Quantum cascade laser element and quantum cascade laser device

Abstract

To provide a quantum cascade laser element and a quantum cascade laser device, capable of suppressing the spread of a molten bonding material toward a main body part of an anti-reflection film.SOLUTION: A quantum cascade laser element 1 includes a semiconductor substrate 2, a semiconductor multilayer body 3 including an active layer 31 and having a first end surface 3a and a second end surface 3b facing each other in a light guide direction A, a first electrode 5, a second electrode 6, and an anti-reflection film 7 formed on the first end surface 3a. The anti-reflection film 7 includes a main body part 70 formed on the first end surface 3a, and a first addition part 71 formed on a surface 5a of the first electrode 5 on the side opposite to the semiconductor multilayer body 3. The first addition part 71 includes a first end part having a thickness reducing as going away from the first end surface 3a in the light guide direction A. The first end part has a first end surface that intersects with the surface 5a of the first electrode 5.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a quantum cascade laser element and a quantum cascade laser device. [Background technology]

[0002] A known conventional quantum cascade laser element includes a semiconductor substrate, a semiconductor laminate formed on the semiconductor substrate, a first electrode formed on the surface of the semiconductor laminate opposite to the semiconductor substrate, and a second electrode formed on the surface of the semiconductor substrate opposite to the semiconductor laminate, in which an anti-reflection film is formed on one of a pair of end faces of the semiconductor laminate including the active layer.

[0003] In recent years, there has been an increasing demand for quantum cascade laser elements capable of emitting laser light having a central wavelength of 7.5 μm or longer. Therefore, for the quantum cascade laser elements described above, it is desirable to realize an antireflection coating that can reliably reduce the reflectance of laser light having a central wavelength of 7.5 μm or longer and ensure sufficient durability. Patent Document 1 describes such an antireflection coating, which includes an insulating film that is a CeO film, a first refractive index film that is a YF film or a CeF film, and a second refractive index film having a refractive index greater than 1.8. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2021-163922 Summary of the Invention [Problem to be solved by the invention]

[0005] The antireflection film described in Patent Document 1 functions effectively against laser light having a central wavelength of 7.5 μm or more, but there is room for improvement in the heat resistance of films other than the CeO 2 film.

[0006] The present invention aims to provide a quantum cascade laser element, a quantum cascade laser device, and a method for manufacturing a quantum cascade laser element that has an anti-reflection coating that functions effectively for laser light having a center wavelength of 7.5 μm or more and has high heat resistance. [Means for solving the problem]

[0007] The quantum cascade laser device of the present invention includes a semiconductor substrate, an active layer having a quantum cascade structure, and a semiconductor laminate formed on the semiconductor substrate, the semiconductor laminate having a first end face and a second end face opposing each other in an optical waveguide direction, a first electrode formed on a surface of the semiconductor laminate opposite to the semiconductor substrate, a second electrode formed on a surface of the semiconductor substrate opposite to the semiconductor laminate, and an anti-reflection film formed on the first end face, wherein the semiconductor laminate is configured to oscillate laser light having a center wavelength of 7.5 μm or longer, and the anti-reflection film includes at least one of at least one layer of CeO2 film formed by continuous sputtering and vacuum deposition and a plurality of layers of CeO2 film formed by discrete sputtering and vacuum deposition.

[0008] In this quantum cascade laser device, the anti-reflection coating includes at least one of at least one CeO film formed by continuous sputtering and vacuum deposition and multiple CeO films formed by discrete sputtering and vacuum deposition. The refractive index of the at least one CeO film formed by continuous sputtering and vacuum deposition can be controlled to a value close to the "square root of the effective refractive index of the optical waveguide structure constituted by the semiconductor laminate," compared to the refractive index of a CeO film formed by sputtering alone or the refractive index of a CeO film formed by vacuum deposition alone. Furthermore, in the multiple CeO films formed by discrete sputtering and vacuum deposition, a CeO film having a refractive index greater than the "square root of the effective refractive index of the optical waveguide structure constituted by the semiconductor laminate" is adjacent to a CeO film having a refractive index smaller than the "square root of the effective refractive index of the optical waveguide structure constituted by the semiconductor laminate." For these reasons, this anti-reflection film can reliably reduce the reflectance of laser light having a central wavelength of 7.5 μm or longer. Furthermore, the CeO2 film can ensure transparency for laser light having a central wavelength of 7.5 μm or longer, prevent short circuits at the first facet, and improve adhesion to the first facet. Moreover, the CeO2 film has high heat resistance. As a result, this quantum cascade laser element can achieve an anti-reflection film that functions effectively for laser light having a central wavelength of 7.5 μm or longer and has high heat resistance.

[0009] In the quantum cascade laser device of the present invention, the anti-reflection coating may include at least one CeO2 film formed by successive sputtering and vacuum deposition, thereby simplifying the structure of the anti-reflection coating.

[0010] In the quantum cascade laser device of the present invention, the anti-reflection coating may be at least one CeO2 film formed by successive sputtering and vacuum deposition, and may include multiple CeO2 film layers having different refractive indices from each other, thereby reliably reducing the reflectance of laser light in a desired wavelength range.

[0011] In the quantum cascade laser device of the present invention, the anti-reflection coating may further include at least one CeO film formed by sputtering, which can reliably reduce the reflectance of laser light in a desired wavelength range.

[0012] In the quantum cascade laser device of the present invention, the antireflection coating may include a multi-layer CeO2 film formed by discrete sputtering and vacuum deposition. This can reliably reduce the reflectance of laser light in a desired wavelength range. For example, the reflectance can be reduced for a broader band of laser light compared to when the antireflection coating is composed of a single CeO2 film.

[0013] In the quantum cascade laser device of the present invention, the antireflection film may include a main body portion formed on the first end face and a first additional portion formed on a surface of the first electrode opposite to the semiconductor laminate, the first additional portion including a first end portion having a thickness that decreases with increasing distance from the first end face in the optical waveguide direction, and the first end portion may have a first side surface intersecting with the surface of the first electrode. According to this, when the first electrode is joined to an electrode pad of another member by a joining member, the molten joining member is blocked by the first side surface of the first additional portion and repelled by the first additional portion, thereby preventing the molten joining member from creeping up toward the main body portion of the antireflection film.

[0014] In the quantum cascade laser device of the present invention, the antireflection film may include a main body portion formed on the first end face and a second additional portion formed on the surface of the second electrode opposite to the semiconductor substrate, the second additional portion including a second end portion having a thickness that decreases with increasing distance from the first end face in the optical waveguiding direction, and the second end portion may have a second side surface intersecting with the surface of the second electrode. In this manner, when the second electrode is joined to an electrode pad of another member by a joining member, the molten joining member is blocked by the second side surface of the second additional portion and repelled by the second additional portion, thereby preventing the molten joining member from creeping up toward the main body portion of the antireflection film.

[0015] The quantum cascade laser device of the present invention may further include an insulating film formed on the surface of the semiconductor laminate, the insulating film being a CeO2 film, and the antireflection film including a main body portion formed on the first facet and a third additional portion formed on the insulating film. This can reduce optical waveguide loss in the semiconductor laminate compared to when the insulating film is a SiN film or a SiO2 film, for example. Furthermore, improved adhesion between the third additional portion of the antireflection film and the insulating film can prevent the antireflection film from peeling off from the first facet.

[0016] The quantum cascade laser device of the present invention comprises the quantum cascade laser element, a support portion that supports the quantum cascade laser element, and a bonding member that bonds the electrode pad of the support portion to the second electrode while the semiconductor substrate is positioned on the support portion side relative to the semiconductor laminate, and the maximum thickness of the bonding member is smaller than the maximum thickness of the second additional portion.

[0017] This quantum cascade laser device can provide an anti-reflection coating that functions effectively for laser light having a center wavelength of 7.5 μm or longer and has high heat resistance. Furthermore, when the second electrode is bonded to the electrode pad of the support portion with a bonding member, the molten bonding member is blocked by the second side surface of the second additional portion and repelled by the second additional portion, thereby preventing the molten bonding member from creeping up toward the main body of the anti-reflection coating.

[0018] In the quantum cascade laser device of the present invention, the quantum cascade laser element may be supported by the support portion such that the distance between the electrode pad and the second electrode decreases with increasing distance from the second addition portion in the optical waveguiding direction. In this way, when the second electrode is bonded to the electrode pad on the support portion with a bonding material, the molten bonding material is reliably filled between the electrode pad and the second electrode, thereby ensuring reliable bonding of the second electrode to the electrode pad.

[0019] The quantum cascade laser device of the present invention comprises the quantum cascade laser element, a support portion that supports the quantum cascade laser element, and a bonding member that bonds the electrode pad of the support portion to the first electrode while the semiconductor stack is positioned on the support portion side relative to the semiconductor substrate, and the maximum thickness of the bonding member is smaller than the maximum thickness of the first additional portion.

[0020] This quantum cascade laser device can provide an anti-reflection coating that functions effectively for laser light having a center wavelength of 7.5 μm or longer and has high heat resistance. Furthermore, when the first electrode is bonded to the electrode pad of the support with a bonding material, the molten bonding material is blocked by the first side surface of the first additional portion and repelled by the first additional portion, thereby preventing the molten bonding material from creeping up toward the main body of the anti-reflection coating.

[0021] In the quantum cascade laser device of the present invention, the quantum cascade laser element may be supported by the support portion such that the distance between the electrode pad and the first electrode decreases with increasing distance from the first addition portion in the optical waveguiding direction. In this way, when the first electrode is bonded to the electrode pad on the support portion with a bonding material, the molten bonding material is reliably filled between the electrode pad and the first electrode, thereby enabling the first electrode to be reliably bonded to the electrode pad.

[0022] The method for manufacturing a quantum cascade laser element of the present invention is a method for manufacturing the above-mentioned quantum cascade laser element, and includes the steps of: preparing a laser bar including a plurality of portions, each of which becomes a set of a semiconductor substrate, a semiconductor laminate, a first electrode, and a second electrode, and which are arranged one-dimensionally in a direction perpendicular to the optical waveguide direction; forming an antireflection layer in the laser bar including a plurality of portions, each of which becomes an antireflection film, by forming at least one of a CeO2 film by continuous sputtering and vacuum deposition and a CeO2 film by discrete sputtering and vacuum deposition on an end face that becomes a first end face of each of the plurality of portions of the laser bar; and dividing the laser bar and the antireflection layer into each of the plurality of portions.

[0023] According to this method for manufacturing a quantum cascade laser device, it is possible to realize an anti-reflection film that functions effectively for laser light having a central wavelength of 7.5 μm or more and has high heat resistance. [Effects of the Invention]

[0024] According to the present invention, it is possible to provide a quantum cascade laser element, a quantum cascade laser device, and a method for manufacturing a quantum cascade laser element, which are provided with an anti-reflection film that functions effectively for laser light having a center wavelength of 7.5 μm or more and has high heat resistance. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a cross-sectional view of a quantum cascade laser device according to one embodiment. [Figure 2] 2 is a cross-sectional view of the quantum cascade laser device taken along line II-II shown in FIG. 1. FIG. [Figure 3] 3 is a cross-sectional view of the quantum cascade laser device taken along line III-III shown in FIG. 2. FIG. [Figure 4] 2 is a perspective view of a portion of the quantum cascade laser device shown in FIG. 1. FIG. [Figure 5] 2 is a cross-sectional view of a main body of the antireflection coating shown in FIG. 1. [Figure 6] 2 is a cross-sectional view of a first additional portion and a second additional portion of the antireflection film shown in FIG. 1. FIG. [Figure 7] 2A to 2C are diagrams illustrating a method for manufacturing the quantum cascade laser device shown in FIG. [Figure 8] 2A to 2C are diagrams illustrating a method for manufacturing the quantum cascade laser device shown in FIG. [Figure 9] 2A to 2C are diagrams illustrating a method for manufacturing the quantum cascade laser device shown in FIG. [Figure 10] 2A to 2C are diagrams illustrating a method for manufacturing the quantum cascade laser device shown in FIG. [Figure 11]FIG. 1 shows an SEM photograph of a CeO2 film formed solely by vacuum deposition. [Figure 12] FIG. 10 is a cross-sectional view of a main body of an antireflection film according to a modified example. [Figure 13] FIG. 1 is a diagram showing the optical characteristics of several types of antireflection coatings. [Figure 14] FIG. 10 is a cross-sectional view of a main body of an antireflection film according to a modified example. [Figure 15] FIG. 10 is a cross-sectional view of a modified quantum cascade laser device. [Figure 16] FIG. 10 is a plan view of a modified quantum cascade laser device. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals, and redundant explanations will be omitted. [Quantum cascade laser device configuration]

[0027] As shown in FIG. 1, quantum cascade laser device 10 includes a quantum cascade laser element 1. As shown in FIGS. 2 and 3, quantum cascade laser element 1 includes a semiconductor substrate 2, a semiconductor stack 3, an insulating film 4, a first electrode 5, a second electrode 6, and an anti-reflection film 7. Semiconductor substrate 2 is, for example, a rectangular plate-shaped N-type InP single crystal substrate. As an example, the length of semiconductor substrate 2 is about 3 mm, the width of semiconductor substrate 2 is about 500 μm, and the thickness of semiconductor substrate 2 is about 150 μm. In the following description, the width direction of semiconductor substrate 2 is referred to as the X-axis direction, the length direction of semiconductor substrate 2 is referred to as the Y-axis direction, and the thickness direction of semiconductor substrate 2 is referred to as the Z-axis direction.

[0028] The semiconductor laminate 3 is formed on the surface 2a of the semiconductor substrate 2. That is, the semiconductor laminate 3 is formed on the semiconductor substrate 2. The semiconductor laminate 3 includes an active layer 31 having a quantum cascade structure. The semiconductor laminate 3 is configured to oscillate laser light having a center wavelength of 7.5 μm or more. As an example, the semiconductor laminate 3 is configured to oscillate laser light having a center wavelength in the mid-infrared region, which is any value between 7.5 and 16 μm. In this embodiment, the semiconductor laminate 3 is configured by laminating a lower cladding layer 32, a lower guide layer (not shown), an active layer 31, an upper guide layer (not shown), an upper cladding layer 33, and a contact layer (not shown) in this order from the semiconductor substrate 2 side. The upper guide layer may have a diffraction grating structure that functions as a distributed feedback (DFB) structure.

[0029] The active layer 31 is, for example, a layer having an InGaAs / InAlAs multiple quantum well structure. The lower cladding layer 32 and the upper cladding layer 33 are, for example, Si-doped InP layers. The lower guide layer and the upper guide layer are, for example, Si-doped InGaAs layers. The contact layer is, for example, Si-doped InGaAs layers.

[0030] The semiconductor laminate 3 has a ridge portion 30 extending along the Y-axis direction. The ridge portion 30 is composed of a portion of the lower cladding layer 32 opposite to the semiconductor substrate 2, a lower guide layer, an active layer 31, an upper guide layer, an upper cladding layer 33, and a contact layer. The width of the ridge portion 30 in the X-axis direction is smaller than the width of the semiconductor substrate 2 in the X-axis direction. The length of the ridge portion 30 in the Y-axis direction is equal to the length of the semiconductor substrate 2 in the Y-axis direction. As an example, the length of the ridge portion 30 is about 3 mm, the width of the ridge portion 30 is about several μm to several tens of μm, and the thickness of the ridge portion 30 is about several μm. The ridge portion 30 is located at the center of the semiconductor substrate 2 in the X-axis direction. The layers that make up the semiconductor laminate 3 are not present on either side of the ridge portion 30 in the X-axis direction.

[0031] The semiconductor laminate 3 has a first end face 3a and a second end face 3b that face each other in the optical waveguide direction A of the ridge portion 30. The optical waveguide direction A is parallel to the Y-axis direction, which is the extension direction of the ridge portion 30. The first end face 3a and the second end face 3b function as light-emitting end faces. The first end face 3a and the second end face 3b are each located on the same plane as both side faces of the semiconductor substrate 2 that face each other in the Y-axis direction.

[0032] The insulating film 4 is formed on the side surfaces 30b of the ridge portion 30 and the surface 32a of the lower cladding layer 32 so that the surface 30a of the ridge portion 30 opposite the semiconductor substrate 2 is exposed. The side surfaces 30b of the ridge portion 30 are both side surfaces of the ridge portion 30 that face each other in the X-axis direction. The surface 32a of the lower cladding layer 32 is the surface of the portion of the lower cladding layer 32 that does not constitute the ridge portion 30 opposite the semiconductor substrate 2. In this embodiment, the insulating film 4 is a CeO2 film.

[0033] The first electrode 5 is formed on a surface 3c of the semiconductor laminate 3 opposite to the semiconductor substrate 2. The surface 3c of the semiconductor laminate 3 is a surface formed by a surface 30a of the ridge portion 30, side surfaces 30b of the ridge portion 30, and a surface 32a of the lower cladding layer 32. When viewed from the Z-axis direction, the outer edge of the first electrode 5 is located inside the outer edges of the semiconductor substrate 2 and the semiconductor laminate 3. The first electrode 5 is in contact with the surface 30a of the ridge portion 30 on the surface 30a of the ridge portion 30, and is in contact with the insulating film 4 on the side surfaces 30b of the ridge portion 30 and the surface 32a of the lower cladding layer 32. As a result, the first electrode 5 is electrically connected to the upper cladding layer 33 via the contact layer.

[0034] The first electrode 5 has a metal underlayer 51 and a metal plating layer 52. The metal underlayer 51 is formed to extend along the surface 3c of the semiconductor laminate 3. The metal underlayer 51 is, for example, a Ti / Au layer. The metal plating layer 52 is formed on the metal underlayer 51 so that the ridge portion 30 is embedded in the metal plating layer 52. The metal plating layer 52 is, for example, an Au plating layer. A surface 52a of the metal plating layer 52 opposite to the semiconductor substrate 2 is a flat surface perpendicular to the Z-axis direction. As an example, the surface 52a of the metal plating layer 52 is a polished surface that has been planarized by chemical mechanical polishing, and polishing marks are formed on the surface 52a of the metal plating layer 52. Here, the ridge portion 30 being embedded in the metal plating layer 52 means that the ridge portion 30 is covered by the metal plating layer 52 in a state where the thickness of the portions of the metal plating layer 52 located on both sides of the ridge portion 30 in the X-axis direction (the thickness of those portions in the Z-axis direction) is greater than the thickness of the ridge portion 30 in the Z-axis direction.

[0035] The second electrode 6 is formed on a surface 2b of the semiconductor substrate 2 opposite to the semiconductor stack 3. The second electrode 6 is, for example, an AuGe / Au film, an AuGe / Ni / Au film, or an Au film. The second electrode 6 is electrically connected to the lower clad layer 32 via the semiconductor substrate 2.

[0036] The antireflection film 7 is formed on the first end face 3a. The antireflection film 7 suppresses resonance of laser light at the first end face 3a and reduces the reflectance of laser light having a center wavelength of 7.5 μm or longer when the laser light is emitted from the first end face 3a. In this embodiment, the antireflection film 7 is formed to extend from the first end face 3a to both the first electrode 5 and the second electrode 6. More specifically, the antireflection film 7 extends from the first end face 3a via the surface of the first electrode 5 facing the first end face 3a to the surface 5a of the first electrode 5 opposite the semiconductor laminate 3. The antireflection film 7 also extends from the first end face 3a to the surface 6a of the second electrode 6 opposite the semiconductor substrate 2 via the surfaces of the semiconductor substrate 2 and the second electrode 6 facing the first end face 3a. [Anti-reflection coating composition]

[0037] As shown in FIGS. 3 and 4, the antireflection coating 7 includes a main body portion 70, a first additional portion 71, a second additional portion 72, and a third additional portion 73. The main body portion 70, the first additional portion 71, the second additional portion 72, and the third additional portion 73 are integrally formed (i.e., continuous). The main body portion 70 is a portion of the antireflection coating 7 formed on the first end face 3a. The main body portion 70 is formed to cover the first end face 3a. The first additional portion 71 is a portion of the antireflection coating 7 formed on the surface 5a of the first electrode 5. The first additional portion 71 is formed along the edge of the surface 5a of the first electrode 5 on the first end face 3a side. The second additional portion 72 is a portion of the antireflection coating 7 formed on the surface 6a of the second electrode 6. The second additional portion 72 is formed along the edge of the surface 6a of the second electrode 6 on the first end face 3a side. The third additional portion 73 is a portion of the antireflection film 7 that is formed on the insulating film 4. The third additional portion 73 is formed on the insulating film 4 along the edge of the surface 3 c of the semiconductor laminate 3 that is closer to the first end face 3 a (i.e., the edge where the first electrode 5 is not formed). The insulating film 4 extends from between the semiconductor laminate 3 and the first electrode 5 to the outer edge of the surface 3 c of the semiconductor laminate 3 (i.e., the outer edge where the first electrode 5 is not formed), and is in contact with the third additional portion 73 at the edge of the surface 3 c of the semiconductor laminate 3 that is closer to the first end face 3 a. Note that in FIG. 4(a) shows a portion of the quantum cascade laser device 1 in a state where the antireflection film 7 is not formed, and (b) shows a portion of the quantum cascade laser device 1 in a state where the antireflection film 7 is formed.

[0038] As shown in FIG. 5, the anti-reflection coating 7 is a single-layer film consisting of only one CeO2 film 7a formed by successive sputtering and vacuum deposition. The CeO2 film 7a is formed directly on the first end facet 3a (i.e., without any other film interposed therebetween). The CeO2 film 7a has sufficient transparency to laser light having a central wavelength of 7.5 μm or longer. An example of the specifications of the anti-reflection coating 7 is as follows: the refractive index of the CeO2 film 7a is 1.786, and the thickness of the CeO2 film 7a in the main body 70 is 1.26 μm. In this case, if the central wavelength of the laser light emitted by the semiconductor laminate 3 is λ=9.0 μm, the effective refractive index of the optical waveguide structure formed by the semiconductor laminate 3 is n'=3.19, the refractive index of the CeO2 film 7a is n=1.786, and the thickness of the CeO2 film 7a in the main body 70 is t=1.26 μm, then n=(n') 1 / 2 and t=λ / 4n are approximately satisfied. Therefore, the anti-reflection coating 7 can reduce the reflectance of the laser light emitted by the semiconductor laminate 3 to less than 0.1%. Note that "forming at least one CeO film by successive sputtering and vacuum deposition" will be described later.

[0039] As shown in FIG. 6, the first additional portion 71 includes a first end portion 71a. The first end portion 71a has a thickness that decreases with increasing distance from the first end face 3a in the optical waveguide direction A (see FIG. 3). The first end portion 71a has a first side surface 71b that intersects with the surface 5a of the first electrode 5. The first side surface 71b is, for example, a flat surface that is perpendicular to the surface 5a of the first electrode 5. The second additional portion 72 includes a second end portion 72a. The second end portion 72a has a thickness that decreases with increasing distance from the first end face 3a in the optical waveguide direction A (see FIG. 3). The second end portion 72a has a second side surface 72b that intersects with the surface 6a of the second electrode 6. The second side surface 72b is, for example, a flat surface that is perpendicular to the surface 6a of the second electrode 6. [Configuration of quantum cascade laser device]

[0040] As shown in FIG. 1, the quantum cascade laser device 10 includes a quantum cascade laser element 1, a support portion 11, a joining member 12, a plurality of wires 15, and a drive portion 14.

[0041] The support 11 supports the quantum cascade laser device 1 with the semiconductor substrate 2 positioned on the support 11 side relative to the semiconductor laminate 3 (i.e., in an epi-side-up state). The support 11 includes a main body 111 and electrode pads 112 formed on the main surface of the main body 111. The main body 111 is formed, for example, from AlN in the shape of a rectangular plate. The electrode pads 112 are, for example, Ti / Pt / Au films or Ti / Pd / Au films, and are formed in the shape of a rectangular film. The support 11 is a submount and is thermally connected to a heat sink (not shown).

[0042] The bonding member 12 bonds the electrode pad 112 of the support 11 to the second electrode 6 of the quantum cascade laser device 1 in an epi-side-up state. The bonding member 12 is, for example, a solder member such as an AuSn member. The maximum thickness of the bonding member 12 is smaller than the maximum thickness of the second additional portion 72 of the antireflection film 7. The quantum cascade laser device 1 is supported by the support 11 such that the distance between the electrode pad 112 and the second electrode 6 decreases with increasing distance from the second additional portion 72 in the optical waveguide direction A. The thickness of the portion of the bonding member 12 disposed between the electrode pad 112 and the second electrode 6 is, for example, about several μm.

[0043] As an example, when viewed from the Z-axis direction, both edges of the electrode pad 112 in the Y-axis direction coincide with both edges of the main body 111 in the Y-axis direction. When viewed from the Z-axis direction, both edges of the electrode pad 112 in the X-axis direction are located inside both edges of the main body 111 in the X-axis direction. When viewed from the Z-axis direction, both edges of the bonding member 12 in the Y-axis direction coincide with both edges of the electrode pad 112 in the Y-axis direction. When viewed from the Z-axis direction, both edges of the bonding member 12 in the X-axis direction are located inside both edges of the electrode pad 112 in the X-axis direction. When viewed from the Z-axis direction, an edge of the quantum cascade laser device 1 on the first end face 3a side in the Y-axis direction coincides with one edge of the bonding member 12 in the Y-axis direction. When viewed from the Z-axis direction, an edge of the quantum cascade laser device 1 on the second end face 3b side in the Y-axis direction is located inside the other edge of the bonding member 12 in the Y-axis direction. When viewed from the Z-axis direction, both edges of the quantum cascade laser device 1 in the X-axis direction are located inside both edges of the bonding member 12 in the X-axis direction.

[0044] A plurality of wires 15 are connected to the first electrode 5. Each wire 15 is formed by wire bonding to the first electrode 5, and a connection end 15a of each wire 15 is joined to the surface 52a of the metal plating layer 52. At least one wire 15 needs to be connected to the first electrode 5.

[0045] The driving unit 14 is electrically connected to the electrode pads 112 and each wire 15. That is, the driving unit 14 is electrically connected to each of the first electrode 5 and the second electrode 6 of the quantum cascade laser device 1. The driving unit 14 drives the quantum cascade laser device 1 so that the quantum cascade laser device 1 continuously oscillates laser light.

[0046] In quantum cascade laser device 10 configured as described above, when drive unit 14 applies a drive voltage to active layer 31 of quantum cascade laser element 1, light is emitted from active layer 31, and laser light having a center wavelength of 7.5 μm or longer among the emitted light is resonated in the distributed feedback structure. At this time, first end face 3a is formed with antireflection coating 7 having the function of reducing the reflectance of laser light having a center wavelength of 7.5 μm or longer. As a result, laser light having a center wavelength of 7.5 μm or longer is continuously oscillated from first end face 3a through antireflection coating 7. [Quantum cascade laser device manufacturing method]

[0047] First, as shown in Fig. 7(a), a wafer 100 is prepared. The wafer 100 includes a plurality of portions 110, each of which becomes a set of a semiconductor substrate 2, a semiconductor laminate 3, an insulating film 4, a first electrode 5, and a second electrode 6. In the wafer 100, the plurality of portions 110 are arranged in a matrix with the X-axis direction as the row direction and the Y-axis direction (i.e., the direction parallel to the optical waveguide direction A in each portion 110) as the column direction. As an example, the wafer 100 is manufactured by the following method.

[0048] First, a semiconductor layer including multiple portions that will become the semiconductor laminate 3 is formed on the surface of a semiconductor wafer including multiple portions that will each become the semiconductor substrate 2. Next, portions of the semiconductor layer are removed by etching so that each of the multiple portions that will become the semiconductor laminate 3 in the semiconductor layer has a ridge portion 30. Next, an insulating layer including multiple portions that will become the insulating film 4 is formed on the semiconductor layer so that the surface 30a of each ridge portion 30 is exposed. Next, a continuous metal underlayer including multiple portions that will become the metal underlayer 51 is formed so as to cover the surface 30a of each ridge portion 30 and to cover the insulating layer. Next, multiple metal plating layers that will each become the metal plating layer 52 are formed on the continuous metal underlayer, and the ridge portion 30 is embedded in each metal plating layer. Next, the surface of each metal plating layer is planarized by polishing to form multiple metal plating layers 52. Next, portions of the continuous metal underlayer that are exposed between adjacent metal plating layers 52 are removed by etching to form multiple metal underlayers 51. Subsequently, the back surface of the semiconductor wafer is polished to thin the semiconductor wafer, and an electrode layer including a plurality of portions each of which becomes a second electrode 6 is formed on the back surface of the semiconductor wafer.

[0049] Once the wafer 100 is prepared as described above, it is cleaved along the X-axis direction to obtain a plurality of laser bars 200, as shown in FIG. 7(b) (step of preparing the laser bars 200). Each laser bar 200 includes a plurality of portions 110. In each laser bar 200, the plurality of portions 110 are aligned one-dimensionally in the X-axis direction (i.e., a direction perpendicular to the optical waveguide direction A in each portion 110). Each laser bar 200 has a pair of end faces 200a, 200b facing each other in the Y-axis direction. The end face 200a includes a plurality of first end faces 3a aligned one-dimensionally along the X-axis direction, and the end face 200b includes a plurality of second end faces 3b aligned one-dimensionally along the X-axis direction.

[0050] Next, as shown in (a) of FIG. 8, an antireflection layer 700 is formed on the surface of a portion 210 of the laser bar 200 that includes the end face 200a (a step of forming the antireflection layer 700 on the laser bar 200). The antireflection layer 700 includes a plurality of portions that will each become the antireflection film 7. Next, the laser bar 200 is cleaved along the Y-axis direction, thereby dividing the laser bar 200 and the antireflection layer 700 into each of the plurality of portions 110, as shown in (b) of FIG. 8, and thereby obtaining a plurality of quantum cascade laser devices 1 (a step of dividing the laser bar 200 and the antireflection layer 700).

[0051] The formation of the anti-reflection layer 700 on the laser bar 200 will be described with reference to FIGS. 9 and 10. First, as shown in FIG. 9, a plurality of laser bars 200 and a plurality of dummy bars 300 are prepared. The length of the dummy bars 300 in the Y-axis direction is shorter than the length of the laser bars 200 in the Y-axis direction. The length of the dummy bars 300 in the X-axis direction is equal to or longer than the length of the laser bars 200 in the X-axis direction. Next, with the end faces 200b of each laser bar 200 and the end faces 300b of each dummy bar 300 arranged on the same plane, the laser bars 200 and the dummy bars 300 are alternately arranged adjacent to each other in the Z-axis direction, and the plurality of laser bars 200 and the plurality of dummy bars 300 are held by a holding member (not shown). This results in a bar unit 400 composed of a plurality of laser bars 200 and a plurality of dummy bars 300. In the bar unit 400, the portion 210 of each laser bar 200 protrudes beyond the end face 300a (the end face opposite the end face 300b in the Y-axis direction) of the adjacent dummy bar 300. In this state, an anti-reflection layer 700 is formed on the surface of the portion 210 of each laser bar 200 by "forming at least one layer of CeO2 film by successive sputtering and vacuum deposition."

[0052] The phrase "forming at least one layer of CeO film by successive sputtering and vacuum deposition" refers to forming at least one layer of CeO film by successively sputtering CeO and vacuum depositing CeO on a target. Formation of at least one layer of CeO film by successive sputtering and vacuum deposition is performed, for example, in a film formation apparatus 500 shown in FIG. 10. As shown in FIG. 10, the film formation apparatus 500 includes a holder 510, a sputtering mechanism 520, and a vacuum deposition mechanism 530. The holder 510 rotates around an axis CL while holding the bar unit 400. The sputtering mechanism 520 sputters CeO onto the end surface 200a of the bar unit 400 introduced into a sputtering chamber (not shown) by the holder 510. The vacuum deposition mechanism 530 performs vacuum deposition of CeO2 on the end surface 200a of the bar unit 400 introduced into a vacuum deposition chamber (not shown) by the holder 510. In the film formation apparatus 500, the ratio between the "intensity of sputtering of CeO2 by the sputtering mechanism 520" and the "intensity of vacuum deposition of CeO2 by the vacuum deposition mechanism 530" is adjusted for each CeO2 film, thereby adjusting the refractive index of at least one CeO2 film formed by successive sputtering and vacuum deposition for each CeO2 film. In other words, at least one CeO2 film is formed by successive sputtering and vacuum deposition so that each CeO2 film has a desired thickness and a desired refractive index. If the "intensity of vacuum deposition of CeO2 by vacuum deposition mechanism 530" is strong relative to the "intensity of sputtering of CeO2 by sputtering mechanism 520," the refractive index of the single layer of CeO2 film will be small, and conversely, if the "intensity of vacuum deposition of CeO2 by vacuum deposition mechanism 530" is weak relative to the "intensity of sputtering of CeO2 by sputtering mechanism 520," the refractive index of the single layer of CeO2 film will be large.

[0053] In the following description, "forming multiple CeO films by discrete sputtering and vacuum deposition" refers to forming multiple CeO films by sequentially forming one CeO film by sputtering alone and another CeO film by vacuum deposition alone. Formation of multiple CeO films by discrete sputtering and vacuum deposition is performed, for example, as follows in the film-forming apparatus 500 shown in FIG. 10 . Specifically, multiple CeO films are formed by sequentially forming one CeO film in which the "intensity of vacuum deposition of CeO by the vacuum deposition mechanism 530" is set to zero relative to the "intensity of sputtering of CeO by the sputtering mechanism 520," and then forming another CeO film in which the "intensity of vacuum deposition of CeO by the sputtering mechanism 520" is set to zero relative to the "intensity of sputtering of CeO by the vacuum deposition mechanism 530." In forming a multi-layer CeO2 film by discrete sputtering and vacuum deposition, as long as the formation of a single layer CeO2 film by sputtering alone and the formation of a single layer CeO2 film by vacuum deposition alone are alternately performed, either may be started or ended at any time, and any number of times may be repeated.

[0054] It is generally impractical to directly characterize a CeO film by its structure or properties, whether it is at least one layer of CeO film formed by continuous sputtering and vacuum deposition or multiple layers of CeO film formed by discrete sputtering and vacuum deposition. However, the refractive index of at least one layer of CeO film formed by continuous sputtering and vacuum deposition can be adjusted to a value different from the refractive index of CeO film formed by sputtering alone and the refractive index of CeO film formed by vacuum deposition alone, e.g., greater than 1.65 and less than 2.1. Furthermore, among multiple layers of CeO film formed by discrete sputtering and vacuum deposition, the refractive index of CeO film formed by sputtering alone is approximately 2.1, while the refractive index of CeO film formed by vacuum deposition alone is approximately 1.65.

[0055] Fig. 11 shows an SEM photograph of a CeO2 film formed solely by vacuum deposition. As shown in Fig. 11, when a CeO2 film is formed solely by vacuum deposition, if the thickness of the CeO2 film exceeds 500 nm, the refractive index changes in the thickness direction due to oxygen deficiency in the deposition source, polycrystallization of the film, and the like. Therefore, when a CeO2 film is formed solely by vacuum deposition, it is preferable to keep the thickness of the CeO2 film to 500 nm or less. [Action and effect]

[0056] In the quantum cascade laser device 1, the anti-reflection coating 7 includes at least one CeO film 7a formed by successive sputtering and vacuum deposition. The refractive index of the at least one CeO film 7a formed by successive sputtering and vacuum deposition can be controlled to a value close to the "square root of the effective refractive index of the optical waveguide structure formed by the semiconductor laminate 3," compared to the refractive index of a CeO film formed by sputtering alone or by vacuum deposition alone. Therefore, the anti-reflection coating 7 can reliably reduce the reflectance of laser light having a center wavelength of 7.5 μm or longer. Furthermore, the CeO film 7a can ensure transparency for laser light having a center wavelength of 7.5 μm or longer, prevent short circuits at the first end facet 3a, and improve adhesion to the first end facet 3a. Furthermore, the CeO film 7a has high heat resistance. As described above, it is possible to realize an anti-reflection coating 7 that functions effectively for laser light having a central wavelength of 7.5 μm or more and that has high heat resistance, using the quantum cascade laser element 1. For the same reason, it is also possible to realize an anti-reflection coating 7 that functions effectively for laser light having a central wavelength of 7.5 μm or more and that has high heat resistance, using a quantum cascade laser device 10 that includes the quantum cascade laser element 1, and using a method for manufacturing the quantum cascade laser element 1.

[0057] In the quantum cascade laser device 1, the anti-reflection coating 7 includes at least one CeO2 film 7a formed by successive sputtering and vacuum deposition, thereby simplifying the configuration of the anti-reflection coating 7.

[0058] In the quantum cascade laser device 1, the antireflection coating 7 includes a second additional portion 72 formed on the surface 6a of the second electrode 6, and the second additional portion 72 includes a second end portion 72a having a thickness that decreases with increasing distance from the first end facet 3a in the optical waveguide direction A, and the second end portion 72a has a second side surface 72b that intersects with the surface 6a of the second electrode 6. As a result, when the second electrode 6 is joined to the electrode pad 112 of the support portion 11 by the joining member 12, the molten joining member 12 is blocked by the second side surface 72b of the second additional portion 72 and repelled by the second additional portion 72, so that the molten joining member 12 can be prevented from creeping up toward the main body portion 70 of the antireflection coating 7.

[0059] Note that a similar effect is achieved when the quantum cascade laser device 1 is supported by the support portion 11 in an epi-side-down state, which will be described later. That is, in the quantum cascade laser device 1, the antireflection coating 7 includes a first additional portion 71 formed on the surface 5a of the first electrode 5, and the first additional portion 71 includes a first end portion 71a having a thickness that decreases with increasing distance from the first end face 3a in the optical waveguide direction A, and the first end portion 71a has a first side surface 71b that intersects with the surface 5a of the first electrode 5. As a result, when the first electrode 5 is joined to the electrode pad 112 of the support portion 11 by the joining member 12, the molten joining member 12 is blocked by the first side surface 71b of the first additional portion 71 and repelled by the first additional portion 71, so that the molten joining member 12 can be prevented from creeping up toward the main body portion 70 of the antireflection coating 7.

[0060] In the quantum cascade laser device 1, the insulating film 4 formed on the surface 3c of the semiconductor laminate 3 is a CeO2 film, and the antireflection film 7 further includes a third additional portion 73 formed on the insulating film 4. This makes it possible to reduce optical waveguide loss in the semiconductor laminate 3 compared to when the insulating film 4 is a SiN film or a SiO2 film, for example. Furthermore, improved adhesion between the third additional portion 73 of the antireflection film 7 and the insulating film 4 can prevent the antireflection film 7 from peeling off from the first end facet 3a.

[0061] In the quantum cascade laser device 10, with the semiconductor substrate 2 positioned on the supporting portion 11 side with respect to the semiconductor laminate 3, the electrode pad 112 of the supporting portion 11 and the second electrode 6 are joined by the joining member 12, and the maximum value of the thickness of the joining member 12 is smaller than the maximum value of the thickness of the second additional portion 72. As a result, when the second electrode 6 is joined to the electrode pad 112 of the supporting portion 11 by the joining member 12, the molten joining member 12 is blocked by the second side surface 72b of the second additional portion 72 and is repelled by the second additional portion 72, so that the molten joining member 12 can be prevented from creeping up toward the main body portion 70 of the antireflection coating 7.

[0062] In the quantum cascade laser device 10, the quantum cascade laser element 1 is supported by the support 11 so that the distance between the electrode pad 112 and the second electrode 6 decreases with increasing distance from the second additional portion 72 in the optical waveguide direction A. As a result, when the second electrode 6 is joined to the electrode pad 112 of the support 11 by the joining member 12, the molten joining member 12 is reliably filled between the electrode pad 112 and the second electrode 6, so that the second electrode 6 can be reliably joined to the electrode pad 112. [Variations]

[0063] The present invention is not limited to the above-described embodiment. For example, the anti-reflection coating 7 may include at least one CeO2 film 7a formed by successive sputtering and vacuum deposition, and multiple CeO2 films 7a having different refractive indices. This can reliably reduce the reflectance of laser light in a desired wavelength range.

[0064] Alternatively, the anti-reflection coating 7 may include at least one CeO2 film 7a formed by successive sputtering and vacuum deposition and at least one CeO2 film formed by sputtering. This ensures a reduction in the reflectance of laser light in a desired wavelength range. As an example, the quantum cascade laser device 1 may include an anti-reflection coating 7A or an anti-reflection coating 7B, as shown in FIGS. 12(a) and 12(b), each of which is composed of at least one CeO2 film 7a formed by successive sputtering and vacuum deposition and at least one CeO2 film 7b formed by sputtering. The anti-reflection coating 7A is a two-layer film composed of only one CeO2 film 7a and one CeO2 film 7b. The anti-reflection coating 7B is a multi-layer film composed of only two CeO2 films 7a and two CeO2 films 7b. The CeO2 film 7b is a CeO2 film formed only by sputtering, and has sufficient transparency to laser light having a center wavelength of 7.5 μm or more.

[0065] As shown in FIG. 12(a), in the antireflection coating 7A, a first-layer CeO2 film 7a is formed directly on the first end facet 3a, and a second-layer CeO2 film 7b is formed directly on the first-layer CeO2 film 7a. An example of the specifications of the antireflection coating 7A is as follows: The refractive index of the first-layer CeO2 film 7a is 1.66, and the thickness of the first-layer CeO2 film 7a in the main body portion 70 is 0.77 μm. The refractive index of the second-layer CeO2 film 7b is 2.1, and the thickness of the second-layer CeO2 film 7b in the main body portion 70 is 0.33 μm. In this case, the thickness of the antireflection coating 7A in the main body portion 70 is 1.10 μm. Alternatively, the first-layer CeO2 film may be the CeO2 film 7b, and the second-layer CeO2 film may be the CeO2 film 7a. Alternatively, a film other than a CeO2 film may be formed between adjacent CeO2 films.

[0066] As shown in FIG. 12(b), in the anti-reflection coating 7B, a first-layer CeO2 film 7b is formed directly on the first facet 3a, and a second-layer CeO2 film 7a is formed directly on the first CeO2 film 7b. Furthermore, a third-layer CeO2 film 7b is formed directly on the second CeO2 film 7a, and a fourth-layer CeO2 film 7a is formed directly on the third CeO2 film 7b. An example of the specifications of the anti-reflection coating 7B is as follows: The refractive index of the first-layer CeO2 film 7b is 2.1, and the thickness of the first-layer CeO2 film 7b in the main body 70 is 1.00 μm. The refractive index of the second-layer CeO2 film 7a is 1.66, and the thickness of the second-layer CeO2 film 7a in the main body 70 is 1.27 μm. The refractive index of the third CeO2 film 7b is 2.1, and the thickness of the third CeO2 film 7b in the main body 70 is 1.01 μm. The refractive index of the fourth CeO2 film 7a is 1.66, and the thickness of the fourth CeO2 film 7a in the main body 70 is 1.28 μm. In this case, the thickness of the anti-reflection film 7B in the main body 70 is 4.55 μm. Note that the first and third CeO2 films may be CeO2 films 7a, and the second and fourth CeO2 films may be CeO2 films 7b. Furthermore, a film other than a CeO2 film may be formed between adjacent CeO2 films.

[0067] FIG. 13 shows the optical characteristics of several types of antireflection coatings 7, 7A, and 7B (simulation results for an example of the specifications of each of the antireflection coatings 7, 7A, and 7B described above). As shown in FIG. 13, it was found that any of the antireflection coatings 7, 7A, and 7B can reliably reduce the reflectance of laser light having a center wavelength of 7.5 μm or longer. In particular, it was found that the antireflection coating 7 (a single-layer coating consisting of only one CeO film 7a) and the antireflection coating 7A (a double-layer coating consisting of only one CeO film 7a and one CeO film 7b) can reduce the reflectance of narrow-band laser light. On the other hand, it was found that the antireflection coating 7B (a multilayer coating consisting of only two CeO films 7a and two CeO films 7b) can reduce the reflectance of broad-band laser light. The antireflection coating 7A, which is a double-layer coating, has the advantage of being thinner than the antireflection coating 7, which is a single-layer coating.

[0068] The anti-reflection coating 7 may also include multiple layers of CeO2 films formed by discrete sputtering and vacuum deposition. This can reliably reduce the reflectance of laser light in a desired wavelength range. For example, the reflectance of laser light over a broader band can be reduced compared to when the anti-reflection coating 7 is composed of a single CeO2 film 7a.

[0069] Furthermore, the anti-reflection coating 7 may include films other than a CeO2 film, as long as it includes at least one CeO2 film. As an example, as shown in FIG. 14, the quantum cascade laser device 1 may include an anti-reflection coating 7C including a Si film 7d and a CeO2 film 7a formed by successive sputtering and vacuum deposition. In the anti-reflection coating 7C, the first Si film 7d is formed directly on the first end facet 3a, and the second CeO2 film 7a is formed directly on the first Si film 7d. In this case, the adhesion between the semiconductor stack 3, the Si film 7d, and the CeO2 film 7a can be improved.

[0070] From the above description, it is sufficient that the anti-reflection coating 7 includes at least one of "at least one CeO film 7a formed by continuous sputtering and vacuum deposition" and "multiple CeO films formed by discrete sputtering and vacuum deposition." The refractive index of the at least one CeO film 7a formed by continuous sputtering and vacuum deposition can be controlled to a value close to the "square root of the effective refractive index of the optical waveguide structure formed by the semiconductor laminate 3," compared to the refractive index of a CeO film formed by sputtering alone or the refractive index of a CeO film formed by vacuum deposition alone. Furthermore, in the multiple CeO films formed by discrete sputtering and vacuum deposition, a CeO film having a refractive index greater than the "square root of the effective refractive index of the optical waveguide structure formed by the semiconductor laminate 3" is adjacent to a CeO film having a refractive index smaller than the "square root of the effective refractive index of the optical waveguide structure formed by the semiconductor laminate 3." For these reasons, the antireflection coating 7 can reliably reduce the reflectance of laser light having a central wavelength of 7.5 μm or longer. Furthermore, each CeO film can ensure transparency to laser light having a central wavelength of 7.5 μm or longer, prevent short circuits at the first facet 3a, and improve adhesion to the first facet 3a. Furthermore, each CeO film has high heat resistance. For these reasons, if the antireflection coating 7 includes at least one of "at least one layer of CeO film 7a formed by continuous sputtering and vacuum deposition" and "multiple layers of CeO films formed by discrete sputtering and vacuum deposition," the antireflection coating 7 can be realized that functions effectively for laser light having a central wavelength of 7.5 μm or longer and has high heat resistance. In the manufacturing method of the quantum cascade laser device 1, in the process of forming the antireflection layer 700 on the laser bar 200, at least one of "forming at least one layer of CeO2 film 7a by continuous sputtering and vacuum deposition" and "forming multiple layers of CeO2 films by discrete sputtering and vacuum deposition" may be performed on the end face 200a of the laser bar 200.

[0071] 15, quantum cascade laser device 10 may be one in which quantum cascade laser element 1 is supported by supporting portion 11 with semiconductor stack 3 positioned on the supporting portion 11 side of semiconductor substrate 2 (i.e., epi-side down state). In this case, at least one wire 15 is connected to second electrode 6.

[0072] 15 , in an epi-side-down state, electrode pad 112 of support portion 11 and first electrode 5 are joined by joining member 12, and the maximum thickness of joining member 12 is smaller than the maximum thickness of first additional portion 71. As a result, when first electrode 5 is joined to electrode pad 112 of support portion 11 by joining member 12, molten joining member 12 is blocked by first side surface 71 b of first additional portion 71 and repelled by first additional portion 71, so that molten joining member 12 can be prevented from creeping up toward main body portion 70 of anti-reflection coating 7.

[0073] 15, the quantum cascade laser element 1 is supported by the support 11 so that the distance between the electrode pad 112 and the first electrode 5 decreases with increasing distance from the first addition portion 71 in the optical waveguide direction A. As a result, when the first electrode 5 is bonded to the electrode pad 112 of the support 11 by the bonding member 12, the molten bonding member 12 is reliably filled between the electrode pad 112 and the first electrode 5, so that the first electrode 5 can be reliably bonded to the electrode pad 112. The thickness of the portion of the bonding member 12 disposed between the electrode pad 112 and the first electrode 5 is, for example, about several μm.

[0074] 16(a) and 16(b), when viewed from the Z-axis direction, the outer edges of the electrode pads 112 may be located inside the outer edges of the main body 111, and the outer edges of the bonding members 12 may be located inside the outer edges of the electrode pads 112. As shown in FIG. 16(a), when viewed from the Z-axis direction, both edges of the quantum cascade laser element 1 in the Y-axis direction may be located outside (outside by about several μm to several hundred μm) both edges of the main body 111 in the Y-axis direction. As shown in FIG. 16(b), when viewed from the Z-axis direction, the edge of the quantum cascade laser element 1 on the side of the first end face 3a in the Y-axis direction may be located outside (outside by about several μm to several hundred μm) one edge of the main body 111 in the Y-axis direction, and the edge of the quantum cascade laser element 1 on the side of the second end face 3b in the Y-axis direction may be located inside the outer edge of the bonding members 12. In both examples shown in (a) and (b) of Figure 16, when viewed from the Z-axis direction, the edge of the quantum cascade laser element 1 on the side of the first end face 3a in the Y-axis direction is located outside one edge of the main body 111 in the Y-axis direction, so that it is possible to prevent part of the laser light emitted from the first end face 3a from being blocked by the main body 111.

[0075] The quantum cascade laser device 1 is not limited to the above-described configuration. For example, a known quantum cascade structure can be applied to the active layer 31. Also, a known stacked structure can be applied to the semiconductor stack 3. As an example, in the semiconductor stack 3, the upper guide layer does not need to have a diffraction grating structure that functions as a distributed feedback structure. Also, the semiconductor stack 3 does not need to have a ridge portion 30 formed therein.

[0076] The insulating film 4 may be formed by successive sputtering and vacuum deposition to adjust the refractive index of the insulating film 4. The insulating film 4 is not limited to a CeO film, but may be a SiN film or a SiO film.

[0077] When viewed from the Z-axis direction, the outer edge of the metal base layer 51 of the first electrode 5 may coincide with the outer edges of the semiconductor substrate 2 and the semiconductor stack 3. When viewed from the Z-axis direction, if the outer edge of the metal base layer 51 of the first electrode 5 coincides with at least the first end face 3 a and the second end face 3 b, heat dissipation at the first end face 3 a and the second end face 3 b can be ensured.

[0078] Furthermore, the metal plating layer 52 of the first electrode 5 does not have to be planarized by polishing. Furthermore, the first electrode 5 does not have the metal plating layer 52, and may be, for example, a metal film formed so as to extend along the surface 3c of the semiconductor laminate 3.

[0079] Furthermore, a metal film may be formed on the second end face 3b via an insulating film in the quantum cascade laser device 1. This allows the metal film to function as a reflective film, thereby enabling efficient optical output from the first end face 3a.

[0080] Furthermore, the antireflection film 7 may include the main body portion 70, but may not include at least one of the first additional portion 71, the second additional portion 72, and the third additional portion 73. As described above, when the quantum cascade laser device 1 is supported by the support portion 11 in an epi-side-up state (see FIG. 1 ), if the antireflection film 7 includes the main body portion 70 and the second additional portion 72, it is possible to prevent the molten bonding member 12 from creeping up toward the main body portion 70 of the antireflection film 7. When the quantum cascade laser device 1 is supported by the support portion 11 in an epi-side-down state (see FIG. 15 ), if the antireflection film 7 includes the main body portion 70 and the first additional portion 71, it is possible to prevent the molten bonding member 12 from creeping up toward the main body portion 70 of the antireflection film 7.

[0081] The quantum cascade laser device 10 is not limited to the above-described configuration. For example, the driver 14 may drive the quantum cascade laser element 1 so that the quantum cascade laser element 1 oscillates laser light in pulses. [Explanation of symbols]

[0082] 1...quantum cascade laser element, 2...semiconductor substrate, 2b...surface, 3...semiconductor laminate, 3a...first end face, 3b...second end face, 3c...surface, 4...insulating film, 5...first electrode, 5a...surface, 6...second electrode, 6a...surface, 7, 7A, 7B, 7C...antireflection film, 7a, 7b...CeO2 film, 10...quantum cascade laser device, 11...support portion, 12...bonding member, 31...active layer, 70...main body portion, 71...first additional portion, 71a...first end portion, 71b...first side surface, 72...second additional portion, 72a...second end portion, 72b...second side surface, 73...third additional portion, 110...portion, 112...electrode pad, 200...laser bar, 700...antireflection layer, A...optical waveguide direction.

Claims

1. a semiconductor substrate; a semiconductor laminate formed on the semiconductor substrate, the semiconductor laminate including an active layer having a quantum cascade structure, the active layer having a first end face and a second end face opposing each other in an optical waveguide direction; a first electrode formed on a surface of the semiconductor laminate opposite to the semiconductor substrate; a second electrode formed on a surface of the semiconductor substrate opposite to the semiconductor laminate; an anti-reflection film formed on the first end surface, The anti-reflection film is a main body portion formed on the first end surface; a first additional portion formed on a surface of the first electrode opposite to the semiconductor laminate, the first additional portion includes a first end portion having a thickness that decreases with increasing distance from the first end face in the optical waveguide direction, The first end has a first side that intersects the surface of the first electrode.

2. a semiconductor substrate; a semiconductor laminate formed on the semiconductor substrate, the semiconductor laminate including an active layer having a quantum cascade structure, the active layer having a first end face and a second end face opposing each other in an optical waveguide direction; a first electrode formed on a surface of the semiconductor laminate opposite to the semiconductor substrate; a second electrode formed on a surface of the semiconductor substrate opposite to the semiconductor laminate; an anti-reflection film formed on the first end surface, The anti-reflection film is a main body portion formed on the first end surface; a second additional portion formed on a surface of the second electrode opposite to the semiconductor substrate, the second additional portion includes a second end portion having a thickness that decreases with increasing distance from the first end face in the optical waveguide direction, The second end has a second side that intersects the surface of the second electrode.

3. The quantum cascade laser device according to claim 1; a support portion that supports the quantum cascade laser element; a bonding member that bonds an electrode pad of the support portion and the first electrode in a state where the semiconductor stacked body is located on the support portion side with respect to the semiconductor substrate, A quantum cascade laser device, wherein a maximum thickness of the joining member between the electrode pad and the first electrode is smaller than a maximum thickness of the first additional portion.

4. 4. The quantum cascade laser device according to claim 3, wherein the quantum cascade laser element is supported by the support portion such that the distance between the electrode pad and the first electrode decreases as the distance from the first additional portion increases in the optical waveguide direction.

5. The quantum cascade laser device according to claim 2; a support portion that supports the quantum cascade laser element; a bonding member that bonds the electrode pad of the support portion and the second electrode in a state in which the semiconductor substrate is positioned on the support portion side with respect to the semiconductor laminate, A quantum cascade laser device, wherein a maximum thickness of the joining member between the electrode pad and the second electrode is smaller than a maximum thickness of the second additional portion.

6. 6. The quantum cascade laser device according to claim 5, wherein the quantum cascade laser element is supported by the support portion so that the distance between the electrode pad and the second electrode becomes smaller as the distance from the second additional portion increases in the optical waveguide direction.

Images