Quantum cascade laser element, quantum cascade laser device, and method for manufacturing quantum cascade laser element

Abstract

To provide a quantum cascade laser element which effectively functions for laser light having a center wavelength of 7.5 μm or longer and having an antireflection film having a high heat resistance, a quantum cascade laser device, and a method for manufacturing such a quantum cascade laser element.SOLUTION: A quantum cascade laser element 1 comprises: a semiconductor substrate 2; a semiconductor laminate 3 including an active layer 31, and having a first end face 3a and a second end face 3b opposed to each other in an optical waveguiding direction A; a first electrode 5; a second electrode 6; and an antireflection film 7 formed on the first end face 3a. The semiconductor laminate 3 is configured so as to oscillate laser light having a center wavelength of 7.5 μm or longer. The antireflection film 7 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 films formed by discrete sputtering and vacuum deposition.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a quantum cascade laser element, a quantum cascade laser device, and a method for manufacturing a quantum cascade laser element.

Background Art

[0002] As a conventional quantum cascade laser element, there is known a quantum cascade laser element including a semiconductor substrate, a semiconductor laminate formed on the semiconductor substrate, a first electrode formed on a surface of the semiconductor laminate opposite to the semiconductor substrate, and a second electrode formed on a surface of the semiconductor substrate opposite to the semiconductor laminate, wherein an antireflection film is formed on one end face of a pair of end faces of the semiconductor laminate including an active layer.

[0003] In recent years, the demand for quantum cascade laser elements capable of oscillating laser light having a center wavelength of 7.5 μm or more has been increasing. Therefore, in the quantum cascade laser element as described above, it is desired to realize an antireflection film capable of reliably reducing the reflectance of laser light having a center wavelength of 7.5 μm or more and ensuring sufficient durability. As such an antireflection film, Patent Document 1 describes an antireflection film including an insulating film which is a CeO2 film, a first refractive index film which is a YF3 film or a CeF3 film, and a second refractive index film having a refractive index greater than 1.8.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The antireflection film described in Patent Document 1 is an antireflection film that functions effectively for 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 CeO2 film.

[0006] An object of the present invention is to provide a quantum cascade laser element, a quantum cascade laser device, and a method for manufacturing a quantum cascade laser element, which include an antireflection film that functions effectively for laser light having a central wavelength of 7.5 μm or more and has high heat resistance.

Means for Solving the Problems

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

[0008] In this quantum cascade laser device, the antireflection film includes at least one layer of CeO₂ film formed by continuous sputtering and vacuum evaporation, and at least one of a plurality of layers of CeO₂ film formed by discrete sputtering and vacuum evaporation. The refractive index of at least one layer of CeO₂ film formed by continuous sputtering and vacuum evaporation 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" as compared with the refractive index of the CeO₂ film formed only by sputtering, and also as compared with the refractive index of the CeO₂ film formed only by vacuum evaporation. Further, in the plurality of layers of CeO₂ film formed by discrete sputtering and vacuum evaporation, a CeO₂ film having a refractive index larger than "the square root of the effective refractive index of the optical waveguide structure constituted by the semiconductor laminate" and a CeO₂ 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" are adjacent to each other. From these facts, according to this antireflection film, the reflectance of laser light having a center wavelength of 7.5 μm or more can be surely reduced. Further, according to the CeO₂ film, it is possible to ensure the transmittance for laser light having a center wavelength of 7.5 μm or more, prevent short-circuiting at the first end face, and improve the adhesion to the first end face. Moreover, the CeO₂ film has high heat resistance. As described above, according to this quantum cascade laser device, it is possible to realize an antireflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more and has high heat resistance.

[0009] In the quantum cascade laser device of the present invention, the antireflection film may include one layer of CeO₂ film which is at least one layer of CeO₂ film formed by continuous sputtering and vacuum evaporation. According to this, the configuration of the antireflection film can be simplified.

[0010] In the quantum cascade laser device of the present invention, the antireflection film may be at least one layer of CeO₂ film formed by continuous sputtering and vacuum evaporation and may include a plurality of layers of CeO₂ film having different refractive indexes from each other. According to this, the reflectance of laser light in a desired wavelength range can be surely reduced.

[0011] In the quantum cascade laser device of the present invention, the antireflection film may further include at least one layer of CeO2 film formed by sputtering. According to this, the reflectance of the laser light can be surely reduced in a desired wavelength range.

[0012] In the quantum cascade laser device of the present invention, the antireflection film may include a plurality of layers of CeO2 films formed by discrete sputtering and vacuum evaporation. According to this, the reflectance of the laser light can be surely reduced in a desired wavelength range. For example, compared with the case where the antireflection film is composed of a single layer of CeO2 film, the reflectance can be reduced for broadband laser light.

[0013] In the quantum cascade laser device of the present invention, the antireflection film includes a main body portion formed on the first end face, and a first additional portion formed on the surface of the first electrode opposite to the semiconductor laminate. The first additional portion includes a first end portion having a thickness that becomes smaller as it moves away from the first end face in the optical waveguide direction. The first end portion may have a first side face that intersects the surface of the first electrode. According to this, when the first electrode is joined to the electrode pad of another member by a joining member, the molten joining member is blocked by the first side face of the first additional portion and repelled by the first additional portion, so that it is possible to suppress the molten joining member from creeping toward the main body portion of the antireflection film.

[0014] In the quantum cascade laser device of the present invention, the antireflection film includes 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 includes a second end portion having a thickness that becomes smaller as it moves away from the first end face in the optical waveguide direction. The second end portion may have a second side face that intersects the surface of the second electrode. According to this, when the second electrode is joined to the electrode pad of another member by a joining member, the molten joining member is blocked by the second side face of the second additional portion and repelled by the second additional portion, so that it is possible to suppress the molten joining member from creeping toward the main body portion of the antireflection film.

[0015] The quantum cascade laser device of the present invention further includes an insulating film formed on the surface of the semiconductor laminate. The insulating film is a CeO2 film, and the antireflection film may include a main body portion formed on the first end face and a third additional portion formed on the insulating film. According to this, for example, compared with the case where the insulating film is a SiN film or a SiO2 film, the optical waveguide loss in the semiconductor laminate can be reduced. Further, since the adhesion between the third additional portion of the antireflection film and the insulating film is improved, peeling of the antireflection film from the first end face can be suppressed.

[0016] The quantum cascade laser device of the present invention includes the above-described quantum cascade laser element, a support portion that supports the quantum cascade laser element, and a joining member that joins the electrode pad of the support portion and the second electrode in a state where the semiconductor substrate is located on the support portion side with respect to the semiconductor laminate. The maximum value of the thickness of the joining member is smaller than the maximum value of the thickness of the second additional portion.

[0017] According to this quantum cascade laser device, an antireflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more and has high heat resistance can be realized. Further, when the second electrode is joined to the electrode pad of the support portion by the 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, so that the molten joining member can be suppressed from creeping toward the main body portion of the antireflection film.

[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 becomes smaller as the distance from the second additional portion in the optical waveguide direction increases. According to this, when the second electrode is joined to the electrode pad of the support portion by the joining member, the molten joining member is surely filled between the electrode pad and the second electrode, so that the second electrode can be surely joined to the electrode pad.

[0019] The quantum cascade laser device of the present invention includes the above-described quantum cascade laser element, a support portion that supports the quantum cascade laser element, and a joining member that joins the electrode pad of the support portion and the first electrode in a state where the semiconductor laminate is positioned on the support portion side with respect to the semiconductor substrate. The maximum value of the thickness of the joining member is smaller than the maximum value of the thickness of the first additional portion.

[0020] According to this quantum cascade laser device, it is possible to realize an antireflection film that effectively functions with respect to laser light having a center wavelength of 7.5 μm or more and has high heat resistance. Further, when the first electrode is joined to the electrode pad of the support portion by the 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, so that it is possible to suppress the molten joining member from creeping toward the main body portion of the antireflection film.

[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 becomes smaller as the distance from the first additional portion increases in the optical waveguide direction. According to this, when the first electrode is joined to the electrode pad of the support portion by the joining member, the molten joining member is surely filled between the electrode pad and the first electrode, so that the first electrode can be surely joined to the electrode pad.

[0022] The method for manufacturing a quantum cascade laser element of the present invention is the method for manufacturing the above-described quantum cascade laser element, and includes a step of preparing a laser bar including a plurality of portions that are respectively a set of semiconductor substrates, semiconductor laminates, first electrodes, and second electrodes and are arranged one-dimensionally in a direction perpendicular to the optical waveguide direction, and a step of forming, on an end surface that becomes the first end surface in each of the plurality of portions of the laser bar, at least one layer of a CeO2 film by continuous sputtering and vacuum deposition, and forming a plurality of layers of a CeO2 film by discrete sputtering and vacuum deposition, thereby forming an antireflection layer including a plurality of portions that respectively become antireflection films on the laser bar, and a step of dividing the laser bar and the antireflection layer for each of the plurality of portions.

[0023] According to the manufacturing method of this quantum cascade laser element, it is possible to realize an antireflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more and has high heat resistance.

Advantages 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 manufacturing method of a quantum cascade laser element, which include an antireflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more and has high heat resistance.

Brief Description of the Drawings

[0025]

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[0026] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In each figure, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions are omitted. [Configuration of Quantum Cascade Laser Element]

[0027] As shown in FIG. 1, the quantum cascade laser device 10 includes a quantum cascade laser element 1. As shown in FIGS. 2 and 3, the quantum cascade laser element 1 includes a semiconductor substrate 2, a semiconductor laminate 3, an insulating film 4, a first electrode 5, a second electrode 6, and an antireflection film 7. The semiconductor substrate 2 is, for example, a rectangular plate-shaped N-type InP single crystal substrate. As an example, the length of the semiconductor substrate 2 is about 3 mm, the width of the semiconductor substrate 2 is about 500 μm, and the thickness of the semiconductor substrate 2 is about one hundred and several tens of μm. In the following description, the width direction of the semiconductor substrate 2 is referred to as the X-axis direction, the length direction of the semiconductor substrate 2 is referred to as the Y-axis direction, and the thickness direction of the 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 that is a wavelength in the mid-infrared region and any value between 7.5 and 16 μm. In the present 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 a multiple quantum well structure of InGaAs / InAlAs. Each of the lower cladding layer 32 and the upper cladding layer 33 is, for example, a Si-doped InP layer. Each of the lower guide layer and the upper guide layer is, for example, a Si-doped InGaAs layer. The contact layer is, for example, a Si-doped InGaAs layer.

[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 on the side opposite to the semiconductor substrate 2, and the lower guide layer, the active layer 31, the upper guide layer, the upper cladding layer 33, and the 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 a dozen or so 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. On both sides of the ridge portion 30 in the X-axis direction, there are no layers constituting the semiconductor laminate 3.

[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 a direction parallel to the Y-axis direction, which is the extending direction of the ridge portion 30. The first end face 3a and the second end face 3b function as light-emitting end faces. Each of the first end face 3a and the second end face 3b is located on the same plane as each of 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 face 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 on the side opposite to the semiconductor substrate 2 is exposed. The side face 30b of the ridge portion 30 is each of both side faces 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 on the side opposite to the semiconductor substrate 2 in the portion of the lower cladding layer 32 that does not constitute the ridge portion 30. In the present embodiment, the insulating film 4 is a CeO2 film.

[0033] The first electrode 5 is formed on the surface 3c of the semiconductor laminate 3 on the side opposite to the semiconductor substrate 2. The surface 3c of the semiconductor laminate 3 is a surface constituted by the surface 30a of the ridge portion 30, the side face 30b of the ridge portion 30, and the 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 face 30b of the ridge portion 30 and the surface 32a of the lower cladding layer 32. Thereby, 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 base layer 51 and a metal plating layer 52. The metal base layer 51 is formed so as to extend along the surface 3c of the semiconductor laminate 3. The metal base layer 51 is, for example, a Ti / Au layer. The metal plating layer 52 is formed on the metal base layer 51 such 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. The surface 52a of the metal plating layer 52 on the side 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 planarized by chemical mechanical polishing, and polishing marks are formed on the surface 52a of the metal plating layer 52. Note that the ridge portion 30 being embedded in the metal plating layer 52 means that the ridge portion 30 is covered with 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 the portion in the Z-axis direction) is larger than the thickness of the ridge portion 30 in the Z-axis direction.

[0035] The second electrode 6 is formed on the surface 2b of the semiconductor substrate 2 on the side opposite to the semiconductor laminate 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 cladding layer 32 via the semiconductor substrate 2.

[0036] The antireflection film 7 is formed on the first end face 3a. The antireflection film 7 has a function of suppressing the resonance of laser light at the first end face 3a, and a function of reducing the reflectance of laser light having a center wavelength of 7.5 μm or more when the laser light is emitted from the first end face 3a. In the present embodiment, the antireflection film 7 is formed so as to reach both the first electrode 5 and the second electrode 6 from the first end face 3a. More specifically, the antireflection film 7 reaches from the first end face 3a to the surface 5a of the first electrode 5 on the side opposite to the semiconductor laminate 3 via the surface of the first electrode 5 on the first end face 3a side. Further, the antireflection film 7 reaches from the first end face 3a to the surface 6a of the second electrode 6 on the side opposite to the semiconductor substrate 2 via the surfaces of the semiconductor substrate 2 and the second electrode 6 on the first end face 3a side, respectively. [Configuration of antireflection film]

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

[0038] As shown in FIG. 5, the antireflection film 7 is a single-layer film composed of only a single layer of CeO2 film 7a formed by continuous sputtering and vacuum evaporation. The CeO2 film 7a is formed directly on the first end face 3a (i.e., without passing through other films or the like). The CeO2 film 7a has sufficient transparency with respect to laser light having a central wavelength of 7.5 μm or more. An example of the specifications of the antireflection film 7 is as follows. That is, the refractive index of the CeO2 film 7a is 1.786, and the thickness of the CeO2 film 7a in the main body portion 70 is 1.26 μm. In this case, assuming that the central wavelength of the laser light oscillated by the semiconductor laminate 3 is λ = 9.0 μm, the effective refractive index of the optical waveguide structure constituted 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 portion 70 is t = 1.26 μm, then n = (n') 1 / 2 and t = λ / 4n is substantially satisfied. Therefore, the antireflection film 7 can reduce the reflectance of the laser light oscillated by the semiconductor laminate 3 to less than 0.1%. Note that "formation of at least one layer of CeO2 film by continuous sputtering and vacuum evaporation" 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 as it moves away from the first end face 3a in the optical waveguide direction A (see FIG. 3). The first end portion 71a has a first side face 71b that intersects the surface 5a of the first electrode 5. The first side face 71b is, for example, a flat face orthogonal 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 as it moves away from the first end face 3a in the optical waveguide direction A (see FIG. 3). The second end portion 72a has a second side face 72b that intersects the surface 6a of the second electrode 6. The second side face 72b is, for example, a flat face orthogonal 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 bonding member 12, a plurality of wires 15, and a driving portion 14.

[0041] The support portion 11 supports the quantum cascade laser element 1 in a state where the semiconductor substrate 2 is located on the support portion 11 side with respect to the semiconductor laminate 3 (that is, in an epi-side-up state). The support portion 11 includes a main body portion 111 and an electrode pad 112 formed on the main surface of the main body portion 111. The main body portion 111 is formed, for example, in a rectangular plate shape by AlN. The electrode pad 112 is, for example, a Ti / Pt / Au film or a Ti / Pd / Au film and is formed in a rectangular film shape. The support portion 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 portion 11 and the second electrode 6 of the quantum cascade laser element 1 in an epi-side-up state. The bonding member 12 is, for example, a solder member such as an AuSn member. The maximum value of the thickness of the bonding member 12 is smaller than the maximum value of the thickness of the second additional portion 72 of the antireflection film 7. The quantum cascade laser element 1 is supported by the support portion 11 such that the distance between the electrode pad 112 and the second electrode 6 decreases as it moves away 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 joining 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 joining 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, the edge on the first end face 3a side of the quantum cascade laser element 1 in the Y-axis direction coincides with one edge of the joining member 12 in the Y-axis direction. When viewed from the Z-axis direction, the edge on the second end face 3b side of the quantum cascade laser element 1 in the Y-axis direction is located inside the other edge of the joining member 12 in the Y-axis direction. When viewed from the Z-axis direction, both edges of the quantum cascade laser element 1 in the X-axis direction are located inside both edges of the joining member 12 in the X-axis direction.

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

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

[0046] In the quantum cascade laser device 10 configured as described above, when a driving voltage is applied to the active layer 31 of the quantum cascade laser element 1 by the driving unit 14, light is emitted from the active layer 31, and laser light having a center wavelength of 7.5 μm or more among the light is resonated in the distributed feedback structure. At this time, an antireflection film 7 having a function of reducing the reflectivity of laser light having a center wavelength of 7.5 μm or more is formed on the first end face 3a. As a result, laser light having a center wavelength of 7.5 μm or more is continuously oscillated from the first end face 3a through the antireflection film 7. [Method for manufacturing quantum cascade laser element]

[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 (that is, 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 a plurality of portions each becoming a semiconductor substrate 2 is formed on the surface of a semiconductor wafer. Subsequently, a part of the semiconductor layer is removed by etching so that each of the plurality of portions becoming the semiconductor laminate 3 in the semiconductor layer has a ridge portion 30. Subsequently, an insulating layer including a plurality of portions each becoming an insulating film 4 is formed on the semiconductor layer so that the surface 30a of each ridge portion 30 is exposed. Subsequently, a continuous metal underlayer including a plurality of portions each becoming a metal underlayer 51 is formed so as to cover the surface 30a of each ridge portion 30 and cover the insulating layer. Subsequently, a plurality of metal plating layers each becoming a metal plating layer 52 are formed on the continuous metal underlayer, and the ridge portions 30 are embedded in each metal plating layer. Subsequently, the surface of each metal plating layer is planarized by polishing to form a plurality of metal plating layers 52. Subsequently, a portion exposed between adjacent metal plating layers 52 in the continuous metal underlayer is removed by etching to form a plurality of 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 becoming a second electrode 6 is formed on the back surface of the semiconductor wafer.

[0049] When the wafer 100 is prepared as described above, the wafer 100 is cleaved along the X-axis direction, whereby a plurality of laser bars 200 are obtained 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 arranged linearly 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 and 200b facing each other in the Y-axis direction. The end face 200a includes a plurality of first end faces 3a arranged linearly along the X-axis direction, and the end face 200b includes a plurality of second end faces 3b arranged linearly along the X-axis direction.

[0050] Subsequently, as shown in Fig. 8(a), an antireflection layer 700 is formed on the surface of the portion 210 including the end face 200a of the laser bar 200 (the step of forming the antireflection layer 700 on the laser bar 200). The antireflection layer 700 includes a plurality of portions each of which becomes an antireflection film 7. Subsequently, by cleaving the laser bar 200 along the Y-axis direction, as shown in Fig. 8(b), the laser bar 200 and the antireflection layer 700 are divided for each of the plurality of portions 110, and a plurality of quantum cascade laser elements 1 are obtained (the step of dividing the laser bar 200 and the antireflection layer 700).

[0051] The formation of the antireflection 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 bar 300 in the Y-axis direction is shorter than the length of the laser bar 200 in the Y-axis direction. The length of the dummy bar 300 in the X-axis direction is equal to or greater than the length of the laser bar 200 in the X-axis direction. Subsequently, with the end face 200b of each laser bar 200 and the end face 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). Thereby, a bar unit 400 composed of a plurality of laser bars 200 and a plurality of dummy bars 300 is obtained. In the bar unit 400, the portion 210 of each laser bar 200 protrudes with respect to the end face 300a of the adjacent dummy bar 300 (the end face on the side opposite to the end face 300b in the Y-axis direction). In this state, "formation of at least one layer of CeO2 film by continuous sputtering and vacuum evaporation" is carried out, so that the antireflection layer 700 is formed on the surface of the portion 210 of each laser bar 200.

[0052] "Formation of at least one layer of CeO2 film by continuous sputtering and vacuum evaporation" means forming at least one layer of CeO2 film by continuously performing sputtering of CeO2 and vacuum evaporation of CeO2 on a target. The formation of at least one layer of CeO2 film by continuous sputtering and vacuum evaporation is carried out, for example, in a film forming apparatus 500 shown in FIG. 10. As shown in FIG. 10, the film forming apparatus 500 includes a holder 510, a sputtering mechanism 520, and a vacuum evaporation mechanism 530. The holder 510 rotates about an axis CL as a center line while holding the bar unit 400. The sputtering mechanism 520 performs sputtering of CeO2 on an end face 200a of the bar unit 400 introduced into a sputtering chamber (not shown) by the holder 510. The vacuum evaporation mechanism 530 performs vacuum evaporation of CeO2 on an end face 200a of the bar unit 400 introduced into a vacuum evaporation chamber (not shown) by the holder 510. In the film forming apparatus 500, the refractive index of at least one layer of CeO2 film formed by continuous sputtering and vacuum evaporation is adjusted for each layer of CeO2 film by adjusting the ratio of "the intensity of sputtering of CeO2 by the sputtering mechanism 520" to "the intensity of vacuum evaporation of CeO2 by the vacuum evaporation mechanism 530". That is, at least one layer of CeO2 film is formed by continuous sputtering and vacuum evaporation so as to have a desired thickness and a desired refractive index for each layer of CeO2 film. When "the intensity of vacuum evaporation of CeO2 by the vacuum evaporation mechanism 530" is stronger than "the intensity of sputtering of CeO2 by the sputtering mechanism 520", the refractive index of a layer of CeO2 film becomes smaller. Conversely, when "the intensity of vacuum evaporation of CeO2 by the vacuum evaporation mechanism 530" is weaker than "the intensity of sputtering of CeO2 by the sputtering mechanism 520", the refractive index of a layer of CeO2 film becomes larger.

[0053] In the following description, "formation of multiple layers of CeO₂ films by discrete sputtering and vacuum evaporation" means forming multiple layers of CeO₂ films by sequentially performing the formation of one layer of CeO₂ film by sputtering only and the formation of one layer of CeO₂ film by vacuum evaporation only. The formation of multiple layers of CeO₂ films by discrete sputtering and vacuum evaporation is carried out, for example, in the film forming apparatus 500 shown in FIG. 10 as follows. That is, the formation of one layer of CeO₂ film with the "intensity of sputtering of CeO₂ by the sputtering mechanism 520" set to 0 with respect to the "intensity of vacuum evaporation of CeO₂ by the vacuum evaporation mechanism 530", and the formation of one layer of CeO₂ film with the "intensity of sputtering of CeO₂ by the vacuum evaporation mechanism 530" set to 0 with respect to the "intensity of vacuum evaporation of CeO₂ by the sputtering mechanism 520" are sequentially carried out, whereby multiple layers of CeO₂ films are formed. In the formation of multiple layers of CeO₂ films by discrete sputtering and vacuum evaporation, if the formation of one layer of CeO₂ film by sputtering only and the formation of one layer of CeO₂ film by vacuum evaporation only are alternately carried out, it may be started and ended in any way and may be repeated any number of times.

[0054] Regarding at least one layer of CeO₂ film formed by continuous sputtering and vacuum evaporation and multiple layers of CeO₂ films formed by discrete sputtering and vacuum evaporation, it is approximately not practical to directly identify the CeO₂ film by its structure or properties. However, the refractive index of at least one layer of CeO₂ film formed by continuous sputtering and vacuum evaporation can be adjusted to a value different from each of the refractive indices of the CeO₂ film formed by sputtering only and the CeO₂ film formed by vacuum evaporation only. For example, it becomes a value greater than 1.65 and less than 2.1. Also, among the multiple layers of CeO₂ films formed by discrete sputtering and vacuum evaporation, the refractive index of the CeO₂ film formed by sputtering only is about 2.1, and the refractive index of the CeO₂ film formed by vacuum evaporation only is about 1.65.

[0055] FIG. 11 is a diagram showing an SEM photograph of a CeO2 film formed only by vacuum evaporation. As shown in FIG. 11, when forming a CeO2 film only by vacuum evaporation, if the thickness of the CeO2 film exceeds 500 nm, the refractive index changes in the thickness direction due to oxygen deficiency in the evaporation source, polycrystallization of the film, etc. Therefore, when forming a CeO2 film only by vacuum evaporation, it is preferable to set the thickness of the CeO2 film to 500 nm or less. [Function and Effect]

[0056] In the quantum cascade laser device 1, the antireflection film 7 includes at least one layer of CeO2 film 7a formed by continuous sputtering and vacuum evaporation. The refractive index of at least one layer of CeO2 film 7a formed by continuous sputtering and vacuum evaporation 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 3" compared with the refractive index of the CeO2 film formed only by sputtering and also compared with the refractive index of the CeO2 film formed only by vacuum evaporation. From this, according to the antireflection film 7, the reflectance of laser light having a center wavelength of 7.5 μm or more can be surely reduced. Further, according to the CeO2 film 7a, it is possible to ensure the transmittance for laser light having a center wavelength of 7.5 μm or more, prevent short-circuiting at the first end face 3a, and improve the adhesion to the first end face 3a. Moreover, the CeO2 film 7a has high heat resistance. As described above, according to the quantum cascade laser device 1, it is possible to realize an antireflection film 7 that effectively functions for laser light having a center wavelength of 7.5 μm or more and has high heat resistance. For the same reason, also by the quantum cascade laser device 10 including the quantum cascade laser device 1 and also by the manufacturing method of the quantum cascade laser device 1, it is possible to realize an antireflection film 7 that effectively functions for laser light having a center wavelength of 7.5 μm or more and has high heat resistance.

[0057] In the quantum cascade laser element 1, the antireflection film 7 includes a single-layer CeO2 film 7a which is at least one layer of CeO2 film formed by continuous sputtering and vacuum evaporation. Thereby, the configuration of the antireflection film 7 can be simplified.

[0058] In the quantum cascade laser element 1, the antireflection film 7 includes a second additional part 72 formed on the surface 6a of the second electrode 6. The second additional part 72 includes a second end part 72a having a thickness that becomes smaller as it moves away from the first end face 3a in the optical waveguide direction A. The second end part 72a has a second side face 72b that intersects the surface 6a of the second electrode 6. Thereby, when the second electrode 6 is joined to the electrode pad 112 of the support part 11 by the joining member 12, the molten joining member 12 is blocked by the second side face 72b of the second additional part 72 and repelled by the second additional part 72. Therefore, it is possible to suppress the molten joining member 12 from creeping toward the main body part 70 of the antireflection film 7.

[0059] Note that, even when the quantum cascade laser element 1 is supported by the support part 11 in the epi-down state described later, the same effect is obtained. That is, in the quantum cascade laser element 1, the antireflection film 7 includes a first additional part 71 formed on the surface 5a of the first electrode 5. The first additional part 71 includes a first end part 71a having a thickness that becomes smaller as it moves away from the first end face 3a in the optical waveguide direction A. The first end part 71a has a first side face 71b that intersects the surface 5a of the first electrode 5. Thereby, when the first electrode 5 is joined to the electrode pad 112 of the support part 11 by the joining member 12, the molten joining member 12 is blocked by the first side face 71b of the first additional part 71 and repelled by the first additional part 71. Therefore, it is possible to suppress the molten joining member 12 from creeping toward the main body part 70 of the antireflection film 7.

[0060] In the quantum cascade laser element 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. Thereby, for example, compared with the case where the insulating film 4 is a SiN film or a SiO2 film, the optical waveguide loss in the semiconductor laminate 3 can be reduced. Further, since the adhesion between the third additional portion 73 of the antireflection film 7 and the insulating film 4 is improved, peeling of the antireflection film 7 from the first end face 3a can be suppressed.

[0061] In the quantum cascade laser device 10, with the semiconductor substrate 2 positioned on the support portion 11 side with respect to the semiconductor laminate 3, the electrode pad 112 of the support portion 11 and the second electrode 6 are joined by a 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. Thereby, 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 face 72b of the second additional portion 72 and repelled by the second additional portion 72, so that the molten joining member 12 can be suppressed from creeping toward the main body portion 70 of the antireflection film 7.

[0062] In the quantum cascade laser device 10, the quantum cascade laser element 1 is supported by the support portion 11 such that the distance between the electrode pad 112 and the second electrode 6 decreases as the distance from the second additional portion 72 increases in the optical waveguide direction A. Thereby, 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 surely filled between the electrode pad 112 and the second electrode 6, so that the second electrode 6 can be surely joined to the electrode pad 112. [Modification Example]

[0063] The present invention is not limited to the above-described embodiments. For example, the antireflection film 7 may be at least one layer of CeO2 film 7a formed by continuous sputtering and vacuum deposition and may include a plurality of layers of CeO2 film 7a having different refractive indexes from each other. According to this, the reflectance of the laser light can be surely reduced in a desired wavelength range.

[0064] Further, the antireflection film 7 may include at least one layer of CeO2 film 7a formed by continuous sputtering and vacuum deposition and at least one layer of CeO2 film formed by sputtering. According to this, the reflectance of the laser light can be surely reduced in a desired wavelength range. As an example, as shown in FIGS. 12(a) and 12(b), the quantum cascade laser element 1 may include an antireflection film 7A or an antireflection film 7B composed only of "at least one layer of CeO2 film 7a formed by continuous sputtering and vacuum deposition" and "at least one layer of CeO2 film 7b formed by sputtering". The antireflection film 7A is a two-layer film composed only of "one layer of CeO2 film 7a" and "one layer of CeO2 film 7b". The antireflection film 7B is a multilayer film composed only of "two layers of CeO2 film 7a" and "two layers of CeO2 film 7b". The CeO2 film 7b is a CeO2 film formed only by sputtering and has sufficient transparency for laser light having a central wavelength of 7.5 μm or more.

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

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

[0067] FIG. 13 is a diagram showing the optical characteristics of a plurality of types of antireflection films 7, 7A, 7B (simulation results for an example of the specifications of each of the above-described antireflection films 7, 7A, 7B). As shown in FIG. 13, it was found that the reflectance of laser light having a central wavelength of 7.5 μm or more can be surely reduced by any of the antireflection films 7, 7A, 7B. In particular, it was found that the antireflection film 7 (a single-layer film composed of only one layer of CeO₂ film 7a) and the antireflection film 7A (a two-layer film composed of only "one layer of CeO₂ film 7a" and "one layer of CeO₂ film 7b") can reduce the reflectance for narrow-band laser light. On the other hand, it was found that the antireflection film 7B (a multilayer film composed of only "two layers of CeO₂ film 7a" and "two layers of CeO₂ film 7b") can reduce the reflectance for wide-band laser light. Note that the two-layer antireflection film 7A has an advantage that its thickness can be made thinner than that of the single-layer antireflection film 7.

[0068] Further, the antireflection film 7 may include a plurality of layers of CeO2 films formed by discrete sputtering and vacuum evaporation. According to this, the reflectance of the laser light can be surely reduced in a desired wavelength range. For example, the reflectance of broadband laser light can be reduced as compared with the case where the antireflection film 7 is composed of a single layer of CeO2 film 7a.

[0069] Also, as long as the antireflection film 7 includes at least one layer of CeO2 film, it may include other films other than the CeO2 film. As an example, the quantum cascade laser element 1 may include an antireflection film 7C including an Si film 7d and a single layer of CeO2 film 7a formed by continuous sputtering and vacuum evaporation, as shown in FIG. 14. In the antireflection film 7C, the first-layer Si film 7d is directly formed on the first end face 3a, and the second-layer CeO2 film 7a is directly formed on the first-layer Si film 7d. In this case, the adhesion between the semiconductor laminate 3, the Si film 7d, and the CeO2 film 7a can be improved.

[0070] From the above description, the antireflection film 7 only needs to include at least one of "at least one layer of CeO2 film 7a formed by continuous sputtering and vacuum evaporation" and "a plurality of layers of CeO2 films formed by discrete sputtering and vacuum evaporation". The refractive index of at least one layer of CeO2 film 7a formed by continuous sputtering and vacuum evaporation 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 3" when compared with the refractive index of the CeO2 film formed by sputtering only and also when compared with the refractive index of the CeO2 film formed by vacuum evaporation only. Further, in the case of a plurality of layers of CeO2 films formed by discrete sputtering and vacuum evaporation, a CeO2 film having a refractive index larger than "the square root of the effective refractive index of the optical waveguide structure constituted by the semiconductor laminate 3" and a CeO2 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 3" will be adjacent to each other. From these facts, according to the antireflection film 7, the reflectance of laser light having a center wavelength of 7.5 μm or more can be surely reduced. Further, according to each CeO2 film, it is possible to ensure the transmittance for laser light having a center wavelength of 7.5 μm or more, prevent short - circuiting at the first end face 3a, and improve the adhesion to the first end face 3a. Moreover, each CeO2 film has high heat resistance. As described above, if the antireflection film 7 includes at least one of "at least one layer of CeO2 film 7a formed by continuous sputtering and vacuum evaporation" and "a plurality of layers of CeO2 films formed by discrete sputtering and vacuum evaporation", an antireflection film 7 that effectively functions for laser light having a center wavelength of 7.5 μm or more and has high heat resistance is realized. In the method for manufacturing the quantum cascade laser element 1 as well, in the step of forming the antireflection layer 700 on the laser bar 200, at least one of "formation of at least one layer of CeO2 film 7a by continuous sputtering and vacuum evaporation" and "formation of a plurality of layers of CeO2 films by discrete sputtering and vacuum evaporation" may be performed on the end face 200a of the laser bar 200.

[0071] Further, as shown in FIG. 15, the quantum cascade laser device 10 may be such that the quantum cascade laser element 1 is supported by the support portion 11 in a state where the semiconductor laminate 3 is located on the support portion 11 side with respect to the semiconductor substrate 2 (i.e., in an epi-down state). In this case, at least one wire 15 is connected to the second electrode 6.

[0072] In the quantum cascade laser device 10 shown in FIG. 15, in the epi-down state, the electrode pad 112 of the support portion 11 and the first electrode 5 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 first additional portion 71. Thereby, 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 it is possible to suppress the molten joining member 12 from creeping toward the main body portion 70 of the antireflection film 7.

[0073] Furthermore, in the quantum cascade laser device 10 shown in FIG. 15, the quantum cascade laser element 1 is supported by the support portion 11 such that the distance between the electrode pad 112 and the first electrode 5 decreases as the distance from the first additional portion 71 increases in the optical waveguide direction A. Thereby, 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 surely filled between the electrode pad 112 and the first electrode 5, so that the first electrode 5 can be surely joined to the electrode pad 112. Note that the thickness of the portion of the joining member 12 disposed between the electrode pad 112 and the first electrode 5 is, for example, about several μm.

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

[0075] The quantum cascade laser element 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 laminated structure can be applied to the semiconductor laminate 3. As an example, in the semiconductor laminate 3, the upper guide layer may not have a diffraction grating structure that functions as a distributed feedback structure. Also, a ridge portion 30 may not be formed in the semiconductor laminate 3.

[0076] Also, the refractive index of the insulating film 4, which is a CeO2 film, may be adjusted by forming it by continuous sputtering and vacuum evaporation. Also, the insulating film 4 is not limited to the CeO2 film and may be a SiN film or a SiO2 film.

[0077] Further, 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 laminate 3. When, when viewed from the Z-axis direction, the outer edge of the metal base layer 51 of the first electrode 5 coincides with at least the first end face 3a and the second end face 3b, heat dissipation at the first end face 3a and the second end face 3b can be ensured.

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

[0079] Also, in the quantum cascade laser element 1, a metal film may be formed on the second end face 3b via an insulating film. Thereby, since the metal film functions as a reflective film, efficient light output can be obtained from the first end face 3a.

[0080] Also, the antireflection film 7 may include the main body portion 70 and 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 element 1 is supported by the support portion 11 in the 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 suppress the molten bonding member 12 from crawling toward the main body portion 70 of the antireflection film 7. Further, when the quantum cascade laser element 1 is supported by the support portion 11 in the 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 suppress the molten bonding member 12 from crawling 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 drive unit 14 may drive the quantum cascade laser element 1 so that the quantum cascade laser element 1 emits laser light in a pulsed manner.

Description of Reference Numerals

[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... Anti-reflection film, 7a, 7b... CeO2 film, 10... Quantum cascade laser device, 11... Support portion, 12... Joining member, 31... Active layer, 70... Main body portion, 71... First additional portion, 71a... First end portion, 71b... First side face, 72... Second additional portion, 72a... Second end portion, 72b... Second side face, 73... Third additional portion, 110... Portion, 112... Electrode pad, 200... Laser bar, 700... Anti-reflection layer, A... Optical waveguide direction.

Claims

1. A semiconductor substrate, a semiconductor laminate including an active layer having a quantum cascade structure, having a first end face and a second end face facing each other in the optical waveguide direction, and formed on the semiconductor substrate, a first electrode formed on the surface of the semiconductor laminate opposite to the semiconductor substrate, a second electrode formed on the surface of the semiconductor substrate opposite to the semiconductor laminate, and an antireflection 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 more, The antireflection film includes at least one layer of CeO formed by continuous sputtering and vacuum evaporation ２ film, and a quantum cascade laser device including at least one of a plurality of layers of CeO ２ film formed by discrete sputtering and vacuum evaporation.

2. The antireflection film is the at least one layer of CeO formed by the continuous sputtering and vacuum evaporation. ２ One layer of CeO film which is a film. ２ The quantum cascade laser device according to claim 1, including a CeO film.

3. The antireflection film is the at least one layer of CeO formed by the continuous sputtering and vacuum evaporation ２ film and includes a plurality of layers of CeO ２ films having different refractive indices from each other, the quantum cascade laser element according to claim 1.

4. The antireflection film further includes at least one layer of CeO film formed by sputtering. ２ The quantum cascade laser device according to claim 2 or 3, further comprising the film.

5. The antireflection film includes the plurality of layers of CeO films formed by the discrete sputtering and vacuum evaporation. ２ The quantum cascade laser device according to claim 1, including the film.

6. The antireflection film, includes a main body portion formed on the first end face, and a first additional portion formed on the surface of the first electrode opposite to the semiconductor laminate, wherein the first additional portion includes a first end portion having a thickness that decreases as it moves away from the first end face in the optical waveguide direction, and the first end portion has a first side face that intersects the surface of the first electrode. The quantum cascade laser element according to any one of Claims 1 to 5.

7. The antireflection film, includes 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, wherein the second additional portion includes a second end portion having a thickness that decreases as it moves away from the first end face in the optical waveguide direction, and the second end portion has a second side face that intersects the surface of the second electrode. The quantum cascade laser element according to any one of Claims 1 to 5.

8. further includes an insulating film formed on the surface of the semiconductor laminate, The insulating film is a CeO ２ film, and wherein the antireflection film, includes a main body portion formed on the first end face, and a third additional portion formed on the insulating film. The quantum cascade laser element according to any one of Claims 1 to 5.

9. The quantum cascade laser element according to Claim 7, a support portion for supporting the quantum cascade laser element, and a joining member for joining the electrode pad of the support portion and the second electrode in a state where the semiconductor substrate is located on the support portion side with respect to the semiconductor laminate, wherein, between the electrode pad and the second electrode, the maximum value of the thickness of the joining member is smaller than the maximum value of the thickness of the second additional portion. A quantum cascade laser device.

10. The quantum cascade laser device according to claim 9, wherein the quantum cascade laser element is supported by the support portion such that the distance between the electrode pad and the second electrode decreases as the distance from the second additional portion increases in the optical waveguide direction.

11. A quantum cascade laser element according to claim 6, a support portion for supporting the quantum cascade laser element, a joining member for joining the electrode pad of the support portion and the first electrode in a state where the semiconductor laminate is located on the support portion side with respect to the semiconductor substrate, and a quantum cascade laser device, wherein a maximum value of the thickness of the joining member is smaller than a maximum value of the thickness of the first additional portion between the electrode pad and the first electrode.

12. The quantum cascade laser device according to claim 11, 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.

13. A method for manufacturing a quantum cascade laser element according to any one of claims 1 to 8, a step of preparing a laser bar including a plurality of portions each serving as a set of the semiconductor substrate, the semiconductor laminate, the first electrode, and the second electrode, the plurality of portions being arranged one-dimensionally in a direction perpendicular to the optical waveguide direction, and In each of the plurality of portions of the laser bar, at least one layer of CeO is formed on the end face that serves as the first end face by continuous sputtering and vacuum evaporation, and a plurality of layers of CeO are formed by discrete sputtering and vacuum evaporation. ２ By performing at least one of film formation and film formation of a plurality of layers of CeO by discrete sputtering and vacuum evaporation, a step of forming an antireflection layer including a plurality of portions each serving as the antireflection film on the laser bar is performed. ２ ​ a step of dividing the laser bar and the antireflection layer for each of the plurality of portions.

Images