Patent Publication Number: US-2023142086-A1

Title: Quantum cascade laser element and quantum cascade laser device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a quantum cascade laser element and 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 a metal film is formed on one end surface of a pair of end surfaces included in the semiconductor laminate including an active layer, with an insulating film interposed therebetween (for example, refer to Patent Literature 1). In such a quantum cascade laser element, since the other end surface of the pair of end surfaces functions as a light-emitting surface while the metal film functions as a reflection film, an efficient light output can be obtained. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Publication No. 2019-9346 
     SUMMARY OF INVENTION 
     Technical Problem 
     When the above-described quantum cascade laser element is mounted on a support portion such as a sub-mount, the first electrode or the second electrode may be joined to an electrode pad of the support portion using a joining member such as a solder member. In that case, when the joining member adheres to the metal film, a light output characteristic of the quantum cascade laser element degrades, which is a concern. As a countermeasure to the degradation, it is conceived that the metal film is covered with an insulating member such that the joining member does not adhere to the metal film. However, in such a configuration, heat generated in the active layer is likely to be trapped, and as a result, the light output characteristic of the quantum cascade laser element degrades, which is a concern. 
     An object of the present disclosure is to provide a quantum cascade laser element and a quantum cascade laser device capable of obtaining an efficient light output while suppressing degradation of a light output characteristic. 
     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; an insulating film continuously formed from the second end surface to a region on a second end surface side of at least one surface of a surface on an opposite side of the first electrode from the semiconductor laminate and a surface on an opposite side of the second electrode from the semiconductor substrate; and a metal film formed on the insulating film to cover at least the active layer when viewed in the light waveguide direction. An outer edge of the metal film does not reach the one surface when viewed in the light waveguide direction. 
     In the quantum cascade laser element, the metal film is provided on the second end surface of the first end surface and the second end surface included in the semiconductor laminate, with the insulating film interposed therebetween. Accordingly, since the first end surface functions as a light-emitting surface while the metal film functions as a reflection film, an efficient light output is obtained. Further, the insulating film is continuously formed from the second end surface of the semiconductor laminate to the region on the second end surface side of at least one surface of the surface of the first electrode and the surface of the second electrode, and the outer edge of the metal film formed on the insulating film does not reach the one surface (namely, an electrode including a region in which the insulating film is formed (hereinafter, referred to as an “electrode around which the insulating film has wrapped”)) when viewed in the light waveguide direction. Accordingly, in order to mount the quantum cascade laser element on a support portion, when the electrode around which the insulating film has wrapped is joined to an electrode pad of the support portion using a joining member, the molten joining member is unlikely to reach the metal film. Moreover, heat generated in the active layer is unlikely to be trapped, for example, as compared to a configuration in which the metal film is covered with an insulating member. For these reasons, the degradation of a light output characteristic of the quantum cascade laser element is suppressed. As described above, according to the quantum cascade laser element, an efficient light output can be obtained while suppressing the degradation of the light output characteristic. 
     In the quantum cascade laser element according to one aspect of the present disclosure, the semiconductor laminate may include a ridge portion. According to this aspect, by the above-described configuration of the insulating film and the metal film, a reduction in the drive current of the quantum cascade laser element and a reduction in the electric power consumption of the quantum cascade laser element can be achieved while securing an efficient light output. At this time, the light density on each of the first end surface and the second end surface increases by the amount that the active layer is narrowed, but heat dissipation is secured by the above-described configuration of the insulating film and the metal film, so that the degradation of the light output characteristic of the quantum cascade laser element can be suppressed. 
     In the quantum cascade laser element according to one aspect of the present disclosure, a thickness of a portion of the metal film formed on the second end surface may be larger than a thickness of a portion of the insulating film formed on the second end surface. According to this aspect, heat dissipation on the second end surface on which the insulating film and the metal film are formed can be improved as compared to when the thickness relationship is reversed. 
     In the quantum cascade laser element according to one aspect of the present disclosure, the insulating film may be continuously formed from the second end surface to at least a region on the second end surface side of the surface of the first electrode, and the outer edge of the metal film may not reach the surface of the first electrode when viewed in the light waveguide direction. According to this aspect, since the first electrode that is an electrode around which the insulating film has wrapped is joined to the electrode pad of the support portion, the active layer can be disposed closer to the support portion as compared to when the second electrode is joined to the electrode pad of the support portion. Therefore, heat generated in the active layer can be efficiently released to a support portion side. 
     In the quantum cascade laser element according to one aspect of the present disclosure, a thickness of a portion of the first electrode corresponding to the active layer in a thickness direction of the semiconductor substrate may be larger than a thickness of a portion of the metal film formed on the second end surface. According to this aspect, when the first electrode that is an electrode around which the insulating film has wrapped is joined to the electrode pad of the support portion, heat generated in the active layer can be more efficiently released to the support portion side. 
     In the quantum cascade laser element according to one aspect of the present disclosure, the insulating film may be an Al 2 O 3  film or a CeO 2  film. According to this aspect, since the molten joining member is unlikely to get wet to the insulating film, the molten joining member can be more reliably prevented from reaching the metal film. 
     A quantum cascade laser device according to one aspect of the present disclosure includes: the quantum cascade laser element; and a drive unit configured to drive the quantum cascade laser element. 
     According to the quantum cascade laser device, an efficient light output can be obtained while suppressing the degradation of the light output characteristic. 
     The quantum cascade laser device according to one aspect of the present disclosure may further include a support portion supporting the quantum cascade laser element; and 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. The insulating film may be continuously formed from the second end surface to at least a region on the second end surface side of the surface of the first electrode, and the outer edge of the metal film may not reach the surface of the first electrode when viewed in the light waveguide direction. According to this aspect, heat generated in the active layer can be efficiently released to the support portion side. 
     In the quantum cascade laser device according to one aspect of the present disclosure, a thickness of a portion of the first electrode corresponding to the active layer in a thickness direction of the semiconductor substrate may be larger than a thickness of a portion of the joining member disposed between the electrode pad and the first electrode. According to this aspect, when the quantum cascade laser element is mounted on the support portion, the distance between the outer edge of the metal film and the surface of the first electrode when viewed in the light waveguide direction can be sufficiently secured such that the molten joining member can be more reliably prevented from reaching the metal film. 
     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 continuously oscillates laser light. When the quantum cascade laser element continuously oscillates laser light, the amount of heat generated in the active layer is increased as compared to when the quantum cascade laser element oscillates laser light in a pulsed manner, so that the above-described configuration of the quantum cascade laser element is particularly effective. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to provide the quantum cascade laser element and the quantum cascade laser device capable of obtaining an efficient light output while suppressing degradation of a light output characteristic. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    a cross-sectional view of a quantum cascade laser element of one embodiment. 
         FIG.  2    is a cross-sectional view of the quantum cascade laser element taken along line II-II shown in  FIG.  1   . 
         FIG.  3    is a view showing a method for manufacturing the quantum cascade laser element shown in  FIG.  1   . 
         FIG.  4    is a view showing the method for manufacturing the quantum cascade laser element shown in  FIG.  1   . 
         FIG.  5    is a view showing the method for manufacturing the quantum cascade laser element shown in  FIG.  1   . 
         FIG.  6    is a cross-sectional view of a quantum cascade laser device including the quantum cascade laser element shown in  FIG.  1   . 
         FIG.  7    is a cross-sectional view of a quantum cascade laser element according to a modification example. 
         FIG.  8    is a view showing a method for manufacturing the quantum cascade laser element shown in  FIG.  7   . 
         FIG.  9    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 Element 
     As shown in  FIGS.  1  and  2   , a 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 , an insulating film  7 , and a metal film  8 . 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 2 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 predetermined center wavelength (for example, a center wavelength of any value of 4 to 11 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 2 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 . 
     The insulating film  7  is continuously formed from the second end surface  3   b  of the semiconductor laminate  3  to a region  5   r  on a second end surface  3   b  side of a surface  5   a  of the first electrode  5  and to a region  6   r  on the second end surface  3   b  side of a surface  6   a  of the second electrode  6 . The surface  5   a  is a surface on an opposite side of the first electrode  5  from the semiconductor laminate  3  (in the present embodiment, the surface  52   a  of the metal plating layer  52 ). The surface  6   a  is a surface on an opposite side of the second electrode  6  from the semiconductor substrate  2 . In the present embodiment, the insulating film  7  is formed to extend along the second end surface  3   b  of the semiconductor laminate  3 , along a region  3   r  on the second end surface  3   b  side of the surface  3   c  of the semiconductor laminate  3 , along a side surface  5   b  on the second end surface  3   b  side of the first electrode  5 , along the region  5   r  on the second end surface  3   b  side of the surface  5   a  of the first electrode  5 , along a side surface  2   c  on the second end surface  3   b  side of the semiconductor substrate  2 , along a side surface  6   b  on the second end surface  3   b  side of the second electrode  6 , and along the region  6   r  on the second end surface  3   b  side of the surface  6   a  of the second electrode  6 . 
     The metal film  8  is formed on the insulating film  7  to cover the active layer  31  when viewed in the light waveguide direction A (namely, to include the active layer  31  when viewed in the light waveguide direction A). In the present embodiment, the metal film  8  is formed only on the insulating film  7  to extend along the second end surface  3   b  of the semiconductor laminate  3 , along the region  3   r  on the second end surface  3   b  side of the surface  3   c  of the semiconductor laminate  3 , along the side surface  2   c  on the second end surface  3   b  side of the semiconductor substrate  2 , along the side surface  6   b  on the second end surface  3   b  side of the second electrode  6 , and along the region  6   r  on the second end surface  3   b  side of the surface  6   a  of the second electrode  6 . An outer edge  80  of the metal film  8  does not reach the surface  5   a  of the first electrode  5  when viewed in the light waveguide direction A. In the present embodiment, a portion  80   a  on a first electrode  5  side of the outer edge  80  of the metal film  8  is located on the region  3   r  on the second end surface  3   b  side of the surface  3   c  of the semiconductor laminate  3 , and a portion  80   b  on a second electrode  6  side of the outer edge  80  of the metal film  8  is located on the region  6   r  on the second end surface  3   b  side of the surface  6   a  of the second electrode  6 . 
     A thickness of a portion of the metal film  8  formed on the second end surface  3   b  is larger than a thickness of a portion of the insulating film  7  formed on the second end surface  3   b.  A thickness of a portion of the first electrode  5  corresponding to the active layer  31  in the Z-axis direction is larger than a thickness of a portion of the metal film  8  formed on the second end surface  3   b.  The thickness of the portion of the insulating film  7  formed on the second end surface  3   b  is, for example, approximately 300 nm. The thickness of the portion of the metal film  8  formed on the second end surface  3   b  is, for example, approximately 500 nm. The thickness of the portion of the first electrode  5  corresponding to the active layer  31  in the Z-axis direction is, for example, 5 μm or more. Incidentally, when a thickness of a portion is not constant, the thickness of the portion means an average value of the thickness of the portion. 
     In the present embodiment, the insulating film  7  is an Al 2 O 3  film or a CeO 2  film, and the metal film  8  is an Au film. When the semiconductor laminate  3  is configured to oscillate laser light having a center wavelength of any value of 4 to 7.5 μm, it is preferable that the insulating film  7  is an Al 2 O 3  film having a property of transmitting light having a wavelength of 4 to 7.5 μm. When the semiconductor laminate  3  is configured to oscillate laser light having a center wavelength of any value of 7.5 to 11 μm, it is preferable that the insulating film  7  is a CeO 2  film having a property of transmitting light having a wavelength of 7.5 to 11 μm. When the semiconductor laminate  3  is configured to oscillate laser light having a center wavelength of any value of 4 to 11 μm, it is preferable that the metal film  8  is an Au film that effectively functions as a reflection film for reflecting light having a wavelength of 4 to 11 μm. 
     In the quantum cascade laser element  1  configured as described above, when a bias voltage is applied to the active layer  31  through the first electrode  5  and through the second electrode  6 , light is emitted from the active layer  31 , and light having a predetermined center wavelength of the light is oscillated in the distributed feedback structure. At this time, the metal film  8  formed on the second end surface  3   b  functions as a reflection film. Accordingly, the first end surface  3   a  functions as a light-emitting surface, and the laser light having the predetermined center wavelength is emitted from the first end surface  3   a.    
     Method for Manufacturing Quantum Cascade Laser Element 
     A method for manufacturing the quantum cascade laser element  1  described above will be described with reference to  FIGS.  3  and  4   . First, as shown in (a) of  FIG.  3   , a wafer  100  including a plurality of portions  110  each of which becomes the quantum cascade laser element  1  is formed. In the wafer  100 , the plurality of portions  110  are arranged in a matrix pattern in the X-axis direction as a row direction and in the Y-axis direction (namely, the light waveguide direction A of the portions  110  each of which becomes the quantum cascade laser element  1 ) as a column direction. As one example, the wafer  100  is manufactured by the following method. 
     Namely, a method for manufacturing the wafer  100  is a method including: a step of forming a semiconductor layer including a plurality of portions each of which becomes the semiconductor laminate  3  on a surface of a semiconductor wafer including a plurality of portions each of which becomes the semiconductor substrate  2 ; a step of removing a part of the semiconductor layer by etching such that the portions of the semiconductor layer each of which becomes the semiconductor laminate  3  include the ridge portions  30 ; a step of forming an insulating layer including a plurality of portions each of which becomes the insulating film  4  on the semiconductor layer such that the surface  30   a  of each of the ridge portions  30  is exposed; a step of forming a metal foundation layer including a plurality of portions each of which becomes the metal foundation layer  51 , to cover the surface  30   a  of each of the ridge portions  30  and to cover the insulating layer; a step of forming a plurality of metal plating layers each of which becomes the metal plating layer  52  on the metal foundation layer and of embedding the ridge portion  30  in each of the metal plating layers; a step of flattening a surface of each of the metal plating layers by polishing; and a step of thinning the semiconductor wafer by polishing a back surface of the semiconductor wafer and of forming an electrode layer including a plurality of portions each of which becomes the second electrode  6  on the back surface of the semiconductor wafer. 
     Subsequently, as shown in (b) of  FIG.  3   , a plurality of laser bars  200  are obtained by cleaving the wafer  100  along the X-axis direction. In each of the laser bars  200 , the plurality of portions  110  are one-dimensionally arranged along the X-axis direction. Each of the laser bars  200  has a pair of end surfaces  200   a  and  200   b  facing each other in the Y-axis direction. The end surface  200   a  includes a plurality of the first end surfaces  3   a  that are one-dimensionally arranged along the X-axis direction, and the end surface  200   b  includes a plurality of the second end surfaces  3   b  that are one-dimensionally arranged along the X-axis direction. 
     Subsequently, as shown in (a) of  FIG.  4   , an insulating layer  700  is formed on a surface of a portion  210  of the laser bar  200 , the portion  210  including the end surface  200   b,  and a metal layer  800  is formed on the insulating layer  700 . The insulating layer  700  is a layer including a plurality of portions each of which becomes the insulating film  7 , and the metal layer  800  is a layer including a plurality of portions each of which becomes the metal film  8 . Subsequently, as shown in (b) of  FIG.  4   , a plurality of the quantum cascade laser elements  1  are obtained by cleaving the laser bar  200  along the Y-axis direction. 
     The formation of the insulating layer  700  and the metal layer  800  on the laser bar  200  will be described with reference to  FIG.  5   . First, as shown in (a) of  FIG.  5   , a plurality of the laser bars  200  and a plurality of dummy bars  300  are prepared. A length of the dummy bars  300  in the Y-axis direction is smaller than a length of the laser bars  200  in the Y-axis direction. A length of the dummy bars  300  in the X-axis direction is equal to or larger than a length of the laser bars  200  in the X-axis direction. 
     Subsequently, in a state where the end surface  200   a  of each of the laser bars  200  and an end surface  300   a  of each of the dummy bars  300  (one end surface of each of the dummy bars  300  in the Y-axis direction) are located on the same plane, the laser bars  200  and the dummy bars  300  are alternately disposed to be 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). Accordingly, the portion  210  of each of the laser bars  200  protrudes from an end surface  300   b  of the dummy bar  300  adjacent thereto (the other end surface of each of the dummy bars  300  in the Y-axis direction). The insulating layer  700  is formed on the surface of the portion  210  of each of the laser bars  200  by performing sputtering of Al 2 O 3  or CeO 2  in this state. 
     Subsequently, as shown in (b) of  FIG.  5   , a plurality of dummy bars  400  are prepared. A length of the dummy bars  400  in the Y-axis direction is larger than the length of the dummy bars  300  in the Y-axis direction and is smaller than the length of the laser bars  200  in the Y-axis direction. A length of the dummy bars  400  in the X-axis direction is equal to or larger than the length of the laser bars  200  in the 
     X-axis direction. Incidentally, in (b) of  FIG.  5   , the illustration of the insulating layer  700  formed on the surface of the portion  210  of each of the laser bars  200  is omitted. 
     Subsequently, in a state where the end surface  200   a  of each of the laser bars  200  and an end surface  400   a  of each of the dummy bars  400  (one end surface of each of the dummy bars  400  in the Y-axis direction) are located on the same plane, the laser bars  200  and the dummy bars  400  are alternately disposed to be adjacent to each other in the Z-axis direction, and the plurality of laser bars  200  and the plurality of dummy bars  400  are held by a holding member (not shown). Accordingly, a portion of the portion  210  of each of the laser bars  200  protrudes from an end surface  400   b  of the dummy bar  400  adjacent thereto (the other end surface of each of the dummy bars  400  in the Y-axis direction), the portion including the end surface  200   b.  The metal layer  800  is formed on the insulating layer  700  by performing sputtering of Au in this state. Incidentally, before the metal layer  800  is formed on the insulating layer  700  by sputtering after the laser bars  200  and the dummy bars  400  are alternately disposed, a plasma activation treatment may be applied to a surface of the insulating layer  700  by so-called reverse sputtering in which ionized inert gas atoms for sputtering (for example, Ar ions) are caused to collide with the surface of the insulating layer  700  by reversing the discharge polarity with respect to a discharge polarity of the sputtering for forming the metal layer  800 . Accordingly, since the surface of the insulating layer  700  is cleaned, the adhesion of the metal layer  800  to the insulating layer  700  can be enhanced. 
     Configuration of Quantum Cascade Laser Device 
     A quantum cascade laser device  10 A including the quantum cascade laser element  1  described above will be described with reference to  FIG.  6   . As shown in  FIG.  6   , the quantum cascade laser device  10 A includes the quantum cascade laser element  1 , a support portion  11 , a joining member  12 , and a CW drive unit (drive unit)  13 . 
     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 AIN. The support portion  11  supports the quantum cascade laser element  1  in a state where the semiconductor laminate  3  is located on a support portion  11  side with respect to the semiconductor substrate  2  (namely, an 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. 
     In the quantum cascade laser device  10 A, the joining member  12  causes the insulating film  7  to wrap around the surface  5   a  of the first electrode  5  joined to the electrode pad  112  of the support portion  11 . However, the outer edge  80  of the metal film  8  does reach the surface  5   a  of the first electrode  5  when viewed in the light waveguide direction A. 
     The thickness of the portion of the first electrode  5  corresponding to the active layer  31  in the Z-axis direction is larger than a thickness of a portion of the joining member  12  disposed between the electrode pad  112  and the first electrode  5 . The thickness of the portion of the first electrode  5  corresponding to the active layer  31  in the Z-axis direction is, for example, 5 μm or more. The thickness of the portion of the joining member  12  disposed between the electrode pad  112  and the first electrode  5  is, for example, approximately 2 to 3 μm. 
     The CW drive unit  13  drives the quantum cascade laser element  1  such that the quantum cascade laser element  1  continuously oscillates laser light. The CW drive unit  13  is electrically connected to each of the electrode pad  112  of the support portion  11  and the second electrode  6  of the quantum cascade laser element  1 . In order to electrically connect the CW drive unit  13  to each of the electrode pad  112  and the second electrode  6 , wire bonding is performed on each of the electrode pad  112  and the second electrode  6 . 
     Actions and Effects 
     In the quantum cascade laser element  1 , the metal film  8  is provided on the second end surface  3   b  of the first end surface  3   a  and the second end surface  3   b  included in the semiconductor laminate  3 , with the insulating film  7  interposed therebetween. Accordingly, since the first end surface  3   a  functions as a light-emitting surface while the metal film  8  functions as a reflection film, an efficient light output is obtained. Further, the insulating film  7  is continuously formed from the second end surface  3   b  of the semiconductor laminate  3  to the region  5   r  on the second end surface  3   b  side of the surface  5   a  of the first electrode  5 , and the outer edge  80  of the metal film  8  formed on the insulating film  7  does not reach the surface  5   a  of the first electrode  5  when viewed in the light waveguide direction A. Accordingly, in order to mount the quantum cascade laser element  1  on the support portion  11 , when the first electrode  5  around which the insulating film  7  has wrapped is joined to the electrode pad  112  of the support portion  11  using the joining member  12 , the molten joining member  12  is unlikely to reach the metal film  8 . Moreover, heat generated in the active layer  31  is unlikely to be trapped, for example, as compared to a configuration in which the metal film  8  is covered with an insulating member. In addition, since the first electrode  5  around which the insulating film  7  has wrapped is joined to the electrode pad  112  of the support portion  11 , the active layer  31  can be disposed closer to the support portion  11  as compared to when the second electrode  6  is joined to the electrode pad  112  of the support portion  11 . Therefore, heat generated in the active layer  31  can be efficiently released to the support portion  11  side. For these reasons, the degradation of a light output characteristic of the quantum cascade laser element  1  is suppressed. As described above, according to the quantum cascade laser element  1 , an efficient light output can be obtained while suppressing the degradation of the light output characteristic. 
     Incidentally, when the quantum cascade laser element  1  is mounted on the support portion  11 , if the molten joining member  12  reaches the metal film  8 , the molten joining member  12  rapidly spread on the metal film  8 , which is a concern. If the molten joining member  12  spreads on the metal film  8 , for example, Sn contained in the joining member  12  diffuses into the metal film  8 , and the reliability of the quantum cascade laser element  1  decreases, which is a concern. In addition, if the molten joining member  12  adheres to the metal film  8 , the metal film  8  peels off from the insulating film  7  because of the shrinkage of the joining member  12  during curing, which is a concern. According to the configuration of the insulating film  7  and the metal film  8  in the quantum cascade laser element  1 , it is prevented that the molten joining member  12  adheres to the metal film  8  to cause a short circuit between the first electrode  5  and the second electrode  6 , and the above-described situation is prevented from occurring. 
     In the quantum cascade laser element  1 , the semiconductor laminate  3  includes the ridge portion  30 . Accordingly, by the above-described configuration of the insulating film  7  and the metal film  8 , a reduction in the drive current of the quantum cascade laser element  1  and a reduction in the electric power consumption of the quantum cascade laser element  1  can be achieved while suppressing an efficient light output. At this time, the light density on each of the first end surface  3   a  and the second end surface  3   b  increases by the amount that the active layer  31  is narrowed, but heat dissipation is secured by the above-described configuration of the insulating film  7  and the metal film  8 , so that the degradation of the light output characteristic of the quantum cascade laser element  1  can be suppressed. In addition, damage to the insulating film  7  caused by heat can be suppressed. 
     In the quantum cascade laser element  1 , the thickness of the portion of the metal film  8  formed on the second end surface  3   b  is larger than the thickness of the portion of the insulating film  7  formed on the second end surface  3   b.  Accordingly, heat dissipation on the second end surface  3   b  on which the insulating film  7  and the metal film  8  are formed can be improved as compared to when the thickness relationship is reversed. 
     In the quantum cascade laser element  1 , the thickness of the portion of the first electrode  5  corresponding to the active layer  31  in the Z-axis direction is larger than the thickness of the portion of the metal film  8  formed on the second end surface  3   b.  Accordingly, when the first electrode  5  around which the insulating film  7  has wrapped is joined to the electrode pad  112  of the support portion  11 , heat generated in the active layer  31  can be more efficiently released to the support portion  11  side. 
     In the quantum cascade laser element  1 , the insulating film  7  is an Al 2 O 3  film or a CeO 2  film. Accordingly, since the molten joining member  12  is unlikely to get wet to the insulating film  7 , the molten joining member  12  can be more reliably prevented from reaching the metal film  8 . 
     According to the quantum cascade laser device  10 A, by the above-described configuration of the quantum cascade laser element  1 , an efficient light output can be obtained while suppressing the degradation of the light output characteristic. 
     In the quantum cascade laser device  10 A, the thickness of the portion of the first electrode  5  corresponding to the active layer  31  in the Z-axis direction is larger than the thickness of the portion of the joining member  12  disposed between the electrode pad  112  and the first electrode  5 . Accordingly, when the quantum cascade laser element  1  is mounted on the support portion  11 , the distance between the outer edge  80  of the metal film  8  and the surface  5   a  of the first electrode  5  when viewed in the light waveguide direction A can be sufficiently secured such that the molten joining member  12  can be more reliably prevented from reaching the metal film  8 . 
     In the quantum cascade laser device  10 A, the CW drive unit  13  drives the quantum cascade laser element  1  such that the quantum cascade laser element  1  continuously oscillates laser light. When the quantum cascade laser element  1  continuously oscillates laser light, the amount of heat generated in the active layer  31  is increased as compared to when the quantum cascade laser element  1  oscillates laser light in a pulsed manner, so that the above-described configuration of the quantum cascade laser element  1  is particularly effective. 
     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, the insulating film  7  may be continuously formed from the second end surface  3   b  of the semiconductor laminate  3  to the region on the second end surface  3   b  side of at least one surface of the surface  5   a  of the first electrode  5  and the surface  6   a  of the second electrode  6 , and in that case, the outer edge  80  of the metal film  8  may not reach the one surface when viewed in the light waveguide direction A. In addition, the metal film  8  may be formed on the insulating film  7  to cover at least the active layer  31  when viewed in the light waveguide direction A. 
     As a modification example, as shown in  FIG.  7   , the outer edge  80  of the metal film  8  may reach neither the surface  5   a  of the first electrode  5  nor the surface  6   a  of the second electrode  6  when viewed in the light waveguide direction A. In the quantum cascade laser element  1  shown in  FIG.  7   , the metal film  8  is formed only on the insulating film  7  to extend along the second end surface  3   b  of the semiconductor laminate  3  and along the side surface  2   c  on the second end surface  3   b  side of the semiconductor substrate  2 . 
     A method for manufacturing the quantum cascade laser element  1  shown in  FIG.  7    is different from the method for manufacturing the quantum cascade laser element  1  shown in  FIGS.  1  and  2    in a way of forming the insulating layer  700  and the metal layer  800  on the laser bar  200 . As shown in (a) of  FIG.  8   , in the method for manufacturing the quantum cascade laser element  1  shown in  FIG.  7   , a plurality of the laser bars  200  and a plurality of the dummy bars  300  are prepared. A length of the dummy bars  300  in the Y-axis direction is smaller than a length of the laser bars  200  in the Y-axis direction. A length of the dummy bars  300  in the X-axis direction is equal to or larger than a length of the laser bars  200  in the X-axis direction. 
     Subsequently, in a state where the end surface  200   a  of each of the laser bars  200  and the end surface  300   a  of each of the dummy bars  300  are located on the same plane, the laser bars  200  and the dummy bars  300  are alternately disposed to be 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). Accordingly, the portion  210  of each of the laser bars  200  protrudes from the end surface  300   b  of the dummy bar  300  adjacent thereto. The insulating layer  700  is formed on the surface of the portion  210  of each of the laser bars  200  by performing sputtering of Al 2 O 3  or CeO 2  in this state. 
     Subsequently, as shown in (b) of  FIG.  8   , a plurality of dummy bars  500  are prepared. A length of the dummy bars  500  in the Y-axis direction is larger than the length of the laser bars  200  in the Y-axis direction. A length of the dummy bars  500  in the X-axis direction is equal to or larger than the length of the laser bars  200  in the X-axis direction. Incidentally, in (b) of  FIG.  8   , the illustration of the insulating layer  700  formed on the surface of the portion  210  of each of the laser bars  200  is omitted. 
     Subsequently, in a state where the end surface  200   a  of each of the laser bars  200  and an end surface  500   a  of each of the dummy bars  500  (one end surface of each of the dummy bars  500  in the Y-axis direction) are located on the same plane, the laser bars  200  and the dummy bars  500  are alternately disposed to be adjacent to each other in the Z-axis direction, and the plurality of laser bars  200  and the plurality of dummy bars  500  are held by a holding member (not shown). Accordingly, a portion of the portion  210  of each of the laser bars  200  is recessed with respect to an end surface  500   b  of the dummy bar  500  adjacent thereto (the other end surface of each of the dummy bars  500  in the Y-axis direction), the portion including the end surface  200   b.  The metal layer  800  is formed on the insulating layer  700  by obliquely performing sputtering of Au in this state. Incidentally, before the laser bars  200  and the dummy bars  500  are alternately disposed, a plasma activation treatment may be applied to a surface of the insulating layer  700  by so-called reverse sputtering in which ionized inert gas atoms for sputtering (for example, Ar ions) are caused to collide with the surface of the insulating layer  700  by reversing the discharge polarity with respect to a discharge polarity of the sputtering for forming the metal layer  800 . Accordingly, since the surface of the insulating layer  700  is cleaned, the adhesion of the metal layer  800  to the insulating layer  700  can be enhanced. 
     In addition, the insulating film  7  is not limited to an Al 2 O 3  film or a CeO 2  film, and the metal film  8  is not limited to an Au film. For example, the metal film  8  may be formed by stacking a Ti film and an Au film in order from an insulating film  7  side. In that case, a thickness of the Ti film is, for example, approximately 30 nm, and a thickness of the Au film is, for example, approximately 500 nm. 
     In addition, as shown in  FIG.  9   , the quantum cascade laser element  1  may be mounted on the support portion  11  in a state where the semiconductor substrate  2  is located on the support portion  11  side with respect to the semiconductor laminate  3  (namely, an epi-side-up state). In the quantum cascade laser element  1  shown in  FIG.  9   , the joining member  12  causes the insulating film  7  to wrap around the surface  6   a  of the second electrode  6  joined to the electrode pad  112  of the support portion  11 . However, the outer edge  80  of the metal film  8  does not reach the surface  6   a  of the second electrode  6  when viewed in the light waveguide direction A. 
     In the quantum cascade laser element  1  shown in  FIG.  9   , in order to mount the quantum cascade laser element  1  on the support portion  11 , when the second electrode  6  around which the insulating film  7  has wrapped is joined to the electrode pad  112  of the support portion  11  using the joining member  12 , the molten joining member  12  is unlikely to reach the metal film  8 . Moreover, heat generated in the active layer  31  is unlikely to be trapped, for example, as compared to a configuration in which the metal film  8  is covered with an insulating member. For these reasons, the degradation of a light output characteristic of the quantum cascade laser element  1  is suppressed. 
     Hereinafter, a quantum cascade laser device  10 B shown in  FIG.  9    will be described. As shown in  FIG.  9   , the quantum cascade laser device  10 B includes the quantum cascade laser element  1 , the support portion  11 , the joining member  12 , and a pulse drive unit (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-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 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, 50 to 500 ns, and a repetition frequency of the laser light is, for example, 1 to 500 kHz. The pulse drive unit  14  is electrically connected to each of the electrode pad  112  of the support portion  11  and the first electrode  5  of the quantum cascade laser element  1 . In order to electrically connect the pulse drive unit  14  to each of the electrode pad  112  and the first electrode  5 , wire bonding is performed on each of the electrode pad  112  and the first electrode  5 . 
     Incidentally, in the quantum cascade laser device  10 A shown in  FIG.  6    and in the quantum cascade laser device  10 B shown in  FIG.  9   , 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.  6   ), 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.  9   ). Therefore, when the quantum cascade laser element  1  is driven to continuously oscillate laser light, the epi-side-down configuration is effective. Particularly, when the semiconductor laminate  3  is configured to oscillate laser light having a relatively short center wavelength (for example, a center wavelength of any value of 4 to 6 μm in a range of 4 to 11 μm) in the mid-infrared region and the quantum cascade laser element  1  is driven to continuously oscillate the laser light, the epi-side-down configuration is effective. However, depending on conditions or the like, in the epi-side-down configuration, the quantum cascade laser element  1  is not limited to being driven to continuously oscillate laser light, and in the epi-side-up configuration, the quantum cascade laser element  1  is not limited to being driven to oscillate laser light in a pulsed manner. 
     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,  5   a : surface,  5   r : region,  6 : second electrode,  6   a : surface,  6   r : region,  7 : insulating film,  8 : metal film,  10 A,  10 B: quantum cascade laser device,  11 : support portion,  12 : joining member,  13 : CW drive unit (drive unit),  14 : pulse drive unit (drive unit),  30 : ridge portion,  31 : active layer,  80 : outer edge,  112 : electrode pad, A: light waveguide direction.