JPWO2008090727A1 - Nitride-based semiconductor optical device - Google Patents

Abstract

A highly reliable high-power semiconductor laser is provided. A semiconductor laser according to an embodiment is a semiconductor laser having an active layer made of a group III nitride semiconductor containing Ga as a constituent element and emitting laser light from an end face of the active layer. This semiconductor laser is provided on the end face from which laser light is emitted, and includes a protective film made of a dielectric film. Carbon is included in a partial region of the dielectric film in contact with the end face.

Description

  The present invention relates to a group III nitride semiconductor optical device.

  Group III nitride semiconductors, typified by gallium nitride, have gained attention as a constituent material for light emitting diodes (LEDs) and laser diodes (LDers) because of their high-efficiency blue-violet emission. I came. In particular, LD is expected as a light source for a large-capacity optical disk apparatus. In recent years, development of a high-power LD as a light source for writing has been vigorously advanced.

  FIG. 7 shows a typical gallium nitride based laser structure. After laminating a laser structure such as an n-type cladding layer 102, light guide layers 103 and 105, an active layer 104, and a p-type cladding layer 106 on a GaN substrate 101, the p-type cladding layer 106 is processed into a ridge shape by dry etching. Is done. The ridge part is covered with an insulating film 107 having a stripe-shaped opening, and a p-type electrode 108 is provided in the opening. The current confinement is made by a striped electrode, and the transverse mode is controlled by adjusting the ridge width and ridge height. Laser light is emitted from a resonant mirror formed by cleavage. In a semiconductor laser, a dielectric protective film is generally formed for protecting the end face. In high power applications, a low reflection (Anti-reflecting, AR) film is formed on the output side end face, and a high reflection (High-reflecting, HR) film is formed on the opposite end face in order to increase the output efficiency.

The requirements for a general end face protective film material include that there is no absorption of laser light, that a desired reflectance can be obtained, and that a close contact with a semiconductor is good. It is also important that film formation with good controllability and productivity is possible from the viewpoint of manufacturing. From this point of view, oxides such as Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , Ta ₂ O ₅ , Nb ₂ O _5, etc., generally formed by a technique such as sputtering, CVD, or vapor deposition, MgF ₂ , Fluorides such as CaF ₂ and nitrides such as AlN and Si ₃ N ₄ are used.

As a prior art document related to the present invention, Patent Document 1 (Japanese Patent Laid-Open No. 10-190139) can be cited.
JP-A-10-190139

  By forming the dielectric film described above as an AR film on the laser emission end face, high output characteristics without COD (Catastrophic Optical Damage) can be realized in the initial stage. However, when a high output long time drive of 100 mW or more is performed, there is a problem in reliability that a sudden failure occurs due to a decrease in the COD level. As a result of detailed studies, the present inventor has found that COD level degradation during high-power long-time driving in a GaN-based semiconductor laser is caused by Ga diffusion into the protective film.

  The nitride-based semiconductor optical device according to the present invention has an active layer made of a group III nitride semiconductor containing gallium (Ga) as a constituent element, and emits laser light from the end face of the active layer. A protective film made of a single-layer or multi-layer dielectric film is provided on the end face from which the laser beam is emitted, and a part of the dielectric film in contact with the end face has a carbon (C) is included.

In the case of a conventional protective film not containing C, COD level deterioration occurs due to the following mechanism. A layer (natural oxide film) mainly composed of an oxide of Ga is formed on the end face of the resonator formed by cleavage. When a laser obtained by depositing a dielectric film as a protective film thereon is operated at a high output, Ga on the semiconductor end face is ionized to become Ga ⁺ . When the gallium ion Ga ⁺ receives and neutralizes electrons induced due to structural defects (oxygen deficiency, etc.) in the protective film at the interface with the protective film, the neutralized Ga is contained in the protective film. Spreads to. As a result, the semiconductor end face is deteriorated and deep levels are generated, which causes a temperature rise due to absorption of laser light. On the other hand, when C is added to the protective film, a deep level (for example, an energy level formed near the center of the forbidden band) is formed in the protective film, and this level traps electrons induced by structural defects. Therefore, the diffusion of Ga ⁺ is suppressed without being neutralized. As a result, a deep level is not generated, and a decrease in COD level is also suppressed.

  According to the present invention, a highly reliable nitride-based semiconductor optical device in which a decrease in the COD level in a high output operation is suppressed is realized.

FIG. 1A and FIG. 1B are cross-sectional views schematically showing the structure of a semiconductor laser according to an embodiment of the present invention. FIG. 2A to FIG. 2C are diagrams illustrating a manufacturing process of the laser structure according to the example. FIG. 3A to FIG. 3C are diagrams illustrating a manufacturing process of the laser structure according to the example. FIG. 4A and FIG. 4B are diagrams illustrating a manufacturing process of the laser structure according to the example. FIG. 5 is a graph showing the relationship between the carbon concentration at the Al ₂ O ₃ / semiconductor interface and the device lifetime. FIG. 6 is a diagram showing the relationship between the oscillation wavelength and the element lifetime when an Al ₂ O ₃ protective film containing no carbon is used. FIG. 7 is a cross-sectional view schematically showing the structure of a conventional semiconductor laser having a ridge-type waveguide structure.

  Hereinafter, preferred embodiments of a nitride-based semiconductor optical device according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same reference numerals are assigned to the same elements, and duplicate descriptions are omitted.

  FIG. 1A and FIG. 1B are cross-sectional views showing an embodiment of a nitride-based semiconductor optical device according to the present invention. FIG. 1A shows a schematic view of an element structure viewed from a cross section perpendicular to the resonator direction. FIG. 1B shows the vicinity of the laser emission end face of a cross section parallel to the resonator direction at the position of the line xx ′ in FIG. In this embodiment, a semiconductor laser is shown as an example of a nitride-based semiconductor optical device.

  This semiconductor laser is a semiconductor laser having an active layer made of a group III nitride semiconductor containing Ga as a constituent element and emitting laser light from the end face of the active layer, on the end face from which the laser light is emitted Provided with a protective film made of a dielectric film. Carbon is contained in a partial region of the dielectric film in contact with the end face.

More specifically, the semiconductor laser includes an Si-doped n-type GaN layer 202 (Si concentration 4 × 10 ¹⁷ cm ⁻³ , thickness 1 μm) on an n-type GaN substrate 201, Si-doped n-type Al _0.1 Ga _{0. .9} N-type cladding layer 203 made of N (Si concentration 4 × 10 ¹⁷ cm ⁻³ , thickness 2 μm), Si-doped n-type GaN (Si concentration 4 × 10 ¹⁷ cm ⁻³ , thickness 0.1 μm) An n-type GaN optical confinement layer 204, In _0.15 Ga _0.85 N (thickness 3 nm) as a well layer, Si-doped In _0.01 Ga _0.99 N (Si concentration 1 × 10 ¹⁸ cm ₋₃ , thickness) 4 nm) as a barrier layer, a three-period multiple quantum well (MQW) layer 205 (active layer), Mg-doped p-type Al _0.2 Ga _0.8 N (Mg concentration 2 × 10 ¹⁹ cm ⁻³ , thickness 10 μm) Cap layer consisting of 06, Mg-doped p-type GaN (Mg concentration 2 × ¹⁰ 19 ^{cm -3,} thickness 0.1 [mu] m) p-type GaN light confining layer 207 made of, p-type _Al 0.1 _{Ga 0.9} N cladding layer 208, Mg A p-type GaN contact layer 209 made of doped p-type GaN (Mg concentration 1 × 10 ²⁰ cm ⁻³ , thickness 0.02 μm) is stacked.

The p-type cladding layer 208 and the p-type contact layer 209 have a ridge structure using dry etching formed in a stripe shape. A p-type electrode 214 is provided on the top surface of the ridge top p-type contact layer 209, and an n-type electrode 216 is provided below the n-type GaN substrate 201. A dielectric protective film is formed on the end face of the resonator formed by cleavage. Here, an AR film 211 (protective film) is formed on the end face on the laser light emission side, and an HR film is formed on the end face on the opposite side.
The reflectance of each end face can be controlled in the range of 1% to 98% by appropriately selecting the refractive index, film thickness, and number of layers of the dielectric material. The reflectivity of the HR film is preferably 70% or more, more preferably 90% or more. The reflectance of the AR film 211 is preferably less than 70%, more preferably 1% to 50%, and even more preferably 5% to 30%.

  The AR film 211 is formed of a multilayer dielectric film, and at least a dielectric film 211a (first dielectric film) in contact with the semiconductor optical element body (hereinafter simply referred to as “semiconductor”) is connected to the semiconductor optical element body. Carbon (C) is added in the vicinity of the interface. The carbon concentration in the region containing carbon in the dielectric film 211a is preferably 0.01 at. % (Atomic concentration) or more, more preferably 0.1 at. % Or more and 10 at. % Or less. This suppresses Ga diffusion into the dielectric film due to high-power laser operation. If the C concentration is too high, carbon clusters are formed, which deteriorates laser characteristics.

  The region containing carbon (C) in the dielectric film 211a may be only near the interface with the semiconductor. The deep level due to carbon (C) absorbs the laser beam and causes a decrease in the initial COD level. Therefore, the C concentration is 1 at. It is preferable that the concentration distribution is such that the region of at least% is limited to a region thinner than ½ of the thickness of the dielectric film 211a. That is, the carbon concentration in the dielectric film 211a is 1 at. % Or more of the region is preferably limited to a region whose distance from the laser emission end face is less than ½ of the thickness of the dielectric film 211a. By doing so, it is possible to drive at a high output for a long time while suppressing a decrease in the initial COD level. As described above, from the viewpoint of suppressing the formation of carbon clusters, the C concentration in the region is 10 at. % Or less is preferable.

As the dielectric film 211a, an oxide, nitride, oxynitride film, or fluoride-based dielectric film can be used. Although the effect of the addition of C does not depend on these film types, it is preferable to use an oxide or an oxynitride from the viewpoint of satisfactorily protecting the natural oxide film formed on the end face. Furthermore, from the viewpoint of improving the adhesion between the film and the semiconductor, an oxide film containing Ti (TiO ₂ , SrTiO ₃ etc.) and an oxynitride film (TiO _1-x N _x etc.) are more preferable.

When a titanium oxide-based material is used for the dielectric film 211a, a multilayer structure in which the outermost surface of the laser emission end face is covered with a dielectric film 211b (second dielectric film) different from the dielectric film 211a is adopted. desirable. The dielectric film 211b can be formed of a dielectric having a larger band gap than the dielectric film 211a. In particular, in a 405 nm band laser, the end face is contaminated by organic impurities contained in the atmosphere and a photochemical reaction by laser light. When the dielectric film 211b is not provided, end face contamination becomes obvious due to the photocatalytic function of titanium oxide. This is suppressed by terminating the outermost surface on the laser emission side with a film having a large band gap. Therefore, the dielectric film 211b is made of an oxide-based material having a larger band gap than the constituent material of the dielectric film 211a, such as Al ₂ O ₃ , SiO ₂ , Nb ₂ O ₅ , Ta ₂ O ₅ , ZrO _2. It is desirable to use it.

  In general, a titanium oxide-based material has a relatively small band gap of about 3.2 eV. Therefore, even in a 405 nm band (395 to 410 nm, 3.1 to 3.0 eV) laser, spontaneous emission light (oscillation) The short wavelength component (light output below the threshold) may be absorbed, and the initial COD level may be lowered. Therefore, when a titanium oxide-based material is used for the dielectric film 211a, a decrease in the initial COD level can be suppressed when the film thickness is as thin as possible. However, since controllability deteriorates if it is too thin, the film thickness is preferably 10 nm or more and 50 nm or less. In addition, by setting the front-surface reflectance to 5% or more, the threshold light output can be reduced, and a decrease in the initial COD level can be further suppressed.

If the dielectric film 211b is too thick, the dielectric film 211a and the semiconductor may peel off due to a difference in thermal expansion coefficient. If the dielectric film 211b is too thin, the film thickness controllability becomes difficult. Is desirable. When a titanium oxide-based dielectric film 211a having a thickness of 10 nm or more and 50 nm or less is used, the dielectric film 211b is preferably a low refractive index material such as Al ₂ O ₃ or SiO ₂ from the viewpoint of reflectance control. Al ₂ O ₃ is more preferable from the viewpoint of the difference in thermal expansion coefficient. For example, in the case of a 405 nm band laser, TiO ₂ (refractive index 2.3 to 2.7) having a thickness of 10 nm to 50 nm is used for the dielectric film 211a, and Al ₂ of 10 nm to 60 nm is used for the dielectric film 211b. By using O ₃ (refractive index of 1.6 to 1.7), the reflectance can be controlled in the range of 1 to 25%.

  According to this embodiment, by providing an end face protective film containing carbon (C) at least near the interface with the semiconductor on the laser emission end face, Ga diffusion into the protective film is suppressed, and a high output of 100 mW or more is achieved. Stable driving for a long time is possible even under. In addition, the semiconductor laser of the present embodiment is formed so as to cover the first dielectric film 211a by providing the first dielectric film 211a containing Ti and oxygen containing C at least near the interface on the laser emission end face. In addition, since the second dielectric film 211b having a larger band gap than the first dielectric film 211a is provided, the film diffusion and the contamination of the end face are reduced while suppressing Ga diffusion into the protective film. High output and high reliability operation is possible.

By the way, with respect to the AlInGaAs-based semiconductor laser (0.98 nm band), a dielectric thin film (Al _x Si _y Ta ₁ ) in which an oxide-based, nitride-based, or carbide-based material is mixed as a protective film material that suppresses a decrease in the COD level. _{-x-y O m n n C} 1-m-n) is reported in Patent Document 1. According to the document, Al ₂ O ₃ (oxide material), SiN _x (nitride material), and TaC (carbide material) alone have the following advantages and disadvantages, and the respective disadvantages are compensated by mixing. You can do that. In this document, the mixed thin film is described as being applicable to an AlInGaN-based semiconductor laser (400 nm band), but there is no description of the composition ratio in that case.
Al ₂ O ₃ (oxide material): Although chemically stable, film deposition damage is easily introduced.
SiN _x (nitride material): little film damage, but poor adhesion at the dielectric / semiconductor interface.
TaC (Carbide material): Although the adhesion at the interface is excellent, interdiffusion at the dielectric / semiconductor interface is likely to occur, and there is a problem in thermal stability.

On the other hand, since the GaN-based semiconductor laser is less affected by film formation damage, even when the above Al ₂ O ₃ (oxide material) is used as a protective film, high output characteristics without COD can be stably obtained in the initial stage. However, a sudden failure can also occur when driving at a high output for a long time. The lifetime of the element does not depend on the film formation damage, but strongly depends on the oscillation wavelength (FIG. 6). From the graph of FIG. 6, it can be seen that the shorter the wavelength at an oscillation wavelength of 410 nm or less, the shorter the lifetime. Since the lifetime becomes shorter as the oscillation wavelength becomes shorter, it has been speculated that there are degradation factors inherent to the short wavelength band. In this regard, according to the present embodiment, it is possible to realize a semiconductor laser having a long element lifetime even when the oscillation wavelength is 410 nm or less.

As a first embodiment, a ridge stripe laser according to the present invention will be described. A method for manufacturing a semiconductor laser according to the present embodiment will be described with reference to FIGS. An n-type GaN (0001) substrate 301 was used as the substrate. A 300 hPa reduced pressure MOVPE apparatus was used to fabricate the element structure. A mixed gas of hydrogen and nitrogen is used as a carrier gas, trimethylgallium (TMG), trimethylaluminum (TMA), trimethylindium (TMI) are used as Ga, Al, and In sources, silane (SiH ₄ ), p is used as an n-type dopant, respectively. Biscyclopentadienyl magnesium (Cp ₂ Mg) was used as the type dopant.

After introducing the n-type GaN substrate 301 into the growth apparatus, the substrate was heated while supplying NH ₃ , and the growth was started when the growth temperature was reached. Si-doped n-type GaN layer 302 (Si concentration 4 × 10 ¹⁷ cm ⁻³ , thickness 1 μm), Si-doped n-type Al _0.1 Ga _0.9 N (Si concentration 4 × 10 ¹⁷ cm ⁻³ , thickness 2 μm) N-type cladding layer 303, Si-doped n-type GaN (Si concentration 4 × 10 ¹⁷ cm ⁻³ , thickness 0.1 μm), n-type optical confinement layer 304, In _0.15 Ga _0.85 N ( Three-cycle multiple quantum well (MQW) layer 305 composed of a well layer and a Si-doped In _0.01 Ga _0.99 N (Si concentration 1 × 10 ¹⁸ cm ⁻³ , thickness 4 nm) barrier layer, Mg-doped A cap layer 306 made of p-type Al _0.2 Ga _0.8 N and a p-type optical confinement layer 307 made of Mg-doped p-type GaN (Mg concentration 2 × 10 ¹⁹ cm ⁻³ , thickness 0.1 μm) are sequentially deposited. did.

Subsequently, a p-type cladding layer 308 made of Mg-doped p-type Al _0.1 Ga _0.9 N (Mg concentration 1 × 10 ¹⁹ cm ⁻³ , thickness 0.5 μm) is deposited, and Mg-doped p-type GaN (Mg concentration) is deposited. A contact layer 309 made of 1 × 10 ²⁰ cm ⁻³ and a thickness of 0.02 μm was deposited (FIG. 2A). The GaN growth is performed at a substrate temperature of 1080 ° C., a TMG supply rate of 58 μmol / min, and an NH ₃ supply rate of 0.36 mol / min, and the AlGaN growth is performed at a substrate temperature of 1080 ° C., a TMA supply rate of 36 μmol / min, and a TMG supply rate of 58 μmol / min. , NH _{3 was} supplied at 0.36 mol / min. The InGaN-based MQW layer is grown at a substrate temperature of 800 ° C., a TMG supply rate of 8 μmol / min, and NH ₃ 0.36 mol / min. At this time, the TMIn supply rate is 48 μmol / min for the well layer and 3 μmol / min for the barrier layer. It was set to min.

An SiO ₂ layer 310 was formed on the laser wafer produced as described above (FIG. 2B), and an SiO ₂ stripe 311 having a width of 1.3 μm was formed by photolithography (FIG. 2C). Using this SiO ₂ stripe 311 as a mask, the p-cladding layer 308 was partially removed by dry etching to form a ridge structure (FIG. 3A). Subsequently, the SiO ₂ mask 311 was removed, and a SiO ₂ layer 312 was newly deposited on the entire surface of the wafer. Next, a thick resist 313 was applied (FIG. 3B), and the top of the ridge was cleaved by etch back in oxygen plasma (FIG. 3C).

After removing SiO ₂ on the ridge top with buffered hydrofluoric acid, a Pd / Pt layer 314 was deposited by an electron beam, and a p-contact was formed by lift-off. Next, RTA (Rapid Thermal Annealing) was performed at 600 ° C. for 30 seconds in a nitrogen atmosphere to form a p-ohmic electrode (FIG. 4A). Thereafter, a Ti film with a thickness of 50 nm, a Pt film with a thickness of 100 nm, and an Au film with a thickness of 2 μm were deposited by sputtering to form a cover electrode 315 (FIG. 4B). After the p-electrode process, the back surface of the wafer is thinned to a thickness of 100 μm, and a titanium (Ti) film having a thickness of 5 nm, an Al film having a thickness of 20 nm, a Ti film having a thickness of 10 nm, and an Au film having a thickness of 500 nm are sequentially formed. Vacuum deposition was performed to form an n-electrode. The sample after electrode formation was cleaved in the direction perpendicular to the stripes to form a laser bar having a resonator length of 600 μm.

The laser bar was introduced into an RF magnetron sputtering apparatus, and an AR film 211 described later was formed on the emission end face. Next, after removing the laser bar from the sputtering apparatus, an HR film having a reflectivity of 95% made of a SiO ₂ / TiO ₂ multilayer film was formed again on the opposite end face by the sputtering apparatus. Thereafter, element isolation was performed to produce a laser chip having an element width of 300 μm.

As the AR coating material, an Al ₂ O ₃ single layer film (22 nm thick) was used. High purity Al ₂ O ₃ was used as the sputtering target. First, a mixed gas of Ar and (O ₂ + CO ₂ ) was used as a plasma gas in order to dope C into the AR film 211 in the vicinity of the interface with the semiconductor. After about 1/5 of the desired film thickness was formed, the plasma was once dropped and the film formation was interrupted. Then, a film was formed again to a desired film thickness using a mixed gas of Ar + O ₂ . Here, the Ar flow rate (30 sccm) and the total flow rate (45 sccm) are constant, and the C amount in the Al ₂ O ₃ film was changed by changing the ratio of O ₂ and CO ₂ . The reflectance of the front end face was estimated to be about 15% at a wavelength of 405 nm.

The laser chip obtained by the above steps was fused to a heat sink to obtain an evaluation element. Typical initial laser characteristics were an oscillation wavelength of 405 nm, an oscillation threshold current density of 3.2 kA / cm ² , and a threshold voltage of 4.0 V. These elements were subjected to a reliability test of 150 mW to evaluate the element lifetime.

FIG. 5 is a diagram showing the relationship between the C concentration at the Al ₂ O ₃ / semiconductor interface and the element lifetime. The C concentration in the film was determined by measuring the deposited film by secondary ion mass spectrometry (SIMS). As is apparent from FIG. 5, the C concentration at the Al ₂ O ₃ / semiconductor interface is 0.1 at. It can be seen that when the ratio is higher than%, sudden deterioration is suppressed and the device life is drastically improved. In order to investigate the factor of this improvement effect, the C concentration in the AR film 211 is 0.03 at. % Element and 0.1 at. % Element was driven at 80 ° C. and an output of 100 mW for 100 hours, and the vicinity of the AR end face was analyzed by cross-sectional observation analysis (cross-section TEM-EDX) using a transmission electron microscope. As a result, the C concentration was 0.03 at. %, Ga was detected in the AR film 211, whereas the C concentration was 0.1 at. Ga was not detected in% elements. From these results, the C concentration of the AR film 211 / semiconductor interface was 0.1 at. It has been found that by setting the ratio to at least%, it is possible to prevent Ga diffusion and suppress an increase in the concentration of non-emissive centers on the semiconductor side.

A semiconductor laser was fabricated in the same manner as in Example 1 except that a TiO ₂ layer (39 nm thickness) was used as the dielectric film 211a and an Al ₂ O ₃ layer (25 nm thickness) was used as the dielectric film 211b on the AR end face. evaluated. For comparison, an element in which the AR protective film was a TiO ₂ single layer (43 nm thickness) was also prepared. Here, the Ga concentration at the TiO ₂ / semiconductor interface was 3% (at.%). The front reflectance was estimated to be about 15% for the two-layer (TiO ₂ / Al ₂ O ₃ ) AR element and about 16% for the single-layer (TiO ₂ ) AR element.

When a reliability test of 150 mW was performed, neither element was suddenly deteriorated after 1000 h. However, in the TiO ₂ single-layer AR element, fluctuations in driving current were confirmed, and when the end face of the deteriorated element was observed with a TEM, deposits containing Si were confirmed on the end face.

A reliability evaluation of 200 mW was performed on the two-layer (TiO ₂ / Al ₂ O ₃ ) AR element and the single-layer (Al ₂ O ₃ ) AR element described in Example 1. As a result, neither element was suddenly deteriorated after 1000 hours. However, among the Al ₂ O ₃ single-layer AR elements, there was an element in which the slope efficiency deteriorated after about 800 hours. As a result of TEM observation of the deteriorated element, it was found that film peeling between Al ₂ O ₃ and the semiconductor occurred.

  The present invention is not limited to the above embodiment, and various modifications can be made. For example, in the above-described embodiment, an example in which the protective film is formed of a multilayer dielectric film is shown, but the protective film may be formed of a single-layer dielectric film.

Claims (7)

A nitride-based semiconductor optical device having an active layer made of a group III nitride semiconductor containing gallium (Ga) as a constituent element and emitting laser light from an end face of the active layer,
Provided on the end face from which the laser beam is emitted, comprising a protective film made of a single-layer or multilayer dielectric film,
A nitride-based semiconductor optical device, wherein carbon is included in a partial region of the dielectric film in contact with the end face. The nitride-based semiconductor optical device according to claim 1,
The dielectric film in contact with the end face has a carbon concentration of 1 at. % Or more of the region is a nitride-based semiconductor optical device in which the distance from the end face is limited to a region that is less than ½ of the thickness of the dielectric film.   3. The nitride based semiconductor optical device according to claim 2, wherein the carbon concentration in the region is 10 at. % Nitride-based semiconductor optical device. The nitride-based semiconductor optical device according to claim 1,
The carbon concentration in the region containing carbon is 0.1 at. % Or more and 10 at. % Nitride-based semiconductor optical device. The nitride-based semiconductor optical device according to any one of claims 1 to 4,
The protective film is
A first dielectric film made of an oxide containing titanium (Ti) in contact with the end face;
A second dielectric film provided on the first dielectric film and made of a dielectric having a larger band gap than the first dielectric film;
A nitride-based semiconductor optical device comprising:   6. The nitride semiconductor optical device according to claim 5, wherein the first dielectric film is made of a titanium oxide-based material, and the thickness of the first dielectric film is in a range of 10 nm to 50 nm. -Based semiconductor optical device.   7. The nitride-based semiconductor optical device according to claim 5, wherein the thickness of the second dielectric film is in the range of 10 nm to 100 nm.

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