Patent Publication Number: US-2023133283-A1

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

Description:
TECHNICAL FIELD 
     The present disclosure relates a quantum cascade laser element, a quantum cascade laser device, and a method for manufacturing a quantum cascade laser device. 
     BACKGROUND ART 
     In the related art, a quantum cascade laser element has been known which includes a semiconductor substrate; a semiconductor laminate formed on the semiconductor substrate; a first electrode formed on a surface on an opposite side of the semiconductor laminate from the semiconductor substrate; and a second electrode formed on a surface on an opposite side of the semiconductor substrate from the semiconductor laminate, in which an anti-reflection film is formed on one end surface of a pair of end surfaces included in the semiconductor laminate including an active layer (for example, refer to Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Publication No. 2013-254764 
     SUMMARY OF INVENTION 
     Technical Problem 
     In recent years, there has been an increasing demand for a quantum cascade laser element capable of oscillating laser light having a center wavelength of 7.5 μm or more. For this reason, in the above-described quantum cascade laser element, it is desired that an anti-reflection film capable of reliably reducing the reflectance of laser light having a center wavelength of 7.5 μm or more and capable of securing sufficient durability is realized. 
     An object of the present disclosure is to provide a quantum cascade laser element including an anti-reflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more, a quantum cascade laser device, and a method for manufacturing a quantum cascade laser device. 
     Solution to Problem 
     A quantum cascade laser element according to one aspect of the present disclosure includes: a semiconductor substrate; a semiconductor laminate formed on the semiconductor substrate to include an active layer having a quantum cascade structure and to have a first end surface and a second end surface facing each other in a light waveguide direction; a first electrode formed on a surface on an opposite side of the semiconductor laminate from the semiconductor substrate; a second electrode formed on a surface on an opposite side of the semiconductor substrate from the semiconductor laminate; and an anti-reflection film formed on the first end surface. The semiconductor laminate is configured to oscillate laser light having a center wavelength of 7.5 μm or more. The anti-reflection film includes an insulating film being a CeO 2  film formed on the first end surface, a first refractive index film being a YF 3  film or a CeF 3  film disposed on a side opposite the first end surface with respect to the insulating film, and a second refractive index film formed on the first refractive index film on a side opposite the first end surface with respect to the first refractive index film and having a refractive index of larger than 1.8. 
     In the quantum cascade laser element, the anti-reflection film includes the first refractive index film that is a YF 3  film or a CeF 3  film and the second refractive index film having a refractive index of larger than 1.8. Accordingly, the reflectance of laser light having a center wavelength of 7.5 μm or more can be reliably reduced. Further, the anti-reflection film includes the insulating film that is a CeO 2  film formed on the first end surface. Accordingly, it is possible to realize securing a property of transmitting laser light having a center wavelength of 7.5 μm or more, preventing a short circuit on the first end surface, and improving the adhesion of the anti-reflection film to the first end surface. Therefore, sufficient durability of the anti-reflection film can be secured. As described above, according to the quantum cascade laser element, the anti-reflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     In the quantum cascade laser element according to one aspect of the present disclosure, the anti-reflection film may further include an intermediate film disposed between the insulating film and the first refractive index film, the intermediate film may be a ZnS film formed on the insulating film, the first refractive index film may be a YF 3  film formed on the intermediate film, and the second refractive index film may be a ZnS film. According to this aspect, adhesion between the films forming the anti-reflection film can be improved. 
     In the quantum cascade laser element according to one aspect of the present disclosure, the anti-reflection film may further include an intermediate film disposed between the insulating film and the first refractive index film, the intermediate film may be a ZnS film formed on the insulating film, the first refractive index film may be a CeF 3  film formed on the intermediate film, and the second refractive index film may be a ZnS film. According to this aspect, adhesion between the films forming the anti-reflection film can be improved. 
     In the quantum cascade laser element according to one aspect of the present disclosure, the first refractive index film may be a CeF 3  film formed on the insulating film, and the second refractive index film may be a ZnS film. According to this aspect, the configuration of the anti-reflection film can be simplified. 
     In the quantum cascade laser element according to one aspect of the present disclosure, a thickness of the insulating film may be 150 nm or less. According to this aspect, since the quality of the insulating film that is a CeO 2  film can be secured, as a result, the quality of each film forming the anti-reflection film can be secured. 
     A quantum cascade laser device according to one aspect of the present disclosure includes: the quantum cascade laser element; a support portion supporting the quantum cascade laser element; a joining member joining an electrode pad included in the support portion and the second electrode in a state where the semiconductor substrate is located on a support portion side with respect to the semiconductor laminate; a wire connected to the first electrode; and a drive unit electrically connected to each of the electrode pad and the wire, and configured to drive the quantum cascade laser element. The anti-reflection film is formed to reach at least one of the support portion and the wire from the first end surface. 
     In the quantum cascade laser device, the anti-reflection film is formed to reach at least one of the support portion and the wire from the first end surface. Accordingly, heat generated on the first end surface is easily released to at least one of the support portion and the wire that the anti-reflection film reaches, for example, as compared to a configuration in which the anti-reflection film reaches neither the support portion nor the wire. Therefore, according to the quantum cascade laser device, the anti-reflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     A quantum cascade laser device according to one aspect of the present disclosure includes: the quantum cascade laser element; a support portion supporting the quantum cascade laser element; a joining member joining an electrode pad included in the support portion and the first electrode in a state where the semiconductor laminate is located on a support portion side with respect to the semiconductor substrate; a wire connected to the second electrode; and a drive unit electrically connected to each of the electrode pad and the wire, and configured to drive the quantum cascade laser element. The anti-reflection film is formed to reach at least one of the support portion and the wire from the first end surface. 
     In the quantum cascade laser device, the anti-reflection film is formed to reach at least one of the support portion and the wire from the first end surface. Accordingly, heat generated on the first end surface is easily released to at least one of the support portion and the wire that the anti-reflection film reaches, for example, as compared to a configuration in which the anti-reflection film reaches neither the support portion nor the wire. Therefore, according to the quantum cascade laser device, the anti-reflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     In the quantum cascade laser device according to one aspect of the present disclosure, the drive unit may drive the quantum cascade laser element such that the quantum cascade laser element oscillates laser light in a pulsed manner. According to this aspect, laser light having a center wavelength of 7.5 μm or more can be oscillated through the anti-reflection film in a pulsed manner. In addition, heat generation on the first end surface can be suppressed, for example, as compared to a configuration in which the quantum cascade laser element continuously oscillates laser light. Therefore, the degradation of the anti-reflection film caused by heat generation on the first end surface can be suppressed. 
     A method for manufacturing the quantum cascade laser device according to one aspect of the present disclosure includes: a first step of preparing the quantum cascade laser element and the support portion including the electrode pad, of joining the electrode pad and the second electrode using the joining member in a state where the semiconductor substrate is located on the support portion side with respect to the semiconductor laminate, and of connecting the wire to the first electrode; and a second step of forming the anti-reflection film on the first end surface after the first step. 
     In the method for manufacturing the quantum cascade laser device, after the electrode pad of the support portion and the second electrode of the quantum cascade laser element are joined by the joining member, the anti-reflection film is formed on the first end surface of the quantum cascade laser element. Accordingly, a situation where heating applied when the electrode pad and the second electrode are joined causes the degradation of the anti-reflection film is avoided. Therefore, according to the method for manufacturing the quantum cascade laser device, the anti-reflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     A method for manufacturing the quantum cascade laser device according to one aspect of the present disclosure includes: a first step of preparing the quantum cascade laser element and the support portion including the electrode pad, of joining the electrode pad and the first electrode using the joining member in a state where the semiconductor laminate is located on the support portion side with respect to the semiconductor substrate, and of connecting the wire to the second electrode; and a second step of forming the anti-reflection film on the first end surface after the first step. 
     In the method for manufacturing the quantum cascade laser device, after the electrode pad of the support portion and the first electrode of the quantum cascade laser element are joined by the joining member, the anti-reflection film is formed on the first end surface of the quantum cascade laser element. Accordingly, a situation where heating applied when the electrode pad and the first electrode are joined causes the degradation of the anti-reflection film is avoided. Therefore, according to the method for manufacturing the quantum cascade laser device, the anti-reflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     In the method for manufacturing the quantum cascade laser device according to one aspect of the present disclosure, in the second step, the anti-reflection film may be formed to reach at least one of the support portion and the wire from the first end surface. According to this aspect, the quantum cascade laser device in which heat generated on the first end surface is easily released to at least one of the support portion and the wire that the anti-reflection film reaches can be easily and reliably obtained. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to provide the quantum cascade laser element including the anti-reflection film that effectively functions for laser light having a center wavelength of 7.5 μm or more, the quantum cascade laser device, and the method for manufacturing a quantum cascade laser device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    a cross-sectional view of a quantum cascade laser device of one embodiment. 
         FIG.  2    is a cross-sectional view of a quantum cascade laser element taken along line II-II shown in  FIG.  1   . 
         FIG.  3    is a cross-sectional view of the quantum cascade laser element taken along line III-III shown in  FIG.  2   . 
         FIG.  4    is an enlarged cross-sectional view of an anti-reflection film shown in  FIG.  1   . 
         FIG.  5    is a table showing one example of numerical values of each film forming the anti-reflection film shown in  FIG.  4   . 
         FIG.  6    is a view showing a method for manufacturing the quantum cascade laser device shown in  FIG.  1   . 
         FIG.  7    is a table showing one example of numerical values of each film forming an anti-reflection film of a first modification example. 
         FIG.  8    is an enlarged cross-sectional view of an anti-reflection film of a second modification example. 
         FIG.  9    is a table showing one example of numerical values of each film forming the anti-reflection film of the second modification example. 
         FIG.  10    is a cross-sectional view of a quantum cascade laser device according to a modification example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Incidentally, in the drawings, the same or equivalent portions are denoted by the same reference signs, and a duplicated description will be omitted. 
     Configuration of Quantum Cascade Laser Device 
     As shown in  FIG.  1   , a 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 , and a second electrode  6 . The semiconductor substrate  2  is, for example, an S-doped InP single crystal substrate having a rectangular plate shape. As one example, a length of the semiconductor substrate  2  is approximately 3 mm, a width of the semiconductor substrate  2  is approximately 500 μm, and a thickness of the semiconductor substrate  2  is approximately one hundred and several tens of μm. In the following description, a width direction of the semiconductor substrate  2  is referred to as an X-axis direction, a length direction of the semiconductor substrate  2  is referred to as a Y-axis direction, and a thickness direction of the semiconductor substrate  2  is referred to as a Z-axis direction. 
     The semiconductor laminate  3  is formed on a surface  2   a  of 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 one example, the semiconductor laminate  3  is configured to oscillate laser light having a center wavelength of any value of 7.5 to 16 μm that is a wavelength in a mid-infrared region. In the present embodiment, the semiconductor laminate  3  is formed by stacking a lower cladding layer  32 , a lower guide layer (not shown), the active layer  31 , an upper guide layer (not shown), an upper cladding layer  33 , and a contact layer (not shown) in order from a semiconductor substrate  2  side. The upper guide layer has a diffraction grating structure functioning as a distributed feedback (DFB) structure. 
     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. 
     The semiconductor laminate  3  includes a ridge portion  30  extending along the Y-axis direction. The ridge portion  30  is formed of a portion on an opposite side of the lower cladding layer  32  from the semiconductor substrate  2 , the lower guide layer, the active layer  31 , the upper guide layer, the upper cladding layer  33 , and the contact layer. A width of the ridge portion  30  in the X-axis direction is smaller than a width of the semiconductor substrate  2  in the X-axis direction. A length of the ridge portion  30  in the Y-axis direction is equal to a length of the semiconductor substrate  2  in the Y-axis direction. As one example, the length of the ridge portion  30  is approximately 3 mm, the width of the ridge portion  30  is approximately several μm to ten and several μm, and a thickness of the ridge portion  30  is approximately several μm. The ridge portion  30  is located at the center of the semiconductor substrate  2  in the X-axis direction. Each layer forming the semiconductor laminate  3  does not exist on both sides of the ridge portion  30  in the X-axis direction. 
     The semiconductor laminate  3  has a first end surface  3   a  and a second end surface  3   b  facing each other in a light waveguide direction A of the ridge portion  30 . The light waveguide direction A is a direction parallel to the Y-axis direction that is an extending direction of the ridge portion  30 . The first end surface  3   a  and the second end surface  3   b  function as light-emitting end surfaces. The first end surface  3   a  and the second end surface  3   b  are located on the same planes as those of both respective side surfaces of the semiconductor substrate  2  in the Y-axis direction. 
     The insulating film  4  is formed on side surfaces  30   b  of the ridge portion  30  and on a surface  32   a  of the lower cladding layer  32  such that a surface  30   a  on an opposite side of the ridge portion  30  from the semiconductor substrate  2  is exposed. The side surfaces  30   b  of the ridge portion  30  are both side surfaces of the ridge portion  30  facing each other in the X-axis direction. The surface  32   a  of the lower cladding layer  32  is a surface of a portion on an opposite side of the lower cladding layer  32  from the semiconductor substrate  2 , the portion not forming the ridge portion  30 . The insulating film  4  is, for example, a SiN film or a SiO 2  film. 
     The first electrode  5  is formed on a surface  3   c  on an opposite side of the semiconductor laminate  3  from the semiconductor substrate  2 . The surface  3   c  of the semiconductor laminate  3  is a surface formed of the surface  30   a  of the ridge portion  30 , the side surfaces  30   b  of the ridge portion  30 , and the surface  32   a  of the lower cladding layer  32 . When viewed in the Z-axis direction, an outer edge of the first electrode  5  is located inside outer edges of the semiconductor substrate  2  and the semiconductor laminate  3 . The first electrode  5  is in contact with the surface  30   a  of the ridge portion  30  on the surface  30   a  of the ridge portion  30  and is in contact with the insulating film  4  on the side surfaces  30   b  of the ridge portion  30  and on the surface  32   a  of the lower cladding layer  32 . Accordingly, the first electrode  5  is electrically connected to the upper cladding layer  33  through the contact layer. 
     The first electrode  5  includes a metal foundation layer  51  and a metal plating layer  52 . The metal foundation layer  51  is formed to extend along the surface  3   c  of the semiconductor laminate  3 . The metal foundation layer  51  is, for example, a Ti/Au layer. The metal plating layer  52  is formed on the metal foundation 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. A surface  52   a  on an opposite side of the metal plating layer  52  from the semiconductor substrate  2  is a flat surface perpendicular to the Z-axis direction. As one example, the surface  52   a  of the metal plating layer  52  is a polished surface that is flattened by chemical mechanical polishing, and polishing marks are formed on surface  52   a  of the metal plating layer  52 . Incidentally, the fact that the ridge portion  30  is 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 a thickness of portions of the metal plating layer  52  (thickness of the portions in the Z-axis direction) is larger than the thickness of the ridge portion  30  in the Z-axis direction, the portions being located on both sides of the ridge portion  30  in the X-axis direction. 
     The second electrode  6  is formed on a surface  2   b  on an opposite side of the semiconductor substrate  2  from 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  through the semiconductor substrate  2 . 
     As shown in  FIG.  1   , the quantum cascade laser device  10  further includes a support portion  11 , a joining member  12 , a plurality of wires  15 , and a pulse drive unit (drive unit)  14 . 
     The support portion  11  includes a body portion  111  and an electrode pad  112 . The support portion  11  is, for example, a sub-mount in which the body portion  111  is made of AlN. The support portion  11  supports the quantum cascade laser element  1  in a state where the semiconductor substrate  2  is located on a support portion  11  side with respect to the semiconductor laminate  3  (namely, an epi-side-up state). 
     The joining member  12  joins the electrode pad  112  of the support portion  11  and the second electrode  6  of the quantum cascade laser element  1  in the epi-side-up state. The joining member  12  is, for example, a solder member such as an AuSn member. A thickness of a portion of the joining member  12  disposed between the electrode pad  112  and the second electrode  6  is, for example, approximately several μm. 
     The plurality of wires  15  are connected to the first electrode  5 . Each of the wires  15  is formed by performing wire bonding on the first electrode  5  (more specifically, the surface  52   a  of the metal plating layer  52 ). At least one wire  15  may be connected to the first electrode  5 . 
     The pulse drive unit  14  is electrically connected to the electrode pad  112  and to each of the wires  15 . Namely, the pulse 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 pulse drive unit  14  drives the quantum cascade laser element  1  such that the quantum cascade laser element  1  oscillates laser light in a pulsed manner. A pulse width of the laser light is, for example, 20 to 1000 ns, a repetition frequency of the laser light is, for example, 1 to 1000 kHz, and a duty is, for example, 10% or less. 
     The quantum cascade laser element  1  further includes an anti-reflection film  7 . The anti-reflection film  7  is formed on the first end surface  3   a.  The anti-reflection film  7  has a function of suppressing the resonance of laser light on the first end surface  3   a  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 surface  3   a.  In the present embodiment, the anti-reflection film  7  is formed to reach both the support portion  11  and each of the wires  15  from the first end surface  3   a.  More specifically, the anti-reflection film  7  reaches a surface on a first end surface  3   a  side of the support portion  11  (namely, a surface on the first end surface  3   a  side of each of the electrode pad  112  and the body portion  111 ) from the first end surface  3   a  via a surface on the first end surface  3   a  side of each of the semiconductor substrate  2 , the second electrode  6 , and the joining member  12 . In addition, the anti-reflection film  7  reaches a surface on the first end surface  3   a  side of a connection end portion  15   a  of each of the wires  15  from the first end surface  3   a  via a surface on the first end surface  3   a  side of the first electrode  5 . Incidentally, in  FIGS.  2  and  3   , the illustration of the anti-reflection film  7  is omitted. 
     As shown in  FIG.  4   , the anti-reflection film  7  includes an insulating film  71 , an intermediate film  72 , a first refractive index film  73 , and a second refractive index film  74 . The insulating film  71  is a CeO 2  film formed on the first end surface  3   a  of the semiconductor laminate  3 . The intermediate film  72  is a ZnS film formed on the insulating film  71  on a side opposite the first end surface  3   a  with respect to the insulating film  71 . The first refractive index film  73  is a YF 3  film formed on the intermediate film  72  on a side opposite the first end surface  3   a  with respect to the intermediate film  72 . The second refractive index film  74  is a ZnS film formed on the first refractive index film  73  on a side opposite the first end surface  3   a  with respect to the first refractive index film  73 . The insulating film  71  is directly (namely, without another film or the like interposed therebetween) provided on the first end surface  3   a.  The intermediate film  72  is directly formed on the insulating film  71 . The first refractive index film  73  is directly formed on the intermediate film  72 . The second refractive index film  74  is directly formed on the first refractive index film  73 . 
       FIG.  5    is a table showing one example of numerical values of each film forming the anti-reflection film  7  shown in  FIG.  4   . In  FIG.  5   , the “wavelengths” in the “film thickness” column indicate center wavelengths of laser light oscillated by the semiconductor laminate  3 , and in the “film thickness” column, the thicknesses of each film that can reduce the reflectance of laser light of each “wavelength” to less than 0.1% are written. Here, the reason that the anti-reflection film  7  includes the first refractive index film  73  and the second refractive index film  74  will be described. 
     In an anti-reflection film including only one type of refractive index film, when a center wavelength of laser light is λ, a refractive index of the one type of refractive index film is n, and a thickness of the one type of refractive index film is t, if a relationship of t=λ/4n is satisfied, the anti-reflection film can reduce the reflectance of the laser light to less than 0.1%. Here, since n corresponds to a square root of a refractive index of the semiconductor laminate  3 , when the refractive index of the semiconductor laminate  3  is 3.2, n=(3.2) 1/2 ≈1.8. However, a material having a property of sufficiently transmitting light having a wavelength of 7.5 μm or more and having a refractive index of 1.8 does not exist. Therefore, in the anti-reflection film  7 , a low reflectance is realized by the first refractive index film  73  having a refractive index of smaller than 1.8 and the second refractive index film  74  having a refractive index of larger than 1.8. 
     Incidentally, the insulating film  71  may have a thickness that can realize securing a property of transmitting laser light having a center wavelength of 7.5 μm or more, preventing a short circuit on the first end surface  3   a,  and improving the adhesion of the anti-reflection film  7  to the first end surface  3   a.  In the present embodiment, from the viewpoint of securing the quality of the insulating film  71 , a thickness of the insulating film  71  is set to 150 nm or less. In addition, the intermediate film  72  may have a thickness that can realize improving adhesion between the insulating film  71  and the first refractive index film  73 . 
     In the quantum cascade laser device  10  configured as described above, when the pulse drive unit  14  applies a bias voltage to the active layer  31  of the quantum cascade laser element  1 , light is emitted from the active layer  31 , and laser light having a center wavelength of 7.5 μm or more of the light is oscillated in a distributed feedback structure. At this time, the anti-reflection film  7  having a function of reducing the reflectance of the laser light having a center wavelength of 7.5 μm or more is formed on the first end surface  3   a.  Accordingly, the laser light having a center wavelength of 7.5 μm or more is oscillated from the first end surface  3   a  through the anti-reflection film  7  in a pulsed manner. 
     Method for Manufacturing Quantum Cascade Laser Device 
     A method for manufacturing the quantum cascade laser device  10  described above will be described. First, as shown in (a) of  FIG.  6   , the quantum cascade laser element  1  and the support portion  11  are prepared. Subsequently, in the epi-side-up state, the electrode pad  112  of the support portion  11  and the second electrode  6  of the quantum cascade laser element  1  are joined by the joining member  12 , and the plurality of wires  15  are connected to the first electrode  5  of the quantum cascade laser element  1  (first step). 
     Subsequently, as shown in (b) of  FIG.  6   , the anti-reflection film  7  is formed on the first end surface  3   a  of the quantum cascade laser element  1  (second step). At this time, the anti-reflection film  7  is formed to reach both the support portion  11  and each of the wires  15  from the first end surface  3   a.  In the present embodiment, the anti-reflection film  7  is formed by forming each film by evaporation in order of the insulating film  71 , the intermediate film  72 , the first refractive index film  73 , and the second refractive index film  74 . Subsequently, as shown in  FIG.  1   , the pulse drive unit  14  is electrically connected to the electrode pad  112  and to each of the wires  15  to obtain the quantum cascade laser device  10 . 
     Incidentally, as one example, the quantum cascade laser element  1  is manufactured as follows. First, a semiconductor layer including a plurality of portions each of which becomes the semiconductor laminate  3  is formed on a surface of a semiconductor wafer including a plurality of portions each of which becomes the semiconductor substrate  2 . Subsequently, a part of the semiconductor layer is removed by etching such that the portions of the semiconductor layer each of which becomes the semiconductor laminate  3  includes the ridge portions  30 . Subsequently, an insulating layer including a plurality of portions each of which becomes the insulating film  4  is formed on the semiconductor layer such that the surface  30   a  of each of the ridge portions  30  is exposed. Subsequently, a metal foundation layer including a plurality of portions each of which becomes the metal foundation layer  51  is formed to cover the surface  30   a  of each of the ridge portions  30  and to cover the insulating layer. Subsequently, a plurality of metal plating layers each of which becomes the metal plating layer  52  is formed on the metal foundation layer, and the ridge portion  30  is embedded in each of the metal plating layers. Subsequently, a surface of each of the metal plating layers is flattened by polishing. Subsequently, the semiconductor wafer is thinned by polishing a back surface of the semiconductor wafer, and an electrode layer including a plurality of portions each of which becomes the second electrode  6  is formed on the back surface of the semiconductor wafer. Subsequently, a plurality of the quantum cascade laser elements  1  are obtained by cleaving the semiconductor wafer. 
     Actions and Effects 
     In the quantum cascade laser element  1 , the anti-reflection film  7  includes the first refractive index film  73  that is a YF 3  film as a refractive index film having a refractive index of smaller than 1.8, and the second refractive index film  74  that is a ZnS film as a refractive index film having a refractive index of larger than 1.8. Accordingly, the reflectance of laser light having a center wavelength of 7.5 μm or more can be reliably reduced. Further, the anti-reflection film  7  includes the insulating film  71  that is a CeO 2  film formed on the first end surface  3   a.  Accordingly, it is possible to realize securing a property of transmitting laser light having a center wavelength of 7.5 μm or more, preventing a short circuit on the first end surface  3   a,  and improving the adhesion of the anti-reflection film  7  to the first end surface  3   a.  Therefore, sufficient durability of the anti-reflection film  7  can be secured. As described above, according to the quantum cascade laser element  1 , the anti-reflection film  7  that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     Incidentally, for example, BaF 2 , MgF 2 , CaF 2 , and the like exist as a material having a refractive index of smaller than 1.8. However, since the roughness of a surface of a film is large when the film is made of BaF 2 , BaF 2  is not suitable as a material for the first refractive index film  73 . In addition, since MgF 2  and CaF 2  absorb light having a wavelength of 10 μm or more, MgF 2  and CaF 2  are not suitable as a material for the first refractive index film  73 . 
     In the quantum cascade laser element  1 , the intermediate film  72  is a ZnS film formed on the insulating film  71 , the first refractive index film  73  is a YF 3  film formed on the intermediate film  72 , and the second refractive index film  74  is a ZnS film formed on the first refractive index film  73 . Accordingly adhesion between the films forming the anti-reflection film  7  can be improved. 
     In the quantum cascade laser element  1 , the thickness of the insulating film  71  is 150 nm or less. Accordingly, since the quality of the insulating film  71  that is a CeO 2  film can be secured, as a result, the quality of each film forming the anti-reflection film  7  can be secured. Incidentally, when the insulating film  71  is formed by evaporation, the quality of the insulating film  71  degrades if the thickness of the insulating film  71  is more than 150 nm. 
     In the quantum cascade laser device  10 , the anti-reflection film  7  is formed to reach both the support portion  11  and each of the wires  15  from the first end surface  3   a  of the quantum cascade laser element  1 . 
     Accordingly, heat generated on the first end surface  3   a  is easily released to both the support portion  11  and each of the wires  15 , for example, as compared to a configuration in which the anti-reflection film  7  reaches neither the support portion  11  nor each of the wires  15 . Therefore, according to the quantum cascade laser device  10 , the anti-reflection film  7  that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     In the quantum cascade laser device  10 , the pulse drive unit  14  drives the quantum cascade laser element  1  such that the quantum cascade laser element  1  oscillates laser light in a pulsed manner. Accordingly, laser light having a center wavelength of 7.5 μm or more can be oscillated through the anti-reflection film  7  in a pulsed manner. In addition, heat generation on the first end surface  3   a  can be suppressed, for example, as compared to a configuration in which the quantum cascade laser element  1  continuously oscillates laser light. Therefore, the degradation of the anti-reflection film  7  caused by heat generation on the first end surface  3   a  can be suppressed. 
     In the method for manufacturing the quantum cascade laser device  10 , after the electrode pad  112  of the support portion  11  and the second electrode  6  of the quantum cascade laser element  1  are joined by the joining member  12 , the anti-reflection film  7  is formed on the first end surface  3   a  of the quantum cascade laser element  1 . Accordingly, a situation where heating applied when the electrode pad  112  and the second electrode  6  are joined causes the degradation of the anti-reflection film  7  is avoided. Therefore, according to the method for manufacturing the quantum cascade laser device  10 , the anti-reflection film  7  that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     In the method for manufacturing the quantum cascade laser device  10 , the anti-reflection film  7  is formed to reach both the support portion  11  and each of the wires  15  from the first end surface  3   a  of the quantum cascade laser element  1 . Accordingly, the quantum cascade laser device  10  in which heat generated on the first end surface  3   a  is easily released to both the support portion  11  and each of the wires  15  can be easily and reliably obtained. 
     Modification Examples 
     The present disclosure is not limited to the above-described embodiment. For example, a known quantum cascade structure can be applied to the active layer  31 . In addition, a known stack structure can be applied to the semiconductor laminate  3 . As one example, in the semiconductor laminate  3 , the upper guide layer may not have a diffraction grating structure functioning as a distributed feedback structure. 
     In addition, when viewed in the Z-axis direction, an outer edge of the metal foundation layer  51  of the first electrode  5  may coincide with the outer edges of the semiconductor substrate  2  and the semiconductor laminate  3 . Incidentally, when the outer edge of the metal foundation layer  51  of the first electrode  5  coincides with at least the first end surface  3   a  and the second end surface  3   b  when viewed in the Z-axis direction, heat dissipation on the first end surface  3   a  and on the second end surface  3   b  can be secured. 
     In addition, in the first electrode  5 , the metal plating layer  52  may not be flattened by polishing. In addition, the first electrode  5  may not include the metal plating layer  52  and may be, for example, a metal film formed to extend along the surface  3   c  of the semiconductor laminate  3 . 
     In addition, in the anti-reflection film  7  shown in  FIGS.  4  and  5   , the first refractive index film  73  may be a CeF 3  film instead of a YF 3  film (hereinafter, referred to as the “anti-reflection film  7  of a first modification example”). Even in the anti-reflection film  7  of the first modification example, for the same reason as when the first refractive index film  73  is a YF 3  film, the anti-reflection film  7  that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. In addition, adhesion between the films forming the anti-reflection film  7  can be improved.  FIG.  7    is a table showing one example of numerical values of each film forming the anti-reflection film  7  of the first modification example. In  FIG.  7   , the “wavelengths” in the “film thickness” column indicate center wavelengths of laser light oscillated by the semiconductor laminate  3 , and in the “film thickness” column, the thicknesses of each film that can reduce the reflectance of laser light of each “wavelength” to less than 0.1% are written. 
     In addition, the anti-reflection film  7  shown in  FIGS.  4  and  5    includes the intermediate film  72  disposed between the insulating film  71  and the first refractive index film  73 , but the anti-reflection film  7  may not include the intermediate film  72 . Namely, the first refractive index film  73  may be disposed on the side opposite the first end surface  3   a  with respect to the insulating film  71 . 
     As one example, as shown in  FIG.  8   , in the anti-reflection film  7 , the first refractive index film  73  may be a CeF 3  film formed on the insulating film  71  on the side opposite the first end surface  3   a  with respect to the insulating film  71 , and the second refractive index film  74  may be a ZnS film formed on the first refractive index film  73  on the side opposite the first end surface  3   a  with respect to the first refractive index film  73  (hereinafter, referred to as the “anti-reflection film  7  of a second modification example”). In the anti-reflection film  7  of the second modification example, the first refractive index film  73  is directly formed on the insulating film  71 , and the second refractive index film  74  is directly formed on the first refractive index film  73 . According to the anti-reflection film  7  of the second modification example, the configuration of the anti-reflection film  7  can be simplified.  FIG.  9    is a table showing one example of numerical values of each film forming the anti-reflection film  7  of the second modification example. In  FIG.  9   , the “wavelengths” in the “film thickness” column indicate center wavelengths of laser light oscillated by the semiconductor laminate  3 , and in the “film thickness” column, the thicknesses of each film that can reduce the reflectance of laser light of each “wavelength” to less than 0.1% are written. 
     Incidentally, sufficient adhesion between the insulating film  71  that is a CeO 2  film and the first refractive index film  73  that is a CeF 3  film can be secured. In addition, a mechanical strength of a CeF 3  film is smaller than a mechanical strength of a YF 3  film, but since the quantum cascade laser element  1  and the support portion  11  are disposed inside a housing, the mechanical strength of the CeF 3  film does not matter practically. 
     In addition, ZnSe, Si, Ge, and the like exist as the material of the second refractive index film  74  having a refractive index of larger than 1.8. However, since ZnSe is toxic, ZnSe is not suitable as a material for the second refractive index film  74 . 
     In addition, in the quantum cascade laser element  1 , a metal film may be formed on the second end surface  3   b  with an insulating film interposed therebetween. Accordingly, the metal film functions as a reflection film, so that an efficient light output from the first end surface is obtained. 
     In addition, as shown in  FIG.  10   , in the quantum cascade laser device  10 , the quantum cascade laser element  1  may be mounted on 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  (namely, an epi-side-down state). 
     A configuration of the quantum cascade laser device  10  shown in  FIG.  10    will be described. As shown in  FIG.  10   , the quantum cascade laser device  10  includes the quantum cascade laser element  1 , the support portion  11 , the joining member  12 , a plurality of the wires  15 , and the pulse drive unit  14 . 
     The support portion  11  includes the body portion  111  and the electrode pad  112 . The support portion  11  is, for example, a sub-mount in which the body portion  111  is made of AIN. The support portion  11  supports the quantum cascade laser element  1  in the epi-side-down state. 
     The joining member  12  joins the electrode pad  112  of the support portion  11  and the first electrode  5  of the quantum cascade laser element  1  in the epi-side-down state. The joining member  12  is, for example, a solder member such as an AuSn member. A thickness of a portion of the joining member  12  disposed between the electrode pad  112  and the first electrode  5  is, for example, approximately several μm. 
     The plurality of wires  15  are connected to the second electrode  6 . Each of the wires  15  is formed by performing wire bonding on the second electrode  6 . At least one wire  15  may be connected to the second electrode  6 . 
     The pulse drive unit  14  is electrically connected to the electrode pad  112  and to each of the wires  15 . Namely, the pulse 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 pulse drive unit  14  drives the quantum cascade laser element  1  such that the quantum cascade laser element  1  oscillates laser light in a pulsed manner. 
     A method for manufacturing the quantum cascade laser device  10  shown in  FIG.  10    will be described. First, the quantum cascade laser element  1  and the support portion  11  are prepared. Subsequently, in the epi-side-down state, the electrode pad  112  of the support portion  11  and the first electrode  5  of the quantum cascade laser element  1  are joined by the joining member  12 , and the plurality of wires  15  are connected to the second electrode  6  of the quantum cascade laser element  1  (first step). 
     Subsequently, the anti-reflection film  7  is formed on the first end surface  3   a  of the quantum cascade laser element  1  (second step). At this time, the anti-reflection film  7  is formed to reach both the support portion  11  and each of the wires  15  from the first end surface  3   a.  Subsequently, as shown in  FIG.  10   , the pulse drive unit  14  is electrically connected to the electrode pad  112  and to each of the wires  15  to obtain the quantum cascade laser device  10 . 
     As described above, in the quantum cascade laser device  10  shown in  FIG.  10   , the anti-reflection film  7  is formed to reach both the support portion  11  and each of the wires  15  from the first end surface  3   a  of the quantum cascade laser element  1 . Accordingly, heat generated on the first end surface  3   a  is easily released to both the support portion  11  and each of the wires  15 , for example, as compared to a configuration in which the anti-reflection film  7  reaches neither the support portion  11  nor each of the wires  15 . Therefore, according to the quantum cascade laser device  10  shown in  FIG.  10   , the anti-reflection film  7  that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     In the quantum cascade laser device  10  shown in  FIG.  10   , the pulse drive unit  14  drives the quantum cascade laser element  1  such that the quantum cascade laser element  1  oscillates laser light in a pulsed manner. Accordingly, laser light having a center wavelength of  7 . 5  μm or more can be oscillated through the anti-reflection film  7  in a pulsed manner. In addition, heat generation on the first end surface  3   a  can be suppressed, for example, as compared to a configuration in which the quantum cascade laser element  1  continuously oscillates laser light. Therefore, the degradation of the anti-reflection film  7  caused by heat generation on the first end surface  3   a  can be suppressed. 
     In the method for manufacturing the quantum cascade laser device  10  shown in  FIG.  10   , after the electrode pad  112  of the support portion  11  and the first electrode  5  of the quantum cascade laser element  1  are joined by the joining member  12 , the anti-reflection film  7  is formed on the first end surface  3   a  of the quantum cascade laser element  1 . Accordingly, a situation where heating applied when the electrode pad  112  and the first electrode  5  are joined causes the degradation of the anti-reflection film  7  is avoided. Therefore, according to the method for manufacturing the quantum cascade laser device  10  shown in  FIG.  10   , the anti-reflection film  7  that effectively functions for laser light having a center wavelength of 7.5 μm or more can be realized. 
     In the method for manufacturing the quantum cascade laser device  10  shown in  FIG.  10   , the anti-reflection film  7  is formed to reach both the support portion  11  and each of the wires  15  from the first end surface  3   a  of the quantum cascade laser element  1 . Accordingly, the quantum cascade laser device  10  in which heat generated on the first end surface  3   a  is easily released to both the support portion  11  and each of the wires  15  can be easily and reliably obtained. 
     Incidentally, in the quantum cascade laser device  10  shown in  FIG.  1    and in the quantum cascade laser device  10  shown in  FIG.  10   , a heat sink (not shown) is provided on the support portion  11  side. For this reason, in a configuration in which the quantum cascade laser element  1  is mounted on the support portion  11  in the epi-side-down state (epi-side-down configuration shown in  FIG.  10   ), heat dissipation of the semiconductor laminate  3  is easily secured as compared to a configuration in which the quantum cascade laser element  1  is mounted on the support portion  11  in the epi-side-up state (epi-side-up configuration shown in  FIG.  1   ). Therefore, when it is examined that the pulse drive unit  14  is replaced with a CW drive unit and the quantum cascade laser element  1  is driven such that the quantum cascade laser element  1  continuously oscillates laser light, the heat resistance of the anti-reflection film needs to be separately examined, but from the viewpoint of heat dissipation of the semiconductor laminate  3 , the epi-side-down configuration is effective. However, depending on conditions or the like, in the epi-side-up configuration or in the epi-side-down configuration, the quantum cascade laser element  1  is not limited to being driven to oscillate laser light in a pulsed manner. 
     In addition, in the quantum cascade laser device  10  shown in 
       FIG.  1    and in the quantum cascade laser device  10  shown in  FIG.  10   , the anti-reflection film  7  may be formed to reach at least one of the support portion  11  and each of the wires  15  from the first end surface  3   a.  Namely, in the method for manufacturing the quantum cascade laser device  10 , the anti-reflection film  7  may be formed to reach at least one of the support portion  11  and each of the wires  15  from the first end surface  3   a.  Accordingly, heat generated on the first end surface  3   a  is easily released to both the support portion  11  and each of the wires  15 , for example, as compared to a configuration in which the anti-reflection film  7  reaches neither the support portion  11  nor each of the wires  15 . 
     However, for example, when the amount of heat generated on the first end surface  3   a  is at a level that does not cause a problem, the anti-reflection film  7  may reach nor the support portion  11  nor the wires  15 . Namely, in the method for manufacturing the quantum cascade laser device  10 , the anti-reflection film  7  may be formed to reach neither the support portion  11  nor each of the wires  15  from the first end surface  3   a.    
     Various materials and shapes can be applied to each configuration in the above-described embodiment without being limited to the materials and shapes described above. In addition, each configuration in one embodiment or the modification examples described above can be arbitrarily applied to each configuration in another embodiment or modification example. 
     REFERENCE SIGNS LIST 
       1 : quantum cascade laser element,  2 : semiconductor substrate,  2   b:  surface,  3 : semiconductor laminate,  3   a:  first end surface,  3   b:  second end surface,  3   c:  surface,  5 : first electrode,  6 : second electrode,  7 : anti-reflection film,  10 : quantum cascade laser device,  11 : support portion,  12 : joining member,  14 : pulse drive unit (drive unit),  15 : wire,  31 : active layer,  71 : insulating film,  72 : intermediate film,  73 : first refractive index film,  74 : second refractive index film,  112 : electrode pad, A: light waveguide direction.