Patent Publication Number: US-2021167579-A1

Title: Surface emitting laser, surface emitting laser device, light source device, and detection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application No. 2019-217393, filed on Nov. 29, 2019, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure relate to a surface emitting laser, a surface emitting laser device, a light source device, and a detection apparatus. 
     Related Art 
     A vertical cavity surface emitting laser (VCSEL) is a semiconductor laser that oscillates a laser beam in a direction perpendicular to a substrate. The surface emitting laser has excellent characteristics capable of low-threshold current oscillation, single longitudinal mode oscillation, and arrangement in a two-dimensional array, unlike an edge-emission semiconductor laser that emits light in a direction parallel to a substrate. 
     SUMMARY 
     In one aspect of this disclosure, there is described a surface emitting laser including a first reflecting mirror; a second reflecting mirror; an active region between the first reflecting mirror and the second reflecting mirror. The first reflecting mirror and the second reflecting mirror each include a plurality of low refractive-index layers having a first refractive index; and a plurality of high refractive-index layers having a second refractive index higher than the first refractive index. The plurality of low refractive-index layers and the plurality of high refractive-index layers are alternated one after another. The plurality of high refractive-index layers of the first reflecting mirror includes a first layer; and a second layer having a higher thermal diffusion property in an in-plane direction than the first layer. 
     In another aspect of this disclosure, there is disclosed a surface emitting laser device including a mount substrate; and the surface emitting laser on the mount substrate. 
     In even another aspect of this disclosure, there is disclosed a light source device including the surface emitting laser device and a driver configured to drive the surface emitting laser device. 
     In still another aspect of this disclosure a detection apparatus including the light source device; and a photosensor configured to detect light emitted outside from the surface emitting laser and reflected by an object. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The aforementioned and other aspects, features, and advantages of the present disclosure would be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  illustrates a layout of a surface emitting laser according to an embodiment; 
         FIG. 2  is a cross-sectional view illustrating an internal structure of the surface emitting laser in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the surface emitting laser in  FIG. 1 ; 
         FIG. 4  is a schematic view of an example case where the surface emitting laser in  FIG. 1  is used; 
         FIG. 5  is a graph of Al composition and thermal conductivity in AlGaAs; 
         FIG. 6  is a cross-sectional view of the surface emitting laser in  FIG. 1  for describing a method of manufacturing the surface emitting laser; 
         FIG. 7  is another cross-sectional view of the surface emitting laser in  FIG. 1  for describing the method of manufacturing the surface emitting laser; 
         FIG. 8  is another cross-sectional view of the surface emitting laser in  FIG. 1  for describing the method of manufacturing the surface emitting laser; 
         FIG. 9  is another cross-sectional view of the surface emitting laser in  FIG. 1  for describing the method of manufacturing the surface emitting laser; 
         FIG. 10  is another cross-sectional view of the surface emitting laser in  FIG. 1  for describing the method of manufacturing the surface emitting laser; 
         FIG. 11  is another cross-sectional view of the surface emitting laser in  FIG. 1  for describing the method of manufacturing the surface emitting laser; 
         FIG. 12  is a cross-sectional view of a surface emitting laser according to another embodiment; 
         FIG. 13  is a cross-sectional view of an internal structure of a surface emitting laser according to another embodiment; 
         FIG. 14  is a cross-sectional view of a mesa structure of the surface emitting laser element in  FIG. 13 ; 
         FIG. 15  is an illustration of a layout of a surface emitting laser according to another embodiment; 
         FIG. 16  is a cross-sectional view of an internal structure of the surface emitting laser in  FIG. 15 ; 
         FIG. 17  is a cross-sectional view of the surface emitting laser in  FIG. 15 ; and 
         FIG. 18  is an illustration of a distance-measuring apparatus as a detection apparatus. 
     
    
    
     The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve similar results. 
     Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and all of the components or elements described in the embodiments of this disclosure are not necessarily indispensable. 
     Referring now to the drawings, embodiments of the present disclosure are described below. In the drawings for explaining the following embodiments, the same reference codes are allocated to elements (members or components) having the same function or shape and redundant descriptions thereof are omitted below. 
     The embodiments of the present disclosure provide a surface emitting laser that achieves higher heat dissipation while increasing the light-emission intensity, a surface emitting laser device incorporating the surface emitting laser, a light source device incorporating the surface emitting laser, and a detection device incorporating the surface emitting laser. 
     Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. Note that, in the specification and the drawings, components having substantially the same functional configuration are denoted by the same reference sign, and redundant description may be omitted. In the following description, a laser oscillation direction (an emission direction of a laser beam) is defined as a Z-axis direction, and two directions orthogonal to each other in a plane perpendicular to the Z-axis direction are defined as an X-axis direction and a Y-axis direction in the right-hand system. A positive Z-axis direction is defined as a downward direction. In the description, a plan view refers to a view in the Z-axis direction, that is, a view in a direction perpendicular to a substrate. However, the surface emitting laser or the like may be used in an upside down state, and may be disposed at any desired angle. 
     A movable device according to a first embodiment is described. The first embodiment relates to a surface emitting laser including a back-emitting surface emitting laser element. 
       FIG. 1  is an illustration of a layout of a surface emitting laser according to the first embodiment.  FIG. 2  is a cross-sectional view of an internal structure of the surface emitting laser in  FIG. 1 .  FIG. 2  is a cross-sectional view taken along line II-II in  FIG. 1 .  FIG. 3  is a cross-sectional view of the surface emitting laser in  FIG. 1 .  FIG. 3  is an enlarged view of a part of  FIG. 2 . 
     As illustrated in  FIG. 1 , a surface emitting laser  100  according to the first embodiment includes, for example, four surface emitting laser elements  151 . The four surface emitting laser elements  151  constitutes a laser element array  153  in which two surface emitting laser elements are arranged in each of the X-axis direction and the Y-axis direction. As illustrated in  FIGS. 2 and 3 , the surface emitting laser elements  151  each emit a laser beam LA toward aback surface  101 A side of a substrate  101 . In other words, the surface emitting laser  100  is configured to emit light through a lower reflecting mirror  102  (i.e., a second reflecting mirror) to be described later. Four pads  156  are provided around the laser element array  153  so as to correspond to the surface emitting laser elements  151 , respectively. 
     The pads  156  are electrically connected to the surface emitting laser elements  151 , respectively. In this arrangement, selecting a surface emitting laser element  151  to be powered between the surface emitting laser elements  151  changes a target surface emitting laser element  151  to emit light. In other words, the laser element array  153  is a 4-channel individually driven array. 
     The surface emitting laser  100  is a surface emitting laser with an oscillation wavelength of 940 nanometer (nm) band. As illustrated in  FIG. 2 , the surface emitting laser  100  includes the substrate  101 , a lower reflecting mirror  102  (i.e., the second reflecting mirror), a lower spacer layer  103 , an active region  104 , an upper spacer layer  105 , an upper reflecting mirror  106  (i.e., a first reflecting mirror), an insulator film  111 , a contact layer  107 , a p-side electrode  112 , an n-side electrode  113 , and an anti-reflection film  115 . 
     As an example, the substrate  101  is an n-GaAs single-crystal semiconductor substrate in which the normal direction of a mirror-polished surface of a surface (principal surface) is inclined by 15 degrees (θ=15 degrees) in a direction of a crystal orientation [111] A direction with respect to a crystal orientation [100] direction. In other words, the substrate  101  is so-called inclined substrate. Note that the substrate is not limited to the one described above. 
     The lower reflecting mirror  102  is stacked on the −Z side (upper side) of the substrate  101  via a buffer layer, and has about 26 pairs of a low refractive-index layer  102 L made of n-Al 0.9 Ga 0.1 As and a high refractive-index layer  102 H made of n-Al 0.2 Ga 0.8 As. A composition-graded layer having a thickness of 20 nm in which the composition gradually changes from one composition to the other composition is provided between the respective refractive-index layers to reduce the electrical resistance between the respective refractive-index layers. Each of the refractive-index layers includes ½ of the adjacent composition-graded layer, and has an optical thickness of λ/4, where λ denotes an oscillation wavelength. Note that when the optical thickness is λ/4, the actual thickness D of the layer is D=λ/4n (where n denotes a refractive index of a medium of that layer). 
     The lower spacer layer  103  is a layer that is stacked on the −Z side (i.e., upper side) of the lower reflecting mirror  102  and is made of non-doped Al 0.2 Ga 0.75 As. The material of the lower spacer layer  103  is not limited to non-doped Al 0.25 Ga 0.75 As, and may be, for example, non-doped AlGaInP. 
     The active region  104  is an active region that is stacked on the −Z side (i.e., the upper side) of the lower spacer layer  103  and has a structure of a multi-quantum well (MQW) including a plurality of quantum well layers and a plurality of barrier layers. The quantum well layers are made of InGaAs, and each barrier layer is made of AlGaAs. 
     The upper spacer layer  105  is a layer that is stacked on the −Z side (i.e., the upper side) of the active region  104  and is made of non-doped Al 0.25 Ga 0.75 As. Like the lower spacer layer  103 , the material of the upper spacer layer  105  is not limited to non-doped Al 0.25 Ga 0.75 As, and may be, for example, non-doped AlGaInP. 
     The portion including the lower spacer layer  103 , the active region  104 , and the upper spacer layer  105  is also referred to as a resonator structure, and the thickness thereof is an optical thickness corresponding to one wavelength. Note that the active region  104  is provided at the center of the resonator structure, which is a position corresponding to the antinode in the standing wave distribution of the electric field, so as to obtain a high induced emission rate. In one example, the thicknesses of the respective layers of the lower spacer layer  103 , the active region  104 , and the upper spacer layer  105  are set so that single longitudinal mode oscillation is obtained at an oscillation wavelength of 940 nm. In another example, the relative relationship (detuning) between the resonance wavelength and the emission wavelength (composition) of the active region  104  is adjusted so that the oscillation threshold current of the surface emitting laser element  151  is the smallest at room temperature. 
     The upper reflecting mirror  106  is stacked on the −Z side (upper side) of the upper spacer layer  105 , and has about 30 pairs of a low refractive-index layer made of p-Al 0.9 Ga 0.1 As and a high refractive-index layer made of p-Al 0.2 Ga 0.8 As. A composition-graded layer in which the composition gradually changes from one composition to the other composition is provided between the respective refractive-index layers to reduce the electrical resistance between the respective refractive-index layers. 
     In one of the low refractive-index layers  106 L of the upper reflecting mirror  106 , a selective oxide layer  108  made of p-AlAs and with a thickness of about 30 nm are inserted. The position of the selective oxide layer  108  corresponds to, for example, the second node from the active region  104  in the standing wave distribution of the electric field. The selective oxide layer  108  includes a non-oxidized region  108   b  and an oxidized region  108   a  surrounding the non-oxidized region  108   b . Any region other than the selective oxide layer  108  in the low refractive-index layers  106 L includes ½ of the adjacent composition-graded layer, and has an optical thickness of λ/4. 
     In one of the high refractive-index layer  106 H of the upper reflecting mirror  106 , a high thermal-conductive layer  109  made of p-GaAs is inserted. The high thermal-conductive layer  109  includes ½ of the adjacent composition-graded layer, and has an optical length of 3V4 where k denotes the oscillation wavelength. The high thermal-conductive layer  109  is provided, for example, on the opposite side of the active region  104  side relative to the selective oxide layer  108 . The high refractive-index layer  106 H except for the high thermal-conductive layer  109  includes ½ of the adjacent composition-graded layer, and has an optical length of λ/4 where λ denotes the oscillation wavelength. 
     The contact layer  107  is a layer that is stacked on the −Z side (i.e., upper side) of the upper reflecting mirror  106  and is made of p-GaAs. 
     The anti-reflection film  115  is formed on the +Z (lower side) surface (back surface  101 A) of the substrate  101 . The anti-reflection film  115  is a non-reflective coating film for the oscillation wavelength of 940 nm. 
     In the surface emitting laser element  151 , the stacked layers of the contact layer  107 , the upper reflecting mirror  106 , the upper spacer layer  105 , and the active region  104  has a mesa structure. The bottom portion of the mesa structure be in the middle of the resonator structure, and may be on the upper surface of the upper spacer layer  105 . The non-oxidized region  108   b  is located at the center of the mesa structure in plan view. 
     In the surface emitting laser element  151 , the insulator film  111  covers the stacked structure of the contact layer  107 , the upper reflecting mirror  106 , the upper spacer layer  105 , the active region  104 , and the lower spacer layer  103 . The insulator film  111  is, for example, a silicon nitride (SiN) film. An opening  111 A is formed in the insulator film  111  to expose a portion of the upper surface of the contact layer  107 . The p-side electrode  112  is formed on the insulator film  111 . The p-side electrode  112  is in contact with the upper surface of the contact layer  107  via the opening  111 A. The p-side electrode  112  includes, for example, a titanium (Ti) film, a platinum (Pt) film, and a gold (Au) film sequentially stacked on the −Z side (upper side). The p-side electrode  112  of the surface emitting laser element  151  is coupled to a p-side electrode of a driver integrated circuit (IC) or a submount by flip-chip mounting. 
     In each pad  156 , the stacked layers of the contact layer  107 , the upper reflecting mirror  106 , the upper spacer layer  105 , and the active region  104  has a mesa structure. Further, a groove  122  is formed in the stacked layers of the lower spacer layer  103  and the lower reflecting mirror  102  around the pad  156 . 
     In the pad  156 , the insulator film  111  covers the contact layer  107 , the upper reflecting mirror  106 , the upper spacer layer  105 , the active region  104 , the lower spacer layer  103 , the lower reflecting mirror  102 , and the substrate  101 . An opening  111 B is formed in the insulator film  111  to expose a portion of a surface  101 B of the substrate  101  at the bottom of the groove  122 . The n-side electrode  113  is formed on the insulator film  111 . Then-side electrode  113  is in contact with the surface  101 B of the substrate  101  inside the opening  111 B. The n-side electrode  113  is partly disposed at the −Z side (i.e., the upper side) relative to the upper reflecting mirror  106  in the pad  156 . The n-side electrode  113  includes, for example, a gold-germanium alloy (AuGe) film, a nickel (Ni) film, and a gold (Au) film sequentially stacked on the −Z side (upper side). The n-side electrode  113  is coupled to an n-side electrode of a driver IC or a submount by flip-chip mounting in the pad  156 . 
     The electric potential difference between the p-side electrode  112  and the n-side electrode  113  cause a voltage to be applied to the active region  104 . The surface emitting laser  100  may be packaged. 
     The surface emitting laser  100  is mounted on, for example, a submount.  FIG. 4  is a schematic view of an example case where the surface emitting laser  100  in  FIG. 1  is used. The submount and the surface emitting laser  100  mounted on the submount are included in a surface emitting laser device  1000 . 
     In this example of use, as illustrated in  FIG. 4 , the surface emitting laser  100  is mounted on a driver IC  300  by flip-chip mounting. The p-side electrode  112  of the surface emitting laser element  151  is electrically coupled to a p-side electrode provided on the driver IC  300  via a conductive body  301 . The n-side electrode  113  of the surface emitting laser element  151  is electrically coupled to an n-side electrode provided on the driver IC  300  via a conductive body  302  in the pad  156 . The surface emitting laser  100  is driven by the driver IC  300 . For example, the driver IC  300  has a larger area than the substrate  101 . The driver IC  300  is an example of a driver for a surface emitting laser. 
     The object on which the surface emitting laser  100  is mounted is not limited to the driver IC  300 . For example, the surface emitting laser  100  may be mounted on a submount. 
     The following describes the action and effects of the surface emitting laser  100 .  FIG. 5  is a graph of Al composition and thermal conductivity in AlGaAs. As illustrated in  FIG. 5 , GaAs used for the high thermal-conductive layer  109  has a higher thermal conductivity than Al 0.2 Ga 0.8 As used for another high refractive-index layer  106 H. Further, the high thermal-conductive layer  109  is thicker than said another high refractive-index layer  106 H. The high thermal-conductive layer  109  is more capable of diffusing heat in the in-plane direction (i.e., a direction perpendicular to the thickness direction) than said another high refractive-index layer  106 H. In other words, the high thermal-conductivity layer  109  has a higher thermal diffusion property in the in-plane direction than said another high refractive-index layer  106 H. In the first embodiment, the heat generated in the active region  104  first diffuses mainly in the thickness direction, that is, in the Z-axis direction. The heat diffused in the −Z direction (i.e., a direction toward the upper side) reaches the high thermal-conductive layer  109 , diffuses in the in-plane direction of the high thermal-conductive layer  109 , and further diffuses in the −Z direction (i.e., direction toward the upper side). In this way, the heat generated in the active region  104  is widely diffused and released to the outside of the surface emitting laser. 
     As the p-side electrode  112  is connected to the p-side electrode on the driver IC  300  via the conductive body  301 , the heat that has reached the p-side electrode  112  is also transferred to the driver IC  300 . The heat that has reached the driver IC  300  is diffused much more widely because the driver IC  300  is wider than the substrate  101 . 
     The configuration according to the first embodiment exhibits better heat dissipation property. 
     As illustrated in  FIG. 5 , AlAs has a higher thermal conductivity than GaAs. In view of such thermal properties, AlAs is used for the low refractive-index layers  106 L to increase the thermal conductivity. However, if AlAs is used for the low refractive-index layers  106 L AlAs would be oxidized during the manufacturing process to be described below, and thus the property of the surface emitting laser elements  151  might decrease. In addition, AlAs are easily corroded, which might reduce reliability of the surface emitting laser elements  151 . In view of this, a material having a high Al composition and good thermal conductivity, such as AlAs, cannot be used for the low refractive-index layers  106 L. 
     In AlGaAs, as the Al composition is lower, the band gap becomes lower and the AlGaAs is more likely to absorb light with a wavelength of 940 nm. Further, the electric field intensity become higher as its position is closer to the active region  104 . In particular, when the resonator structure has an optical length of nλ (n is a natural number), the electric field intensity is higher because the resonator structure has its end at the antinode of the standing wave. For this reason, if the high thermal-conductive layer  109  of a low Al composition is closer to the active region  104  than the upper reflecting mirror  106 , the high thermal-conductive layer  109  would be more likely to absorb light generated in the active region  104 . In the first embodiment, however, the high thermal-conductive layer  109  is disposed within the upper reflecting mirror  106  to be away from the active region  104 . This configuration enables the high thermal-conductive layer  109  to be less likely to absorb the light. Thus, the optical loss due to the light absorption is reduced or prevented, and the surface emitting laser can emit light with a higher intensity. 
     A method of manufacturing the surface emitting laser  100  is described next. Note that a structure in which a plurality of semiconductor layers are stacked on the substrate  101  as described above is also referred to as a stacked layers or a stacked body in the following description.  FIGS. 6 to 11  are cross-sectional views for describing a method of manufacturing the surface emitting laser  100  according to the first embodiment. 
     First, as illustrated in  FIG. 6 , the stacked body is formed by crystal growth by a metal organic chemical vapor deposition (MOCVD) method or a molecular beam epitaxy (MBE) method. 
     In the case of the MOCVD method, trimethyl aluminum (TMA), trimethyl gallium (TMG), or trimethyl indium (TMI) is used as the raw material of the group III; and phosphine (PH 3 ) or arsine (AsH 3 ) is used as the raw material of the group V. Carbon tetrabromide (CBr 4 ) or dimethyl zinc (DMZn) is used as the raw material of the p-type dopant; and hydrogen selenide (H 2 Se) is used as the raw material of the n-type dopant. 
     As illustrated in  FIG. 7 , a mesa structure is formed in the regions corresponding to the surface emitting laser elements  151  and the regions corresponding to the pads  156  by etching the contact layer  107 , the upper reflecting mirror  106 , the upper spacer layer  105 , and the active region  104 . As the etching, for example, inductively coupled plasma (ICP) dry etching or electron cyclotron resonance (ECR) dry etching can be performed. 
     Thereafter, as illustrated in  FIG. 8 , the stacked layers are subjected to heat treatment in water vapor. As a result, Al (aluminum) in the selective oxide layer  108  is selectively oxidized from the outer peripheral portion of the mesa structure, and the non-oxidized region  108   b  surrounded by the oxidized region  108   a  of Al remains in the center portion of the mesa structure. That is, a so-called oxidized confinement structure is formed to restrict the path of the driving current of a light emitter to the center portion of the mesa structure. The non-oxidized region  108   b  is a current passing region. 
     When the low refractive-index layers  106 L is formed from material, such as AlAs that is high Al composition with a high thermal conductivity, the low refractive-index layers  106 L might be oxidized like the selective oxide layer  108  during the formation of the oxidized region  108   a . The high thermal-conductive layer  109 , however, which is formed from GaAs, is not oxidized but remains unchanged as illustrated in  FIG. 8 . 
     As illustrated in  FIG. 9 , a groove  122  is formed by etching the lower spacer layer  103  and the lower reflecting mirror  102  around the pad  156 . The etching for forming the groove  122  after the selective oxidation of the selective oxide layer  108  prevents damage to the selective oxide layer  108  before the selective oxidation. 
     As illustrated in  FIG. 10 , the insulator film  111  is formed on the entire surface on the front surface  101 B side of the substrate  101 . The insulator film  111  is formed by, for example, a chemical vapor deposition (CVD) method. Subsequently, the openings  111 A and  111 B are formed in the insulator film  111 . The openings  111 A and  111 B are formed by wet etching using, for example, buffered hydrofluoric acid (BHF). 
     As illustrated in  FIG. 11 , the p-side electrode  112  is formed in the region corresponding to the surface emitting laser elements  151 , and the n-side electrode  113  is formed in the region corresponding to the pad  156 . The p-side electrode  112  and the n-side electrode  113  are formed by, for example, a lift-off method. Either of the p-side electrode  112  and the n-side electrode  113  may be formed first. To form the p-side electrode  112  and the n-side electrode  113 , after the film formation, heat treatment is performed in a reducing atmosphere or an inert atmosphere, and ohmic conduction is established by the eutectic reaction of the semiconductor material and the electrode material. 
     Thereafter, the back surface  101 A of the substrate  101  is polished and mirror-finished, and the anti-reflection film  115  is formed on the back surface  101 A (see  FIG. 2 ). 
     In this way, the surface emitting laser  100  is manufactured. 
     The high thermal-conductive layer  10 ) may be disposed closer to the active region  104  than the selective oxide layer  108 , or may be farther away from the active region  104  than the selective oxide layer  108 . 
     To achieve intended performance, at least one set of one low refractive-index layer  106 L and another high refractive-index layer  106 H is disposed between the high thermal-conductive layer  109  and the active region  104 . This arrangement enables the high thermal-conductive layer  109  to be separate from the active region  104  so as to prevent light absorption of the high thermal-conductive layer  109  more effectively. 
     The composition of the high thermal-conductive layer  109  is not limited to GaAs and may be AlGaAs having a lower Al composition than another high refractive-index layer  106 H. For example, when another high refractive-index layer  106 H is formed from material of Al 0.2 Ga 0.8 As, the high thermal-conductive layer  109  may have a composition of Al 0.1 Ga 0.9 As or Al 0.05 Ga 0.95 As. 
     When the high thermal-conductive layer  109  is thicker than another high refractive-index layer  106 H, the high thermal-conductive layer  109  may be formed from material with the approximately same thermal conductivity as the material of the high refractive-index layer  106 H. When the high thermal-conductive layer  109  has a higher thermal conductivity in material than the high refractive-index layer  106 H, the high thermal-conductive layer  109  may have approximately same thickness as another high refractive-index layer  106 H. The high thermal-conductive layer  109  may have an optical thickness of (2n+1) λ/4, for example, where n is a natural number. In the present disclosure, when the composition gradient layer is provided adjacent to the high thermal-conductive layer  109 , the optical thickness of the high thermal-conductive layer  109  includes ½ of the thickness of the composition gradient layer. 
     The number of the high thermal-conductive layer  109  is not limited to one, and a plurality of high thermal-conductive layers  109  may be included in the upper reflecting mirror  106 . 
     The number of the surface emitting laser elements  151  is not limited. Further, the present disclosure is not limited to the configuration in which a plurality of surface emitting laser elements  151  are individually driven. In some examples, a plurality of surface emitting laser elements  151  is collectively driven. One pad  156  may be provided for the plurality of surface emitting laser elements  151 . For example, one pad  156  may be commonly provided for four surface emitting laser elements  151 . 
     In some example, an electrode film is replaced with the pad  156 , to be provided on the back surface  101 A from which a laser beam LA is emitted. 
     The following describes a second embodiment. The second embodiment differs from the first embodiment in that the lower reflecting mirror also includes a high thermal-conductive layer.  FIG. 12  is a cross-sectional view of the surface emitting laser in  FIG. 2 . 
     In the second embodiment of  FIG. 12 , one of the high refractive-index layer  102 H of the upper reflecting mirror  102  includes a high thermal-conductive layer  209  made of n-GaAs inserted. The high thermal-conductive layer  209  includes ½ of the adjacent composition-graded layer, and has an optical length of 3λ/4 where λ denotes the oscillation wavelength. The high refractive-index layer  102 H except for the high thermal-conductive layer  209  includes ½ of the adjacent composition-graded layer, and has an optical length of λ/4 where λ denotes the oscillation wavelength. 
     The other configurations are similar to those in the first embodiment. 
     In the second embodiment, the heat generated in the active region  104  first diffuses mainly in the thickness direction, that is, in the Z-axis direction. Same as in the first embodiment, the heat diffused in the −Z direction (i.e., direction toward the upper side) reaches the high thermal-conductive layer  109 , diffuses in the in-plane direction of the high thermal-conductive layer  109 , and further diffuses in the −Z direction (i.e., direction toward the upper side). The heat diffused in the +Z direction (i.e., direction toward the lower side) reaches the high thermal-conductive layer  209 , and diffuses in the in-plane direction of the high thermal-conductive layer  209 , and further diffuses in the +Z direction (i.e., direction toward the lower side). In this way, the heat generated in the active region  104  is widely diffused and released to the outside of the surface emitting laser. 
     The configuration according to the second embodiment exhibits better heat dissipation property. 
     To achieve intended performance, at least one set of one low refractive-index layer  102 L and another high refractive-index layer  102 H is disposed between the high thermal-conductive layer  209  and the active region  104 . This arrangement enables the high thermal-conductive layer  209  to be separate from the active region  104  so as to prevent light absorption of the high thermal-conductive layer  209  more effectively. 
     The number of the high thermal-conductive layer  209  is not limited to one, and a plurality of high thermal-conductive layers  209  may be included in the lower reflecting mirror  102 . 
     The following describes a third embodiment. The third embodiment differs from the second embodiment in the mesa structure.  FIG. 13  is a cross-sectional view of an internal structure of the surface emitting laser according to the third embodiment.  FIG. 14  is a cross-sectional view of a mesa structure of the surface emitting laser element in  FIG. 13 .  FIG. 14  is an enlarged view of a part of  FIG. 13 . 
     In the third embodiment as illustrated in  FIG. 13 , the stacked layers of the contact layer  107 , the upper reflecting mirror  106 , the upper spacer layer  105 , the active region  104 , the lower spacer layer  103 , and the lower reflecting mirror  102  has a mesa structure. In other words, the bottom of the mesa structure is in the middle of the lower reflecting mirror  102 . More specifically, as illustrated in  FIG. 14 , the bottom of the mesa structure is closer to the substrate  101  than the high thermal-conductive layer  209 , and a separate high thermal-conductive layer  209  is provided in each mesa structure of the surface emitting laser elements  151 . 
     The other configurations are similar to those in the second embodiment. 
     The third embodiment also attains effects similar to those of the second embodiment. 
     The following describes a fourth embodiment. The fourth embodiment differs from the third embodiment in that a metal film is provided around the mesa structure in the fourth embodiment.  FIG. 15  is an illustration of a layout of a surface emitting laser according to the fourth embodiment.  FIG. 16  is a cross-sectional view of an internal structure of the surface emitting laser in  FIG. 15 .  FIG. 16  corresponds to a cross-sectional view taken along line XVI-XVI in  FIG. 15 .  FIG. 17  is a cross-sectional view of the surface emitting laser in  FIG. 15 .  FIG. 17  is an enlarged view of a part of  FIG. 16 . 
     In the fourth embodiment as illustrated in  FIGS. 15 to 17 , a metal film  460  is provided surrounding the mesa structure of each surface emitting laser element  151 . The metal film  460  is in contact with the insulator film  111  and the p-side electrode  112  and covers the side surface of the mesa structure. The metal film  460  includes, for example, a gold (Au) film. 
     The other configurations are similar to those in the third embodiment. 
     The fourth embodiment also attains effects similar to those of the third embodiment. Further, the heat that has reached the substrate  101  through the high thermal-conductive layer  209  is transferred to the mount substrate such as the driver IC  300  through the metal film  460 . This configuration exhibits better heat dissipation property. 
     The metal film  460  is formed by, for example, plating. The metal film  460  may be formed at the same time as the p-side electrode  112 , or may be formed in a separate process from the p-side electrode  112 . The metal film  460  may include a copper (Cu) film instead of the gold (Au) film. 
     When each surface emitting laser element  151  is driven at the same time, a metal film  460  may cover the side surface of the mesa structure of each surface emitting laser element  151 . For example, the space between the mesa structures of the surface emitting laser elements  151  may be filled with the metal film  460 . 
     The following describes a fifth embodiment. The fifth embodiment relates to a light source device and a detection apparatus including the surface emitting laser  100  according to any one of the first embodiment to the fourth embodiment.  FIG. 18  is an illustration of a distance-measuring apparatus  10  as a detection apparatus. 
     The distance measuring apparatus  10  includes a light source device  11  as an example of the light source device. The distance measuring apparatus  10  is a time-of-flight (TOF) distance detection apparatus that provides projection (irradiation) with pulsed light from the light source device  11  to an object to be detected  12 , receives the reflected light from the object to be detected  12  by a photosensor  13 , and measures the distance from the object to be detected  12  based on the time required for receiving the reflected light. 
     As illustrated in  FIG. 18 , the light source device  11  includes a light source  14  and an optical system  15 . The light source  14  includes the surface emitting laser  100  according to the first embodiment, and the emission of light of the light source  14  is controlled based on electric current sent from a light-source drive circuit  16 . The light-source drive circuit  16  transmits a signal to a signal control circuit  17  when the light source  14  is caused to emit light. The optical system  15  includes an optical element, such as a lens, a diffractive-optical element (DOE), or a prism, that adjusts the angle of divergence or direction of the light emitted from the light source  14 , and irradiates the object to be detected  12  with the light. 
     The light that is projected from the light source device  11  and then reflected by the object to be detected  12  is guided to the photosensor  13  through a light receiving optical system  18  that has a light focusing effect. The photosensor  13  includes a photoelectric conversion element. The light that is received by the photosensor  13  is photoelectrically converted, and the photoelectrically-converted light is sent to the signal control circuit  17  as an electrical signal. The signal control circuit  17  calculates the distance to the object to be detected  12  based on the time difference between the timing of light projection (i.e., the timing at which a light emission signal is input from the light-source drive circuit  16 ) and the timing of light reception (i.e., the timing at which a light reception signal is input from the photosensor  13 ). Accordingly, in the distance measuring apparatus  10 , the light receiving optical system  18  and the photosensor  13  function as a detection system on which the light emitted from the light source device  11  and reflected by the object to be detected  12  is incident. The signal control circuit  17  may be configured so as to obtain, for example, information about the presence or absence of the object to be detected  12  and the relative velocity of the object to be detected  12 , based on a signal sent from the photosensor  13 . 
     The present embodiment provides the light source device  11  and the distance measuring apparatus  10  that exhibit better heat dissipation property while increasing the light-emission intensity. 
     Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that, within the scope of the above teachings, the present disclosure may be practiced otherwise than as specifically described herein. With some embodiments having thus been described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the present disclosure and appended claims, and all such modifications are intended to be included within the scope of the present disclosure and appended claims.