JP7646147B2 - Laser Diode - Google Patents

Description

This disclosure relates to laser diodes.

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 has been disclosed for oscillating a current-injection type laser diode in the ultraviolet region (for example, Non-Patent Document 1).

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

The ultraviolet laser diodes described above are pulse driven, and continuous oscillation is required for practical applications. This continuous oscillation requires a reduction in the oscillation threshold current. When the light-emitting layer relaxes during the film formation and processing processes of the laser diode, the oscillation threshold current deteriorates significantly, i.e., the threshold current required for oscillation increases. The oscillation threshold current also deteriorates when dislocations are introduced into the light-emitting layer.

The objective of this disclosure is to provide a laser diode with a low oscillation threshold current.

In order to solve the above-mentioned problems, a laser diode according to an embodiment of the present disclosure includes a nitride semiconductor substrate containing Al, and a semiconductor laminate portion disposed on the nitride semiconductor substrate, the semiconductor laminate portion having: a first conductivity type cladding layer disposed on the substrate and including a nitride semiconductor layer of a first conductivity type; a nitride semiconductor light emitting layer disposed on the first conductivity type cladding layer and including one or more quantum wells; and a second conductivity type cladding layer disposed on the light emitting layer and including a nitride semiconductor layer of a second conductivity type, at least a portion of the semiconductor laminate portion has a mesa structure for optical resonance and emission, and is characterized in that the mesa structure including the first conductivity type cladding layer has a plurality of dislocation lines extending in the crystal orientation <1-100> of the nitride semiconductor substrate only in a first region having a distance L of less than 15 μm from a side surface of the mesa structure.
It should be noted that the above summary of the invention does not list all of the features of the present disclosure.

According to one aspect of the present disclosure, it is possible to provide a laser diode with a low oscillation threshold current.

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

1. Embodiment 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 including a first conductive type nitride semiconductor layer, a light emitting layer disposed on the first conductive type cladding layer and formed of a nitride semiconductor including one or more quantum wells, and a second conductive type cladding layer disposed on the light emitting layer and including a second conductive type nitride semiconductor layer. At least a part of the semiconductor laminate has a mesa structure for optical resonance and emission. The semiconductor laminate includes a plurality of dislocation lines extending in the crystal orientation <1-100> of the nitride semiconductor substrate only in a first region in the mesa structure including the first conductive type cladding layer, the first region being at a distance L from the side surface of less than 15 μm.
Each layer of the laser diode will now be described in detail.

(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 of the composition of other layers, the wording "comprises" has a similar meaning. Also, 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 a substrate. Here, for example, the word "on" in the expression "the first conductive cladding layer is formed on a 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 a second conductive cladding layer is formed on the first conductive waveguide layer described later via an electron block layer. In addition, in the description of this embodiment, the "first conductive type" and the "second conductive type" mean semiconductors exhibiting 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 the purpose of controlling the longitudinal conductivity. In this case, the above-mentioned limitation on the Al composition may be the Al composition obtained by averaging the Al composition at a position in the film thickness direction within the first conductivity type cladding layer over the film thickness of 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., and impurities such as 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 viewpoints of lattice relaxation within the first conductivity type cladding layer and 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 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 b=0.52 and c is 0.57 or more. In addition, from the viewpoint of light confinement and layer resistance, the thickness of the waveguide layer is preferably 70 nm or more and 150 nm or less, in terms of the total thickness of the first conductive type waveguide layer and the second conductive type waveguide layer.

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 below, the Al composition of the second conductivity type waveguide layer may be higher than the Al composition of the first conductivity type waveguide layer. For the same purpose, the film thickness of the second conductivity type waveguide layer may be thicker than the film thickness of the first conductivity type waveguide.

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

(Second Conductive Type Cladding Layer)
The second-conductivity-type cladding layer is 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 in the shape of the waveguide layer (second-conductivity-type waveguide layer). This allows the second-conductivity-type cladding layer to easily lattice-match with the light-emitting layer or the waveguide layer, and makes it possible to suppress threading dislocation density.

The second 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 composition gradient layer (second conductivity type vertical conduction layer) formed of Al _e Ga _(1-e) N in which 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 e 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 e does not decrease in the direction away from the substrate, but does not include a portion in which the Al composition e 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 conductive 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 constituting a region of the second-conductivity-type cladding layer opposite to the light-emitting layer, and is formed on the second-conductivity-type vertical conduction layer.
The second-conductivity-type 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 even more 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, the mesa structure is preferably rectangular in plan view having long and short sides, with the long sides extending in the <1-100> direction. This is because, when obtaining the resonator mirror end faces of the laser resonator by various methods such as cleavage and etching, the atomically flat (1-100) plane is the easiest to form.
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 semiconductor laminate portion has a plurality of dislocation lines extending in the <1-100> crystal orientation of the nitride semiconductor substrate in a first region having a distance L of less than 15 μm from the side surface of the mesa structure. The dislocation lines are formed only in the first region of the mesa structure. Here, "extending in the <1-100>direction" means that the dislocation lines do not need to be linear and extend completely in the <1-100> direction, but rather that the dislocation lines extend in the <1-100> direction as a whole. In addition, the dislocation lines do not need to be a single line, and may branch off midway or meet with other dislocation lines.
In addition, the distance between the side surfaces of the mesa structure is correlated with the threshold voltage and the oscillation threshold current, and therefore, in a rectangular mesa structure, it is preferable that the ratio of the distance between the end faces of the resonator mirror in a plan view to the distance between the side surfaces of the mesa structure is 5:1 to 500:1.

The semiconductor laminate may have a plurality of mesa structures arranged in the <11-20> direction. That is, the semiconductor laminate may have a form having a plurality of discontinuous side surfaces. From the viewpoint of efficiently injecting power into the light emitting layer, it may be preferable to have a mesa structure (also called a ridge portion) in which a part or all of the layers above the second conductive cladding layer are removed except for the vicinity of the second electrode. Examples of the layers above the second conductive cladding layer include a second conductive contact layer, or a second conductive contact layer and a second conductive cladding layer. In this case, the ridge portion is different from the mesa structure for exposing the first conductive cladding layer, and it is understood that, for example, the side surface of the ridge portion is different from the side surface of the mesa structure.
From the viewpoint of reducing the oscillation threshold current, it is preferable that the mesa structure having a ridge structure has no dislocation lines horizontal to the <1-100> direction in a region that is less than 15 μm away from the side surface of the ridge at a distance L. In other words, it is preferable that the ridge is formed so as not to overlap with the first region of the mesa structure in a plan view.

The mesa structure can be formed by etching the semiconductor laminate with inductively coupled plasma (ICP) or the like. Depending on the etching conditions at this time, dislocations are introduced from the end face (resonator mirror end face) of the mesa structure. This is because the semiconductor laminate grows pseudomorphically in which the semiconductor laminate grows lattice-matched to the substrate, and distortion accumulates in the semiconductor laminate. By etching the semiconductor laminate, the distortion is released and dislocations are introduced throughout the semiconductor laminate. In this case, dislocations are also introduced into the light-emitting layer formed above the semiconductor laminate. In particular, when dislocations are introduced into the region of the light-emitting layer located below the second electrode, the oscillation threshold current is significantly deteriorated. In order to avoid the introduction of dislocations into the light-emitting layer, it is preferable to adopt etching conditions that introduce dislocations only into the end of the mesa structure.

From the viewpoint of preventing dislocations from being introduced into the light-emitting layer, it is preferable that the dislocations introduced into the end of the mesa structure are formed only in a region less than 15 μm from the end, and that the dislocations extend in a direction along the <1-100> direction. In other words, it is preferable that dislocations exist only in the first region, which is a region 15 μm from the end of the mesa structure, and that no dislocation lines extend in the <1-100> direction or the <11-20> direction exist in the region 15 μm or more and less than 20 μm from the end of the mesa structure.

At this time, it is preferable that the number of dislocation lines extending in the <1-100> direction in the first region is greater than the number of dislocation lines extending in directions other than the <1-100> direction. It is also preferable that the Burgers vector of the dislocation line has a component in the <11-20> direction perpendicular to the <1-100> direction. It is also preferable that the number of dislocation lines extending in the <1-100> direction per unit length in the first region is 4.5 lines/μm to 11 lines/μm, and more preferably 4.5 lines/μm to 7.5 lines/μm. Furthermore, the average spacing of dislocation lines extending in the <1-100> direction in the first region is preferably 1/100 atomic plane to 1/5000 atomic plane, and more preferably 1/300 atomic plane to 1/700 atomic plane, relative to the (11-20) atomic plane.

These dislocations can be efficiently alleviated by being introduced into the first conductivity type cladding layer, which is the thickest and has the most accumulated strain. Therefore, it is preferable that the dislocation lines extending in the <1-100> direction are located in the first conductivity type cladding layer.

(electrode)
The laser diode can be formed by a second electrode disposed on the second conductivity type cladding layer and a first electrode disposed on the first conductivity type cladding layer, where the first electrode is formed in electrical contact with the first conductivity type cladding layer and the second electrode is formed in electrical contact with the second conductivity type cladding layer.
The first electrode can be disposed, for example, on the back side of the substrate, or on the first conductive type cladding layer exposed by removing layers above the first conductive type cladding layer of the semiconductor laminate by, for example, chemical etching or dry etching.

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-conductivity-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 a small number of defects is formed on the buffer layer. Furthermore, by providing the buffer layer, the first-conductivity-type cladding layer can be grown under compressive stress, and the occurrence of cracks in the first-conductivity-type cladding layer can be suppressed. Therefore, even if the substrate is formed of a nitride semiconductor such as AlN or AlGaN, a nitride semiconductor layer having a small number of 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 μm. 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.

(1.2) Method for Manufacturing Ultraviolet Light Emitting Device The laser diode of this embodiment is manufactured through the steps of forming each layer on a substrate.

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

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

Here, among the layers formed on the substrate, the nitride semiconductor layer can be formed using, for example, an Al source containing trimethylaluminum (TMAl), a Ga source containing trimethylgallium (TMGa) or triethylgallium (TEGa), or the like, or a N source containing ammonia (NH ₃ ).

Laser diodes are manufactured through a process in which unnecessary portions of each layer of a semiconductor laminate formed on a substrate are removed by etching. Removal of unnecessary portions of each layer of a semiconductor laminate can be performed, for example, by inductively coupled plasma (ICP) etching.

(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, for example, 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.

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 that constitutes 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 relaxation)
The presence or absence of relaxation can be obtained by the lattice constant calculated from the lattice diffraction image of the TEM. The strain can be expressed by how much the lattice constant in the in-plane direction of the a-axis of each layer of the laser diode has changed from the original lattice constant. If the lattice constant is the same as the lattice constant of the substrate, it is strained, and if there is a difference, it can be said to be relaxed. Here, relaxation calculated by calculation is 20% or more. The difference in lattice constant can be measured by automatic mapping of the lattice diffraction image at each spot and displayed as a mapping by analysis software. Specifically, the measurement can be performed using analysis software (ASTER manufactured by NanoMEGAS).

(Method of measuring atomic concentration of each layer)
A method for measuring the atomic concentration in each layer constituting a laser diode includes reciprocal space mapping (RSM) using X-ray diffraction (XRD). Specifically, the lattice relaxation rate and Al composition relative to the underlayer can be obtained by analyzing reciprocal space 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 oblique polishing. The composition of each layer can also be measured by SEM-EDX measurement of a cross section exposed by oblique polishing.

(Method of measuring dislocation lines at the edge of a mesa structure)
The dislocation lines formed at the end of the mesa structure can be measured by cutting out a predetermined cross section parallel to the surface of the substrate and observing it with a transmission electron microscope (TEM). As an example, a primary sample including an observation range of a 20 μm square or larger area (i.e., a 20 μm square or larger area including a part of the first area) including the end surface ES and side surface SS of the mesa structure including the first conductive cladding layer is cut out from the substrate by a focused ion beam method. Next, a secondary sample is prepared by thinning the primary sample to a thickness of 0.5 μm by polishing or a focused ion beam method. A planar TEM image is obtained for the secondary sample by an electron beam incident on the <0001> zone axis. During this process, a cross-sectional TEM image can be simultaneously obtained using the primary sample or a sample obtained by the same method as the primary sample.

Dislocations are counted using a planar TEM image of the <0001> zone axis incidence observed at a magnification of 10k to 20k with an applied voltage of 200 kV. The observation area is moved from the edge of the mesa structure to a region 20 μm inward, and dislocation lines are observed. The extension direction of the obtained dislocation lines does not extend completely in the <1-100> direction throughout the entire field of view, and the number of lines may increase or decrease by meeting other dislocation lines. Therefore, the number of dislocation lines is counted by imagining a sampling line, which is a straight line extending in the <1-100> direction, and determining the number of intersections between the sampling line and the dislocation line by the statistical average value of multiple sampling lines. Specifically, multiple sampling lines are imaginary at intervals of 1 μm or less, and the measurement location (position of the sampling line) is moved within a 20 μm field of view, and the number of intersections between the sampling line and the dislocation line at each measurement position is measured 20 times or more. The average value of these 20 or more measurements is taken as the number of dislocation lines. Hereinafter, this measurement is referred to as a planar TEM measurement.

Burgers spectrum analysis of dislocation lines further involves acquiring planar TEM dark-field images under systematic reflection conditions of reciprocal lattice vectors <-1100> and <11-20>, and identifying the direction of the Burgers vector from the disappearance of the contrast of the dislocation lines. As an example, if the Burgers vector has a component in the <11-20> direction, the contrast of the dislocation lines disappears in the planar TEM dark-field image under systematic reflection conditions of <11-20>.

(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/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).
A region in the mesa structure 201 that is at a distance L of less than 15 μm from the side surface SS is a first region 202 (shown by a dashed line in FIG. 2). The laser diode 1 has a plurality of dislocation lines extending in the <1-100> crystal orientation of the substrate 10 only in the first region 202.

(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.

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

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

(3.4) Fourth embodiment Fig. 5 is a schematic diagram for explaining a laser diode 4 according to a fourth embodiment. Fig. 5 is a schematic cross-sectional view of the laser diode 4.
The laser diode 4 differs from the laser diode 1 according to the first embodiment in that it has a semiconductor laminate portion 20C including a second conductivity type cladding layer 23 composed of a second conductivity type vertical conduction layer 23A and a second conductivity type lateral 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.

(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 conductivity type lateral conduction layer 23B and the second conductivity type contact layer 26 with complete distortion while improving the conductivity of carriers in the intermediate layer 27.

(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 further includes a first-conductivity-type waveguide layer 24, a second-conductivity-type waveguide layer 25, and a second-conductivity-type contact layer 26, and the mesa structure of the laser diode 6 is a two-stage structure, and differs from the laser diode 1 according to the first embodiment in that it includes a semiconductor laminate portion 20E including a mesa structure 201 including a first-conductivity-type cladding layer 21 and a mesa structure 203 not including 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. From the viewpoint of reducing the oscillation threshold current, it is preferable that the mesa structure 203 not including the first-conductivity-type cladding layer 21 does not include dislocation lines extending in the crystal orientation <1-100> direction of the substrate 10.

4. Effects The above-described laser diode has the following effects.
(1) In the laser diode, at least a portion of a semiconductor laminate disposed on a substrate has a mesa structure for optical resonance and emission, and the laser diode has a plurality of dislocation lines extending in the <1-100> crystal orientation of the nitride semiconductor substrate only in a first region having a distance L of less than 15 μm from a side surface of the mesa structure including a first conductivity type cladding layer.
As a result, the laser diode can efficiently relax each layer of the semiconductor stack while avoiding the introduction of dislocations into the light-emitting layer, which would significantly deteriorate the oscillation threshold current, thereby reducing the oscillation threshold current.
(2) It is preferable that the Burgers vector of a dislocation line extending in the <1-100> crystal orientation direction formed in the first region of the mesa structure of the laser diode has a component in the <11-20> direction.
This makes it possible to avoid the introduction of dislocations into the light-emitting layer, which would otherwise significantly deteriorate the lasing threshold current, and therefore to reduce the lasing threshold current.

(3) It is preferable that the mesa structure of the laser diode is a rectangle having long and short sides in a plan view, and is disposed so that the long sides extend in the <1-100> direction.
This allows the resonator mirror end faces of the laser resonator to be formed into atomically flat (1-100) faces, making it easy to form the resonator mirror end faces.
(4) In the first region of the mesa structure, the number of dislocation lines extending in the <1-100> direction is preferably greater than the number of dislocation lines extending in directions other than the <1-100> direction.
This makes it possible to suppress the introduction of dislocations into the light emitting layer, which would otherwise significantly deteriorate the lasing threshold current, while also allowing relaxation to be performed in the first conductivity type cladding layer, thereby enabling the lasing threshold current to be efficiently reduced.

(5) The number of dislocation lines per unit length extending in the <1-100> direction in the first region of the mesa structure is preferably 4.5 lines/μm to 11 lines/μm inclusive. Also, the average spacing of dislocation lines extending in the <1-100> direction in the first region of the mesa structure is preferably 1/100 atomic planes to 1/5000 atomic planes inclusive with respect to the (11-20) atomic plane.
This allows dislocations to be appropriately introduced into the first conductivity type cladding layer, which is the thickest and has accumulated strain, and the first conductivity type cladding layer can be efficiently relaxed.

(6) In a region where the distance L from the side surface of the mesa structure of the laser diode is 15 μm or more and less than 20 μm, there are no dislocation lines extending in the <1-100> direction and the <11-20> direction.
This makes it difficult for dislocations to be introduced into the region of the light emitting layer located below the second electrode, making it possible to suppress a significant decrease in the oscillation threshold current.
(7) It is preferable that the ratio of the distance between the end faces of the resonator mirrors when the laser diode is viewed in a plan view to the distance between the side faces of the mesa structure is 5:1 to 500:1.
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.

(8) 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.
(9) 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.

(10) 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 0.5 μm, 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.
(11) 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.

(12) 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.
(13) 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.

(14) 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.
(15) 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.

(16) 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.
(17) It is preferable that dislocation lines extending in the <1-100> direction formed in the mesa structure exist only in the first conductivity type cladding layer in the first region.
This allows the layers of the semiconductor laminate to be efficiently relaxed by introducing dislocation lines into the first conductivity type cladding layer where the most strain is accumulated.

The following describes examples and comparative examples of this disclosure.

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

Next, an AlN layer, which is a homoepitaxial layer, was formed on the substrate as a buffer layer. The AlN layer was formed to a thickness of 500 nm in an environment 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 degree of vacuum 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 laminated structure including a second conductive vertical conductive layer and a second conductive horizontal conductive layer, and is a graded layer with a gradient Al composition ratio. The second conductive vertical conductive layer was an AlGaN layer having a thickness of 20 nm, in which the Al composition is distributed in the direction away from the substrate, varying from Al=0.63 to 1.0. The second conductive horizontal conductive layer was a p-type AlGaN layer having a thickness of 320 nm, 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 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 source, and triethylgallium (TEGa) was used as the Ga source.

Next, a p-type contact layer, which is a second conductive type contact layer, was formed on the second conductive type cladding layer. Here, the p-type contact layer was formed from an AlGaN layer and a GaN layer. The AlGaN layer was a p-type nitride semiconductor layer with a layer thickness of 30 nm, using Mg as a dopant impurity and with an Al composition distribution in the direction away from the substrate, varying from Al = 0.7 to 0.4. The GaN layer was formed from GaN (i.e. Al: 0%) with a thickness of 10 nm.

The p-type contact layer was formed at a temperature of 950°C, with a degree of vacuum set to 150 mbar and a V/III ratio of 3650. The growth rate of the p-type contact layer was 0.2 μm/hr.

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

The semiconductor laminated portion 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 n-type cladding layer was exposed by dry etching with a gas containing _Cl2 and _BCl3 using an ICP. The n-type cladding layer was formed in a rectangular region long in the <1-100> direction in a plan view. At this time, the ICP conditions were an antenna power of 320 W, a bias power of 30 W, an automatic pressure adjustment (APC: Adaptive Pressure Control valve) of 2 Pa, a process pressure of 600 Pa, and a gas flow rate of 20 sccm for _Cl2 gas and 5 sccm for _BCl3 gas.
The formed mesa structure had a length of 700 μm in the <1-100> direction and a length of 40 μm in the <11-20> direction, where the length of the mesa structure in the <1-100> direction is the distance between the end faces of the resonator mirror in a plan view, and the length in the <11-20> direction is the distance between the side faces of the mesa structure.

On the p-type contact layer of the mesa structure, Ni and Au were sequentially deposited in a rectangular shape elongated in the <1-100> direction to form multiple electrode metal regions as second electrodes. At this time, the width of the second electrode was 5 μm and the length was 600 μm or more. 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 multiple electrode metal regions as first electrodes. Furthermore, within the electrode metal regions, the substrate was divided into stripes by cleaving multiple times parallel to the <11-20> direction to form individual laser diodes. The final length of the mesa structure in the <1-100> direction was 600 μm.

When a current-facet emission intensity measurement was carried out on the laser diode thus obtained by current injection, the threshold voltage was 9 V and the oscillation threshold current was 10 kA/cm ² .
Plane TEM measurements of this laser diode showed that the number of dislocation lines extending in the <1-100> direction in the first region, which had a distance L from the side surface of the mesa structure of less than 15 μm, was 66 in total, and the number of dislocation lines per unit length was 7.3/μm. The average spacing of the dislocation lines was 1/440 atomic planes. The number of dislocation lines extending in directions other than the <1-100> direction was 12, and the number of dislocation lines extending in the <1-100> direction was greater. The Burgers vector of the dislocation lines had a component in the <11-20> direction. The first region was also relaxed.
Furthermore, in the region where the distance L from the side surface of the mesa structure is 15 μm or more and less than 20 μm, there are no dislocation lines extending in the <1-100> direction and the <11-20> direction, and this region is not relaxed.

[Example 2]
A laser diode of Example 2 was formed in the same manner as in Example 1, except that the APC was set to 1.5 Pa among the ICP conditions when exposing the first conductivity type cladding layer.
When the current-facet emission intensity of the obtained laser diode was measured by current injection, the threshold voltage was 9.2 V and the oscillation threshold current was 10.5 kA/cm ² .
Plane TEM measurements were performed on the mesa structure of this laser diode, and the total number of dislocation lines extending in the <1-100> direction in the first region, which had a distance L from the side surface of the mesa structure of less than 15 μm, was 80, and the number of dislocation lines per unit length was 8.9/μm. The average spacing of the dislocation lines was 1/360 atomic planes. The number of dislocation lines extending in directions other than the <1-100> direction was 18, and the number of dislocation lines extending in the <1-100> direction was greater. The Burgers vector of the dislocation lines had a component in the <11-20> direction. The first region was relaxed.
Furthermore, in the region where the distance L from the side surface of the mesa structure is 15 μm or more and less than 20 μm, there are no dislocation lines extending in the <1-100> direction and the <11-20> direction, and this region is not relaxed.

[Example 3]
A laser diode of Example 3 was formed in the same manner as in Example 1, except that, among the ICP conditions when exposing the first conductive type cladding layer, the APC was set to 0.75 Pa and the gas flow rate was set to 20 sccm of Cl ₂ gas.
When a current-facet emission intensity measurement was carried out on the obtained laser diode by current injection, the threshold voltage was 9 V and the oscillation threshold current was 10.2 kA/cm ² .
Plane TEM measurements were performed on the mesa structure of this laser diode, and the total number of dislocation lines extending in the <1-100> direction in the first region, which had a distance L from the side surface of the mesa structure of less than 15 μm, was 100, and the number of dislocation lines per unit length was 10.5/μm. The average spacing of the dislocation lines was 1/290 atomic planes. The number of dislocation lines extending in directions other than the <1-100> direction was 15, and the number of dislocation lines extending in the <1-100> direction was greater. The Burgers vector of the dislocation lines had a component in the <11-20> direction. The first region was relaxed.
Furthermore, in the region where the distance L from the side surface of the mesa structure is 15 μm or more and less than 20 μm, there are no dislocation lines extending in the <1-100> direction and the <11-20> direction, and this region is not relaxed.

[Example 4]
A laser diode of Example 4 was formed in the same manner as in Example 1, except that the gas flow rates of the ICP used to expose the first conductive type cladding layer were 20 sccm of _Cl2 gas, 5 sccm of _BCl3 gas, and 5 sccm of _SF6 gas.
When the current-facet emission intensity of the obtained laser diode was measured by current injection, the threshold voltage was 9.1 V and the oscillation threshold current was 10 kA/cm ² .
Plane TEM measurements of the mesa structure of this laser diode revealed that the number of dislocation lines extending in the <1-100> direction in the first region, which had a distance L from the side surface of the mesa structure of less than 15 μm, was 70 in total, and the number of dislocation lines per unit length was 4.6/μm. The average spacing of the dislocation lines was 1/700 atomic planes. The number of dislocation lines extending in directions other than the <1-100> direction was 12, and the number of dislocation lines extending in the <1-100> direction was greater. The Burgers vector of the dislocation lines had a component in the <11-20> direction. The first region was relaxed.
Furthermore, in the region where the distance L from the side surface of the mesa structure is 15 μm or more and less than 20 μm, there are no dislocation lines extending in the <1-100> direction and the <11-20> direction, and this region is not relaxed.

[Comparative Example 1]
A laser diode of Comparative Example 1 was formed in the same manner as in Example 1, except that the ICP conditions for exposing the first conductive type cladding layer were an antenna power of 150 W, an APC of 1 Pa, and a gas flow rate of _Cl2 gas of 25 sccm.
When the current-facet emission intensity of the obtained laser diode was measured by current injection, the threshold voltage was 10.5 V and the oscillation threshold current was 25 kA/cm ² .
Plane TEM measurements of the mesa structure of this laser diode revealed that the number of dislocation lines extending in the <1-100> direction in the first region, which had a distance L from the side surface of the mesa structure of less than 15 μm, was 82 in total, and the number of dislocation lines per unit length was 9.1/μm. The average spacing of the dislocation lines was 1/355 atomic planes. The number of dislocation lines extending in directions other than the <1-100> direction was 15, and the number of dislocation lines extending in the <1-100> direction was greater. The Burgers vector of the dislocation lines had a component in the <11-20> direction. The first region was relaxed.
Furthermore, in the region where the distance L from the side surface of the mesa structure was 15 μm or more and less than 20 μm, the number of dislocation lines extending in the <1-100> direction was 80, and the number of dislocation lines extending in a direction other than the <1-100> direction was 14. In addition, this region was relaxed.

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

As shown in Table 1, the laser diodes of the examples, which do not have dislocation lines extending in the <1-100> and <11-20> directions in the region where the distance L from the side of the mesa structure is 15 μm or more and less than 20 μm, have a significantly lower oscillation threshold than the laser diode of the comparative example, which has dislocation lines in the region. In addition, the laser diodes of the examples also have a lower threshold voltage than the laser diode of the comparative example.

Furthermore, the laser diodes of Examples 1 and 4, in which the number of dislocation lines extending in the <1-100> direction per unit length in the first region was 7.5 lines/μm or less and the average spacing of the dislocation lines was 1/400 atomic planes or more relative to the (11-20) atomic plane, had a lower oscillation threshold than the laser diodes of Examples 2 and 3.
From the above, it was found that it is necessary to appropriately set the positions of dislocation lines in the mesa structure, as well as the number and average spacing of dislocation lines.

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

10 Substrate 20 Semiconductor laminate 21 First conductivity type cladding layer 22 Light emitting layer 23 Second conductivity type cladding layer 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 (16)

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 layer;
a light emitting layer disposed on the first conductive type cladding layer and formed of a nitride semiconductor including one or more quantum wells;
a second conductivity type cladding layer disposed on the light emitting layer and including a second conductivity type nitride semiconductor layer;
having
At least a part of the semiconductor laminate has a mesa structure for optical resonance and emission,
The laser diode has a plurality of dislocation lines extending in the <1-100> crystal orientation of the nitride semiconductor substrate only in a first region having a distance L of less than 15 μm from a side surface of the mesa structure including the first conductivity type cladding layer. 2. The laser diode according to claim 1, wherein the mesa structure has a resonator in which a mesa end face parallel to a (1-100) crystal plane of the nitride semiconductor substrate in a plan view serves as a resonator mirror end face, and the laser diode is an edge-emitting laser diode that emits light in the <1-100> direction. The ratio of the distance between the end faces of the resonator mirrors in a plan view to the distance between the side faces of the mesa structure is 5:1 to 500:1.
3. The laser diode of claim 2. 4. The laser diode according to claim 1, wherein the mesa structure is rectangular in plan view having long and short sides, and the long sides are arranged to extend in the <1-100> direction. 5. The laser diode according to claim 1, wherein the number of dislocation lines extending in the <1-100> direction per unit length in the first region is 4.5 lines/μm or more and 11 lines/μm or less. The average spacing of dislocation lines extending in the <1-100> direction in the first region is 1/5000 atomic planes or more and 1/100 atomic planes or less with respect to the (11-20) atomic plane.
6. A laser diode according to claim 1. 7. The laser diode according to claim 1, wherein there are no dislocation lines extending in the <1-100> direction and the <11-20> direction in a region where the distance L from the side surface of the mesa structure is 15 μm or more and less than 20 μm. 8. The laser diode according to claim 1, wherein the nitride semiconductor substrate is an AlN single crystal substrate. a first conductivity type waveguide layer disposed between the first conductivity type cladding layer and the light emitting layer to confine light to the light emitting layer;
a second conductive type waveguide layer disposed between the second conductive type cladding layer and the light emitting layer to confine light to the light emitting layer;
9. A laser diode according to any one of claims 1 to 8 . a second conductive type contact layer disposed on the second conductive type cladding layer and formed of a nitride semiconductor containing GaN;
The second conductivity type cladding layer includes a second conductivity type vertical conduction layer having a thickness of less than 0.5 μm, and a second conductivity type lateral conduction layer including Al _f Ga _{1 -f} N (0<f≦1), the second conductivity type vertical conduction layer having a composition gradient in which the Al _composition e decreases with increasing distance from the nitride semiconductor substrate.
10. A laser diode according to any one of claims 1 to 9 . the second conductivity type cladding layer includes a second conductivity type vertical conduction layer having a thickness of less than 0.5 μm, the second conductivity type vertical conduction layer including Al _e Ga 1 _- e N (0.1≦e≦1) and a composition gradient in which the Al composition e decreases with increasing distance from the nitride semiconductor substrate, and the second conductivity type cladding layer includes 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);
10. The laser diode of claim 9 . The second conductive type vertical conductive layer has a thickness of 250 nm or more and 450 nm or less.
12. The laser diode of claim 11 . 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;
13. A laser diode according to claim 11 or 12 . the first conductive type cladding layer is made of AlaGa1-aN (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;
14. A laser diode according to any one of claims 10 to 13 . 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;
15. A laser diode according to any one of claims 10 to 14 . a second electrode provided on the second conductive type cladding layer in a region spaced apart from the side surface of the mesa structure by 5 μm or more in a plan view;
16. The laser diode of claim 1 , comprising:

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