JP7545142B2 - Optical Devices - Google Patents

Description

The present invention relates to an optical device.

Patent document 1 describes a laser device equipped with a laser diode and a photodetector that monitors the output of the laser diode. Non-patent documents 1 and 2 also describe ultraviolet laser diodes.

JP 2018-82045 A

Zhang et al. , Applied Physics Express 12, 124003 (2019) Zhang et al. , Applied Physics Letters 117, 152104 (2020)

Monitoring the output of a laser diode in real time using a photodetector and adjusting the operating conditions of the laser diode based on the monitoring results is important for the stable light emission operation of the laser diode. In particular, to achieve a configuration that is inexpensive and suitable for mass production, it is desirable to have the laser diode and photodetector monolithically mounted on the same substrate.

In the ultraviolet laser diode structures described in Non-Patent Documents 1 and 2, the depletion layer formed by the pn junction does not coincide with the active layer. For this reason, if the ultraviolet laser diode structures described in Non-Patent Documents 1 and 2 are used as they are as a light receiving element, the electron-hole pairs generated by receiving light in the active layer will recombine and disappear before being extracted as a photocurrent, because no electric field is applied to them. This causes the problem that the output of the laser diode cannot be monitored by the light receiving element.

In other words, the objective of this disclosure is to provide an optical device that can monitor the output of a laser diode in real time using a light receiving element.

In order to achieve the above object, an optical device according to one aspect of the present invention includes a nitride semiconductor substrate containing aluminum, a laser diode formed on the nitride semiconductor substrate and having a first nitride semiconductor stack, and a light-receiving element formed on the nitride semiconductor substrate and having a second nitride semiconductor stack, each of the first nitride semiconductor stack and the second nitride semiconductor stack including a first conductive type cladding layer including a nitride semiconductor layer having a first conductive type conductivity, an active layer disposed on the first conductive type cladding layer and formed of a nitride semiconductor layer including one or more quantum wells, and a second conductive type cladding layer disposed on the active layer and formed of a nitride semiconductor layer including one or more quantum wells. and a second conductivity type contact layer arranged on the second conductivity type cladding layer, the laser diode having a first conductivity type contact electrode arranged in a portion of an exposed region of the first conductivity type cladding layer of the first nitride semiconductor stack, and a second conductivity type contact electrode arranged on the second conductivity type contact layer of the first nitride semiconductor stack, and the light receiving element has at least a first electrode and a second electrode in contact with the second conductivity type contact layer of the second nitride semiconductor stack, and at least one of the first electrode and the second electrode is a Schottky electrode.

According to one aspect of the present invention, the output of a laser diode can be monitored in real time using a light receiving element.

1 is a schematic cross-sectional view showing an example of a schematic configuration of an optical device according to a first embodiment of the present invention. FIG. 5 is a schematic cross-sectional view showing an example of a schematic configuration of an optical device according to a second embodiment of the present invention. FIG. 11 is a schematic cross-sectional view showing an example of a schematic configuration of an optical device according to a third embodiment of the present invention. FIG. 13 is a schematic cross-sectional view showing an example of a schematic configuration of an optical device according to a fourth embodiment of the present invention.

The present invention will be described below through embodiments of the invention, 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.

<Optical Devices>
The optical device according to the present embodiment includes a nitride semiconductor substrate including aluminum (Al), a laser diode formed on the nitride semiconductor substrate and having a first nitride semiconductor stack, and a light receiving element formed on the nitride semiconductor substrate and having a second nitride semiconductor stack, each of the first nitride semiconductor stack and the second nitride semiconductor stack including a first conductive type cladding layer including a nitride semiconductor layer having a first conductive type conductivity, an active layer disposed on the first conductive type cladding layer and formed of a nitride semiconductor layer including one or more quantum wells, and a second conductive type cladding layer disposed on the active layer and having a second conductive type conductivity. the laser diode has a first conductivity type contact electrode arranged in a part of an exposed region of the first conductivity type cladding layer of the first nitride semiconductor stack and a second conductivity type contact electrode arranged on the second conductivity type cladding layer of the first nitride semiconductor stack, and the light receiving element has at least a first electrode and a second electrode on the second conductivity type contact layer of the second nitride semiconductor stack, and at least one of the first electrode and the second electrode is a Schottky electrode. A structure from the first conductivity type cladding layer to the second conductivity type contact layer of the first nitride semiconductor stack is identical to a structure from the first conductivity type cladding layer to the second conductivity type contact layer of the second nitride semiconductor stack.

(substrate)
The substrate provided in the optical device according to the present embodiment includes a nitride semiconductor containing Al. The nitride semiconductor containing Al is, for example, aluminum nitride (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, aluminum gallium nitride (AlGaN). For example, when the substrate is a nitride semiconductor single crystal substrate such as AlN or AlGaN, the difference in lattice constant between the substrate and the nitride semiconductor layer formed on the substrate becomes small, and the nitride semiconductor layer can be grown in a lattice matching system. This reduces threading dislocations in the substrate. 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 current threshold, it is more preferable that the density is 1×10 ³ cm ⁻² or more and 1×10 ⁴ cm ⁻² or less.

Here, the "comprises" in the expression "the substrate 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 the nitride semiconductor layer is slightly changed by adding small amounts of other elements (for example, a few percent or less of elements such as gallium (Ga) (when Ga is not the main element), indium (In), arsenic (As), phosphorus (P), or antimony (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 ), silicon (Si), silicon carbide (SiC), magnesium oxide (MgO), gallium oxide ( _Ga2O3 ), zinc oxide (ZnO), gallium nitride (GaN), or indium nitride ( _InN ).

As an example, the substrate preferably has a layer thickness of 100 μm or more and 600 μm or less. The surface orientation of the substrate may be c-plane (0001), a-plane (11-20), m-plane (10-10), etc., with a c-plane (0001) substrate being more preferable. Furthermore, the laser diode and the light receiving element can 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 this is not limited thereto.

(First Conductive Type Cladding Layer)
The first conductive cladding layer provided in the optical device according to the present embodiment is a layer of a nitride semiconductor containing Al and Ga. 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 the substrate. The above expression also includes the case where another layer exists 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 case where the second conductive cladding layer is formed on the p-type waveguide layer via an electron block layer is also included in the expression "the second conductive cladding layer is formed on the p-type waveguide layer".

In addition, in the description of this embodiment, the "first conductivity type" and the "second conductivity type" mean semiconductors exhibiting different conductivity types, and for example, when one of the "first conductivity type" and the "second conductivity type" is n-type conductivity, the other of the "first conductivity type" and the "second conductivity type" is p-type conductivity. Although not particularly limited, the first conductivity type cladding layer is formed of, for example, Al _a Ga _(1-a) N (0<a<1). This makes it possible to increase the crystallinity of the active layer and improve the luminous efficiency when a material corresponding to the band gap energy in the deep ultraviolet region is formed as the active layer. From the viewpoint of realizing high luminous efficiency, the nitride semiconductor constituting the first conductivity type cladding layer is preferably a mixed crystal of AlN and GaN.

From the viewpoint of growing with complete strain on the substrate, the first conductivity type cladding layer is more preferably formed of Al _a Ga _(1-a) N (0.6≦a≦0.8). When the first conductivity type cladding layer is an n-type conductive semiconductor layer, impurities such as Group V elements other than N, such as P, As, Sb, carbon (C), hydrogen (H), fluorine (F), oxygen (O), magnesium (Mg), and Si may be mixed in, but the types of impurity elements are not limited to these.

From the viewpoints 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 less.

From the viewpoints of relaxation 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 300 nm or more and 750 nm or less, and even more preferably 300 nm or more and 500 nm or less.

The first conductivity type cladding layer may be a gradient layer in which the Al composition ratio increases in the direction away from the substrate, for the purpose of controlling its longitudinal conductivity. In this case, the above-mentioned limitation on the Al composition ratio can be the Al composition ratio averaged over the thickness of the first conductivity type cladding layer at each thickness direction position in the first conductivity type cladding layer, and a similar embodiment can be adopted.

(Waveguide layer)
In the optical device according to the present embodiment, from the viewpoint of light confinement, each of the first nitride semiconductor stack and the second nitride semiconductor stack may have a first conductivity type waveguide layer disposed between the first conductivity type cladding layer and the active layer and confining light in the active layer, and a second conductivity type waveguide layer disposed between the second conductivity type cladding layer and the active layer and confining light in the active layer.

The waveguide layers are formed above and below the active layer, sandwiching it between them, and have the effect of confining the light emitted from the active layer within the active layer. In other words, the waveguide layer is composed of two layers: a portion on the first conductivity type cladding layer side of the active layer (first conductivity type waveguide layer), and a portion on the second conductivity type cladding layer side of the active layer (second conductivity type waveguide layer).

From the viewpoint of light confinement, the waveguide layer is preferably a nitride semiconductor containing at least one of Al and Ga having a band gap with a higher energy than that of the active layer. The waveguide layer preferably has an Al composition ratio and a film thickness that can increase the overlap between the electric field intensity distribution of light standing in the device and the active layer. In particular, the film thickness ratio of the first conductive type waveguide layer and the second conductive type waveguide layer can be variously taken depending on the light confinement in the active layer and the Al composition ratio of AlGaN in the first conductive type cladding layer and the second conductive type cladding layer. From the viewpoint of carrier confinement in the active layer, when the active 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 the Al composition ratio is b<c, and c≧b+0.05. For example, in the case of an active layer having an emission wavelength of 265 nm, it is preferable that b=0.52 and c is 0.57 or more. From the viewpoints of light confinement and layer resistance, it is preferable that the thickness of each of the first conductivity type waveguide layer and the second conductivity type waveguide layer is 70 to 150 nm.

The Al composition ratio 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 present on the second conductivity type contact, the Al composition ratio of the second conductivity type waveguide layer may be higher than the Al composition ratio 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.

(Active layer)
The active layer is a layer of a nitride semiconductor containing Al and Ga sandwiched between a first conductivity type waveguide layer and a second conductivity type waveguide layer. From the viewpoint of realizing high light emission efficiency, the nitride semiconductor contained in the active layer is preferably a mixed crystal of, for example, AlN and GaN, and is formed, for example, of Al _b Ga _(1-b) N (0<b<1). In addition to N, the active 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 active layer may have either a multiple quantum well structure or a single quantum well structure. The number of quantum well structures is preferably 1 to 3, depending on the longitudinal conductivity of the first and second conductive cladding layers. For the purpose of reducing the influence of crystal defects in the active layer, a part or the whole of the active layer may contain elements such as Si, Sb, P, etc. at an impurity concentration of 1×10 ¹⁵ cm ^-3 or more.

(Second Conductive Type Cladding Layer)
The second conductivity type cladding layer is formed on the active layer and the waveguide layer, and is a nitride semiconductor layer having second conductivity type conductivity, containing Al and Ga. The second conductivity type cladding layer contains, for example, A _d Ga _(1-d) N (0<d<1). This allows the second conductivity type cladding layer to easily lattice match with the waveguide layer, and makes it possible to suppress threading dislocation density.

The second conductive cladding layer is not particularly limited as long as it has sufficient conductivity to inject holes into the active layer and can increase the overlap between the electric field intensity distribution of the optical mode present in the optical device and the active layer (i.e., increase the optical confinement). For example, it may be p-type AlGaN doped with Mg. In addition, when the second conductive cladding layer is a p-type conductive semiconductor layer, it may contain impurities such as group V elements other than N, such as P, As, and Sb, and C, H, F, O, Mg, and Si, but the types of impurity elements are not limited to these.

From the viewpoint of more efficient hole injection into the active layer, the second conductive type cladding layer is a compositionally graded layer (a second conductive type vertical conductive layer (e.g., a p-type vertical conductive layer) and a compositionally graded layer (a second conductive type vertical conductive layer (e.g., a p-type vertical conductive layer)) formed of Al _e Ga _(1-e) N having a compositional gradient such that the Al compositional ratio e decreases in the direction away from the upper surface of the substrate (i.e., the Al compositional _ratio decreases as it moves away from the substrate ₎ . N (0<f≦1), and a second-conductivity-type lateral conduction layer (e.g., a p-type lateral conduction layer). The profile (slope) of the Al composition ratio f 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 ratio e is the same. In other words, the second-conductivity-type vertical conduction layer may include a portion in which the Al composition ratio e does not decrease in the direction away from the substrate, but does not include a portion in which the Al composition ratio e increases. The second-conductivity-type vertical conduction layer may have a thickness of less than 0.5 μm.

From the viewpoint of efficiently converting light into electron-hole pairs in the light receiving element, the second conductivity type vertical conductive layer should have a band gap narrower than that of the active layer, i.e., the second conductivity type vertical conductive layer is preferably made of _{AleGa (1-e)} N (0.1≦e≦b) and has a band gap at least smaller than that of the active layer.

From the viewpoint of lattice matching, the second conductive type vertical conductive layer is preferably 500 nm or less. From the viewpoint of light confinement and carrier injection, the second conductive type vertical conductive 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-conductivity-type vertical conduction layer is not intentionally mixed with impurities such as H, Mg, Be, Zn, Si, B, etc. in the region close to the second-conductivity-type waveguide layer, that is, it is preferable that the second-conductivity-type vertical conduction layer is in an undoped state. That is, the second-conductivity-type vertical conduction layer may have an undoped region in a certain range including the interface with the second-conductivity-type waveguide layer in the thickness direction of the second-conductivity-type cladding layer. Here, the term "undoped" means that the above-mentioned elements are not intentionally supplied to the target layer 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, for example, in a range of 10 ¹⁶ cm ^-3 or less. In addition, the region of the second-conductivity-type vertical conduction layer that is in an undoped state includes at least the boundary with the second-conductivity-type waveguide layer, but the size of the region is not limited. For example, the entire region of the second-conductivity-type vertical conduction layer may be in an undoped state. As another example, 50% of the second-conductivity-type vertical conduction layer may be undoped, the region being closer to the second-conductivity-type waveguide layer than the second-conductivity-type lateral conduction layer in the thickness direction of the second-conductivity-type cladding layer. As another example, about 10% of the second-conductivity-type vertical conduction layer may be undoped, the region being closer to the second-conductivity-type waveguide layer in the thickness direction of the second-conductivity-type cladding layer. Even in this undoped state, holes are generated in the second-conductivity-type vertical conduction layer due to polarization.

The second-conductivity-type transverse conduction layer is a layer formed on the second-conductivity-type vertical conduction layer and contains Al _f Ga _(1-f) N (0<f≦1). 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.

From the viewpoint of facilitating quantum transmission of the penetrating carriers, the second conductivity type lateral conduction layer preferably has a thickness of 20 nm or less, more preferably a thickness of 10 nm or less, and even more preferably a thickness of 5 nm or less.

The second-conductivity-type lateral conduction layer may have an Al composition ratio f greater than the minimum value of the Al composition ratio e in the second-conductivity-type vertical conduction layer at the adjacent surface (i.e., interface) with the second-conductivity-type vertical conduction layer. Furthermore, the second-conductivity-type lateral conduction layer is preferably completely distorted with respect to the substrate. Such a second-conductivity-type lateral conduction layer can improve the lateral conductivity by induced holes at the interface as the net internal electric field accumulated on the surface and inside near the surface of the second-conductivity-type lateral conduction layer becomes negative.

Each of the first nitride semiconductor laminate and the second nitride semiconductor laminate may have an Al _g Ga _1-g N (0<g≦1.0) layer disposed between the second conductivity type vertical conduction layer and the second conductivity type waveguide layer. The intermediate layer formed of the Al _g Ga _1-g N (0<g≦1.0) layer provided between (for example, in the middle) the second conductivity type vertical conduction layer and the second conductivity type waveguide layer may be configured so that the Al composition ratio g increases in the direction away from the upper surface of the substrate. By the intermediate layer having such an Al composition ratio g, at least one of the effects of improving the conductivity between the second conductivity type vertical conduction layer and the second conductivity type waveguide layer and the effect of forming the second conductivity type lateral conduction layer and the second conductivity type contact layer with complete distortion can be obtained. The intermediate layer provided between the second conductivity type vertical conduction layer and the second conductivity type waveguide layer may be a non-bandgap mixed crystal that does not absorb light of the desired emission wavelength, and further preferably has a thickness of 50 nm or less, and may be an undoped layer.

The second conductivity type vertical conductive layer has the effect of generating holes by the polarization doping effect and efficiently injecting the holes into the active layer within the active layer. Therefore, by providing the second conductivity type vertical conductive layer between the active layer and the second conductivity type contact layer, the carrier injection efficiency of the laser diode can be increased and the threshold voltage can be reduced. The second conductivity type horizontal conductive layer has the effect of spreading the hole distribution in the horizontal direction, which is narrowed by the electric field concentrated under the electrode. This effect can increase the carrier injection efficiency in the same way as the second conductivity type vertical conductive layer.

(Second Conductive Type Contact Layer)
The nitride semiconductor laminate of the optical device according to the present embodiment may further include a second conductive contact layer disposed on the second conductive cladding layer. The nitride semiconductor constituting the second conductive contact layer is formed of, for example, GaN, AlN, or InN, and a mixed crystal containing them. The second conductive contact layer may contain impurities such as V group elements other than N, such as P, As, and Sb, carbon (C), hydrogen (H), fluorine (F), oxygen (O), Mg, Si, and beryllium (Be). From the viewpoint of versatility of the raw material gas, it is preferable that the impurity contained in the second conductive contact layer is Mg. From the viewpoint of reducing the contact resistance between the second conductive contact layer and the second conductive cladding layer, it is preferable that the concentration of Mg is 8×10 ¹⁹ cm ⁻³ or more and 5×10 ²¹ cm ⁻³ or less. More preferably, the concentration of Mg contained is 5×10 ²⁰ cm ⁻³ or more and 5×10 ²¹ cm ⁻³ or less.

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

(Electron Blocking Layer)
The nitride semiconductor stack of the optical device according to this embodiment may further include an electron blocking layer having a band gap larger than that of the second conductivity type waveguide layer, inside the second conductivity type waveguide layer, between the second conductivity type waveguide layer and the active layer (for example, at the middle), between the second conductivity type waveguide layer and the second conductivity type vertical conductive layer (for example, at the middle), or in a part of the second conductivity type waveguide layer. The film 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 holes can easily quantum penetrate the electron blocking layer.

(Mesa structure)
The optical device according to this embodiment has a first mesa structure formed to electrically isolate a second conductivity type layer and a first conductivity type layer in a laser diode having a first nitride semiconductor stack, and a second mesa structure formed to isolate the first nitride semiconductor stack and the second nitride semiconductor stack, respectively.

From the viewpoint of current confinement of the laser and amplification by reflection at the end facet, the first mesa structure is preferably rectangular in plan view, has long and short sides, and the long sides are preferably parallel to the <1-100> direction. The reason why the long sides of the first mesa structure are preferably parallel to the <1-100> direction is that when obtaining the resonator mirror end facet 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, it is preferable that the first mesa structure has a resonator in which the resonator mirror end facet is a mesa end face that is parallel to the (1-100) plane in plan view with respect to the crystal orientation of the nitride semiconductor substrate, and the laser diode is an edge-emitting laser diode that emits light in the <1-100> direction.

The nitride semiconductor laminate may have multiple mesa structures. That is, the nitride semiconductor laminate may have multiple discontinuous side surfaces. From the viewpoint of efficiently injecting power into the active layer, it may be preferable to have a mesa structure (also called a ridge portion) in which the second conductivity type contact layer, or the second conductivity type contact layer and the second conductivity type cladding layer are partially or entirely removed except for the vicinity of the second electrode. In this case, the ridge portion is different from the mesa structure for exposing the first conductivity type cladding layer, and for example, the side surface of the ridge portion is understood to be different from the side surface of the mesa structure.

The second mesa structure is not particularly limited as long as it can electrically separate the first nitride semiconductor laminate and the second nitride semiconductor laminate. From the viewpoint of reducing the influence of heat generated from the first nitride semiconductor laminate, it is preferable that at least a part of the n-clad is exposed. More preferably, a part of the substrate is exposed. That is, the first mesa structure can be formed first, and then a part of the laminate can be covered with a protective layer, and then the second mesa structure can be formed, or the first mesa structure can be formed by forming the second mesa structure first and then covering a part of it with a protective layer. By having such a first mesa structure and a second mesa structure in the optical device, the laser diode and the light receiving element are electrically separated from each other except for a part of the nitride semiconductor substrate and the first conductive clad layer (an example of an n-type clad layer).

From the viewpoint of monitoring the intensity of the laser light irradiated from the first nitride semiconductor stack, the second mesa structure is preferably installed on the opposite side of the surface irradiated with the laser and receives the leakage light of the reflection. From the viewpoint of receiving more laser light, the second mesa structure is preferably a rectangle long in the direction of laser irradiation. From the viewpoint of chip yield, the second mesa structure is preferably the same width as the mesa structure of the laser diode, but is not limited thereto. In addition, the second mesa structure may have a material or shape processed on the side to facilitate absorption of the laser light. From the viewpoint of absorption of the laser light, the distance between the first mesa structure and the second mesa structure is preferably 1 μm or more and 1000 μm or less. In other words, the distance between the laser diode and the light receiving element may be 1 μm or more and 1000 μm or less. In this case, the distance means the shortest distance from the reflection surface located opposite the laser irradiation surface included in the first mesa structure to the second mesa structure when measured horizontally.

(electrode)
Electrical contact to the optical device according to the present embodiment is achieved in the first nitride semiconductor stack by a fourth electrode disposed on the second conductivity type cladding layer and a third electrode disposed in contact with the first conductivity type cladding layer. For example, the electrical contact can be made by disposing an electrode on the back side of the substrate. Alternatively, the electrical contact can be made by removing, for example by chemical or dry etching, various upper layers of the laser diode to expose the first conductivity type cladding layer in one or more regions adjacent the second conductivity type cladding layer, and disposing an electrode on the exposed first conductivity type cladding layer.

When the first conductive type cladding layer is an n-type cladding layer, the third electrode is formed of a metal such as Al, titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), molybdenum (Mo), tungsten (W), cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), platinum (Pt), copper (Cu), silver (Ag), gold (Au), Zr, or a mixed crystal thereof, or a conductive oxide such as _indium oxide (ITO) or _Ga2O3 .

When the second conductive type cladding layer is a p-type cladding layer, the fourth 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 electrode are not limited as long as electrical contact is obtained with each of the first conductivity type cladding layer and the second conductivity type contact layer. From the viewpoint of internal loss, it is preferable that the third electrode be arranged in an area on the second conductivity type cladding layer that is 5 μm or more away from the side of the mesa structure in a plan view.

The light receiving element has at least a first electrode and a second electrode on the second conductive contact layer of the second nitride semiconductor laminate, and at least one of the first electrode and the second electrode is a Schottky electrode. Therefore, electrical contact with the second nitride semiconductor laminate is made by at least one Schottky electrode (e.g., a first electrode) formed on the second conductive contact layer and an electrode (e.g., a second electrode) formed opposite the first electrode. The term "Schottky electrode" here means that when the voltage-current characteristics between two electrodes are measured, the voltage obtained by extrapolating from the slope at the point where the change is most linear is 1V or more.

On the premise that at least one of the first electrode and the second electrode is a Schottky electrode, the first electrode and the second electrode are preferably any combination of the following (1) to (4).
(1) At least one of the first electrode and the second electrode is a first conductivity type electrode. Or, both the first electrode and the second electrode are first conductivity type electrodes. For example, the first electrode is an n-type Schottky electrode, and the second electrode is an n-type Schottky electrode.
(2) The first electrode is a first conductivity type electrode, and the second electrode is a second conductivity type ohmic electrode, for example, the first electrode is an n-type Schottky electrode, and the second electrode is a p-type ohmic electrode.
(3) At least one of the first electrode and the second electrode is a second conductivity type electrode. Or, both the first electrode and the second electrode are second conductivity type electrodes. For example, the first electrode is a p-type Schottky electrode, and the second electrode is a p-type Schottky electrode.
(4) The first electrode is a second conductivity type electrode, and the second electrode is a second conductivity type ohmic electrode. For example, the first electrode is a p-type Schottky electrode, and the second electrode is a p-type ohmic electrode.

In this case, the material of the n-type electrode is preferably an alloy of Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zr, etc., and more preferably an alloy containing any two of Ti, Al, Au, Ni, V, Mo, and Zr. In other words, the Schottky electrode may be an n-type (an example of a first conductivity type) electrode containing at least two of Ti, Al, Au, Ni, V, Mo, and Zr.

The material of the p-type electrode is preferably an alloy of Ni, Au, Pt, Ag, Rh, Pd, Pt, Cu, Al, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Co, Ir, Zr, etc., and more preferably an alloy containing Ni or Au. Therefore, for example, the second conductivity type contact electrode may contain at least Ni or Au. In the case of a p-type electrode, a Schottky electrode can be obtained for the p-type cladding layer by setting the heat treatment temperature to 800°C or higher during the alloying process and performing heat treatment in a nitrogen atmosphere.

From the viewpoint of efficiently extracting the photocurrent, the distance between the first electrode and the second electrode is preferably 2 μm or more and 20 μm or less. The shape of each of the first electrode and the second electrode is not particularly limited, and is not limited to a circular, rectangular, comb-shaped, etc. The number of first electrodes and second electrodes is also not particularly limited as long as a voltage can be applied between the electrodes, and multiple first electrodes and second electrodes may be connected in series. From the viewpoint of the manufacturing process, it is preferable to arrange one first electrode and one second electrode opposite each other.

(Light receiving element)
The light receiving element provided in the optical device according to this embodiment has two or more second conductive type contact electrodes on the second conductive type cladding layer. In general laser diodes and light emitting diodes (LEDs), a depletion layer is often formed in the active layer sandwiched between a p-type layer and an n-type layer. Therefore, two or more identical laser diodes can be arranged on the same substrate, and one of them can be used as a light receiving element. In other words, by injecting a current into the pn junction of the laser diode and applying a voltage between the pn junction of the laser diode used as the light receiving element, the light irradiated from the laser diode can be received and extracted as a photocurrent. On the other hand, the optical device according to this embodiment has a structure having a composition gradient layer in the second conductive type cladding layer, so that the depletion layer is formed on the second conductive type cladding layer side, not the active layer. Since the light emitted from the laser diode corresponds to the band gap of the active layer, if a depletion layer is not formed in the active layer, the light is received by the active layer, and the generated electron-hole pairs are recombined and disappear without being extracted as a photocurrent. When a plurality of electrodes including at least one Schottky electrode are arranged on the second-conductivity-type contact layer and a voltage is applied between these electrodes, a depletion layer is formed at the interface between the contact layer including the second-conductivity-type cladding layer and the Schottky electrode, and an electric field proportional to the voltage is applied to the depletion layer. When the depletion layer is irradiated with light from the active layer, the second-conductivity-type contact layer has a band gap narrower than the band gap of the active layer, and the second-conductivity-type cladding layer is a composition-graded layer and has a band gap narrower than the band gap of the active layer. Therefore, electron-hole pairs are excited in the depletion layer and are generated. The electron-hole pairs are extracted as a current from each electrode by the electric field, and can be read out as a photocurrent. That is, it can be used as a metal-semiconductor-metal (MSM) type light receiving element that monitors an electromotive current generated between electrodes, that is, the light receiving element is an MSM type sensor.

By forming the light receiving element and the laser diode on the same substrate, it is possible to significantly reduce manufacturing costs. Furthermore, the positioning of the laser diode and the light receiving element, which is important in optical devices that have a light receiving element and a laser diode monolithically, is also very simple, as it is uniquely determined by the photomask during processing. Furthermore, with regard to temperature characteristics, since they are formed on the same substrate, there is the advantage that corrections can be easily made using offset processing, etc.

(Method of manufacturing optical devices)
The optical device according to this embodiment is manufactured through the process of forming each layer on a 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.

The process of forming each layer on the substrate can be carried out by, for example, molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), or metal organic chemical vapor deposition (MOCVD).

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

Optical devices are manufactured through a process in which unnecessary parts of each layer formed on a substrate are removed by etching. This process can be carried out, for example, by inductively coupled plasma (ICP) etching.

Optical devices can also be manufactured through a process of forming electrodes. This process can be carried out in a variety of ways, for example by evaporating metal using the electron beam deposition (EB) method.

Here, optical devices are manufactured by dividing the substrate, on which each layer has been formed through the above process, into individual pieces by dicing.

The electrodes are formed by resistance heating deposition, electron gun deposition, sputtering, or the like, but are not limited to these methods. The electrodes may be formed as a single layer or as a multi-layer laminate. After the layers are formed, the electrodes may be heat-treated in an oxygen, nitrogen, or air atmosphere.

Next, the optical device according to this embodiment will be described in more detail with reference to the drawings.

First Embodiment
1 is a schematic cross-sectional view of an optical device 1A according to a first embodiment of the present invention. A laser diode 10 includes a first nitride semiconductor stack 11 on a nitride semiconductor substrate 100. A light-receiving element 20A includes a second nitride semiconductor stack 21 on the nitride semiconductor substrate 100. The first nitride semiconductor stack 11 and the second nitride semiconductor stack 21 have the same layer configuration.

The first nitride semiconductor laminate 11 has an n-type cladding layer 111 including a nitride semiconductor layer having n-type conductivity (an example of a first conductivity type), an active layer 113 disposed on the n-type cladding layer 111 and formed of a nitride semiconductor layer including one or more quantum wells, a p-type cladding layer 115 disposed on the active layer 113 and including a nitride semiconductor layer having p-type conductivity (an example of a second conductivity type), and a p-type contact layer 116 disposed on the p-type cladding layer 115.

The first nitride semiconductor stack 11 also has an n-type waveguide layer 112 that is disposed between the n-type cladding layer 111 and the active layer 113 and confines light in the active layer 113, and a p-type waveguide layer 114 that is disposed between the p-type cladding layer 115 and the active layer 113 and confines light in the active layer 113. The n-type waveguide layer 112 and the p-type waveguide layer 114 are suitable for operating the first nitride semiconductor stack 11 as part of a laser diode.

The second nitride semiconductor stack 21 has an n-type cladding layer 211 including a nitride semiconductor layer having n-type conductivity, an active layer 213 disposed on the n-type cladding layer 211 and formed of a nitride semiconductor layer including one or more quantum wells, a p-type cladding layer 215 disposed on the active layer 213 and including a nitride semiconductor layer having p-type conductivity, and a p-type contact layer 216 disposed on the p-type cladding layer 215.

The second nitride semiconductor stack 21 has an n-type waveguide layer 212 that is disposed between the n-type cladding layer 211 and the active layer 213 and confines light in the active layer 213, and a p-type waveguide layer 214 that is disposed between the p-type cladding layer 215 and the active layer 213 and confines light in the active layer 213. If the first nitride semiconductor stack 11 does not have the n-type waveguide layer 112 and the p-type waveguide layer 114, the second nitride semiconductor stack 21 also does not have the n-type waveguide layer 212 and the p-type waveguide layer 214.

The n-type cladding layers 111 and 211 correspond to the first conductivity type cladding layers described above. The n-type waveguide layers 112 and 212 correspond to the first conductivity type waveguide layers described above. The active layers 113 and 213 correspond to the active layers described above. The p-type waveguide layers 114 and 214 correspond to the second conductivity type waveguide layers described above. The p-type cladding layers 115 and 215 correspond to the second conductivity type cladding layers described above. The p-type contact layers 116 and 216 correspond to the second conductivity type contact layers described above.

The laser diode 10 has an n-type contact electrode 12 arranged in a part of the exposed region of the n-type cladding layer 111 of the first nitride semiconductor laminate 11, and a p-type contact electrode 13 arranged on the p-type cladding layer 115 of the first nitride semiconductor laminate 11. The n-type contact electrode 12 corresponds to the third electrode described above, and the p-type contact electrode 13 corresponds to the fourth electrode described above.

The light receiving element 20A has an n-type Schottky electrode 23a (an example of a first electrode) and an n-type Schottky electrode 23b (an example of a second electrode) on the p-type contact layer 216 of the second nitride semiconductor laminate 21. Thus, the light receiving element 20A has at least a first electrode and a second electrode on the p-type contact layer 216 of the second nitride semiconductor laminate 21, and at least one of the first electrode and the second electrode is a Schottky electrode. In the first embodiment, at least one of the first electrode and the second electrode is an n-type electrode. More specifically, both the first electrode and the second electrode are n-type electrodes (i.e., n-type Schottky electrodes 23a and 23b), and the n-type Schottky electrode 23a and the n-type Schottky electrode 23b are arranged opposite each other.

By applying a bias between the n-type contact electrode 12 and the p-type contact electrode 13, the laser diode 10 supplies electrons and holes to the active layer 113, where they recombine and generate light with a wavelength according to the band gap of the active layer 113. The light generated in the active layer 113 of the laser diode 10 causes stimulated emission, which is amplified and oscillated within the resonator structure.

The p-type cladding layer 215 has a dopant concentration of about 1×10 ¹⁸ cm ⁻³ . Therefore, when the n-type Schottky electrodes 23a and 23b are formed on the p-type contact layer 216 by, for example, deposition, depletion layers are formed between the n-type Schottky electrodes 23a and 23b and the p-type contact layer 216 and the p-type cladding layer 215. An electric field is applied to these depletion layers by a voltage. When the light emitted from the laser diode 10 is incident on the p-type contact layer 216 and the p-type cladding layer 215 of the light receiving element 20A, electrons and holes are generated in these depletion layers, and a photocurrent flows between the n-type Schottky electrodes 23a and 23b. 1, for example, when a higher voltage is applied to the n-type Schottky electrode 23b than to the n-type Schottky electrode 23a, electrons generated in the depletion layer move toward the n-type Schottky electrode 23b, and holes generated in the depletion layer move toward the n-type Schottky electrode 23a. As a result, a photocurrent flows from the n-type Schottky electrode 23a toward the n-type Schottky electrode 23b. It becomes possible to monitor the output of the laser diode 10 from changes in the amount of this photocurrent.

Second Embodiment
2 is a schematic cross-sectional view of an optical device 1B according to a second embodiment. The optical device 1B has a similar configuration to the optical device 1A according to the first embodiment, except that a light-receiving element 20B includes one n-type Schottky electrode 23 and one p-type ohmic electrode 24 on a p-type contact layer 216.

The light receiving element 20B has an n-type Schottky electrode 23 (an example of a first electrode) and a p-type ohmic electrode 24 (an example of a second electrode) on the p-type contact layer 216 of the second nitride semiconductor stack 21. Thus, the light receiving element 20B has at least a first electrode and a second electrode on the p-type contact layer 216 of the second nitride semiconductor stack 21, and at least one of the first electrode and the second electrode is a Schottky electrode. In the second embodiment, the first electrode is an n-type electrode (i.e., the n-type Schottky electrode 23), and the second electrode is a p-type ohmic electrode (i.e., the p-type ohmic electrode 24). The n-type Schottky electrode 23 and the p-type ohmic electrode 24 are arranged opposite each other.

The p-type cladding layer 215 has a success of about 1×10 ¹⁸ cm ⁻³ . Therefore, when the n-type Schottky electrode 23 and the p-type ohmic electrode 24 are formed on the p-type contact layer 216 by, for example, deposition, a depletion layer is formed between the n-type Schottky electrode 23 and the p-type contact layer 216 and the p-type cladding layer 215. On the other hand, no depletion layer is formed between the p-type ohmic electrode 24 and the p-type contact layer 216 and the p-type cladding layer 215. An electric field due to a voltage is applied to the depletion layer formed on the n-type Schottky electrode 23 side. When the light emitted from the laser diode 10 is incident on the p-type contact layer 216 and the p-type cladding layer 215 of the light receiving element 20B, electrons and holes are generated in the depletion layer, and a photocurrent flows between the n-type Schottky electrode 23 and the p-type ohmic electrode 24. 2, for example, when a higher voltage is applied to the p-type ohmic electrode 24 than to the n-type Schottky electrode 23, electrons generated in the depletion layer move toward the p-type ohmic electrode 24, and holes generated in the depletion layer move toward the n-type Schottky electrode 23. As a result, a photocurrent flows from the n-type Schottky electrode 23 toward the p-type ohmic electrode 24. It becomes possible to monitor the output of the laser diode 10 from changes in the amount of this photocurrent.

Third Embodiment
3 is a schematic cross-sectional view of an optical device 1C according to a third embodiment of the present invention. The optical device 1C has a similar configuration to the optical device 1A according to the first embodiment, except that a light-receiving element 20C includes two p-type Schottky electrodes 25a (an example of a first electrode) and 25b (an example of a second electrode) on a p-type contact layer 216.

The light receiving element 20C has a p-type Schottky electrode 25a (an example of a first electrode) and a p-type Schottky electrode 25b (an example of a second electrode) on the p-type contact layer 216 of the second nitride semiconductor laminate 21. Thus, the light receiving element 20C has at least a first electrode and a second electrode on the p-type contact layer 216 of the second nitride semiconductor laminate 21, and at least one of the first electrode and the second electrode is a Schottky electrode. In the third embodiment, at least one of the first electrode and the second electrode is a p-type electrode. More specifically, both the first electrode and the second electrode are p-type electrodes (i.e., p-type Schottky electrodes 25a, 25b), and the p-type Schottky electrode 25a and the p-type Schottky electrode 25b are arranged opposite each other.

The p-type cladding layer 215 has a carrier concentration of about 1×10 ¹⁸ cm ⁻³ . Therefore, when the p-type Schottky electrodes 25a and 25b, which have a carrier concentration different from that of the p-type cladding layer 215, are formed on the p-type contact layer 216 by, for example, deposition, a depletion layer is formed between the p-type Schottky electrode 25a and the p-type Schottky electrode 25b and between the p-type contact layer 216 and the p-type cladding layer 215. An electric field due to a voltage is applied to these depletion layers. When the light emitted from the laser diode 10 is incident on the p-type contact layer 216 and the p-type cladding layer 215 of the light receiving element 20C, electrons and holes are generated in these depletion layers, and a photocurrent flows between the p-type Schottky electrode 25a and the p-type Schottky electrode 25b. 3, for example, when a higher voltage is applied to the p-type Schottky electrode 25b than to the p-type Schottky electrode 25a, electrons generated in the depletion layer move toward the p-type Schottky electrode 25b, and holes generated in the depletion layer move toward the p-type Schottky electrode 25a. As a result, a photocurrent flows from the p-type Schottky electrode 25a toward the p-type Schottky electrode 25b. It becomes possible to monitor the output of the laser diode 10 from a change in the amount of this photocurrent.

Fourth Embodiment
4 is a schematic cross-sectional view of an optical device 1D according to a fourth embodiment. The optical device 1D has a similar configuration to the optical device 1A according to the first embodiment, except that a light-receiving element 20D includes one p-type Schottky electrode 25 (an example of a first electrode) and one p-type ohmic electrode 24 (an example of a second electrode) on a p-type contact layer 216.

The light receiving element 20D has a p-type Schottky electrode 25 (an example of a first electrode) and a p-type ohmic electrode 24 (an example of a second electrode) on the p-type contact layer 216 of the second nitride semiconductor stack 21. Thus, the light receiving element 20C has at least a first electrode and a second electrode on the p-type contact layer 216 of the second nitride semiconductor stack 21, and at least one of the first electrode and the second electrode is a Schottky electrode. In the fourth embodiment, the first electrode is a p-type electrode (i.e., the p-type Schottky electrode 25), and the second electrode is a p-type ohmic electrode (i.e., the p-type ohmic electrode 24). The p-type Schottky electrode 25 and the p-type ohmic electrode 24 are arranged opposite each other.

The p-type cladding layer 215 has a carrier concentration of about 1×10 ¹⁸ cm ⁻³ . Therefore, when the p-type Schottky electrode 25 and the p-type ohmic electrode 24, which have a carrier concentration different from that of the p-type cladding layer 215, are formed on the p-type contact layer 216 by, for example, deposition, a depletion layer is formed between the p-type Schottky electrode 25 and the p-type contact layer 216 and the p-type cladding layer 215. On the other hand, no depletion layer is formed between the p-type ohmic electrode 24 and the p-type contact layer 216 and the p-type cladding layer 215. An electric field due to a voltage is applied to the depletion layer formed on the p-type Schottky electrode 25 side. When the light emitted from the laser diode 10 is incident on the p-type contact layer 216 and the p-type cladding layer 215 of the light receiving element 20D, electrons and holes are generated in the depletion layer, and a photocurrent flows between the p-type Schottky electrode 25 and the p-type ohmic electrode 24. 4, for example, when a voltage lower than that applied to the p-type ohmic electrode 24 is applied to the p-type Schottky electrode 25, holes generated in the depletion layer move toward the p-type ohmic electrode 24, and electrons generated in the depletion layer move toward the p-type Schottky electrode 25. As a result, a photocurrent flows from the p-type ohmic electrode 24 toward the p-type Schottky electrode 25. It becomes possible to monitor the output of the laser diode 10 from changes in the amount of this photocurrent.

(Measurement of impurity and doping concentrations)
The concentrations of dopants and impurities contained in each layer constituting the nitride semiconductor substrate 100, the first nitride semiconductor stack 11, and the second nitride semiconductor stack 21 provided in each of the optical devices 1A, 1B, 1C, and 1D can be measured by secondary ion mass spectrometry (SIMS).

When the concentration of dopants and impurities contained in each layer is measured by SIMS after processing into each of the optical devices 1A, 1B, 1C, and 1D, the measurement can be performed in a state where the n-type contact electrode 12, the p-type contact electrode 13, the first electrode, and the second electrode are removed by chemical etching or physical polishing. The concentration of dopants and impurities contained in each layer can also be measured by sputtering from the front side of both sides of the nitride semiconductor substrate 100 on which the n-type contact electrode 12, the p-type contact electrode 13, the first electrode, and the second electrode are not formed.

Specifically, SIMS measurements are performed under the measurement conditions provided by Evans Analytical Group (EAG). A cesium (Cs) ion beam with an energy of 14.5 keV is used to sputter the sample during the measurements.

(Method of measuring layer thickness)
The thickness of each layer constituting each of the optical devices 1A, 1B, 1C, and 1D can be measured by cutting out a predetermined cross section perpendicular to the nitride semiconductor substrate 100, observing the 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 (i.e., the surface on which the first nitride semiconductor stack 11 and the second nitride semiconductor stack 21 are formed) of each of the nitride semiconductor substrates 100 of the optical devices 1A, 1B, 1C, and 1D is observed using the 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 nitride semiconductor substrate 100 in a TEM image showing a cross section perpendicular to the main surface of each of the nitride semiconductor substrates 100 of the optical devices 1A, 1B, 1C, and 1D. Within this observation width, contrast is observed at the interface between two layers with different compositions, so the thickness up to this interface is observed in a continuous observation region with a width of 200 nm. The thickness of each layer can be obtained by calculating the average value of the thickness of each layer included in this 200 nm observation region from five points arbitrarily selected from the observation width of 2 μm or more described above.

(Method of measuring atomic concentration of each layer)
A method for measuring the atomic concentration in each layer and hillock constituting each of the optical devices 1A, 1B, 1C, and 1D includes reciprocal space mapping (RSM) using an X-ray diffraction (XRD) method. Specifically, the lattice relaxation rate and the 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.

For layers that do not provide sufficient reflection intensity by XRD, such as the compositionally graded layers and hillocks contained in the active layers 113, 213 and the p-type cladding layers 115, 215, the layer thicknesses 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-sectioned sample used in TEM observations. 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 the layer thickness calculation method 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 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, the depth direction can be evaluated by performing XPS measurement while performing sputter etching using an ion beam. Although Ar+ is generally used as the ion beam, other ion species such as Ar cluster ions can be used as long as they can be irradiated with an 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 nitride semiconductor substrate 100 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.

(Optical device application fields)
The optical devices 1A, 1B, 1C, and 1D according to the present embodiment are applicable to devices in, for example, the medical and life science fields, the environmental fields, the industrial and manufacturing fields, the lifestyle and home appliance fields, the agricultural fields, and other fields. The optical devices are applicable to devices for synthesizing and decomposing medicines or chemical substances, devices for sterilizing liquids, gases, and solids (containers, food, medical equipment, etc.), devices for cleaning semiconductors, etc., devices for modifying the surfaces of films, glass, metals, etc., exposure devices for manufacturing semiconductors, FPDs, PCBs, and other electronic products, printing and coating devices, bonding and sealing devices, devices for transferring and forming films, patterns, mockups, etc., and devices for measuring and inspecting 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.

Next, examples and comparative examples relating to any of the optical devices 1A, 1B, 1C, and 1D of the first to fourth embodiments will be described. Table 1 is a list summarizing the main parts of Examples 1 to 9 and Comparative Examples 1 and 2 relating to any of the optical devices 1A, 1B, 1C, and 1D.

[Example 1]
An annealing process was performed using a metal-organic chemical vapor deposition (MOCVD) apparatus on an AlN single crystal substrate having a thickness of 550 μm as the nitride semiconductor substrate 100. The annealing process was performed in two sets, each set consisting of annealing for 5 minutes in an _NH3 atmosphere and annealing for 5 minutes in an _H2 atmosphere, in an environment of 1300° C.

Next, an AlN layer, which is a homoepitaxial layer, was formed on the AlN single crystal substrate. The AlN layer was formed to a thickness of 500 nm in an environment of 1200° C. At this time, the ratio (V/III ratio) of the supply rate of the group III 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 was set to 0.5 μm/hr. Trimethylaluminum (TMAl) was used as the Al source. Ammonia (NH ₃ ) was used as the N source.

On the AlN layer formed as described above, a first conductive type cladding layer, a part of which will eventually become the n-type cladding layer 111, 211, was formed. The first conductive type 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 type 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 type 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, a first-conductivity-type waveguide layer, a part of which will eventually become the n-type waveguide layer 112, 212, was formed on the first-conductivity-type cladding layer. The first-conductivity-type cladding 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 first-conductivity-type waveguide layer was formed to a thickness of 60 nm under 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-conductivity-type cladding 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, a light-emitting layer, a part of which will eventually become the active layer 113, 213, was formed on the first conductive type cladding layer. The active layer was formed by depositing a film so as 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 Al0.52Ga0.48N} layer) having a thickness of 3.0 nm. The barrier layer was an AlGaN layer (Al: 63%, i.e., _{an Al0.63Ga0.37N} layer) having a thickness of 6.0 nm.

The active layer was formed under conditions of a vacuum of 50 mbar and a V/III ratio of 4000. The growth rate of the quantum well layer was 0.18 μm/hr. The growth rate of the barrier layer was 0.15 μm/hr.

Next, a second conductive type waveguide layer, a part of which will eventually become the p-type waveguide layer 114, 214, was formed on the active layer. The second conductive type waveguide layer was an AlGaN layer (Al: 63%, i.e., _{an Al0.63Ga0.37N} layer) containing no dopant. The second conductive type waveguide layer was formed to a thickness of 60 nm under 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 second conductive 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-conductivity-type cladding layer (graded layer), a part of which will eventually become the p-type cladding layer 115, 215, was formed on the second-conductivity-type waveguide layer. The second-conductivity-type cladding layer has a laminated structure of an AlGaN layer having a thickness of 20 nm, in which the Al composition is distributed in the direction away from the AlN single crystal substrate and changes from Al=0.63 to 1.0, and an AlGaN layer having a thickness of 350 nm, in which the Al composition is distributed in the direction away from the AlN single crystal substrate and changes from Al=1.0 to 0.3. The second-conductivity-type cladding layer was formed under 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 second-conductivity-type cladding layer at this time was 0.3 to 0.5 μm/hr. Trimethylaluminum (TMAl) was used as the Al raw material. In addition, triethylgallium (TEGa) was used as the Ga source material.

Next, a nitride semiconductor layer having p-type conductivity (p-type contact layer) was formed on the second conductivity type cladding layer, a portion of which would eventually become the p-type contact layers 116, 216. Here, the p-type contact layer had a laminated structure of a 30 nm thick AlGaN layer in which Mg was used as a dopant impurity and Al varied from 0.7 to 0.4, and a 10 nm thick GaN layer (i.e., Al: 0%).

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 at this time was 0.2 μm/hr.

In this manner, a nitride semiconductor stack was formed on the AlN single crystal substrate, a portion of which would eventually become the first nitride semiconductor stack 11 and the second nitride semiconductor stack 21. When reciprocal lattice mapping measurements were performed on this nitride semiconductor stack by XRD, it was found that pseudomorphic growth without relaxation had occurred from the first conductivity type cladding layer to the p-type contact layer.

The nitride semiconductor laminate fabricated as described above was annealed in an N ₂ atmosphere at 700° C. for 10 minutes or more, whereby the resistance of the p-type contact layer was further reduced.

A first nitride semiconductor laminate 11 having a first mesa structure in which the first conductive type cladding layer is exposed in a rectangular region parallel to the <1-100> direction and long in the <1-100> direction was formed by performing dry etching with a gas containing _Cl2 using ICP. The formed first mesa structure had a length of 600 μm in the <1-100> direction and a length of 40 μm in the <11-20> direction. The end surface of the first mesa structure was treated with a TMAH solution heated to 80°C. This treatment formed the end surface of the first mesa structure at an angle of 90°±3° with respect to the AlN single crystal substrate.

Next, with the first mesa structure protected by a mask, dry etching was performed with a gas containing _Cl2 using ICP to form a second nitride semiconductor stack 21 having a second mesa structure that was parallel to the <1-100> direction and exposed the AlN single crystal substrate within a rectangular region that was long in the <1-100> direction. At this time, the distance between the first mesa structure and the second mesa structure was 20 μm.

On the p-type contact layer 116 in the first mesa structure, a rectangular electrode metal region (fourth electrode) containing Ni or Au that is parallel to the <1-100> direction and long in the <1-100> direction was formed as the p-type contact electrode 13. At this time, the width of the fourth electrode was 5 μm and the length was 600 μm.

In addition, on the p-type contact layer 216 in the second mesa structure, n-type Schottky electrodes 23a (first electrode) and 23b (second electrode) containing V, Al, Mo, and Au were formed facing each other and parallel to the <1-100> direction and rectangular in shape and elongated in the <1-100> direction. At this time, the width of the n-type Schottky electrodes 23a and 23b was 15 μm and the length was 500 μm, respectively, and the distance between the n-type Schottky electrodes 23a and 23b was 5 μm.

In addition, in the area where the first conductive type cladding layer is exposed, multiple rectangular electrode metals (third electrodes) made of V, Al, Ni, Ti or Au and parallel to the <1-100> direction and elongated in the <1-100> direction are formed as n-type contact electrodes 12.

In this way, an optical device 1A according to Example 1 of the first embodiment was obtained, which has a monolithic laser diode structure having the same structure as the laser diode 10 and a sensor structure having the same structure as the light receiving element 20A.

The end face emission intensity of the obtained optical device 1A was measured by injecting a current into the laser diode 10. At this time, a voltage of 20 V was applied between the n-type Schottky electrode 23a and the n-type Schottky electrode 23b provided on the light receiving element 20A. In addition, the current value and light receiving sensitivity of the light receiving element 20A were calibrated using a Xe lamp and a spectrometer whose emission intensity is known in advance.

As shown in Table 1, in the optical device 1A according to Example 1, the dark current was 1×10 ⁻¹⁰ A when the laser was not oscillating. When the injection current density was 15 kA/cm ² , the photocurrent was 1×10 ⁻⁶ A (see Table 1), and the converted peak emission intensity was 10 mW. The peak emission intensity was calculated from the correlation between the light emitted from the laser irradiation surface of the laser diode 10 and the leakage light from the reflection surface located opposite to the laser irradiation surface, which was obtained as a calibration curve using a pre-calibrated power meter. Furthermore, as shown in Table 1, the ratio of the photocurrent to the dark current (S/N ratio) was 1×10 ^4. In this way, the optical device 1A according to Example 1 was able to monitor the output of the laser diode 10 in real time using the light receiving element 20A.

[Example 2]
The optical device 1B according to Example 2 of the second embodiment was formed in the same manner as Example 1, except that an n-type Schottky electrode 23 (first electrode) containing V, Al, Mo, and Au and a p-type ohmic electrode 24 (second electrode) containing Ni and Au were formed facing each other on the p-type contact layer in the second mesa structure, the n-type Schottky electrode 23 (first electrode) being parallel to the <1-100> direction and having a rectangular shape elongated in the <1-100> direction.

The obtained optical device 1B was subjected to edge emission intensity measurement by injecting a current into the laser diode 10. At this time, a voltage of 5 V was applied between the n-type Schottky electrode 23 and the p-type ohmic electrode 24 provided on the light-receiving element 20B. As shown in Table 1, in the optical device 1B according to Example 2, the dark current was 5×10 ⁻¹⁰ A when the laser was not oscillating. Furthermore, when the injection current density was 15 kA/cm ² , the photocurrent was 9×10 ⁻⁶ A (see Table 1), and the converted peak emission intensity was 10 mW. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 1.8×10 ⁴ (see Table 1). Thus, the optical device 1B according to Example 2 was able to monitor the output of the laser diode 10 in real time by the light-receiving element 20B.

[Example 3]
An optical device 1C according to Example 3 of the third embodiment was formed in the same manner as Example 1, except that a p-type Schottky electrode 25 a (first electrode) and a p-type Schottky electrode 25 b (second electrode) containing Ni and Au and having a rectangular shape parallel to the <1-100> direction and elongated in the <1-100> direction were formed facing each other on the p-type contact layer in the second mesa structure.

The end face emission intensity of the obtained optical device 1C was measured by injecting a current into the laser diode 10. At this time, a voltage of 5 V was applied between the p-type Schottky electrode 25a and the p-type Schottky electrode 25b provided on the light receiving element 20C. As shown in Table 1, in the optical device 1C according to Example 3, the dark current was 1×10 ⁻¹⁰ A when the laser was not oscillating. Furthermore, when the injection current density was 15 kA/cm ² , the photocurrent was 9×10 ⁻⁷ A (see Table 1), and the converted peak emission intensity was 10 mW. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 9×10 ³ (see Table 1). Thus, the optical device 1C according to Example 3 was able to monitor the output of the laser diode 10 in real time by the light receiving element 20C.

[Example 4]
An optical device 1D according to Example 4 of the fourth embodiment was formed in the same manner as Example 1, except that a rectangular p-type Schottky electrode 25 (first electrode) containing Ni and Au and elongated in the <1-100> direction and a p-type ohmic electrode 24 (second electrode) containing Ni and Au were formed facing each other on the p-type contact layer in the second mesa structure, and were parallel to the <1-100> direction.

The obtained optical device 1D was subjected to edge emission intensity measurement by injecting a current into the laser diode 10. At this time, a voltage of 5 V was applied between the p-type Schottky electrode 25 and the p-type ohmic electrode 24 provided on the light-receiving element 20D. As shown in Table 1, in the optical device 1D according to Example 4, the dark current was 4×10 ⁻¹⁰ A when the laser was not oscillating. When the injection current density was 15 kA/cm ² , the photocurrent was 7×10 ⁻⁶ A (see Table 1), and the converted peak emission intensity was 10 mW. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 1.8×10 ⁴ (see Table 1). Thus, the optical device 1A according to Example 4 was able to monitor the output of the laser diode 10 in real time by the light-receiving element 20A.

[Example 5]
An optical device 1B according to Example 5 of the second embodiment was formed in the same manner as Example 2, except that the distance between the first mesa structure and the second mesa structure was 50 μm.

The end face emission intensity of the obtained optical device 1B was measured by injecting a current into the laser diode 10. At this time, a voltage of 5 V was applied between the n-type Schottky electrode 23 and the p-type ohmic electrode 24 provided on the light receiving element 20B. As shown in Table 1, in the optical device 1B according to Example 5, the dark current was 5×10 ⁻¹⁰ A when the laser was not oscillating. When the injection current density was 15 kA/cm ² , the photocurrent was 2×10 ⁻⁶ A (see Table 1), and the converted peak emission intensity was 10 mW. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 4×10 ³ (see Table 1). Thus, the optical device 1B according to Example 5 was able to monitor the output of the laser diode 10 in real time by the light receiving element 20B.

[Example 6]
An optical device 1B according to Example 6 of the second embodiment was formed in the same manner as Example 2, except that the distance between the first mesa structure and the second mesa structure was 5 μm.

The end face emission intensity of the obtained optical device 1B was measured by injecting a current into the laser diode 10. At this time, a voltage of 5 V was applied between the n-type Schottky electrode 23 and the p-type ohmic electrode 24 provided on the light receiving element 20B. When the laser was not oscillating, the dark current was 5×10 ⁻¹⁰ A. When the injection current density was 15 kA/cm ² , the photocurrent was 1×10 ⁻⁵ A (see Table 1), and the converted peak emission intensity was 10 mW. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 2×10 ⁴ (see Table 1). Thus, the optical device 1B according to Example 6 was able to monitor the output of the laser diode 10 in real time by the light receiving element 20B.

[Example 7]
The optical device 1B according to Example 7 of the second embodiment was formed in the same manner as Example 2, except that the distance between the n-type Schottky electrode 23 (first electrode) and the p-type ohmic electrode 24 (second electrode) was 1 μm.

The end face emission intensity of the obtained optical device 1B was measured by injecting a current into the laser diode 10. At this time, a voltage of 5 V was applied between the n-type Schottky electrode 23 and the p-type ohmic electrode 24 provided on the light receiving element 20B. As shown in Table 1, the dark current was 7×10 ⁻¹⁰ A when the laser was not oscillating. When the injection current density was 15 kA/cm ² , the photocurrent was 1×10 ⁻⁵ A (see Table 1), and the converted peak emission intensity was 10 mW. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 1.4×10 ⁴ (see Table 1). Thus, the optical device 1B according to Example 7 was able to monitor the output of the laser diode 10 in real time by the light receiving element 20B.

[Example 8]
The optical device 1B according to Example 8 of the second embodiment was formed in the same manner as Example 2, except that the distance between the n-type Schottky electrode 23 (first electrode) and the p-type ohmic electrode 24 (second electrode) was 10 μm.

The edge emission intensity of the obtained optical device 1B was measured by injecting a current into the laser diode 10. At this time, a voltage of 5 V was applied between the n-type Schottky electrode 23 and the p-type ohmic electrode 24 provided on the light-receiving element 20B. As shown in Table 1, the dark current was 5×10 ⁻¹⁰ A when the laser was not oscillating. When the injection current density was 15 kA/cm ² , the photocurrent was 4×10 ⁻⁶ A (see Table 1), and the converted peak emission intensity was 10 mW. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 8×10 ³ (see Table 1). In this way, the optical device 1B according to Example 8 was able to monitor the output of the laser diode 10 in real time by the light-receiving element 20B.

[Example 9]
The optical device 1B according to Example 8 of the second embodiment was formed in the same manner as Example 2, except that the distance between the n-type Schottky electrode 23 (first electrode) and the p-type ohmic electrode 24 (second electrode) was 20 μm.

The end face emission intensity of the obtained optical device 1B was measured by injecting a current into the laser diode 10. At this time, a voltage of 5 V was applied between the n-type Schottky electrode 23 and the p-type ohmic electrode 24 provided on the light receiving element 20B. As shown in Table 1, the dark current was 5×10 ⁻¹⁰ A when the laser was not oscillating. When the injection current density was 15 kA/cm ² , the photocurrent was 2×10 ⁻⁶ A (see Table 1), and the converted peak emission intensity was 10 mW. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 4×10 ³ (see Table 1). Thus, the optical device 1B according to Example 9 was able to monitor the output of the laser diode 10 in real time by the light receiving element 20B.

[Comparative Example 1]
The optical device according to Comparative Example 1 was formed in the same manner as Example 1, except that the first mesa structure and the second mesa structure had the same shape, and the second mesa structure had a third electrode and a fourth electrode like the first mesa structure in each embodiment. That is, the optical device according to Comparative Example 1 has a part of the n-type cladding layer exposed, a third electrode arranged on the exposed part of the n-type cladding layer, and one fourth electrode arranged on the p-type contact electrode of the light receiving element.

The edge emission intensity of the obtained optical device was measured by injecting a current into the laser diode. At this time, a voltage of 5 V was applied between the third electrode and the fourth electrode provided on the light receiving element. As shown in Table 1, the dark current was 1×10 ⁻¹⁰ A when the laser was not oscillating. When the injection current density was 15 kA/cm ² , the photocurrent was also 1×10 ⁻¹⁰ A (see Table 1), and the peak emission intensity could not be detected. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 1.0 (see Table 1). Thus, the optical device according to Comparative Example 1 was unable to monitor the output of the laser diode in real time.

[Comparative Example 2]
The optical device of Comparative Example 2 was formed in the same manner as Example 1, except that a p-type contact layer provided in the second mesa structure contained rectangular Ni and Au that were parallel to the <1-100> direction and elongated in the <1-100> direction, and two ohmic electrode metal regions corresponding to the third electrode and fourth electrode in each embodiment were formed facing each other.

The edge emission intensity of the obtained optical device was measured by injecting a current into the laser diode. At this time, a voltage of 5 V was applied between two ohmic electrode metal regions provided on the light receiving element. As shown in Table 1, the dark current was 1×10 ⁻³ A when the laser was not oscillating. When the injection current density was 15 kA/cm ² , the photocurrent was also 1×10 ⁻³ A (see Table 1), and the peak emission intensity could not be detected. Furthermore, the ratio of the photocurrent to the dark current (S/N ratio) was 1.0 (see Table 1). Thus, the optical device according to Comparative Example 2 was unable to monitor the output of the laser diode in real time.

As described above, the optical devices according to the first to fourth embodiments and examples 1 to 9 include a nitride semiconductor substrate 100 containing Al, a laser diode 10 formed on the nitride semiconductor substrate 100 and having a first nitride semiconductor stack 11, and one of the light-receiving elements 20A to 20D formed on the nitride semiconductor substrate 100 and having a second nitride semiconductor stack 21. Each of the light-receiving elements 20A to 20D has at least a first electrode and a second electrode on the p-type contact layer 216 of the second nitride semiconductor stack 20, and at least one of the first electrode and the second electrode is a Schottky electrode.

As a result, the optical devices according to the first to fourth embodiments and examples 1 to 9 can monitor the output of the laser diode 10 in real time using the light receiving elements 20A to 20D.

Reference Signs List 1A, 1B, 1C, 1D Optical device 10 Laser diode 11 First nitride semiconductor stack 12 N-type contact electrode 13 P-type contact electrode 20 Second nitride semiconductor stack 20A, 20B, 20C, 20D Light receiving element 21 Second nitride semiconductor stack 23, 23a, 23b N-type Schottky electrode 24 P-type ohmic electrode 25, 25a, 25b P-type Schottky electrode 100 Nitride semiconductor substrate 111, 211 N-type cladding layer 112, 212 N-type waveguide layer 113, 213 Active layer 114, 214 P-type waveguide layer 115, 215 P-type cladding layer 116, 216 P-type contact layer

Claims (21)

a nitride semiconductor substrate containing aluminum;
a laser diode formed on the nitride semiconductor substrate and having a first nitride semiconductor stack;
a light receiving element formed on the nitride semiconductor substrate and having a second nitride semiconductor stack;
Equipped with
Each of the first nitride semiconductor stack and the second nitride semiconductor stack is
a first conductivity type cladding layer including a nitride semiconductor layer having a first conductivity type;
an active layer formed of a nitride semiconductor layer including one or more quantum wells and disposed on the first conductive type cladding layer;
a second conductive type cladding layer disposed on the active layer and including a nitride semiconductor layer having a second conductive type conductivity;
a second conductive type contact layer disposed on the second conductive type cladding layer;
the laser diode has a first conductivity type contact electrode disposed in a part of an exposed region of the first conductivity type cladding layer of the first nitride semiconductor stack, and a second conductivity type contact electrode disposed on the second conductivity type contact layer of the first nitride semiconductor stack,
the light receiving element has at least a first electrode and a second electrode in contact with the second conductivity type contact layer of the second nitride semiconductor stack,
At least one of the first electrode and the second electrode is a Schottky electrode. 2. The optical device according to claim 1, wherein the laser diode and the light receiving element are electrically isolated from each other except for the nitride semiconductor substrate and a part of the first conductivity type cladding layer. The optical device according to claim 1 , wherein at least one of the first electrode and the second electrode is an electrode of a first conductivity type. both the first electrode and the second electrode are first conductivity type electrodes;
The optical device according to claim 1 , wherein the first electrode and the second electrode are disposed opposite to each other. The optical device according to claim 1 , wherein the Schottky electrode is a first conductivity type electrode containing at least two of titanium, aluminum, gold, nickel, vanadium, molybdenum, and zirconium. The optical device according to claim 1 , wherein at least one of the first electrode and the second electrode is an electrode of a second conductivity type. both the first electrode and the second electrode are second conductivity type electrodes;
The optical device according to claim 1 , 2 or 6 , wherein the first electrode and the second electrode are disposed opposite to each other. The optical device according to claim 6 , wherein the second conductivity type contact electrode contains at least nickel or gold. the first electrode is a first conductivity type electrode,
The optical device according to claim 1 , wherein the second electrode is an ohmic electrode of a second conductivity type. the first electrode is a second conductivity type electrode,
The optical device according to claim 1 , wherein the second electrode is an ohmic electrode of a second conductivity type. The optical device according to claim 1 , wherein the light receiving element is a metal-semiconductor-metal type sensor. The optical device according to claim 1 , wherein the distance between the first electrode and the second electrode is not less than 2 μm and not more than 20 μm. The optical device according to claim 1 , wherein the distance between the laser diode and the light receiving element is not less than 1 μm and not more than 1000 μm. The optical device according to claim 1 , wherein the nitride semiconductor substrate is an aluminum nitride single crystal substrate. the second conductive type cladding layer has a band gap narrower than at least the active layer;
15. An optical device according to any one of claims 1 to 14. Each of the first nitride semiconductor stack and the second nitride semiconductor stack is
a first conductivity type waveguide layer disposed between the first conductivity type cladding layer and the active layer to confine light to the active layer;
a second conductivity type waveguide layer disposed between the second conductivity type cladding layer and the active layer to confine light to the active layer;
Further,
15. An optical device according to any one of claims 1 to 14. the second conductivity type cladding layer includes a second conductivity type vertical conduction layer that includes Al _e Ga _{1- e} N (0.1≦ e ≦1) and has a composition gradient in which the Al composition ratio e decreases with increasing distance from the nitride semiconductor substrate, the second conductivity type vertical conduction layer being disposed on the nitride semiconductor substrate side, and a second conductivity type lateral conduction layer that includes Al _f Ga _1-f N (0<f≦1) and is disposed on the second conductivity type contact electrode side;
Each of the first nitride semiconductor laminate and the second nitride semiconductor laminate is disposed between the second conductive type vertical conduction layer and the second conductive type waveguide layer and includes an Al _g Ga _(1-g) N (0<g≦1.0) layer.
17. The optical device of claim 16. The thickness of the second conductive type vertical conductive layer is 250 nm or more and 450 nm or less.
20. The optical device of claim 17. the second conductivity type vertical conduction layer has an undoped region in a certain range including an interface with the second conductivity type waveguide layer in a thickness direction of the second conductivity type cladding layer;
19. An optical device according to claim 17 or 18. 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 fully strained with respect to the nitride semiconductor substrate;
20. An optical device according to any one of claims 17 to 19. the second conductive type lateral conduction layer has an Al composition ratio f that is greater than a minimum value of the Al composition ratio e at a surface adjacent to the second conductive type vertical conduction layer;
21. An optical device according to any one of claims 17 to 20.

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