Semiconductor light emitting element and external cavity laser module

Abstract

To provide a semiconductor light emitting element having AR films with long-term reliability and low reflectance.SOLUTION: A semiconductor light emitting element 100 has a semiconductor substrate 110, a stacked growth layer 120 composed of AlGaInP-based material, and an AR (Anti-Reflection) film 160. The AR film 160 includes a first layer 162 including Al2O3 formed on the outgoing end surface S1 of the stacked growth layer 120, and a second layer 164 formed on top of the first layer 162. The content of rare gas elements in the first layer 162 is greater than the content of rare gas elements in the second layer 164.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a semiconductor light emitting device for use in an external cavity semiconductor laser. [Background technology]

[0002] In recent years, external cavity semiconductor lasers have come into use. In external cavity semiconductor lasers, a semiconductor laser element is used as a semiconductor optical amplifier, and a laser cavity is formed between a diffraction grating provided outside the element and the rear end face of the semiconductor laser. By installing a mirror so that the diffraction grating reflects only a specific wavelength, oscillation occurs in only a single longitudinal mode, realizing a narrow linewidth of the emitted light.

[0003] In addition, compared to distributed feedback (DFB) lasers that have a diffraction grating structure inside the semiconductor laser element, or distributed Bragg reflector (DBR) lasers, it is easier to achieve high output because the mirror is external. Due to these characteristics, it is used in applications that require high-output light of specific wavelengths, such as optical communications, optical measurement, and laser cooling.

[0004] In the above-mentioned external cavity semiconductor laser, the reflectance of the output end face of the semiconductor laser element that functions as a semiconductor optical amplifier is increased by applying an AR (anti-reflection) coating. Since the lower the reflectance, the less loss there is as an external cavity, it is required that the reflectance be as close to 0% (approximately 0.01%) as possible. Specifically, a reflectance of 0.2% or less, preferably 0.1% or less, is required at the desired wavelength.

[0005] Therefore, an AR coating is designed to have a minimum reflectance at a desired wavelength, but variations in film thickness shift the wavelength at which the reflectance is at its minimum, thereby increasing the reflectance at the desired wavelength.

[0006] The accuracy required for the thickness of the AR film becomes more stringent as the laser oscillation wavelength becomes shorter. In the case of a laser in the 600 nm to 700 nm band, in order to obtain a low-reflective film with a reflectance of 0.1% or less, film thickness control on the order of 1 nm or less is required, and it is very difficult to achieve this by simply controlling the flow conditions of the film-forming material and the film-forming time. In order to solve this problem, a method has been proposed in which the reflectance is measured while the laser is oscillating during film formation (Patent Document 1). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 6-125149 [Patent Document 2] U.S. Patent No. 6,297,066 [Patent Document 3] Japanese Patent Application Publication No. 4-299591 [Patent Document 4] Patent No. 4621791 Summary of the Invention [Problem to be solved by the invention]

[0008] For lasers in the 600nm to 700nm bands, AlGaInP-based materials are used as semiconductor materials. AlGaInP-based materials are chemically unstable compared to GaAs (AlGaAs)-based materials. Specifically, AlGaInP-based materials are vulnerable to deterioration due to oxidation, so the films that can be used for AR coating are limited. Also, to ensure the long-term reliability of the laser, it is necessary to prevent water intrusion.

[0009] An embodiment of the present disclosure has been made in view of the above problems, and one of its exemplary objectives is to provide a semiconductor light-emitting element including an AR film having long-term reliability and low reflectance. [Means for solving the problem]

[0010] A semiconductor light emitting device according to an embodiment of the present disclosure includes a semiconductor substrate, and an AR (Anti-Reflection) film including a multilayer growth layer formed on the semiconductor substrate, the multilayer growth layer including an N-type cladding layer, an active layer, and a P-type cladding layer, the multilayer growth layer including an AlGaInP-based material, a first layer including Al2O3 formed on an emission end face of the multilayer growth layer, and a second layer formed on the first layer. The content of the rare gas element in the first layer is higher than the content of the rare gas element in the second layer.

[0011] Any combination of the above components, or mutual substitution of components or expressions between methods, devices, systems, etc., are also effective as aspects of the present invention or the present disclosure. Furthermore, the description in this section (Means for Solving the Problems) does not explain all essential features of the present invention, and therefore, subcombinations of the described features may also constitute the present invention. Effect of the Invention

[0012] According to an embodiment of the present disclosure, a semiconductor light emitting element including an AR film having long-term reliability and low reflectance can be provided. [Brief description of the drawings]

[0013] [Figure 1] 1 is a cross-sectional view of a semiconductor light-emitting element according to a first embodiment. [Diagram 2] FIG. 4 is a diagram showing the reflectance characteristics of the AR film according to the first embodiment. [Diagram 3] FIG. 6 is a cross-sectional view of a semiconductor light-emitting element according to a second embodiment. [Figure 4] FIG. 11 is a diagram showing the reflectance characteristics of the AR film according to the second embodiment. [Diagram 5] FIG. 11 is a cross-sectional view of a semiconductor light-emitting element according to a third embodiment. [Figure 6] FIG. 1 is a diagram showing an external cavity type laser module equipped with a semiconductor light emitting element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] (Overview of the embodiment) A summary of some exemplary embodiments of the present disclosure is provided. This summary is intended as a prelude to the more detailed description that follows, or as a basic understanding of the embodiments. This summary is intended to provide a simplified description of some concepts of one or more embodiments, and is not intended to limit the scope of the invention or disclosure. Additionally, this summary is not intended to be a comprehensive overview of all possible embodiments, nor is it intended to limit essential elements of the embodiments. For convenience, the term "one embodiment" may refer to one embodiment (example or variant) or multiple embodiments (examples or variants) disclosed in this specification.

[0015] A semiconductor light-emitting element according to one embodiment comprises a semiconductor substrate, a multilayer growth layer formed on the semiconductor substrate, the multilayer growth layer including an N-type cladding layer, an active layer, and a P-type cladding layer, the multilayer growth layer being made of an AlGaInP-based material, a first layer including Al2O3 (alumina) formed on the emission side end face of the multilayer growth layer, and a second layer formed on the first layer, and an AR (Anti-Reflection) film including the first layer having a higher rare gas element content than the second layer.

[0016] In this configuration, the AR film has a multi-layer structure including a first layer and a second layer. AlGaInP is vulnerable to oxidation, but the Al2O3 that constitutes the first layer traps oxygen and protects AlGaInP from oxidation. On the other hand, as can be seen from its use as a porous ceramic material, amorphous Al2O3 can become low-density depending on manufacturing conditions and other factors, and can have the property of being easily permeable by water. In that case, the long-term reliability of AlGaInP cannot be guaranteed. By intentionally including rare gas elements in Al2O3, a dense film can be formed, which can prevent water from passing through.

[0017] In one embodiment, the rare gas element may be Ar (argon).

[0018] In one embodiment, the reflectance of the output end face may be 0.2% or less at the same wavelength as the peak wavelength of the output light.

[0019] In one embodiment, the rare gas element content in the first layer may be 0.2 at% or more. This allows the formation of a denser film. If the rare gas element content exceeds 5 at%, the crystal state of the material constituting the film is destroyed, making it difficult to form a dense film. Therefore, the rare gas element content in the first layer is desirably 5 at% or less.

[0020] In one embodiment, the rare gas element content in the second layer may be less than 0.2 at %.

[0021] In one embodiment, the emission peak wavelength of the semiconductor light emitting element is λ P When the refractive index of the first layer is n1, the thickness d1 of the first layer is d1≦λ P / (2n1) may be satisfied.

[0022] In one embodiment, the emission peak wavelength of the semiconductor light emitting element is λ P When the refractive index of the first layer is n1, the thickness d1 of the first layer is d1≦λ P When the refractive index of the second layer is n2, the thickness d2 of the second layer satisfies d2≦λ P / (4n2) may be satisfied.

[0023] In one embodiment, the thickness d1 of the first layer and the thickness d2 of the second layer may satisfy d1>d2.

[0024] In one embodiment, the thickness d1 of the first layer may satisfy d1>5 nm.

[0025] An external cavity laser module according to an embodiment includes any one of the semiconductor light emitting elements described above and a grating element.

[0026] (Embodiment) Hereinafter, the present disclosure will be described with reference to the drawings based on preferred embodiments. The same or equivalent components, parts, and processes shown in each drawing will be given the same reference numerals, and duplicated descriptions will be omitted as appropriate. In addition, the embodiments are not intended to limit the disclosure, but are merely examples, and all features and combinations thereof described in the embodiments are not necessarily essential to the disclosure.

[0027] The dimensions (thickness, length, width, etc.) of each component shown in the drawings may be enlarged or reduced as appropriate for ease of understanding. Furthermore, the dimensions of multiple components do not necessarily represent their relative sizes. Even if a component A is drawn thicker than another component B on the drawings, component A may actually be thinner than component B.

[0028] 1 is a cross-sectional view of a semiconductor light-emitting device 100 according to embodiment 1. The semiconductor light-emitting device 100 is used as a light source or gain medium of an external cavity laser module. The semiconductor light-emitting device 100 includes a semiconductor substrate 110, a multilayer growth layer 120, a P-side electrode 150, an N-side electrode 152, an AR film 160, and an HR film 170.

[0029] The semiconductor substrate 110 is, for example, an N-type GaAs substrate. The laminated growth layer 120 is formed on the semiconductor substrate 110 by epitaxial growth. The laminated growth layer 120 includes an N-type cladding layer 122, an active layer 130, and a P-type cladding layer 124, and forms a waveguide. In this embodiment, a P-type contact layer 126 made of GaAs is formed on the laminated growth layer 120 to enable contact with an electrode. The material of each layer of the laminated growth layer 120 is selected according to the desired wavelength, and in this embodiment, an AlGaInP-based material is used. In this specification, the AlGaInP-based material refers to a material having a general formula (Al 1-x Ga x ) y In 1-yP (0≦x≦1, 0≦y≦1) and collectively represents binary, ternary, and quaternary materials. In the multilayer growth layer 120, the layers in contact with the AR film over a wide area are the N-type cladding layer and the P-type cladding layer, which occupy most of the multilayer thickness, and these layers have a large effect on reliability. Therefore, the multilayer growth layer 120 made of an AlGaInP material mainly refers to the N-type cladding layer and the P-type cladding layer. For example, the N-type cladding layer 122 is AlGaInP, the active layer 130 is GaInP, and the P-type cladding layer 124 is AlInP. The active layer is selected according to the desired wavelength, and may be GaInAsP.

[0030] A P-side electrode 150 is formed on the P-type contact layer 126 , and an N-side electrode 152 is formed on the back surface of the semiconductor substrate 110 .

[0031] When the semiconductor light emitting device 100 is incorporated into an external cavity laser module, an AR film 160 is formed on an end face (emission side end face, front end face) S1 which becomes the emission side.

[0032] The AR film 160 is composed of two layers, including a first layer 162 and a second layer 164. The first layer 162 is formed on the emission side end surface S1 and is mainly made of Al2O3 (alumina). The first layer 162 is formed in a state in which a rare gas element is mixed in amorphous Al2O3. Specific examples of the rare gas element include He (helium), Ar (argon), Ne (neon), Kr (krypton), Xe (xenon), and Rn (radon), with Ar being preferred from the viewpoint of compatibility with semiconductor manufacturing processes.

[0033] The thickness d1 of the first layer 162, which is the crystal protection layer, is preferably 5 to 120 nm. If the thickness d1 is 5 nm or less, a uniform film cannot be formed, the surface cannot be sufficiently terminated, and intrusion of moisture from the outside cannot be prevented. If the thickness d1 is too thick, the band (wavelength range) in which the reflectance of the AR film is reduced becomes narrow, making it extremely difficult to control the thickness of the reflectance adjustment layer when attempting to obtain a desired reflectance.

[0034] The second layer 164 is formed on the first layer 162. The second layer 164 may be made of an insulating material other than Al2O3, and may be made of an insulating material that is highly stable in air, such as AlN or SiN.

[0035] Regarding the second layer 164, which is a reflectance adjusting layer, if the refractive index fluctuates due to the incorporation of rare gas elements, the optical thickness expressed as the product of the refractive index and the film thickness fluctuates, so even if the film can be formed with an accurate film thickness, the optical thickness will deviate from the optimal value, making it impossible to sufficiently reduce the reflectance. Therefore, it is better to reduce the rare gas element content of the reflectance adjusting layer as much as possible.

[0036] That is, in this embodiment, the content of the rare gas element in the first layer 162 is configured to be greater than the content of the rare gas element in the second layer 164. More specifically, the content of the rare gas element in the first layer 162 is preferably 0.2 at % or more and 5 at % or less.

[0037] For example, the first layer 162 can be formed by a sputtering method. In this case, a desired amount of rare gas element can be mixed into Al2O3 by adjusting the concentration of the atmospheric gas for sputtering.

[0038] It has been reported that in ordinary semiconductor laser elements, when the content of rare gas elements is high, heat is generated due to light absorption by the rare gas elements, and the element deteriorates due to COD (Patent Document 4). On the other hand, in a semiconductor light-emitting element with an AR coating having an end face reflectance of 1% or less, very little light is reflected at the exit face and returns to the inside of the element, so the light density at the exit facet is small and heat generation due to light absorption is not a major issue. On the other hand, if the reflectance of an AR-coated element increases due to some cause over a long period of use, the residual reflectance loss at the front end facet of the element increases when used as a light source for an external resonator, and if the reflectance increases beyond a certain level, it may cause a failure. This type of failure is caused by the alteration of the crystal at the end face of the semiconductor crystal due to the diffusion of moisture from the outside. In an AR-coated element designed to have the lowest reflectance in the initial state, the reflectance increases when the refractive index changes due to the alteration of the crystal.

[0039] In this embodiment, the outermost surface of the semiconductor crystal is coated with a dense crystal protection layer, which can suppress moisture from penetrating through the end face film to the crystal end face during operation, thereby suppressing such a failure mode. Therefore, the crystal protection layer in contact with the semiconductor crystal is better if it is a dense film containing a certain amount of Ar, and the Ar concentration in the film is preferably 0.2 at% or more. On the other hand, if the Ar concentration is too high, the crystalline state of the material that forms the crystal protection layer will collapse, and conversely, it will not be possible to form a dense film, so the Ar concentration is preferably 5 at% or less.

[0040] On the other hand, the content of the rare gas element in the second layer 164 is 0.2 at % or less, and more preferably 0.1 at % or less. Note that the rare gas element content here is a concept that includes 0, and the second layer 164 may not contain a rare gas element.

[0041] The first layer 162 functions as a protective layer for the multilayer growth layer 120, which is an AlGaInP-based material. On the other hand, the second layer 164 functions as a reflectance adjustment layer that adjusts the reflectance of the AR film 160 as a whole. Therefore, the thickness of the second layer 164 needs to be controlled with high precision so as to minimize the reflectance, and it is preferable to form the second layer 164 by using a deposition method. More preferably, during the formation of the second layer 164, it is preferable to control the thickness of the second layer 164 so as to minimize the intensity of the emitted light while supplying power to the semiconductor light emitting device 100 and monitoring in situ the light emitted from the emission side end face S1 while the semiconductor light emitting device 100 is in the light emitting state.

[0042] When the semiconductor light emitting element 100 is incorporated into an external cavity laser module, an HR (High-Reflection) film 170 is formed on the end face (rear end face) S2 opposite to the emission side. The structure of the HR film 170 is not particularly limited, and any known technology may be used. It may have a two-layer structure of a first layer 172 and a second layer 174. The first layer 172 may function as a protective layer. The second layer 174 is, for example, a dielectric multilayer film, and the number of layers and the thickness of each layer are controlled so as to obtain a high reflectance at the oscillation wavelength of the laser.

[0043] The semiconductor light emitting device 100 has the above configuration.

[0044] In this embodiment, the AR film 160 has a multilayer structure including a first layer 162 and a second layer 164. Although AlGaInP-based materials are susceptible to oxidation, Al in Al2O3 constituting the first layer 162 traps oxygen, thereby protecting the AlGaInP from oxidation.

[0045] On the other hand, as can be seen from its use as a ceramic material, Al2O3 has a property of being easily permeable to water, and if left as is, the long-term reliability of AlGaInP cannot be guaranteed. In this embodiment, a dense Al2O3 film can be formed by intentionally including a rare gas element in Al2O3, thereby preventing water permeation. In particular, a film formed by a sputtering method using a rare gas element has high adhesion to the crystal and is dense. This terminates defects present on the semiconductor crystal surface, suppressing deterioration originating from the defects, and suppresses the permeation of moisture from the outside, thereby suppressing alteration of the crystal and reducing the deterioration of long-term reliability.

[0046] By controlling the thickness of the second layer 164 with high precision on the order of 1 nm or less, a reflectance of 0.2% or less, more preferably 0.1% or less, can be achieved in the wavelength range of 600 nm to 700 nm. If the second layer 164 contains a rare gas element, the refractive index of the film itself will deviate from the theoretical value. The reflectance of the end face film can be effectively reduced by designing the optical thickness, which is the product of the refractive index and the film thickness, to be λ / 4. However, if the refractive index of the second layer 164 fluctuates due to the inclusion of a rare gas element, the optical thickness, which is the product of the refractive index and the film thickness, will change, resulting in a deviation from the calculated value. Therefore, in order to improve the controllability of the reflectance, it is necessary to suppress the fluctuation in the refractive index and deposit a material with a stable refractive index. In this embodiment, the content of the rare gas element in the second layer 164, which is a reflectance adjustment layer, is intentionally made smaller than that of the first layer 162, which is a crystal protection layer. This suppresses the refractive index fluctuation caused by the rare gas element, and by depositing a predetermined film thickness, a desired reflectance can be stably obtained, and good AR coating characteristics can be obtained.

[0047] As described above, according to this embodiment, it is possible to provide a semiconductor light emitting device 100 including an AR film 160 having long-term reliability and low reflectance.

[0048] 2 is a diagram showing the reflectance characteristics of the AR film 160 according to the first embodiment. In the case of an AlGaInP-based material, the refractive index n sis between 3.0 and 3.6. Now, the refractive index of the waveguide, n s Let us assume that the first layer 162, which is a crystal protection layer, is made of Al2O3 with a refractive index n1=1.75, and the second layer 164, which is a reflectance adjustment layer, is made of AlN with a refractive index n2=2.17.

[0049] The emission peak wavelength of the semiconductor light emitting element is λ P Then, the thickness d1 of the first layer 162 is d1≦λ P It is preferable that / (2n1) is satisfied.

[0050] Furthermore, when the AR film 160 has a two-layer structure of a first layer 162 and a second layer 164, the thickness d1 of the first layer is set to be d1≦λ P / (4n1), and the thickness d2 of the second layer satisfies d2≦λ P It is preferable that / (4n2) is satisfied.

[0051] The thickness of the first layer 162 is d1=62.2 nm, and the thickness of the second layer 164 is d2=17.2 nm. The relationship of the refractive index at this time is as follows: n1<√n s <n2 This holds true.

[0052] This AR coating provides an anti-reflection effect of 10 -6 The following reflectances are obtained:

[0053] (Embodiment 2) 3 is a cross-sectional view of a semiconductor light emitting device 100A according to embodiment 2. In the semiconductor light emitting device 100A, the AR film 160A has a three-layer structure, specifically including a first layer 162, a second layer 164, and an intermediate layer 166 inserted therebetween.

[0054] The first layer 162 can be Al2O3 as described above, and the second layer 164 can be AlN. The intermediate layer 166 can be Al2O3 like the first layer 162, but the refractive index n3 of the intermediate layer 166 is different from the refractive index n2 of the first layer 162.

[0055] 4 is a diagram showing the reflectance characteristics of the AR film 160A according to embodiment 2. The calculation was performed under the following conditions. Refractive index of the waveguide, n s =3.3 First layer 162 Material: Al2O3 Refractive index: n1 = 1.75 Film thickness: d1 = 39.3 nm Intermediate layer 166 Material: Al2O3 Refractive index: n3 = 1.70 Film thickness: d3 = 20.2 nm Second layer 164 Material: AlN Refractive index: n2=2.17 Film thickness: d2=19.5 nm In this case, the same 10 -6 The following reflectances are obtained:

[0056] (Embodiment 3) 5 is a cross-sectional view of a semiconductor light emitting device 100B according to the third embodiment. In the semiconductor light emitting device 100B, an AR film 180 is also formed on the rear end face S2. The AR film 180 can have a two-layer structure, that is, a first layer 182 and a second layer 184, similar to the AR film 160. The first layer 182 can function as a crystal protection layer. The second layer 184 can function as a reflectance adjustment layer. The second layer 184 is made of, for example, an insulating material or a dielectric multilayer film, and is adjusted to obtain a low reflectance for the oscillation wavelength of the laser.

[0057] (Laser module) The semiconductor light emitting elements 100, 100A, and 100B (hereinafter collectively referred to as 100) are incorporated into an external cavity type laser module.

[0058] The laser module 200 includes a semiconductor light-emitting element 100, a grating element 210, and a lens 220. In the laser module 200, a resonator is formed between the HR film 170 of the semiconductor light-emitting element 100 and the grating element 210. The light L1 emitted from the emission side end face S1 of the semiconductor light-emitting element 100 is collimated by the lens 220, and the collimated light L2 is incident on the grating element 210. The zeroth-order diffracted light L3 of the grating element 210 is extracted to the outside as the emission light L4 of the laser module 200. The first-order diffracted light L5 of the grating element 210 is collected by the lens 220 and incident on the waveguide inside the semiconductor light-emitting element 100 from the emission side end face S1 of the semiconductor light-emitting element 100. The light L6 incident on the waveguide of the semiconductor light-emitting element 100 is amplified in the semiconductor light-emitting element 100 and is totally reflected by the HR film 170.

[0059] In the laser module 200 including the semiconductor light emitting element 100B described in the third embodiment, a reflective optical system such as a grating or a mirror is also disposed on the rear end face side.

[0060] The embodiments merely illustrate the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments without departing from the spirit of the present invention defined in the claims. [Explanation of symbols]

[0061] 100 Semiconductor light emitting element 110 Semiconductor substrate 120 Layered Growth 122 N-type cladding layer 124 P-type cladding layer 126 P-type contact layer 130 Active layer 150 P side electrode 152 N side electrode 160 AR membrane 162 1st layer 164 2nd layer 166 Middle Class 170 HR membrane 172 1st layer 174 2nd layer 180 AR membrane 182 1st layer 184 2nd layer 200 Laser Module 210 Grating element 220 Lens

Claims

1. A semiconductor substrate; a layered growth layer formed on the semiconductor substrate, the layered growth layer including an N-type cladding layer, an active layer, and a P-type cladding layer, the layered growth layer being made of an AlGaInP-based material; The Al formed on the emission side face of the laminated growth layer ２ O ３ and a second layer formed on the first layer; a content of a rare gas element in the first layer is greater than a content of a rare gas element in the second layer.

2. 2. The semiconductor light emitting device according to claim 1, wherein the reflectance of the light emitting end face is 0.2% or less at the same wavelength as the peak wavelength of the emitted light.

3. 3. The semiconductor light emitting device according to claim 1, wherein the content of the rare gas element in the first layer is 0.2 at % or more and 5 at % or less.

4. 3. The semiconductor light emitting device according to claim 1, wherein the content of the rare gas element in the second layer is less than 0.2 at %.

5. The emission peak wavelength of the semiconductor light emitting element is λ Ｐ , the refractive index of the first layer is n １ When the thickness of the first layer is d １ , d １ ≦λ Ｐ / (2n １ 3. The semiconductor light emitting device according to claim 2, wherein

6. The emission peak wavelength of the semiconductor light emitting element is λ Ｐ , the refractive index of the first layer is n １ When the thickness of the first layer is d １ , d １ ≦λ Ｐ / (4n １ ) is satisfied, The refractive index of the second layer is n ２ When the thickness of the second layer is d ２ , d ２ ≦λ Ｐ / (4n ２ 2. The semiconductor light emitting device according to claim 1, wherein

7. The thickness d of the first layer １ and the thickness d of the second layer ２ By that I mean, １ >d ２ 7. The semiconductor light emitting device according to claim 6, wherein

8. The thickness d of the first layer １ , d １ 3. The semiconductor light-emitting element according to claim 1, wherein the thickness satisfies >5 nm.

9. A semiconductor light emitting device according to claim 1 or 2, A grating element; 1. An external cavity laser module comprising:

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