for ice makers, freezers, ice makers, humidifiers, dehumidifiers, cold water tanks, hot water tanks and flow pipes of water servers, freestanding water purifiers, portable water purifiers, water supply units, hot water heaters, wastewater treatment devices, garbage disposers, toilet drain traps, washing machines, dialysis water sterilization modules, peritoneal dialysis connector sterilizers, disaster water storage systems, etc.

Examples of gas sterilization devices include, but are not limited to, air purifiers, air conditioners, ceiling fans, floor or bedding vacuum cleaners, futon dryers, shoe dryers, washing machines, clothes dryers, indoor germicidal lamps, storage ventilation systems, shoe boxes, chests of drawers, etc.
Examples of solid sterilization equipment (including surface sterilization equipment) include, but are not limited to, vacuum packing machines, belt conveyors, hand tool sterilization equipment for medical/dental use, barber shops/beauty salons, toothbrushes, toothbrush holders, chopstick cases, cosmetic pouches, drain covers, toilet spot cleaners, toilet lids, etc.

3. Specific Examples of Ultraviolet Light Emitting Devices The laser diode of the present embodiment will be described in more detail below with reference to Figures 1 to 8. Note that the detailed configuration of each layer in each of the following embodiments is as described above.

(3.1) First embodiment Figures 1 and 2 are schematic diagrams for explaining a laser diode 1 according to a first embodiment. Figure 1 is a schematic plan view of the laser diode 1, and Figure 2 is a schematic cross-sectional view of the laser diode 1. In Figure 1, <1-100>, <11-20>, and <0001> respectively indicate crystal orientations.
As shown in Fig. 2, the laser diode 1 includes a substrate 10, a semiconductor laminate 20 disposed on the substrate 10, a first electrode 31, and a second electrode 32. The semiconductor laminate 20 includes a first conductive type cladding layer 21, a light emitting layer 22, and a second conductive type cladding layer 23. A part of the semiconductor laminate 20 has a mesa structure 201. An end surface ES (see Fig. 1) of the mesa structure 201 is a resonator structure for optical resonance and emission, and laser light is emitted in a direction perpendicular to the end surface ES (the direction of the arrow in Fig. 1).

Furthermore, hillocks (not shown) are formed on the surface of the first conductivity type cladding layer 21 of the laser diode 1 that faces the first electrode 31. This allows the threshold voltage of the laser diode 1 to be reduced.
Also, on the first-conductivity-type cladding layer 21, a hillock periphery (not shown) is formed along the outer periphery of the hillock. The hillock and the hillock periphery protruding from the surface of the first-conductivity-type cladding layer 21, i.e., the hillock structure, are covered by the first electrode and are not exposed, but the external shape of the hillock structure can be observed from above the first electrode. The hillock is formed starting from the surface of the substrate 10 and protrudes from the surface of the first-conductivity-type cladding layer 21. The hillock periphery is a protruding shape formed by etching to remove a part of the semiconductor laminate 20 on the first-conductivity-type cladding layer 21 when the mesa structure 201 is formed. The hillock periphery is a history of the protruding part of the hillock formed on the second-conductivity-type cladding layer 23 of the semiconductor laminate 20 removed from the first-conductivity-type cladding layer 21, and has the same maximum diameter, height, and shape as the hillock formed on the second-conductivity-type cladding layer 23.

A region in the mesa structure 201 that is at a distance L of less than 5 μm from the side surface SS is the first region 202 (shown by a dashed line in FIGS. 1 and 2). The laser diode 1 has a plurality of dislocation lines that extend in the <1-100> crystal orientation direction of the substrate 10 only in the first region 202. For this reason, the laser diode 1 has the second electrode 32 in a region on the second conductivity type cladding layer 23 other than the first region 202, that is, in a region on the second conductivity type cladding layer 23 that is 5 μm away from the side surface SS of the mesa structure 201 in a plan view.
In addition, in the laser diode 1, no hillocks are formed on the surface of the second conductive type cladding layer 23 facing the second electrode 32. This allows the injected current to be efficiently coupled in the light emitting layer in the laser diode 1, thereby reducing the oscillation threshold current.

(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 of the laser diode 2 is improved.
Also in the laser diode 2, similarly to the laser diode 1, hillocks (not shown) are formed on the surface of the first conductive type cladding layer 21 facing the first electrode 31, and no hillocks are formed on the surface of the second conductive type cladding layer 23 facing the second electrode 32. As a result, the oscillation threshold current and threshold voltage can also be reduced in the laser diode 2.

(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 1 according to the first 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 of the laser diode 3 is improved.
Also in the laser diode 3, similarly to the laser diode 1, hillocks (not shown) are formed on the surface of the first conductivity type cladding layer 21 facing the first electrode 31. Also in the laser diode 3, no hillocks are formed on the surface of the second conductivity type contact layer 26 facing the second electrode 32. As a result, the oscillation threshold current and threshold voltage can also be reduced in the laser diode 3.
In addition, in the laser diode 3, the hillock outer periphery formed on the first conductivity type cladding layer 21 is a history of the protruding portion of the hillock that was formed on the second conductivity type contact layer 26 of the semiconductor laminate portion 20B that was removed from the first conductivity type cladding layer 21.

The second conductive type 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 type contact layer 26 disposed on the second conductive type 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 of the laser diode 4 is improved.
Also in the laser diode 4, similarly to the laser diode 3, hillocks (not shown) are formed on the surface of the first conductivity type cladding layer 21 facing the first electrode 31, and no hillocks are formed on the surface of the second conductivity type contact layer 26 facing the second electrode 32. As a result, the oscillation threshold current and threshold voltage can be reduced in the laser diode 4 as well.

(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.
Such a laser diode 5 can improve the lateral conductivity of carriers by forming the second conductive type lateral conduction layer 23B and the second conductive type contact layer 26 with complete distortion while improving the conductivity of carriers in the intermediate layer 27.
Also in the laser diode 5, similarly to the laser diode 1, hillocks (not shown) are formed on the surface of the first conductivity type cladding layer 21 facing the first electrode 31, and no hillocks are formed on the surface of the second conductivity type cladding layer 23 (specifically, the second conductivity type lateral conduction layer 23B) facing the second electrode 32. As a result, the oscillation threshold current and threshold voltage can be reduced in the laser diode 5 as well.

(3.6) Sixth embodiment Fig. 7 and Fig. 8 are schematic diagrams for explaining a laser diode 6 according to a sixth embodiment. Fig. 7 is a schematic plan view of the laser diode 6, and Fig. 8 is a schematic cross-sectional view of the laser diode 6.
The laser diode 6 differs from the laser diode 1 according to the first embodiment in that it has a semiconductor laminate 20E further including 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 has a mesa structure 201 including a first-conductivity-type cladding layer 21 and a mesa structure 203 not including a first-conductivity-type cladding layer 21. At this time, the mesa structure 201 including the first-conductivity-type cladding layer 21 includes a plurality of dislocation lines extending in the crystal orientation <1-100> direction of the substrate 10. The mesa structure 203 does not include a first region 202 including dislocation lines extending in the crystal orientation <1-100> direction of the substrate 10. Therefore, it is possible to reduce the oscillation threshold current.
Also in the laser diode 6, similarly to the laser diode 3, hillocks (not shown) are formed on the surface of the first conductivity type cladding layer 21 facing the first electrode 31, and no hillocks are formed on the surface of the second conductivity type contact layer 26 facing the second electrode 32. As a result, the oscillation threshold current and threshold voltage can also be reduced in the laser diode 6.

4. Effects The above-described laser diode and the method for manufacturing the laser diode have the following effects.
(1) A method for manufacturing a laser diode, comprising: exposing a portion of a surface of a nitride semiconductor substrate containing Al; forming a resist mask covering the remaining portion of the surface; etching the portion of the surface of the nitride semiconductor substrate by an etching method to form protrusions having a diameter of 300 nm to 1000 nm and a density of ¹⁰² / ^cm2 to ¹⁰⁵ / ^cm2 in the portion of the surface; forming a first-conductivity-type cladding layer on the nitride semiconductor substrate, the first-conductivity-type cladding layer including a nitride semiconductor having one or more quantum wells and having hillocks formed above the protrusions on the substrate and exposed on the surface; forming a light-emitting layer on the first-conductivity-type cladding layer using a nitride semiconductor having one or more quantum wells; forming a second-conductivity-type cladding layer on the light-emitting layer, the second-conductivity-type cladding layer including a nitride semiconductor having a second conductivity type and having the hillocks formed above the protrusions and exposed on the surface; and etching the portion of the semiconductor laminate including the first-conductivity-type cladding layer, the light-emitting layer, and the second-conductivity-type cladding layer where the hillocks are formed, to expose the first-conductivity-type cladding layer and form a mesa structure having a resonator. Furthermore, a first electrode is formed in the region where the hillocks are exposed on the exposed surface of the first conductivity type cladding layer, and a second electrode is formed on the second conductivity type cladding layer.
As a result, by not forming hillocks on the surface of the second conductivity type cladding layer facing the second electrode, the laser diode can efficiently inject current into the light emitting layer, thereby reducing the oscillation threshold current. Furthermore, by forming hillocks having a lower electrical resistance than other regions in the parallel plane on the surface of the first conductivity type cladding layer facing the first electrode, a laser diode can be produced in which the threshold voltage can be reduced.
(2) A method for manufacturing a laser diode, comprising: exposing a partial region of a surface of a nitride semiconductor substrate containing Al; forming a resist mask covering the remaining region of the surface; etching the partial region of the surface of the nitride semiconductor substrate by an etching method to form protrusions having a diameter of 300 nm to 1000 nm and a density of ¹⁰² / ^cm2 to ¹⁰⁵ / ^cm2 in the partial region; forming a first-conductivity-type cladding layer on the nitride semiconductor substrate, the first-conductivity-type cladding layer including a nitride semiconductor having one or more quantum wells and having hillocks formed above the protrusions on the substrate and exposed on the surface; forming a light-emitting layer on the first-conductivity-type cladding layer using a nitride semiconductor having one or more quantum wells; forming a second-conductivity-type cladding layer on the light-emitting layer, the second-conductivity-type cladding layer including a nitride semiconductor having a second conductivity type and having the hillocks formed above the protrusions and exposed on the surface; and etching the region where the hillocks are formed in the semiconductor laminate including the first-conductivity-type cladding layer, the light-emitting layer, and the second-conductivity-type cladding layer to expose the first-conductivity-type cladding layer, thereby forming a mesa structure having a resonator. Furthermore, a first electrode is formed in the region where the hillocks are exposed on the exposed surface of the first conductivity type cladding layer, and a second electrode is formed on the second conductivity type cladding layer.
As a result, by not forming hillocks on the surface of the second conductivity type cladding layer facing the second electrode, current can be efficiently injected into the light emitting layer, thereby reducing the oscillation threshold current. Furthermore, by forming hillocks having a lower electrical resistance than other regions in the parallel plane on the surface of the first conductivity type cladding layer facing the first electrode, a laser diode can be produced in which the threshold voltage can be reduced.
(3) A laser diode 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 disposed on the nitride semiconductor substrate and containing a nitride semiconductor of a first conductivity type; a light-emitting layer disposed on the first-conductivity-type cladding layer and formed of a nitride semiconductor including one or more quantum wells; and a second-conductivity-type cladding layer disposed on the light-emitting layer and containing a nitride semiconductor of a second conductivity type, wherein a part of the first-conductivity-type cladding layer, the light-emitting layer, and the second-conductivity-type cladding layer of the semiconductor laminate portion form a mesa structure having a resonator, a first electrode is disposed on the first-conductivity-type cladding layer in a region not forming the mesa structure, one or more hillocks are present on a surface of the first-conductivity-type cladding layer facing the first electrode, and a second electrode is disposed on the second-conductivity-type cladding layer, and no hillocks are present on a surface of the second-conductivity-type cladding layer facing the second electrode.
As a result, the laser diode can efficiently inject current into the light-emitting layer by not forming hillocks on the surface facing the second electrode on the second conductivity type cladding layer, thereby reducing the oscillation threshold current, and further, by forming hillocks having a lower electrical resistance than other regions in the parallel plane on the surface facing the first electrode on the first conductivity type cladding layer, the threshold voltage can be reduced.
(4) In the laser diode, a hillock periphery is formed along the outer surface of the hillock on the first conductivity type cladding layer, and the diameter of the hillock structure composed of the hillock and the hillock periphery is preferably 1 μm or more and 30 μm or less.
This allows the diameter of the hillock structure, which is correlated with the threshold voltage, to be appropriately adjusted, thereby making it possible to further reduce the threshold voltage.
(5) The hillock structure preferably has a polygonal shape in a plan view.
This makes it possible to more reliably reduce the threshold voltage by appropriately adjusting the diameter of the hillock structure, which is correlated with the threshold voltage.
(6) In the laser diode, the height of the hillock structure is preferably 10 nm or more and 1000 nm or less when taken from the surface of the first conductivity type cladding layer facing the first electrode.
This can improve the adhesion between the first conductivity type cladding layer and the first electrode.
(7) In the laser diode, the amount of Ga contained in the hillocks is preferably greater than the amount of Ga contained in regions other than the hillocks in a plane of the first conductivity type cladding layer parallel to the surface of the nitride semiconductor substrate.
This ensures that the threshold voltage is reduced.
(8) In a laser diode, the electric resistance of the hillocks is preferably lower than the electric resistance of regions other than the hillocks in a plane parallel to the surface of the nitride semiconductor substrate.
This makes it possible to further reliably reduce the threshold voltage.
(9) In the laser diode, the hillocks preferably have an n-sided pyramid shape (n is an integer of 3 or more) whose diameter increases upward from the surface of the nitride semiconductor substrate.
This makes it possible to more reliably reduce the threshold voltage by appropriately adjusting the diameter of the hillock structure, which is correlated with the threshold voltage.
(10) In the laser diode, the mesa structure preferably has a ratio of the length of an end face of the cavity to the length of a side face of the mesa structure in a plan view of 1:5 or more and 1:500 or less.
This makes it possible to appropriately adjust the distance between the side surfaces of the mesa structure, which is correlated with the threshold voltage and the oscillation threshold current, thereby lowering the threshold voltage and the oscillation threshold current.
(11) In the laser diode, the nitride semiconductor substrate is preferably an AlN single crystal substrate.
This reduces the difference in lattice constant between the substrate and the nitride semiconductor layer formed on the substrate, and by growing the nitride semiconductor layer in a lattice-matched system, threading dislocations can be reduced, making it possible to form a highly stable nitride semiconductor layer.
(12) The laser diode preferably comprises a first conductivity type waveguide layer disposed between the first conductivity type cladding layer and the light emitting layer, and a second conductivity type waveguide layer disposed between the second conductivity type cladding layer and the light emitting layer.
This improves the effect of confining light in the light emitting layer, thereby improving the luminous efficiency.
(13) The laser diode preferably includes a second-conductivity-type contact layer disposed on the second-conductivity-type cladding layer and formed of a nitride semiconductor containing GaN. In this case, the second-conductivity-type cladding layer preferably includes a second-conductivity-type vertical conduction layer containing Al _d Ga _1-d N (0.1≦d≦1) and having a composition gradient in which the Al composition e decreases with increasing distance from the nitride semiconductor substrate, the second-conductivity-type vertical conduction layer having a film thickness of less than 500 nm, and a second-conductivity-type lateral conduction layer containing Al _f Ga _1-f N (0<f≦1).
This allows carriers to be more efficiently injected into the light emitting layer, improving the light emitting efficiency.
(14) 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 formed of _{AlgGa1 -} gN (0<g≦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.
(15) The thickness of the second conductivity type vertical conductive 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.
(16) 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.
(17) 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 distortion with respect to the 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.
(18) The Al composition f of the second conductivity type lateral conduction layer at a surface facing the second conductivity type vertical conduction layer is preferably larger than the minimum value of the Al composition e of the second conductivity type vertical conduction layer.
This allows the carriers to be diffused in the lateral direction, and the carrier injection efficiency can be improved.
(19) The laser diode preferably includes a second electrode provided on the second conductivity type cladding layer in a region that is 5 μm or more away from a side surface of the mesa structure in a plan view.
This prevents internal loss in the laser diode, and improves the light emission efficiency.
(20) In the first conductivity type cladding layer, hillocks are preferably present on the surface facing the first electrode at a density of 10 ² /cm ² or more and 10 ⁵ /cm ² or less.
This makes it possible to more reliably reduce the threshold voltage by appropriately adjusting the number of hillocks on the surface of the first conductivity type cladding layer facing the first electrode, which is correlated with the threshold voltage.

The following describes examples and comparative examples of this disclosure.

[Example 1]
A (0001) AlN single crystal substrate having a thickness of 550 μm was used as the substrate, and protrusions were formed on the substrate by a lithography process and a sputtering process.
First, a convex portion that becomes the starting point of hillocks was formed on the substrate. Specifically, the region on the substrate corresponding to the region where the mesa structure is formed in the laser diode was protected with a mask made of SiO _2. Next, alumina (Al ₂ O ₃ ) particles with a diameter of 0.8 μm were sprayed on the substrate for 1 minute using a micro air blaster device (manufactured by Syscom, "M6500"). Then, the SiO ₂ on the substrate was removed with CF ₄ gas using an inductively coupled plasma device (ICP). As a result, a convex portion made of alumina particles was formed only in the region on the substrate where the mesa structure was not formed. At this time, when the substrate was observed using an optical microscope (manufactured by Keyence, "LM1000"), convex portions made of alumina particles with a diameter of 0.8 μm were formed at a density of 1.7×10 ⁴ pieces/cm ² in the region where the mesa structure was not formed. In addition, there were no microparticles in the region where the mesa structure was formed, and no convex portions were formed.

The substrate with the protrusions was subjected to an annealing process using a metal organic chemical vapor deposition (MOCVD) apparatus. The annealing process was performed in two sets of annealing at 1300° C., each set consisting of annealing for 5 minutes in an NH ₃ atmosphere and 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 at 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 is an AlGaN layer with a thickness of 320 nm, in which the Al composition is distributed in the direction away from the substrate, varying from Al=1.0 to 0.7. The second conductive horizontal conductive layer is an AlGaN layer with Al=0.9 and a thickness of 2 nm. The second conductive cladding layer was formed at a temperature of 1080°C, with a vacuum degree 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. Trimethylaluminum (TMAl) was used as the Al raw material. Triethylgallium (TEGa) was used as the Ga raw material.

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 the manner described above, a semiconductor laminate was formed on an AlN substrate. When reciprocal lattice mapping measurement was performed on the semiconductor laminate by XRD, it was found that the semiconductor laminate had grown pseudomorphically without relaxation up to the p-type contact layer.
Furthermore, hillocks were formed protruding from the surface of the p-type contact layer, which had grown from microparticles on the substrate.

The semiconductor laminate formed as described above was annealed in an _N2 atmosphere at 700°C for 10 minutes or more to further reduce the resistance of the p-type contact layer. The first conductive clad layer (n-type clad layer) was exposed by dry etching with a gas containing _Cl2 using ICP. At this time, a resist mask was arranged so that the region where hillocks were formed in the semiconductor laminate was removed to expose the n-type clad layer, and a mesa structure including the p-type contact layer was formed in the region where hillocks were not formed. The n-type clad layer was formed in a rectangular region long in the <1-100> direction in a plan view.
Furthermore, a hillock periphery, which had a history of the hillock on the p-type contact layer, was formed on the periphery of the hillock on the exposed surface of the n-type cladding layer by the ICP etching, thereby forming a hillock structure composed of the hillock and the hillock periphery on the n-type cladding layer.

After the mesa structure was formed, the hillock structures formed on the n-type cladding layer were observed using a laser microscope (Keyence Corporation, "VK-X3000"). As a result, all of the hillock structures formed had six sides in a plan view. Furthermore, when the maximum diameter and height of the hillock structures were measured, the hillock structures on the n-type cladding layer had a median maximum diameter of 10 μm and a median height of 200 nm.

Furthermore, after the formation of the mesa structure, the Ga concentration (Ga content) of the hillocks in the n-type cladding layer was compared with the Ga concentration of the areas other than the hillocks by EDX measurement. It was found that the Ga concentration of the hillocks was 5% or more higher than that of the areas other than the hillocks in a plane parallel to the substrate surface.

Furthermore, TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and exposed n-type cladding layer in the thickness direction, and it was found that the hillocks formed in the n-type cladding layer originated from convex parts, i.e., microparticles, located on the substrate. The hillocks were also observed to have a larger diameter in the direction away from the substrate, that is, they were seen to extend upward from the convex parts on the substrate surface while expanding in diameter.

In addition, Ni and Au were sequentially deposited on the p-type contact layer in the mesa structure 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 700 μm. In addition, 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 rectangular shape elongated in the <1-100> direction to form a plurality of electrode metal regions as first electrodes. Next, a passivation layer containing SiO ₂ was formed on the surface of the nitride semiconductor laser diode using a chemical vapor deposition (CVD) device.

Furthermore, the substrate was divided into stripes by cleaving the electrode metal region multiple times in parallel to the <11-20> direction to form individual laser diodes. At this time, the width of the second electrode was 5 μm and the length was 600 μm. In the laser diode, the area of the electrode metal region (the bottom area of the first electrode) was 0.0003 ^cm2 on average.

In addition, when the number of hillock structures appearing on the first electrode as the number of hillock structures in the first electrode region of the laser diode of Example 1 was measured, the average was 5. That is, the density of hillocks on the surface of the n-type cladding layer facing the first electrode was 1.7×10 ⁴ /cm ² (=5 /0.0003 cm ² ), which was the same as the density of the convex portions on the substrate. Also, since the mesa structure did not include hillocks originating from the convex portions on the substrate, no hillock structures appeared on the second electrode.
When a current-facet emission intensity measurement was carried out by current injection on the fabricated laser diode of Example 1, the threshold voltage was 9 V and the oscillation threshold current was 10 kA/cm ² .

[Example 2]
A (0001)-face AlN single crystal substrate having a thickness of 550 μm was used, and a resist mask was formed on this substrate using a mask having a circular pattern with a diameter of 1 μm. At this time, the region on the substrate corresponding to the region where the mesa structure is formed in the laser diode was protected with resist. Then, the substrate was etched with Cl ₂ gas using an inductively coupled plasma device (ICP). As a result, a convex portion corresponding to the circular pattern was formed only in the region on the substrate where the mesa structure was not formed. At this time, when the substrate was observed using an optical microscope in the same manner as in Example 1, no convex portion was present in the region where the mesa structure was formed, and convex portions with a diameter of 1 μm were formed at a density of 2×10 ⁴ pieces/cm ² in the other regions. In addition, no convex portion due to the circular pattern was formed in the region where the mesa structure was formed.
A laser diode of Example 2 was fabricated in the same manner as in Example 1, except that a convex portion was formed on the substrate by etching.

As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed with a laser microscope. The measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. In addition, the mesa structure did not include hillocks originating from the convex portion on the substrate.

Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode of Example 2, the average number of hillock structures appearing on the first electrode was 6. The density of hillocks on the surface of the n-type cladding layer facing the first electrode was 2×10 ⁴ /cm ² (=6 / volume 0.0003 cm ² ), which was the same as the density of the convex parts on the substrate. Since the mesa structure did not include hillocks originating from the convex parts on the substrate, no hillock structures appeared on the second electrode.
For the laser diode of Example 2 thus fabricated, a current-facet emission intensity measurement was carried out by current injection, and the threshold voltage was 9.5 V and the oscillation current threshold was 10 kA/cm ² .

[Example 3]
Microparticles were sprayed onto the AlN single crystal substrate for 30 seconds using the same micro air blaster device as in Example 1, and protrusions made of the microparticles were formed on the AlN single crystal substrate at a density of 3.3 x ¹⁰³ particles/ ^cm2 . Otherwise, the laser diode of Example 3 was obtained in the same manner as in Example 1.

As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed with a laser microscope. The measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. In addition, the mesa structure did not include hillocks originating from the convex portion on the substrate.

Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode according to Example 3, the number of hillock structures appearing on the first electrode was 1 on average. The density of hillocks on the surface of the n-type cladding layer facing the first electrode was 3.3×10 ³ /cm ² (=1 /0.0003 cm ² ), which was the same as the density of the convex parts on the substrate. Since the mesa structure did not include hillocks originating from the convex parts on the substrate, no hillock structures appeared on the second electrode.
For the laser diode fabricated according to Example 3, a current-facet emission intensity measurement was carried out by current injection, and the threshold voltage was 11 V and the oscillation current threshold was 10 kA/cm ² .

[Example 4]
Microparticles were sprayed onto the AlN single crystal substrate for 2 minutes using the same micro air blaster device as in Example 1, and protrusions made of the microparticles were formed on the AlN single crystal substrate at a density of 5.0 x ¹⁰⁴ particles/ ^cm2 . Otherwise, the laser diode of Example 4 was obtained in the same manner as in Example 1.

As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed with a laser microscope. The measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. In addition, the mesa structure did not include hillocks originating from the convex portion on the substrate.

Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode fabricated according to Example 4, the average number of hillock structures appearing on the first electrode was 15. The density of hillocks on the surface of the n-type cladding layer facing the first electrode was 5.0×10 ⁴ /cm ² (=15 /0.0003 cm ² ), which was the same as the density of the convex parts on the substrate. In addition, since the mesa structure did not include hillocks originating from the convex parts on the substrate, no hillock structures appeared on the second electrode.
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Example 4, the threshold voltage was 8 V and the oscillation current threshold was 10 kA/cm ² .

[Example 5]
Microparticles were sprayed onto the AlN single crystal substrate for 4 minutes using the same micro air blaster device as in Example 1, and protrusions made of the microparticles were formed on the AlN single crystal substrate at a density of 1.0 x ¹⁰⁵ particles/ ^cm2 . Otherwise, the laser diode of Example 5 was obtained in the same manner as in Example 1.

As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed with a laser microscope. The measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. In addition, the mesa structure did not include hillocks originating from the convex portion on the substrate.

Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode according to Example 5, the average number of hillock structures appearing on the first electrode was 30. The density of hillocks on the surface of the n-type cladding layer facing the first electrode was 1.0×10 ⁵ /cm ² (=30 /0.0003 cm ² ), which was the same as the density of the convex parts on the substrate. In addition, since the mesa structure did not include hillocks originating from the convex parts on the substrate, no hillock structures appeared on the second electrode.
For the fabricated laser diode structure according to Example 5, a current-facet emission intensity measurement was carried out by current injection, and the threshold voltage was 8.5 V and the oscillation current threshold was 10 kA/cm 2 .

[Example 6]
The growth V/III ratio of the n-type cladding layer and the p-type cladding layer was set to 12000. Other than that, the laser diode of Example 6 was obtained in the same manner as in Example 1.
As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed by a laser microscope. The hillock structure thus formed had a median maximum diameter of 1 μm, and the other measurement results (shape and height in plan view) by the laser microscope were the same as those in Example 1. Furthermore, no hillocks originating from the convex portion on the substrate were formed in the mesa structure.

In addition, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the areas other than the hillocks by EDX measurement, the Ga concentration of the hillocks was 5% or more higher than that of the areas other than the hillocks in a plane parallel to the substrate surface, as in the first embodiment. Furthermore, when TEM measurement (observation of the a-plane) was performed on the cross section in the thickness direction of the substrate and the exposed n-type cladding layer, it was found that the hillocks originated from the convex parts arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode according to Example 6, the number of hillock structures appearing on the first electrode was 5 on average, similar to Example 1, and the density of hillocks on the surface of the n-type cladding layer facing the first electrode was 1.7×10 ⁴ /cm ^2. In addition, since no hillocks were formed in the mesa structure, no hillock structures appeared on the second electrode.
When a current-facet emission intensity measurement was carried out by current injection on the laser diode structure according to Example 6, the threshold voltage was 10 V and the oscillation current threshold was 10 kA/cm ² .

[Example 7]
The growth V/III ratio of the n-type cladding layer and the p-type cladding layer was set to 8000. Other than that, the laser diode of Example 7 was obtained in the same manner as in Example 1.
As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed by a laser microscope. The hillock structure thus formed had a median maximum diameter of 5 μm. Other measurement results by the laser microscope (shape and height in plan view) were the same as those in Example 1.
Furthermore, in the mesa structure, no hillocks originating from the protrusions on the substrate were formed.

In addition, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the areas other than the hillocks by EDX measurement, the Ga concentration of the hillocks was 5% or more higher than that of the areas other than the hillocks in a plane parallel to the substrate surface, as in the first embodiment. Furthermore, when TEM measurement (observation of the a-plane) was performed on the cross section in the thickness direction of the substrate and the exposed n-type cladding layer, it was found that the hillocks originated from the convex parts arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode according to Example 7, the number of hillock structures appearing on the first electrode was 5 on average, similar to Example 1, and the density of hillocks on the surface of the n-type cladding layer facing the first electrode was 1.7×10 ⁴ /cm ^2. Since no hillocks were formed in the mesa structure, no hillock structures appeared on the second electrode.
For the fabricated laser diode structure according to Example 7, a current-facet emission intensity measurement was carried out by current injection, and the threshold voltage was 9.5 V and the oscillation current threshold was 10 kA/cm ² .

[Example 8]
The growth V/III ratio of the n-type cladding layer and the p-type cladding layer was set to 2000. Other than that, the laser diode of Example 8 was obtained in the same manner as in Example 1.
As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed by a laser microscope. The hillock structure thus formed had a median maximum diameter of 15 μm. Other measurement results by the laser microscope (shape and height in plan view) were the same as those in Example 1.
Furthermore, in the mesa structure, no hillocks were formed starting from the protrusions on the substrate.

In addition, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the areas other than the hillocks by EDX measurement, the Ga concentration of the hillocks was 5% or more higher than that of the areas other than the hillocks in a plane parallel to the substrate surface, as in the first embodiment. Furthermore, when TEM measurement (observation of the a-plane) was performed on the cross section in the thickness direction of the substrate and the exposed n-type cladding layer, it was found that the hillocks originated from the convex parts arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode according to Example 8, the number of hillock structures appearing on the first electrode was 5 on average, similar to Example 1, and the density of hillocks on the surface of the n-type cladding layer facing the first electrode was 1.7×10 ⁴ /cm ^2. Since no hillocks were formed in the mesa structure, no hillock structures appeared on the second electrode.
For the fabricated laser diode structure according to Example 8, a current-facet emission intensity measurement was carried out by current injection, and the threshold voltage was 8.5 V and the oscillation current threshold was 10 kA/cm ² .

[Example 9]
The growth V/III ratio of the n-type cladding layer and the p-type cladding layer was set to 1000. Other than that, the laser diode of Example 9 was obtained in the same manner as in Example 1.
As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed by a laser microscope. The hillock structure thus formed had a median maximum diameter of 30 μm. Other measurement results by the laser microscope (shape and height in plan view) were the same as those in Example 1.
Furthermore, in the mesa structure, no hillocks originating from the protrusions on the substrate were formed.

In addition, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the areas other than the hillocks by EDX measurement, the Ga concentration of the hillocks was 5% or more higher than that of the areas other than the hillocks in a plane parallel to the substrate surface, as in the first embodiment. Furthermore, when TEM measurement (observation of the a-plane) was performed on the cross section in the thickness direction of the substrate and the exposed n-type cladding layer, it was found that the hillocks originated from the convex parts arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode according to Example 9, the number of hillock structures appearing on the first electrode was 5 on average, similar to Example 1, and the density of hillocks on the surface of the n-type cladding layer facing the first electrode was 1.7×10 ⁴ /cm ^2. In addition, since no hillocks were formed in the mesa structure, no hillock structures appeared on the second electrode.
When a current-facet emission intensity measurement was carried out by current injection on the laser diode structure according to Example 9, the threshold voltage was 10 V and the oscillation current threshold was 10 kA/cm ² .

[Example 10]
The growth rate of the n-type cladding layer was set to 0.15 μm/h. Other than that, the laser diode of Example 10 was obtained in the same manner as in Example 1.
As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed by a laser microscope. As a result, the hillock structure formed had a median height of 20 nm. Other measurement results by the laser microscope (shape in plan view, maximum diameter) were the same as in Example 1. Furthermore, no hillocks originating from the convex portion on the substrate were formed in the mesa structure.
In addition, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface, as in the first embodiment. Furthermore, when TEM measurement (observation on the a-plane) was performed on the cross section in the thickness direction of the substrate and the exposed n-type cladding layer, it was found that the hillocks were generated from the convex parts arranged on the substrate, and the diameter expanded in the direction away from the substrate from the cross section.

In the fabricated laser diode structure according to Example 10, the number of hillock structures appearing on the first electrode was 5 on average, similar to Example 1, and the density of hillocks on the surface of the n-type cladding layer facing the first electrode was 1.7×10 ⁴ /cm ^2. In addition, since hillocks were not formed in the mesa structure, no hillock structures appeared on the second electrode.
When a current-facet emission intensity measurement was carried out by current injection on the laser diode structure according to Example 10, the threshold voltage was 10 V and the oscillation current threshold was 10 kA/cm ² .

[Example 11]
The growth rate of the n-type cladding layer was set to 0.25 μm/h. Other than that, the laser diode of Example 11 was obtained in the same manner as in Example 1.

As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed by a laser microscope. The hillock structure thus formed had a median height of 100 nm. Other measurement results by the laser microscope (shape in plan view, maximum diameter) were the same as those in Example 1. Furthermore, no hillocks originating from the convex portion on the substrate were formed in the mesa structure.
In addition, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface, as in the first embodiment. Furthermore, when TEM measurement (observation on the a-plane) was performed on the cross section in the thickness direction of the substrate and the exposed n-type cladding layer, it was found that the hillocks were generated from the convex parts arranged on the substrate, and the diameter expanded in the direction away from the substrate from the cross section.

In the fabricated laser diode structure according to Example 11, the number of hillock structures appearing on the first electrode was 5 on average, similar to Example 1, and the hillock density on the surface of the n-type cladding layer facing the first electrode was 1.7×10 ⁴ /cm ^2. In addition, since no hillocks were formed in the mesa structure, no hillock structures appeared on the second electrode.
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Example 11, the threshold voltage was 9.5 V and the oscillation current threshold was 10 kA/cm 2 .

[Example 12]
The growth rate of the n-type cladding layer was set to 0.45 μm/h. Other than that, the laser diode of Example 12 was obtained in the same manner as in Example 1.

As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed by a laser microscope. As a result, the hillock structure formed had a median height of 500 nm. Other measurement results by the laser microscope (shape in plan view, maximum diameter) were the same as in Example 1. Furthermore, no hillocks originating from the convex portion on the substrate were formed in the mesa structure.
Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode structure fabricated according to Example 12, the average number of hillocks was 5, similar to Example 1, and the hillock density on the surface of the n-type cladding layer facing the first electrode was ^7.4 .
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Example 12, the threshold voltage was 8.5 V and the oscillation current threshold was 10 kA/cm ² .

[Example 13]
The growth rate of the n-type cladding layer was set to 0.55 μm/h. Other than that, the laser diode of Example 13 was obtained in the same manner as in Example 1.

As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed by a laser microscope. The hillock structure thus formed had a median height of 900 nm. Other measurement results by the laser microscope (shape in plan view, maximum diameter) were the same as in Example 1. Furthermore, no hillocks originating from the convex portion on the substrate were formed in the mesa structure.
Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode structure fabricated according to Example 13, the number of hillock structures appearing on the first electrode was five on average, similar to Example 1, and the hillock density on the surface of the n-type cladding layer facing the first electrode was 1.7× ¹⁰ / ^cm .
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Example 13, the threshold voltage was 10 V and the oscillation current threshold was 10 kA/cm ² .

[Example 14]
A laser diode of Example 14 was obtained in the same manner as in Example 1, except that the length of the second electrode was 200 μm and the width was 5 μm.

As in Example 1, after the formation of the mesa structure, the hillock structure formed on the n-type cladding layer was observed by a laser microscope, and the measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. Moreover, the mesa structure did not include hillocks originating from the convex portion on the substrate.
Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode structure fabricated according to Example 14, the number of hillock structures appearing on the first electrode was five on average, similar to Example 1, and the hillock density on the surface of the n-type cladding layer facing the first electrode was 1.7× ¹⁰ / ^cm .
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Example 14, the threshold voltage was 11 V and the oscillation current threshold was 15 kA/cm ² .

[Example 15]
A laser diode of Example 15 was obtained in the same manner as in Example 1, except that the length of the p-type electrode was 1500 μm and the width was 5 μm.

As in Example 1, after the formation of the mesa structure, the hillock structure formed on the n-type cladding layer was observed by a laser microscope, and the measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. Moreover, the mesa structure did not include hillocks originating from the convex portion on the substrate.
Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode structure fabricated according to Example 15, the number of hillock structures appearing on the first electrode was five on average, similar to Example 1, and the hillock density on the surface of the n-type cladding layer facing the first electrode was 1.7× ¹⁰ / ^cm .
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Example 15, the threshold voltage was 10.5 V and the oscillation current threshold was 8 kA/cm ² .

[Example 16]
A laser diode of Example 16 was obtained in the same manner as in Example 1, except that the length of the p-type electrode was 600 μm and the width was 2 μm.

As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed with a laser microscope. The measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. In addition, the mesa structure did not include hillocks originating from the convex portion on the substrate.

Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode structure fabricated according to Example 16, the number of hillock structures appearing on the first electrode was five on average, similar to Example 1, and the hillock density on the surface of the n-type cladding layer facing the first electrode was 1.7× ¹⁰ / ^cm .
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Example 16, the threshold voltage was 7.2 V and the oscillation current threshold was 10 kA/cm ² .

[Example 17]
A laser diode of Example 17 was obtained in the same manner as in Example 1, except that the length of the p-type electrode was 600 μm and the width was 10 μm.

As in Example 1, after the formation of the mesa structure, the hillock structure formed on the n-type cladding layer was observed by a laser microscope, and the measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. Moreover, the mesa structure did not include hillocks originating from the convex portion on the substrate.
Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode structure fabricated according to Example 17, the number of hillock structures appearing on the first electrode was five on average, similar to Example 1, and the hillock density on the surface of the n-type cladding layer facing the first electrode was 1.7× ¹⁰ / ^cm .
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Example 17, the threshold voltage was 11 V and the oscillation current threshold was 10 kA/cm ² .

[Comparative Example 1]
The laser diode of Comparative Example 1 was obtained in the same manner as in Example 1, except that no protrusions were formed (processed) on the AlN single crystal substrate.
After the mesa structure was formed, observation with a laser microscope revealed that no hillocks were formed on the n-type cladding layer or on the mesa structure.
When a current-facet emission intensity measurement was carried out by current injection on the laser diode according to Comparative Example 1, the threshold voltage was 17 V and the oscillation current threshold was 10 kA/cm ² .

[Comparative Example 2]
Microparticles were sprayed onto the AlN single crystal substrate for 5 minutes using the same micro air blaster device as in Example 1, and protrusions made of the microparticles were formed on the AlN single crystal substrate at a density of 1.2 x ¹⁰⁵ particles/ ^cm2 . Otherwise, the laser diode of Comparative Example 2 was obtained in the same manner as in Example 1.

As in Example 1, after the formation of the mesa structure, the hillock structure formed on the n-type cladding layer was observed by a laser microscope, and the measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. Moreover, the mesa structure did not include hillocks originating from the convex portion on the substrate.
Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode structure fabricated according to Comparative Example 2, the average number of hillock structures appearing on the first electrode was 35. The density of hillocks on the surface of the n-type cladding layer facing the first electrode was 1.2×10 ⁵ /cm ² (=35 /0.0003 cm ² ), which was the same as the density of protrusions on the substrate.
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Comparative Example 2, the threshold voltage was 13.5 V and the oscillation current threshold was 10 kA/cm ² .

[Comparative Example 3]
Using the same micro air blaster device as in Example 1, convex portions made of microparticles were formed on the AlN single crystal substrate at a density of 2.3 x ¹⁰⁴ particles/ ^cm2 . In addition, when forming the convex portions on the substrate by the lithography process and the sputtering process, the region on the substrate corresponding to the formation region of the mesa structure in the laser diode was not protected with _SiO2 . Otherwise, the laser diode of Comparative Example 3 was fabricated in the same manner as in Example 1.

As in Example 1, after the mesa structure was formed, the hillock structure formed on the n-type cladding layer was observed with a laser microscope. The measurement results (shape in plan view, maximum diameter, height) were the same as in Example 1. Hillocks originating from the convex portion on the substrate were also formed on the p-type contact layer of the mesa structure.

Furthermore, when the Ga content (Ga concentration) of the hillocks was compared with the Ga concentration of the regions other than the hillocks by EDX measurement, it was found that, similarly to the first embodiment, the Ga concentration of the hillocks was 5% or more higher than that of the regions other than the hillocks in a plane parallel to the substrate surface.
Furthermore, when TEM measurements (observation of the a-plane) were performed on the cross sections of the substrate and the exposed n-type cladding layer in the thickness direction, it was found that the hillocks originated from convex portions arranged on the substrate, and the cross section showed that the diameter expanded in the direction away from the substrate.

In the laser diode structure according to Comparative Example 3, the average number of hillock structures appearing on the first electrode was 7. The density of hillocks on the surface of the n-type cladding layer facing the first electrode was 2.3×10 ⁴ /cm ² (= number of hillocks: 7 /0.0003 cm ² ), which was the same as the density of the protrusions on the substrate. The number of hillock structures appearing on the second electrode was 2.
When a current-facet emission intensity measurement was carried out by current injection on the laser diode of Comparative Example 3, the threshold voltage was 9 V and the oscillation current threshold was 25 kA/cm ² .

The evaluation results for each example and comparative example are shown in Table 1 below.

As shown in Table 1, in the laser diode according to each embodiment, a first electrode is disposed on the first conductivity type cladding layer (n-type cladding layer in each embodiment and each comparative example) in a region where the mesa structure is not formed, and 1 to 30 hillocks are present on the surface of the first conductivity type cladding layer facing the first electrode. Furthermore, in the laser diode according to each embodiment, a second electrode is disposed on the second conductivity type cladding layer (p-type cladding layer in each embodiment and each comparative example), and no hillocks are present on the surface of the second conductivity type cladding layer facing the second electrode.
As a result, the laser diodes in each example had a threshold voltage of 11 V or less and an oscillation threshold of 15 KA/cm ² or less. In other words, the laser diodes in each example were able to suppress the oscillation threshold current and threshold voltage to relatively low values.

Moreover, the laser diodes in the respective examples had a significantly reduced threshold voltage compared to the laser diode according to Comparative Example 1, which had no hillocks on the surface of the first conductive type cladding layer facing the first electrode, and the laser diodes in the respective examples had a significantly reduced threshold voltage compared to the laser diode according to Comparative Example 2, which had more than 30 hillocks on the surface of the first conductive type cladding layer facing the first electrode.
Furthermore, the laser diodes in the examples had a significantly reduced oscillation threshold current, as compared with the laser diode of Comparative Example 3 having hillocks on the surface of the second conductivity type cladding layer facing the second electrode.

In addition, the laser diodes of Examples 4 and 5, in which the number of hillocks on the surface of the first conductivity type cladding layer facing the first electrode was 15 or more, had a lower threshold voltage than the laser diode of Example 3, in which the number of hillocks was one.
In addition, the laser diodes of Examples 7 and 8, in which the maximum diameter of the hillocks on the surface of the first conductivity type cladding layer facing the first electrode was in the range of 5 μm to 15 μm, had a lower threshold voltage than Examples 6 and 9.

From the above, it was found that it is necessary to appropriately set the position where hillocks are formed in the laser diode, and the number of hillocks and the maximum diameter of the hillocks on the surface of the first conductivity type cladding layer facing the first electrode.

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.

1, 2, 3, 4, 5, 6 Laser diode 10 Substrate 20, 20A, 20B, 20C, 20D, 20E Semiconductor laminate portion 21 First conductivity type cladding layer 22 Light emitting layer 23 Second conductivity type cladding layer 231 Second conductivity type vertical conductive layer 232 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 31 First electrode 32 Second electrode 201 Mesa structure including first conductivity type cladding layer 202 First region 203 of mesa structure including first conductivity type cladding layer Mesa structure not including first conductivity type cladding layer (ridge portion)
L: Distance from the side of the mesa structure SS: Side of the mesa structure ES: End surface of the mesa structure

Claims (20)

forming a protrusion made of microparticles having a diameter of 300 nm or more and 1000 nm or less and a density of 10 ² particles/cm ² or more and 10 ⁵ particles/cm ² or less on a partial region of a surface of an Al-containing nitride semiconductor substrate;
forming a first conductivity type cladding layer on the nitride semiconductor substrate, the first conductivity type cladding layer including a nitride semiconductor and having hillocks formed above the protrusions and exposed on a surface thereof;
forming a light emitting layer on the first conductive type cladding layer using a nitride semiconductor including one or more quantum wells;
forming a second conductive type cladding layer on the light emitting layer, the second conductive type cladding layer including a second conductive type nitride semiconductor and having the hillocks formed above the protrusions and exposed on a surface thereof;
a region of a semiconductor laminate including the first conductive type cladding layer, the light emitting layer, and the second conductive type cladding layer, in which the hillock is formed, is etched to expose the first conductive type cladding layer, thereby forming a mesa structure having a resonator;
forming a first electrode in a region where the hillock is exposed on the exposed surface of the first conductive type cladding layer;
forming a second electrode on the second conductive type cladding layer;
A method for manufacturing a laser diode. a resist mask is formed on a surface of an Al-containing nitride semiconductor substrate so as to expose a partial area of the surface and cover the remaining area of the surface;
etching the partial region of the surface of the nitride semiconductor substrate by an etching method to form protrusions having a diameter of 300 nm to 1000 nm and a density of 10 ² /cm ² to 10 ⁵ /cm ² in the partial region;
forming a first conductivity type cladding layer on the nitride semiconductor substrate, the first conductivity type cladding layer including a nitride semiconductor and having hillocks formed above the protrusions and exposed on a surface thereof;
forming a light emitting layer on the first conductive type cladding layer using a nitride semiconductor including one or more quantum wells;
forming a second conductive type cladding layer on the light emitting layer, the second conductive type cladding layer including a second conductive type nitride semiconductor and having the hillocks formed above the protrusions and exposed on a surface thereof;
a region of a semiconductor laminate including the first conductive type cladding layer, the light emitting layer, and the second conductive type cladding layer, in which the hillock is formed, is etched to expose the first conductive type cladding layer, thereby forming a mesa structure having a resonator;
forming a first electrode in a region where the hillock is exposed on the exposed surface of the first conductive type cladding layer;
forming a second electrode on the second conductive type cladding layer;
A method for manufacturing a laser diode. 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 disposed on the nitride semiconductor substrate and including a first conductivity type nitride semiconductor;
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 disposed on the light emitting layer and including a second conductivity type nitride semiconductor;
a part of the first conductive type cladding layer, the light emitting layer, and the second conductive type cladding layer of the semiconductor laminate portion form a mesa structure having a resonator;
a first electrode is disposed on the first conductive type cladding layer in a region where the mesa structure is not formed, and 1 to 30 hillocks are present on a surface of the first conductive type cladding layer facing the first electrode;
a second electrode is disposed on the second conductive type clad layer, and no hillocks are present on a surface of the second conductive type clad layer facing the second electrode;
Laser diode. a hillock periphery is formed on the first conductive type cladding layer along a peripheral surface of the hillock;
a diameter of a hillock structure formed by the hillock and the hillock periphery is 1 μm or more and 30 μm or less;
4. The laser diode of claim 3. The hillock structure has a polygonal shape in a plan view.
5. The laser diode of claim 4. a height of the hillock structure is 10 nm or more and 1000 nm or less based on a surface of the first conductive type cladding layer facing the first electrode;
6. The laser diode according to claim 4 or 5. 7. The laser diode according to claim 3, wherein the amount of Ga contained in the hillocks is greater than the amount of Ga contained in regions other than the hillocks in a plane parallel to the surface of the nitride semiconductor substrate in the first conductivity type cladding layer. 8. The laser diode according to claim 3, wherein the hillocks have a lower electrical resistance than a region other than the hillocks in a plane parallel to the surface of the nitride semiconductor substrate. 9. The laser diode according to claim 3, wherein the hillock has an n-sided pyramid shape (n is an integer of 3 or more) whose diameter increases upward from the surface of the nitride semiconductor substrate. 10. The laser diode according to claim 3, wherein the mesa structure has a ratio of a length of an end face of the resonator to a length of a side face of the mesa structure in a plan view of 1:5 or more and 1:500 or less. The laser diode according to claim 3 , 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;
12. A laser diode according to any one of claims 3 to 11. a second conductive type contact layer disposed on the second conductive type cladding layer and formed of a nitride semiconductor containing GaN;
The second conductive type cladding layer is
13. The laser diode according to claim 12, comprising: a second conductivity type vertical conduction layer having a thickness of 500 nm or less, the second conductivity type vertical conduction layer including Al _e Ga ₁ -e N (0.1≦e≦1) and having a composition gradient in which the Al composition e decreases with increasing distance from the nitride semiconductor substrate; and a second conductivity type lateral conduction layer including Al _f Ga _1-f N (0<f≦1). an intermediate layer disposed between the second conductive type vertical conductive layer and the second conductive type waveguide layer and made of _{AlgGa1 -gN} (0<g≦1.0);
14. The laser diode of claim 13. The thickness of the second conductive type vertical conductive layer is 250 nm or more and 450 nm or less.
15. A laser diode according to claim 13 or 14. 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;
16. A laser diode according to any one of claims 13 to 15. 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;
17. A laser diode according to any one of claims 13 to 16. an Al composition f 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 e of the second conductive type vertical conduction layer;
18. A laser diode according to any one of claims 13 to 17. the second electrode is provided in a region on the second conductive type cladding layer that is spaced apart from a side surface of the mesa structure by 5 μm or more in a plan view;
19. A laser diode according to any one of claims 3 to 18. 20. The laser diode according to claim 3, wherein the hillocks are present on the surface of the first conductive type cladding layer facing the first electrode at a density of ¹⁰² pieces/cm2 or ^more and ¹⁰⁵ pieces/ ^cm2 or less.

Images