Patent Publication Number: US-2007104237-A1

Title: Semiconductor laser apparatus and semiconductor laser device

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims priority to Japanese Patent Application No. JP 2005-322678, which was filed on Nov. 7, 2005, the contents of which, are incorporated herein by reference, in their entirety.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a semiconductor laser apparatus and a semiconductor laser device.  
      2. Description of the Related Art  
      In a semiconductor laser apparatus formed by mounting a semiconductor laser chip on a heat sink, a difference in thermal expansion coefficient between the semiconductor laser device and the heat sink generates stress, causing a problem that an internal stress is generated in the semiconductor laser chip, that is, a problem that a semiconductor layer constituting the semiconductor laser chip suffers from strain.  
      In view of such a problem, there has been provided a semiconductor laser apparatus of related art, in which the internal stress of the semiconductor laser chip is reduced by virtue of a shape of electrode of the semiconductor laser chip.  
      The above-described semiconductor laser device is disclosed on Japanese Examined Patent Publication JP-B2 3461632.  FIG. 11  is a sectional view of a semiconductor laser apparatus  1  of related art as described above.  FIG. 12  is a plan view of a semiconductor laser chip  2  in the semiconductor laser apparatus  1  when seen from a lower electrode  3  side. The semiconductor laser apparatus  1  has a semiconductor laser chip  2 , a solder layer  4 , and a heat sink  5 . The semiconductor laser chip  2  is mounted on the heat sink  5  via the solder layer  4 .  
      The heat sink  5  has one surface in a thickness direction thereof on which a heat sink upper electrode  6  is formed, and the other surface in a thickness direction thereof on which a heat sink lower electrode  7  is formed. To the heat sink upper electrode  6  is applied the solder layer  4  on which the semiconductor laser chip  2  is stacked so that the lower electrode  3  faces the solder layer  4 .  
      The semiconductor laser chip  2  is constituted in a manner that an active layer  13  and a cap layer  15  are stacked in this order on the other surface in a thickness direction of a substrate  11 , and an ohmic electrode layer  16  and a non-alloying electrode layer  17  are stacked in this order on the cap layer  15 , while a semiconductor laser chip upper electrode  18  is formed on one surface in a thickness direction of the substrate  11 . On a part of the non-alloying electrode layer  17  is stacked an alloying electrode layer  19  which forms the lower electrode  3  of the semiconductor laser chip  2  together with the non-alloying electrode layer  17 .  
      In a surface of the lower electrode  3  of the semiconductor laser chip  2  facing the semiconductor layer  4  stacked on the heat sink upper electrode  6  of the heat sink  5  is formed a region  21  having a predetermined width ranging from a surface portion located right under a central line in a longitudinal direction of a light-emitting region  8  of the semiconductor laser chip  2  to both sides of the central line. The region  21  is formed in the non-alloying electrode layer  17  which is not alloyed with the solder layer. A surface of the lower electrode  3  excluding the region  21  is alloyed with the solder layer  4  and thereby adhered to the solder layer  4 . The region  21  is formed from one end to the other end of the longitudinal direction of the light-emitting region  8 , that is, between one end and the other end of a longitudinal direction of the semiconductor laser chip  2 .  
      Upon heat-sealing the semiconductor laser chip  2  and the heat sink  5  by use of a solder material, the alloying electrode layer  19  shown in  FIG. 11  is alloyed with a solder material applied to the heat sink  5  and thereby adhered solidly to the solder layer  4  while the non-alloying electrode layer  17  shown in  FIG. 11  is not alloyed with the solder material applied to the heat sink  5  and therefore not adhered solidly to the solder layer  4 . Accordingly, the internal stress in the non-alloying electrode layer  17  is lower than that in the alloying electrode layer  19 . Since the light-emitting region  8  is formed in a non-alloying region where the non-alloying electrode layer  17  contacts the solder layer  4 , the internal stress on the light-emitting region  8  can be reduced, with the result that reliability of the semiconductor laser apparatus  1  can be enhanced.  
      In the semiconductor laser apparatus  1  of related art, a part of the lower electrode  3  of the semiconductor laser chip  2 , which is stacked on the light-emitting region  8 , is provided with the non-alloying electrode layer  17  to thereby decrease the strength of bonding between the lower electrode  3  and the solder layer  4  applied to the heat sink  5 . This helps prevent the semiconductor laser chip  2  from having the internal stress generated therein. However, the weak bonding between the non-alloying electrode  17  and the solder layer  4  leads a problem such that heat generated by the light-emitting region  8  is hard to be conducted from the non-alloying electrode layer  17  to the solder layer  4 , resulting in deterioration in efficiency of dissipating heat toward the heat sink  5  and an increase in operating current at a high temperature, which leads deterioration in reliability at a high temperature.  
     SUMMARY OF THE INVENTION  
      An object of the invention is to provide a semiconductor laser device in which reduced stress is generated under the condition that it is mounted on a mount and an efficiency of dissipating heat toward the mount is enhanced to prevent a rise of operating current at high temperature, and to a semiconductor laser apparatus having the laser device mounted on a mount.  
      The invention provides a semiconductor laser apparatus comprising a semiconductor laser device having a stripe-shaped light emitting region, and a mount to which the semiconductor laser device is adhered via a solder layer,  
      wherein an outermost surface of the semiconductor laser device to which the solder layer is applied is electrically conductive and the outermost surface has an incomplete adherent region which is incompletely adhered to the solder layer, extends in a width direction perpendicular to a longitudinal direction of the light-emitting region and a stacking direction of the semiconductor laser device, the solder layer and the mount, outwardly to either side by a predetermined distance from a virtual plane which passes through a center of the light-emitting region and is perpendicular to the width direction, and which incomplete adherent region has a longitudinal length shorter than that of the light-emitting region, and the outermost surface has a complete adherent region which is adhered to the solder layer and extends over an area of the outermost surface excluding the incomplete adherent region.  
      According to the invention, the incomplete adherent layer is formed in the outermost surface of a semiconductor laser device to which outermost surface the solder layer is applied. The incomplete adherent region extends outwardly to either side by a predetermined distance from a virtual plane which passes through a center of the light-emitting region and is perpendicular to the width direction as viewed in a width direction perpendicular to the longitudinal direction of the light-emitting region and the stacking direction of the semiconductor laser device and the solder layer onto the mount. In the incomplete adherent region, the outermost surface is not or incompletely adhered to the solder layer. Accordingly, it is possible to reduce the stress imparted to the light-emitting region, which is caused by differences in thermal expansion coefficient among the semiconductor laser apparatus, the solder layer, and the mount during operation. As a result, the degree of strain in the light-emitting region can be. Moreover, when viewed in the longitudinal direction of the light-emitting region, a length of the incomplete adherent region is shorter than a length of the light-emitting region, with the result that a part of the above-stated area is completely adhered to the solder layer at a part of the outermost surface stacked over the light-emitting region. This facilitates conduction of heat generated by the light-emitting region also in the above-stated area adjacent to the light-emitting region, resulting in enhancement in efficiency of dissipating heat toward the mount. Consequently, the operating current at a high temperature is prevented from increasing so that the reliability at a high temperature can be enhanced.  
      Further, the complete adherent region formed in an area of the outermost surface excluding the incomplete adherent region allows a solid mechanical coupling between the semiconductor laser device and the mount.  
      Further, in the invention, it is preferable that the incomplete adherent region is formed at a light-emitting end of the semiconductor laser device.  
      According to the invention, in a part of the light-emitting region where the complete adherent layer is stacked on the outermost surface, light passing through the light-emitting region is influenced by refractive index fluctuation caused by internal stress, that is, strain due to stress, whereas in a part of the light-emitting region where the incomplete adherent layer is stacked on the outermost surface, light passing through the light-emitting region is less easily influenced by the refractive index fluctuation by virtue of a small amount of the internal stress, allowing reduction of the distortions arising in the radiation pattern. The light passing through the light-emitting region thus passes through a part which is susceptible to the refractive index fluctuation caused by internal stress, and a part which is less easily influenced by the refractive index fluctuation. Owing to the incomplete adherent region formed at a light-emitting end, the light-emitting end is less easily influenced by the refractive index fluctuation and therefore, it is possible to prevent the distortions from arising in the radiation pattern of the laser light being emitted.  
      Further, in the invention, it is preferable that a ratio of the longitudinal length of the incomplete adherent region to that of the light-emitting region is 20% or more and 80% or less.  
      According to the invention, if a ratio of the longitudinal length of the incomplete adherent region to that of the light-emitting region is less than 20%, an increase of internal crystal defects caused by the strain of the light-emitting region drastically deteriorates the service life of the semiconductor laser apparatus, resulting in a short service life. If a ratio of the longitudinal length of the incomplete adherent region to that of the light-emitting region exceeds 80%, the efficiency of dissipating heat from the complete adherent region is low, resulting in a drastic increase of the operating current at a high temperature. According to the invention, by adjusting the ratio of the longitudinal length of the incomplete adherent region to that of the light-emitting region to be 20% or more and 80% or less, it is possible to provide a semiconductor laser apparatus having enhanced service life and operating reliability at a high temperature, in which a service life of a semiconductor laser device can be prevented from decreasing and an operating current at a high temperature can be prevented from increasing.  
      Further, in the invention, it is preferable that a part included in the incomplete adherent region of the outermost surface is made of one or more substances selected from a group consisting of Mo, Pt, and Ti, and  
      wherein a part included in the complete adherent region of the outermost surface is made of alloy of a material containing Au and a solder material made of AuSn, and  
      wherein the solder layer is made of a solder material made of AuSn.  
      According to the invention, a part included in the incomplete adherent region of the outermost surface is made of one or more substances selected from a group consisting of Mo, Pt, and Ti, and a part included in the complete adherent region of the outermost surface is made of alloy of a material containing Au and a solder material made of AuSn, and the solder layer is made of a solder material made of AuSn, with the result that a semiconductor laser apparatus achieving the above-described effects can be easily realized.  
      Further, in the invention, it is preferable that in the incomplete adherent region, a-void is formed between the outermost surface and the solder layer.  
      According to the invention, in the incomplete adherent region, a void formed between the outermost surface and the solder layer allows further reduction of the internal stress imposed on the light-emitting region of the semiconductor laser device, with the result that the service life of the semiconductor laser device can be enhanced furthermore.  
      Further, in the invention, it is preferable that a part included in the incomplete adherent region of the outermost surface is made of Mo, and  
      wherein a part included in the complete adherent region of the outermost surface is made of alloy of a material containing Au and a solder material made of AuSn, and  
      wherein the solder layer is made of a solder material made of AuSn.  
      According to the invention, a part included in the incomplete adherent region of the outermost surface is made of Mo, and a part included in the complete adherent region of the outermost surface is made of alloy of a material containing Au and a solder material made of AuSn, and the solder layer is made of a solder material made of AuSn. Since Mo is not alloyed with AuSn, the outermost surface made of Mo does not make intimate contact with the solder layer made of AuSn, with the result that the above-described void can be easily attained. Moreover, the part included in the complete adherent region is made of alloy of a material containing Au and a solder material made of AuSn and therefore, solidly adhered to the solder layer made of AuSn.  
      Further, in the invention, it is preferable that the incomplete adherent region and the complete adherent region are formed alternately along the longitudinal direction of the semiconductor laser device in an area ranging outward by the predetermined distance on either side from the virtual plane which passes through the center in the width direction of the light-emitting region and which is perpendicular to the width direction.  
      According to the invention, the incomplete adherent region and the complete adherent region are formed alternately along a longitudinal direction of the semiconductor laser device in an area ranging outward by a predetermined distance on either side from a virtual plane which passes through a center in the width direction of the light-emitting region and which is perpendicular to the width direction, with the result that, in the area, the internal stress of the semiconductor laser device can be dispersed in the longitudinal direction of the light-emitting region and furthermore, heat transmission paths of high heat conduction can be dispersed in the longitudinal direction of the light-emitting region. Accordingly, the internal stress imparted to the light-emitting region is equalized as much as possible when viewed in the longitudinal direction so that the distortions arising in the radiation pattern can be reduced. Further, the heat conduction from the light-emitting region to the mount is equalized as much as possible when viewed in the longitudinal direction so that a temperature of the light-emitting region can be made as uniform as possible. As a result, it is possible to further prevent the operating current at a high temperature from increasing.  
      Further, the invention provides a semiconductor laser device that is adhered to a mount via a solder layer, comprising:  
      a stripe-shaped light-emitting region; and  
      an outermost surface which is electrically conductive and to which the solder layer is applied, the outermost surface having an incomplete adherent region and a complete adherent region,  
      wherein the incomplete adherent region is incompletely adhered to the solder layer, extends in a width direction perpendicular to a longitudinal direction of the light-emitting region and a stacking direction of the semiconductor laser device, the solder layer and the mount, outwardly to either side by a predetermined distance from a virtual plane which passes through a center of the light-emitting region and is perpendicular to the width direction, and the incomplete adherent region has a longitudinal length shorter than that of the light-emitting region, and the complete adherent region is adhered to the solder layer and extends over an area of the outermost surface excluding the incomplete adherent region.  
      According to the invention, the incomplete adherent layer is not or incompletely adhered to the solder layer, and it is therefore possible to reduce the stress which is imparted to the semiconductor laser device by thermal expansion and thermal contraction of a solder material upon heat-sealing the semiconductor laser device onto the heat sink by soldering, with the result that the degree of strain in the light-emitting region can be reduced. Moreover, during operation of the semiconductor laser device already mounted on the mount, it is possible to reduce the stress caused by differences in thermal expansion coefficient among the semiconductor laser apparatus, the solder layer, and the mount so that the degree of strain in the light-emitting region can be reduced. Furthermore, when viewed in the longitudinal direction of the light-emitting region, a length of the incomplete adherent layer is shorter than a length of the light-emitting region, with the result that a part of the above-stated area of the outermost surface stacked on the light-emitting region is completely adhered to the solder layer, thus effecting easier conduction of heat generated by the light-emitting region to the mount through the solder layer. By so doing, the efficiency of dissipating heat to the mount can be enhanced so that the operating current at a high temperature is prevented from increasing, allowing enhancement in reliability at a high temperature.  
      Further, the complete adherent region formed in an area of the outermost surface excluding the incomplete adherent region allows a solid mechanical coupling between the semiconductor laser device and the mount.  
      Further, in the outermost surface region of the semiconductor laser device to which the solder layer is applied are formed the incomplete adherent layer and the complete adherent layer. That is, the semiconductor laser device is mounted onto the mount via the solder layer applied to the incomplete adherent layer and the complete adherent layer. In this case, the solder layer can be applied to the entire stacking surface of the outermost surface without the necessity of being subjected to processing in some way, thus facilitating the mounting of the semiconductor laser device onto the mount. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein:  
       FIG. 1  is a plan view of a semiconductor laser device provided in a semiconductor laser apparatus according to one embodiment of the invention when seen from a side to be mounted onto a mount;  
       FIG. 2  is a sectional view of the semiconductor laser device taken along the line II-II of  FIG. 1 ;  
       FIG. 3  is a sectional view of the semiconductor laser device adhered to a mount via a solder layer when seen from the line II-II of the semiconductor laser apparatus;  
       FIG. 4  is a sectional view of the semiconductor laser device adhered to the mount via the solder layer when seen from the line III-III of the semiconductor laser apparatus;  
       FIG. 5  is a graph showing the relationship between a ratio of a length of an incomplete adherent region to a length of a light-emitting region when viewed in a longitudinal direction X, and a service life of the semiconductor laser apparatus;  
       FIG. 6  is a graph showing the relationship between the ratio of the length of the incomplete adherent region to that of the light-emitting region when viewed in the longitudinal direction X, and an operating current of the semiconductor laser apparatus;  
       FIG. 7  is a graph showing a radiation pattern of emitted light of the semiconductor laser apparatus according to the embodiment;  
       FIG. 8  is a graph showing a radiation pattern of emitted light of a semiconductor laser apparatus of a comparative example;  
       FIG. 9  is a sectional view showing a semiconductor laser apparatus according to another embodiment of the invention;  
       FIG. 10  is a plan view of a semiconductor laser device provided in a semiconductor laser apparatus according to still another embodiment of the invention when seen from a side to be mounted onto the mount;  
       FIG. 11  is a sectional view of a semiconductor laser apparatus of the related art; and  
       FIG. 12  is a plan view of a semiconductor laser chip in the semiconductor laser apparatus when seen from a lower electrode side. 
    
    
     DETAILED DESCRIPTION  
      Now referring to the drawings, preferred embodiments of the invention are described below.  
       FIG. 1  is a plan view of a semiconductor laser device  32  provided in a semiconductor laser apparatus  31  according to one embodiment of the invention when seen from a side to be mounted onto a mount  72 .  FIG. 2  is a sectional view of the semiconductor laser device  32  taken along the line II-II of  FIG. 1 . Note that a complete adherent layer  53  shown in  FIG. 1  is diagonally shaded for the sake of simplifying an understanding of the illustration.  
      The semiconductor laser device  32  is built as a semiconductor laser chip. In the present embodiment, the semiconductor laser device  32  has a ridge structure. The semiconductor laser device  32  is composed of a semiconductor substrate  42 , a first clad layer  43 , an active layer  44 , a second clad layer  45 , a cap layer  46 A, a terrace portion carrying layer  46 B, an insulating layer  47 , an ohmic electrode layer  48 , a plate electrode layer  49 , a metal layer  52  including an incomplete adherent layer  51 , a complete adherent layer  53 , and a back-side electrode layer  54  serving as a second electrode. The semiconductor laser device  32  is formed into a schematic rectangular parallelepiped shape.  
      The semiconductor substrate  42  can stack thereon semiconductor layers made of a compound semiconductor. In the present embodiment, the semiconductor substrate  42  is made of n-type gallium arsenide (GaAs). The semiconductor substrate  42  has a quadrilateral cross-sectional profile when viewed in a thickness direction Z. The thickness of the semiconductor substrate  42  is adjusted to fall in a range of from 50 μm to 130 μm, for example.  
      The first clad layer  43  is formed on one surface  42   a  in a thickness direction Z of the semiconductor substrate  42  so as to be stacked over the entire one surface  42   a  with use of n-type (Al x Ga 1-x )In 1-Y P, wherein the following conditions have to be satisfied: 0&lt;X&lt;1 and 0&lt;Y&lt;1. In the present embodiment, the value of X is set at 0.7, and the value of Y is set at 0.5. That is, the first clad layer  43  is made of n-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P. The thickness of the first clad layer  43  is set at 2.0 μm, for example.  
      The active layer  44  is formed on one surface  43   a  in a thickness direction Z of the first clad layer  43  so as to be stacked on the entire one surface  43   a . The active layer  44  takes on a quantum well structure composed of: a first guide layer stacked on Z direction-wise one surface  43   a  of the first clad layer  43 ; a first well layer stacked on Z direction-wise one surface of the first guide layer; a first barrier layer stacked on Z direction-wise one surface of the first well layer; a second well layer stacked on Z direction-wise one surface of the first barrier layer; a second barrier layer stacked on Z direction-wise one surface of the second well layer; a third well layer stacked on Z direction-wise one surface of the second barrier layer; and a second guide layer stacked on Z direction-wise one surface of the third well layer. Each of the first, second, and third well layers is made of In 0.5 Ga 0.5 P, the thickness of which is set at 60 Å, for example. Each of the first and second barrier layers is made of (Al 0.5 Ga 0.5 ) 0.5 In 0.5 P, the thickness of which is set at 50 Å, for example. Each of the first and second guide layers is made of (Al 0.5 Ga 0.5 ) 0.5 In 0.5 P, the thickness of which is set at 50 Å, for example.  
      The second clad layer  45  is formed on one surface  44   a  in a thickness direction Z of the active layer  44  so as to be stacked on the entire one surface  44   a  with use of p-type (Al x Ga 1-x ) Y In 1-Y P, wherein the following conditions have to be satisfied: 0&lt;X&lt;1 and 0&lt;Y&lt;1. In the present embodiment, the value of X is set at 0.7, and the value of Y is set at 0.5. That is, the second clad layer  45  is made of p-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P. The thickness of the second clad layer  45  is adjusted to fall in a range of from 1.0 μm to 2.0 μm, for example.  
      On the second clad layer  45  are formed a ridge portion  61  and a terrace portion  62 . The ridge portion  61  is provided in a direction in which laser light is emitted, that is, provided at a center portion of a width direction Y perpendicular to each of a longitudinal direction X and a thickness direction Z of the semiconductor laser device  32 . On both sides in the width direction Y of the ridge portion  61  are formed clad layer groove portions  63 , each of which extends along the longitudinal direction X. On outer sides in the width direction Y of the clad layer groove portion  63  are formed the terrace portions  62 . The ridge portion  61  and the terrace portion  62  form protrusions extending from a bottom surface  63   a  of the clad layer groove portion  63  to one side in the thickness direction Z.  
      The semiconductor laser device  32  is designed substantially surface-symmetrical about a virtual plane passing through the Y direction-wise center in parallel with the thickness direction Z. Each of the ridge portion  61  and the terrace portion  62  is formed into a substantial rectangular parallelepiped shape, and extends along the longitudinal direction X across both ends of the semiconductor laser device  32 . That is to say, the ridge portion  61  is formed in a striped shape. Each thickness of the ridge portion  61  and the terrace portion  62  is selected to fall in a range of from 1.0 μm to 2.0 μm. The ridge portion  61  forms a ridge waveguide through which laser light is directed.  
      The ridge portion  61  is formed so as to have a predetermined length L 1  when viewed in the width direction Y, which predetermined length L 1  is selected to fall in a range of from 1.0 μm to 3.0 μm. More specifically, when viewed in the width direction Y, the dimension of Z direction-wise one end of the ridge portion  61 , namely one end of the ridge portion  61  located away from the semiconductor substrate  42  is selected to fall in a range of from 0.5 μm to 2.5 μm, whereas the dimension of the other Z direction-wise end of the ridge portion  61  is selected to fall in a range of from 1.0 μm to 3.0 μm. When viewed in the direction perpendicular to the direction X in which the ridge portion  61  extends, the ridge portion  61  has a trapezoidal cross-sectional profile, the lower side of the trapezoid facing the semiconductor substrate  42 . Note that in  FIG. 2 , the ridge portion  61  is shown with a quadrilateral cross-sectional profile for the sake of simplifying an understanding of the illustration.  
      When viewed in the width direction Y, the terrace portion  62  is formed on either side of the ridge portion  61 , that is, the ridge waveguide. A predetermined first distance L 2  is secured between the terrace portion  62  and the ridge portion  61 . The predetermined first distance L 2  is selected to fall in a range of from 10 μm to 20 μm. When viewed in the width direction Y, the terrace portion  62  is so formed as to extend outwardly from a position which is located the predetermined first distance L 2  away from the ridge portion  61 , to the edge of the semiconductor laser device  32 .  
      By virtue of the terrace portion  62 , it is possible to reduce the hazard of mechanical damage to the ridge portion  61  at the time of working with a wafer on which is formed a precursor of the semiconductor laser device  32  during the process of manufacture of the semiconductor laser device  32 , as well as the time of mounting the semiconductor laser device  32 .  
      In a region included in the active layer  44 , on which the ridge portion  61  is stacked, is formed the stripe-shaped light-emitting region  40  extending in the longitudinal direction X. The light-emitting region  40  indicates a part which emits light by laser oscillation when the semiconductor laser device  32  is supplied with electric current. A flowing trace of carrier passing through the ridge portion  61  spreads out in the width direction Y from the ridge portion  61 , so that a dimension of the light-emitting region  40  is a slightly larger than that of the ridge portion  61  when viewed in the width direction Y. The light-emitting region  40  which extends in the longitudinal direction X along the ridge portion  61 , is formed between one end and the other end of the longitudinal direction X of the semiconductor laser device  32 .  
      The cap layer  46 A is formed on one surface  61   a  in a thickness direction Z of the ridge portion  61  of the second clad layer  45  so as to be stacked on the entire one surface  61   a . The cap layer  46 A is made of p-type gallium arsenide (GaAs), the thickness of which is selected to fall in a range of from 0.2 μm to 0.5 μm, for example. The cap layer  46 A is employed to gain an ohmic contact with the ohmic electrode layer  48 .  
      The terrace portion carrying layer  46 B is formed on one surface  62   a  in a thickness direction Z of the terrace portion  62  so as to be stacked on the entire one surface  62   a . The terrace portion carrying layer  46 B is the same in material and thickness as the cap layer  46 A.  
      The insulating layer  47  is stacked on one side of each of the cap layer  46 A, the terrace portion carrying layer  46 B, and the second clad layer  45 , except for one surface  46   a  in a thickness direction Z of the cap layer  46 A. The insulating layer  47  covers a surface  61   b  of the ridge portion  61  facing the terrace portion  62  and a surface  62   b  of the terrace portion  62  facing the ridge portion  61  when viewed in the thickness direction Z. The insulating layer  47  is formed of, for example, SiO 2 , the thickness of which is selected to fall in a range of from 500 Å to 2000 Å. By virtue of the insulating layer  47 , a flow of the electric current is enabled to be focused on the cap layer  46 A and the ridge portion  61 .  
      The ohmic electrode layer  48  is formed on one surface  47   a  in a thickness direction Z of the insulating layer  47  and one surface  46   a  in the thickness direction Z of the cap layer  46 A so as to be stacked on the entire one surface  46   a ,  47   a . The ohmic electrode layer  48  is made of AuZn, the thickness of which is selected to fall in a range of from 300 Å to 700 Å, for example.  
      The plate electrode layer  49  is electrically conductive and formed on one surface  48   a  in a thickness direction Z of the ohmic electrode layer  48  so as to be stacked on the entire one surface  48   a . The plate electrode layer  49  is made of gold (Au), the thickness of which is selected to be 0.5 μm or more and less than 5.0 μm. By thus setting the thickness of the plate electrode layer  49 , the heat generated by the light-emitting region  40  can be conducted outwardly in the width direction Y through the plate electrode layer  49  formed of Au having a high thermal conductivity, thus allowing by-passing of the heat transmission path whereby the operating current at a high temperature can be reduced. If the thickness of the plate electrode layer  49  is less than 0.5 μm, a satisfactory heat-transmission effect cannot be attained. By way of contrast, if the thickness of the plate electrode layer  49  exceeds 5.0 μm, the wafer will suffer from some warping during the formation of metal layers thereon, wherefore a stress is generated in the ridge portion  61 , resulting in strain in the ridge waveguide. In light of the foregoing, by adjusting the thickness of the plate electrode layer  49  to be 0.5 μm or more and 5.0 μm or less, it is possible to attain enhancement in the effect of heat-transmission directed outward from a center portion in the width direction Y of the semiconductor laser device  32 , as well as reduction of the stress given to the ridge portion  61 .  
      The metal layer  52  including the incomplete adherent layer  51  is electrically conductive and formed on a one surface  49   a  in a thickness direction Z of the plate electrode layer  49  so as to be stacked on the entire one surface  49   a . The metal layer  52  is made of a material of which melting point is higher than that of the solder material constituting an after-mentioned solder layer  71 . The metal layer  52  is made of one or two or more substances which are selected from molybdenum (Mo), platinum (Pt), and titanium (Ti) . In the present embodiment, the metal layer  52  is made of Pt. The incomplete adherent layer  51  constitutes a part of the metal layer  52 . A thickness of the metal layer  52  is selected to fall in a range of from 0.05 μm to 0.30 μm.  
      The complete adherent layer  53  is formed on one surface  52   a  in a thickness direction Z of the metal layer  52  so as to be stacked on a predetermined region of the one surface  52   a . The complete adherent layer  53  is made of gold (Au). A thickness of the complete adherent layer  53  is selected to fall in a range of from 0.1 μm to 0.4 μm. The complete adherent layer  53  is formed over a region in the outermost surface of one side in the thickness direction Z of the semiconductor laser device  32 , which region excludes a region where the incomplete adherent layer  51  is formed. The incomplete adherent layer  51  and the complete adherent layer  53  constitute a surface electrode of the semiconductor laser device  32 .  
      With reference to  FIG. 1 , the incomplete adherent layer  51  and the complete adherent layer  53  will be described in more detail. The semiconductor laser device  32  has a light-emitting end face  32 A and a light-reflecting end face  32 B. The light-emitting end face  32 A is formed on one end of the semiconductor laser device  32  while the light-reflecting end face  32 B is formed on the other end of the semiconductor laser device  32 , when viewed in a direction in which laser light is emitted, that is, in the longitudinal direction X. During operation of the semiconductor laser device  32 , the laser light travels back and forth more than once between the light-emitting end face  32 A and the light-reflecting end face  32 B, and then is emitted from the light-emitting face  32 A to outside.  
      The light-reflecting end face  32 B is constituted by vapor-deposition in the longitudinal direction X of 10 films composed of alternate Al 2 O 3  films and TiO 2  films. A thickness of each Al 2 O 3  film is selected to be 100 nm while a thickness of each TiO 2  film is selected to be 75 nm. After the vapor-deposition of the 10 films, the formation of reflecting film is completed by vapor-depositing the Al 2 O 3  film. A thickness of the last (outermost) Al 2 O 3  film is selected to be 200 nm. A reflectivity of the laser light on the light-reflecting end face  32 B is 95%. The light-emitting end face  32 A is constituted by vapor-deposition of an Al 2 O 3  film. A thickness of the Al 2 O 3  film is selected to be 120 nm. A reflectivity of the laser light on the light-emitting end face  32 A is 6%.  
      The incomplete adherent layer  51  is formed on a part of the outermost surface  55  of the semiconductor laser device  32 , which outermost surface  55  is to be attached to the mount  72 , that is, the outermost surface located the farthest away from the semiconductor substrate  42 , of the deposition formed on the Z direction-wise one surface  42   a  of the semiconductor substrate  42 . The incomplete adherent layer  51  extends over an area  60  ranging outward by a predetermined second distance L 3  on either side from a virtual plane which passes through the Y direction-wise center of the light-emitting region  40  and which is perpendicular to the width direction Y. When viewed in the longitudinal direction X, a length L 5  of the incomplete adherent layer  51  is shorter than a length L 6  of the light-emitting region  40 . A longitudinal direction of the light-emitting region  40  corresponds to the longitudinal direction X of the semiconductor laser device  32 . To be more specific, the incomplete adherent layer  51  is formed on the light-emitting end where the light-emitting end face  32 A is formed, and extends by the length L 5  from the light-emitting end face  32 A. When viewed in the longitudinal direction X, the length L 6  of the light-emitting region  40  is equal to a length of the ridge portion  61 .  
      The predetermined second distance L 3  is selected to be 10 μm, so as to be 2 μm or more and less than 20 μm, for example. If the predetermined second distance L 3  is 20 μm or more, the efficiency of dissipating heat is deteriorated and the operating current at a high temperature is increased, resulting in deterioration in reliability. By way of contrast, if the predetermined second distance L 3  is less than 2 μm, the light-emitting region  40  suffers from strain, resulting in deterioration in reliability.  
      In the outermost surface  55  acting as the Z direction-wise outermost surface of the semiconductor laser device  32  located the farthest away from the semiconductor substrate  42  is formed a surface groove portion  82  which extends along the longitudinal direction X. The surface groove portion  82  is formed on either side of a Y direction-wise center portion where the ridge portion  61  is formed. The surface groove portion  82  is constituted by irregularities generated on the Z direction-wise one surface of the second clad layer  45  attributable to the ridge portion  61  and the terrace portion  62  formed on the second clad layer  45  and further the cap layer  46 A stacked on the ridge portion  61  and the terrace portion carrying layer  46 B stacked on the terrace portion  62 , on which surface being irregular are stacked the insulating layer  47 , the ohmic electrode layer  48 , the plate electrode layer  49 , the metal layer  52 , and the complete adherent layer  53 . A part located at the Y direction-wise center portion where the ridge portion  61  is formed, which part protrudes to one side in the thickness direction Z from a bottom of the surface groove portion  82 , is referred to as a ridge protrusion  83 . A part located on at the Y direction-wise both ends where the terrace portion  62  is formed, which part protrudes to one side in the thickness direction Z from a bottom of the surface groove portion  82 , is referred to as a terrace protrusion  84 .  
      When viewed in the width direction Y, the incomplete adherent layer  51  of the metal layer  52  is provided so as to cover at least the ridge protrusion  83 . Accordingly, when viewed in the width direction Y, the incomplete adherent layer  51  of the metal layer  52  is provided so as to cover at least a part above the ridge structure  56 . The incomplete adherent layer  51  extends to a Y direction-wise center of the surface groove portion  82 . The ridge structure  56  contains the ridge portion  61  and a portion located at a region where the ridge portion  61  is formed, which portion is stacked on the ridge portion  61 , in the semiconductor laser device  32 . The ridge structure  56  ranges over an area across the both ends on the semiconductor substrate  42 -side of the ridge portion  61  in the width direction Y, that is, an area indicated by a reference symbol L 1  in  FIG. 2 . The incomplete adherent layer  51  included in the surface groove portion  82  extends from the ridge protrusion  83  by a predetermined third distance L 4 .  
      The predetermined third distance L 4  is selected to be 1 μm or more and less than 19 μm. The predetermined second distance L 3  is determined firstly and then, an approximate value of the predetermined third distance L 4  is obtained by a calculation (L 3 −1 μm).  
      Since a length of an incomplete adherent region is shorter than the X direction-wise length L 6  of the light-emitting region  40  when viewed in the longitudinal direction X of the light-emitting region  40 , a part of the area  60  included in the outermost surface  55  is adhered to the solder layer  71  at a position of deposition formed on the light-emitting region  40  included in the outermost surface  55  of the semiconductor laser device  32 . By so doing, also in the range  60  adjacent to the light-emitting region  40 , the heat generated by the light-emitting region  40  is made to be more easily conducted to the mount  72  through the solder layer  71  to thereby enhance the efficiency of dissipating heat to the mount  72 , so that the operating current at a high temperature is prevented from increasing, allowing enhancement in reliability at a high temperature.  
      The length L 5  and the length L 6  are selected so as to satisfy the following relation: 
 
0.2 ×L 6 ≦L 5≦0.8× L 6  . . . (1) 
 
 wherein L 5  represents an X direction-wise length of the incomplete adherent layer  51  and L 6  represents an X direction-wise length of the light-emitting region  40 . 
 
      For example, the X direction-wise length L 6  of the light-emitting region  40  is selected to be 1500 μm, and the X direction-wise length L 5  of the incomplete adherent layer  51  is selected to be 1000 μm.  
      The back-side electrode layer  54  is formed on the other surface portion in the thickness direction Z of the semiconductor substrate  42  so as to be stacked on the entire surface of the other surface  42   b  in the thickness direction Z of the semiconductor substrate  42 . The back-side electrode layer  54  is made of gold (Au). A thickness of the back-side electrode layer  54  is different from the thickness of the plate electrode layer  52 , and selected to fall in a range of from 1000 Å to 3000 Å.  
      Next, a method of preparing the semiconductor laser device  32  will be described. At the outset, on one surface of a precursor of the semiconductor substrate  42  ranging in thickness from 300 μm to 350 μm are successively stacked the 2.0 μm-thick first clad layer  43 , the active layer  44 , a 1.5 μm-thick first precursor layer made of p-type (Al 0.7 Ga 0.3 ) 0.5 In 0.5 P for forming the second clad layer  45 , and a 0.5 μm-thick second precursor layer made of GaAs for forming the cap layer  46 A and the terrace portion carrying layer  46 B in the order named by means of epitaxial growth technique using a metalorganic chemical vapor deposition (MOCVD for short) apparatus or a molecular beam epitaxial (MBE for short) apparatus. In forming the active layer  44 , the first, second and third well layers are each set at 60 Å in thickness, the first and second barrier layer are each set at 50 Å in thickness, and the first and second guide layers are each set at 500 Å in thickness.  
      Next, as shown in  FIG. 2 , parts of the first and second precursor layers are removed by means of photolithography and etching techniques to create the ridge portion  61 , the terrace portion  62 , the cap layer  46 A, and the terrace portion carrying layer  46 B.  
      Next, a layer made of SiO 2  is stacked on the second clad layer  45 , the cap layer  46 A, and the terrace portion carrying layer  46 B. Subsequently, of the layer made of SiO 2 , the portion located on the Z direction-wise one surface  46   a  of the cap layer  46 A is removed by means of photolithography and etching techniques to create the insulating layer  47 .  
      Next, the ohmic electrode layer  48  is stacked on the insulating layer  47  and the cap layer  46 A by vapor-deposition.  
      Next, the other Z direction-wise surface of the precursor of the semiconductor substrate  42  is polished to the thickness ranging from 50 μm to 130 μm, thereby constituting the semiconductor substrate  42 .  
      Next, on the other Z direction-wise surface  42   b  of the semiconductor substrate  42  is formed the back-side electrode layer  54 . Then, the ohmic electrode layer  48  and the back-side electrode layer  54  are subjected to an alloying process in an atmosphere of nitrogen gas.  
      Next, the ohmic electrode layer  48  is subjected to power feeding to carry out electrolytic Au plating for a predetermined period of time. In this way, there is formed the plate electrode layer  49  having a thickness of 0.5 μm or more and less than 5.0 μm. By adjusting the thickness of the back-side electrode layer  54  to the above-stated level, it is possible to alleviate the stress generated in stacking the plate electrode layer  49  on an opposite side of the semiconductor substrate  42  from a side on which the back-side electrode layer  54  is formed.  
      Next, Pt is vapor-deposited onto Z direction-wise one surface  49   a  of the plate electrode layer  49  to create the metal layer  52 . Subsequently, Au is vapor-deposited onto Z direction-wise one surface  52   a  of the metal layer  52  to create a third precursor layer.  
      Next, a resist is applied onto Z direction-wise one surface of the third precursor layer. After that, part of the resist overlying the metal layer  52  is removed by means of photolithography and etching techniques to expose the part of the third precursor layer which is stacked on a certain region of the metal layer  52  to be formed into the incomplete adherent layer  51 . In this way, a resist pattern layer is formed.  
      Next, the part of the third precursor layer which is not covered with the resist pattern layer is removed by means of etching technique so that part of the metal layer  52  is exposed. The exposed part of the metal layer  52 , which is not covered with the third precursor layer, constitutes the incomplete adherent layer  51 . Moreover, upon part of the third precursor layer and the resist pattern layer being removed, the complete adherent layer  53  is created in a region excluding the incomplete adherent layer  51 . Next, the light-emitting face  32 A and the light-reflecting end face  32 B will be formed.  
       FIG. 3  is a sectional view of the semiconductor laser device  32  adhered to the mount  72  via the solder layer  71  when seen from the line II-II of the semiconductor laser apparatus  31 .  FIG. 4  is a sectional view of the semiconductor laser device  32  adhered to the mount  72  via the solder layer  71  when seen from the line III-III of the semiconductor laser apparatus  31 .  FIG. 3  is a sectional view taken along a direction perpendicular to the longitudinal direction X, relating to a part where an incomplete adherent region  68  is formed on the outermost surface of semiconductor laser device  32  to which the solder layer  71  is applied.  FIG. 4  is a sectional view taken along a direction perpendicular to the longitudinal direction X, relating to a part where only a complete adherent region  69  is formed on the outermost surface of the semiconductor layer device  32 . A direction in which the semiconductor laser device  32 , the solder layer  71 , and the mount  72  are stacked, is the thickness direction Z.  
      The semiconductor laser device  32  can be mounted onto the mount  72  by applying a solder material to the outermost surface  55 , that is, by applying a solder material to the incomplete adherent layer  51  and the complete adherent layer  53 , by means of die bonding. The solder material is made of AuSn. In the present embodiment, the solder material has an Au content of 70% and a Sn content of 30%. The solder layer  71  is made of the solder material.  
      The mount  72  is constituted by a heat sink. The mount  72  is composed of a mount main body  73 , a first mounting electrode layer  74 , and a second mounting electrode layer  75 . The first mounting electrode layer  74  is formed on one surface  73   a  of a thickness direction Z of the mount main body  73  so as to be stacked on the entire one surface  73   a . The second mounting electrode layer  75  is formed on the other surface  73   b  of the thickness direction Z of the mount main body  73  so as to be stacked on the entire surface of the other surface  73   b . Each of the one surface  73   a  and the other surface  73   b  is formed into a plane when viewed in the thickness direction X of the mount main body  73 . Each of the first mounting electrode  74  and the second mounting electrode  75  is formed so as to have a predetermined thickness. The Z direction-wise one surface  74   a  of the first mounting electrode layer  74  is formed into a plane. The mount main body  73  is made of a material having high electrical conductivity and also high thermal conductivity, such as aluminum nitride (AlN) and silicon carbide (SiC), the material of which thermal expansion coefficient is approximate to that of the semiconductor substrate  42 . The first mounting electrode layer  74  and the second mounting electrode layer  75  are made of a material having high electrical conductivity and also high thermal conductivity, such as Au, including a metallic material which can be alloyed with the solder material. By use of the mount main body  73  made of a material having a thermal expansion coefficient approximate to that of the semiconductor substrate  42 , the respective layers of the semiconductor sandwiched between the mount main body  73  and the semiconductor substrate  42  have reduced stress imparted thereto, which is caused by a difference in thermal expansion coefficient between the semiconductor laser apparatus  31  and the mount main body  73  when the semiconductor layer apparatus  31  is attached to the mount main body  73  by heat. By so doing, the degree of strain in the light-emitting region  40  can be reduced.  
      The semiconductor laser device  32  is die-bonded to the mount  72  under predetermined die-bonding conditions including a loading condition as to the level of load application required to mount the semiconductor laser device  32  onto the mount  72  and a heating condition as to the level of heat application required to mount the semiconductor laser device  32  onto the mount  72 .  
      Application of a physical load is necessary to press the semiconductor laser device  32  against the solder material applied to the mount  72 . However, if an unduly heavy load, for example, a load of 1.0 N (newton) is imposed on the semiconductor laser device  32 , the inner structure thereof such as the ridge waveguide will be subjected to a high pressing stress, thus causing the strain in the ridge waveguide and, as the worst case, there may occur breakage of the semiconductor laser device  32  in itself. By way of contrast, if an unduly light load, for example, a load of 0.05 N is imposed, the semiconductor laser device  32  cannot be pressed sufficiently against the solder material applied to the mount  72 , thus causing a failure of bonding and eventually causing separation. Although it will thus be seen that the mounting load is preferably adjusted to be more than 0.05 N and less than 1.0 N, from the standpoint of achieving mounting successfully with minimum loading, it is more preferable that the mounting load is adjusted to fall in a range of from, for example, 0.1 N to 0.3 N so that not a heavy load region but a light load region is set.  
      Moreover, application of heat is necessary to cause the solder material applied onto the mount  72  to melt so that the Au-made complete adherent layer  53  present on the outermost die-bonded surface of the semiconductor laser device  32  can be alloyed with the solder material. The mount  72  is placed on a heater to effect heating. At this time, if the mount  72  is heated excessively, for example, if it is heated at 360° C. (degree) for 30 s (seconds) and is thereafter forcibly cooled down for one second to approximately 200° C. with use of a blower, then a stress will be developed in the layer stacking arrangement existing within the semiconductor laser device  32  due to layer peeling and separation resulting from differences in thermal expansion coefficient, variation in physical properties, an alloying reaction, or other factors. This results in occurrence of strain. By way of contrast, if the mount  72  is heated insufficiently, for example, if it is heated at 280° C. for 0.3 s and is thereafter forcibly cooled down for one second to approximately 200° C. with use of a blower, then the semiconductor laser device  32  cannot be die-bonded properly to the solder material applied to the mount  72  because of a failure of alloying, thus causing separation. In light of the foregoing, it is preferable that the mount  72  is heated at a temperature of more than 200° C. and less than 360° C. for more than 0.3 second and less than 30 seconds. From the standpoint of achieving bonding successfully with minimum heating, the heating condition should preferably be at 300° C. and for approximately 2 seconds.  
      The heating temperature condition depends to a large degree on the thickness of the complete adherent layer  53  present on the outermost die-bonded surface of the semiconductor laser device  32 . By setting the heating temperature at 300° C. and the heating duration at approximately 2 seconds in consideration of minimum heating, it is possible to reduce the thickness of the complete adherent layer  53  to, for example, 0.12 μm, and thereby allow the complete adherent layer  53  to be alloyed in a shorter period of time.  
      An alloying reaction between the solder material AuSn and Au constituting the complete adherent layer  53  starts with a heat application onto the mount  72  while the semiconductor laser device  32  is pressed against the solder material under the predetermined loading and heating conditions. In an alloying process of AuSn and Au, at first the solder material made of AuSn is caused to melt by heating, and the molten AuSn is adhered to the surface of the complete adherent layer  53 , and then, as the heating process continues, the adherent AuSn is diffused into the complete adherent layer  53 . As to the direction of diffusion, AuSn travels in the direction of thickness of the complete adherent layer  53 , and then starts to diffuse at certain several points (diffusion points) on the surface of the complete adherent layer  53 . As the heating process continues further, the number of the diffusion points is increased and simultaneously the diffusion point changes its shape from a spot to a circle. The speed and depth at which AuSn travels in the thickness direction Z of the complete adherent layer  53  depend upon the ratio in absolute amount between the solder material AuSn and Au constituting the complete adherent layer  53 , namely the mass ratio, and the level of heating. The time to be spent in completing the diffusion also depends upon the aforementioned factors. By increasing the amount of the solder material relatively to the amount of Au constituting the complete adherent layer  53  and also raising the level of heating, it is possible to allow the complete adherent layer  53  to be alloyed instantly on contact with AuSn. Accordingly, the complete adherent layer  53  present on the outermost die-bonded surface of the semiconductor laser device  32  is formed in the manner as described hereinabove, and the amount of the solder material is increased. In this state, the heating operation is discontinued at the instant when AuSn starts to diffuse, and the diffusion is thereupon no longer in process.  
      Of one outermost surface of the semiconductor laser device  32  in the thickness direction Z of the semiconductor substrate  42 , the outermost surface which is the farthest away from the semiconductor substrate  42 , the incomplete adherent region  68  where the Pt-made incomplete adherent layer  51  is formed, contains no Au and therefore, the solder material made of AuSn makes intimate contact with the incomplete adherent layer  61  whereas almost no alloying reaction takes place therebetween. The solder material AuSn applied to the mount  72  is alloyed only with the entire complete adherent layer  53  to thereby form an alloyed layer  53 A which is obtained by an alloying process between the complete adherent layer  53  and the solder material.  
      There are formed the Pt-made metal layer  52 , the Au-made plate electrode layer  49  and ohmic electrode layer  48 , and the like layers as base layers for the complete adherent layer  53 . These base layers are also subjected to the influence of stress given to the complete adherent layer  53  when alloyed with the solder material. That is, pressing force and pulling force are applied to the base layers. Of the stress, the pressing force arises when the solder material is caused to expand through application of heat, whereas the pulling force arises when the heated solder material is cooled down after the solder material has been heated. Accordingly, so long as the solder material is caused to expand, makes contact with the semiconductor laser device  32  and is caused to contract in a uniform manner, the semiconductor laser device  32  is subjected to a uniform stress, which results in reduction in the degree of strain. This makes it possible to bond the semiconductor laser device  32  in substantially bare chip (raw chip) form to the mount  72 . In reality, however, the solder material is caused to expand and contract differently from part to part. Therefore, during the heating process, the semiconductor laser device  32  is subjected partly to a strong pressing force and partly to a weak pressing force. This gives rise to lack of uniformity in the alloying reaction, wherefore a stress is generated locally. As the uneven alloying reaction is going on, the heating is discontinued to effect cooling, and the solder material thereupon starts to contract. At this time, the semiconductor laser device  32  is subjected partly to a strong pulling force and partly to a weak pulling force, in addition to strong and weak pressing forces.  
      Note that AuSn is a solder material of one type that is hard to be alloyed with the Pt-made incomplete adherent layer  51  in a temperature range of from 300° C. to 400° C. in which AuSn is thermally bondable. Accordingly, of the outermost surface  55  of the semiconductor laser device  32 , the complete adherent region  69  where the complete adherent layer  53  and the solder material are alloyed with each other, has an increased strength of bonding with the solder layer  71  as described above when the semiconductor laser device  32  is being mounted onto the mount  72 , causing large stress. Of the outermost surface  55  of the semiconductor laser device  32 , by way of contrast, the incomplete adherent region  68  where the incomplete adherent layer  51  is formed, has a decreased strength of bonding with the solder layer  71  because the incomplete adherent layer  51  and the solder material are hardly alloyed with each other under the above-described die-bonding conditions when semiconductor laser device  32  is being mounted onto the mount  72 , allowing reduction in stress imparted to the light-emitting region  40  when the solder material undergoes the thermal expansion and thermal contraction.  
      During operation of the semiconductor laser apparatus  31 , the semiconductor laser device  32  generates heat which is then conducted to the solder layer  71  and the mount  72 , with the result that the semiconductor laser device  32 , the solder layer  71 , and the mount  72  undergo the thermal expansion. At the time, differences in thermal expansion coefficient among the semiconductor laser device  32 , the solder layer  71 , and the mount  72  cause the stress imposed on the light-emitting region  40 . In this case, by virtue of an incomplete bonding between the incomplete adherent layer  51  and the solder layer  71 , it is possible to reduce the stress generated by the difference in thermal expansion coefficient between the incomplete adherent layer  51  and the solder layer  71 , thereby reducing the stress being imparted from the incomplete adherent layer  51  to the light-emitting region  40 , with the result that the degree of strain in the light-emitting region  40  can be reduced.  
       FIG. 5  is a graph showing the relationship between a ratio of a length of incomplete adherent region  68  to a length of light-emitting region  40  when viewed in the longitudinal direction X, and a service life of the semiconductor laser apparatus  31 . At the outset, the above-described semiconductor laser apparatus  31  was produced, and its service life was measured with different ratios of the length of incomplete adherent region  68  to that of light-emitting region  40  when viewed in the longitudinal direction X, that is, different ratios of the X direction-wise length of incomplete adherent layer  51  to the X direction-wise length of light-emitting region  40 . In  FIG. 5 , a horizontal axis represents the ratio of the length of incomplete adherent region  68  to that of light-emitting region  40  when viewed in the longitudinal direction X, that is, a value obtained by a calculation L 5 /L 6 ×100 (%), and a vertical axis represents the length of service life (h). The measurement was conducted in a manner that the produced semiconductor laser apparatus  31  was placed in an atmosphere of 75° C. to supply the semiconductor laser apparatus  31  with a pulse current so that optical output of 300 mW can be obtained.  
      As the incomplete adherent region  68  is larger, that is, as the value L 5 /L 6  is larger, the service life of the apparatus tends to be enhanced. However, if the value obtained by the calculation L 5 /L 6 ×100 (%) is less than 20%, the service life of the apparatus is drastically deteriorated to be short. A reason for the shortened service life of the apparatus is that the value obtained by the calculation L 5 /L 6 ×100 (%) less than 20% results in increased strain of the light-emitting region  40 , leading an increase of crystal defects in the light-emitting region  40  during application of current. Accordingly, in order to enhance the service life of the apparatus, the ratio of the length of incomplete adherent region  68  to that of the light-emitting region  40  when viewed in the longitudinal direction X only needs to be 20% or more.  
       FIG. 6  is a graph showing the relationship between the ratio of the length of incomplete adherent region  68  to that of light-emitting region  40  when viewed in the longitudinal direction X, and an operating current of the semiconductor laser apparatus  31 . The above-described semiconductor laser apparatus was produced, and its operating current was measured with different ratios of the length of incomplete adherent region  68  to that of light-emitting region  40  when viewed in the longitudinal direction X. In  FIG. 6 , a horizontal axis represents the ratio of the length of incomplete adherent region  68  to that of light-emitting region  40  when viewed in the longitudinal direction X, that is, a value obtained by a calculation L 5 /L 6 ×100 (%), and a vertical axis represents a level of a driving current (mA) . The measurement of the driving current was conducted in a manner that the produced semiconductor laser apparatus  31  was placed in an atmosphere of 75° C. to supply the semiconductor laser apparatus  31  with a pulse current so that optical output of 300 mW can be obtained.  
      As the incomplete adherent region  68  is larger, that is, as the value L 5 /L 6  is larger, the operating current of the apparatus tends to be increased. However, it can be seen that, if the value obtained by the calculation L 5 /L 6  ×100 (%) is made so large as to exceed 80%, the operating current is drastically increased, with the result that a problem arises in reliability at a high temperature. In order to prevent the operating current at a high temperature from increasing, the ratio of the length of incomplete adherent region  68  to that of the light-emitting region  40  when viewed in the longitudinal direction X only needs to be 80% or less.  
      In the semiconductor laser device  32  according to the embodiment, the ratio of the length of incomplete adherent region  68  to that of light-emitting region  40  when viewed in the longitudinal direction X, that is, the value obtained by the calculation L 5 /L 6 ×100 (%), is selected to be 20% or more and 80% or less, in other words, the above-stated formula (1) is satisfied, whereby the service life of the apparatus can be prevented from decreasing and the operating current during operation at a high temperature can be prevented from increasing. As a result, it is possible to provide a semiconductor laser apparatus having a longer service life, of which operating reliability at a high temperature is enhanced.  
       FIG. 7  is a graph showing a radiation pattern of emitted light of the semiconductor laser apparatus  31  according to the embodiment.  FIG. 8  is a graph showing a radiation pattern of emitted light of a semiconductor laser apparatus of a comparative example.  FIG. 7  and  FIG. 8  respectively show radiation patterns at optical outputs of 90 mW, 100 mW, 110 mW, and 120 mW. The radiation pattern is represented in a form of a far field pattern (FFP for short) along a horizontal direction, that is, a direction which is parallel to a Z direction-wise surface of the semiconductor substrate  42 . In  FIG. 7  and  FIG. 8 , the radiation pattern obtained at optical output of 90 mW is shown in a solid line, the radiation pattern obtained at optical output of 100 mW is shown in a chain line, the radiation pattern obtained at optical output of 110 mW is shown in a two-dot chain line, and the radiation pattern obtained at optical output of 120 mW is shown in a dotted line. In this case, a value obtained by the calculation L 5 /L 6 ×100 (%) is 67%. In each of  FIG. 7  and  FIG. 8 , a horizontal axis represents a radiation angle, and a vertical axis represents light intensity.  
      The semiconductor laser apparatus  31  according to the embodiment and the semiconductor laser apparatus of the comparative example are different only in a region where the incomplete adherent region  68  is formed. In the semiconductor laser apparatus of the comparative example, the incomplete adherent region  68  is formed on the light-reflecting end face  32 B-side, and the complete adherent region  69  is formed on the light-emitting end face  32 A-side.  
      It can be seen that the radiation pattern shown in  FIG. 8  obtained by the semiconductor laser apparatus of the comparative example includes distortions which are larger at a larger output, whereas the radiation pattern shown in  FIG. 7  obtained by the semiconductor laser apparatus  31  according to the embodiment includes no distortions. By providing the incomplete adherent region  68  at the light-emitting end, that is, by designing a L 5 -long region extending from the light-emitting end face  31 A to be the incomplete adherent region  68 , it is possible to prevent the distortions from arising in the radiation pattern. In a part of the light-emitting region  40  extending in a resonance direction, of the semiconductor laser device  32 , where the incomplete adherent region  69  is formed, that is, a part on which the alloyed layer  53 A is stacked, light passing through the light-emitting region  40  is influenced by refractive index fluctuation caused by internal stress, with the result that the distortions arise in the radiation pattern. By contrast, in a part of the light-emitting region  40  on which the incomplete adherent layer  51  is stacked, light passing through the light-emitting region  40  is less easily influenced by the refractive index fluctuation by virtue of a small amount of the internal stress, allowing reduction of the distortions arising in the radiation pattern.  
      Although a part of the light-emitting region  40  is influenced by the refractive index fluctuation, the influence of the refractive index fluctuation is alleviated while the light passes through the part of the light-emitting region  40  where the incomplete adherent region  68  is formed. As a result, the light being emitted from the emitting end has no influence of the refractive index fluctuation, causing no distortions in the radiation pattern.  
      When the length of L 5  is short, the radiation pattern has large distortions. In order not to generate distortions in the radiation pattern, it is desirable that the incomplete adherent region  68  range from the light-emitting end face  32 A by 100 μm or more.  
      As described above, in the semiconductor laser apparatus  31 , the above-stated incomplete adherent region  68  is formed on the outermost surface  55  to which the solder layer  71  is to be applied, thereby allowing reduction of the internal stress of the semiconductor laser device  32 , with the result that the service life of the apparatus can be enhanced. Furthermore, the complete adherent region  69  is also formed so that there can be an enhanced efficiency of dissipating heat from the semiconductor laser device  32  to the mount  72  via the solder layer  71 , with the result that the operating current at a high temperature can be reduced.  
      Further, in the semiconductor laser apparatus  31 , upon mounting the semiconductor laser device  32  onto the mount  72 , the solder material is applied to the entire Z direction-wise surface of the semiconductor laser device  32  and adhered thereto. Accordingly, the solder material need not be applied partly to the outermost surface  55  so that the manufacturing process is simplified.  
      Although the complete adherent layer  53  is made of Au in the present embodiment, the complete adherent layer  53  in another embodiment of the invention may be made of a material predominantly containing Au, of which Au content is 60% to 90%. Also in this case, the same effects can be obtained. And furthermore, the stress caused by the alloying process between the complete adherent layer  53  and the solder material can be prevented so that the degree of strain in the light-emitting region  40  can be reduced.  
       FIG. 9  is a sectional view showing a semiconductor laser apparatus  131  according to another embodiment of the invention. The semiconductor laser apparatus  131  according to the present embodiment has basically the same structure as the semiconductor laser apparatus  31  according to the preceding embodiment. Accordingly, the constituent components that play the same or corresponding roles as in the semiconductor laser apparatus  31  will be denoted by the same reference symbols, and descriptions thereof will be omitted while only different parts will be described.  
      In the semiconductor laser apparatus  131 , a cavity  91  is created between the incomplete adherent layer  51  and the solder layer  71 . The cavity  91  is formed in the surface groove portion  82  of the semiconductor laser device  32 . The Z direction-wise one surface of the ridge protrusion  83  contacts the solder layer  71 , but a part of the surface groove portion  82  where the incomplete adherent layer  51  is formed, does not contact the solder layer  71 . The cavity  91  is formed between the incomplete adherent layer  51  and the solder layer  71  so as to extend from one end to the other end in the longitudinal direction X of the incomplete adherent layer  51 .  
      In the embodiment, the incomplete adherent layer  51  is made of Mo, the complete adherent layer  53  is made of Au, and the solder layer  71  is made of AuSn. Upon mounting the semiconductor laser device  32  onto the mount  72 , the mount  72  is placed at a lower position in a direction of gravitational force and then, the semiconductor laser device  32  is mounted onto the mount  72  from an upper position in the direction of gravitational force. The semiconductor laser device  32  having a ridge structure is adhered to the mount  72  by use of the solder material under the above-mentioned die-bonding conditions, and the Au-made complete adherent layer  53  and the AuSn-made solder material are easily alloyed with each other to thereby form the alloyed layer  53 A whereas the Mo-made incomplete adherent layer  51  and the AuSn-made solder material are not alloyed with each other at all. Since the wettability between Mo and AuSn is low, the solder material moves to the mount  72 -side by gravity before coagulated in the surface groove portion  82 , whereby the cavity  91  can be formed between the incomplete adherent layer  51  and the solder layer  71  in the surface groove portion  81 . The cavity  91  is a space surrounded by a Y direction-wise surface portion of the ridge protrusion  83 , a surface portion of the bottom of surface groove portion  82  where the incomplete adherent layer  51  is formed, and the solder layer  71 . A surface of the solder layer  71  facing the cavity  91  is inclined in a substantially linear form from the Z direction-wise surface portion of the ridge protrusion  83  to an end portion on the ridge protrusion  83 -side of the complete adherent layer  53 . In the embodiment, the incomplete adherent layer is a non-alloying layer, and the incomplete adherent region is a non-alloying region.  
      Also in the semiconductor laser apparatus  131  according to the embodiment, it is possible to obtain effects which are the same as those obtained in the above-described semiconductor laser apparatus  31 . Moreover, the cavity  91  contributes to the reduction of the stress imparted laterally in the width direction Y to the ridge protrusion  83  of the semiconductor laser device  32 . This makes it possible to further reduce the internal stress of the semiconductor laser device  32 , allowing further extension of the length of service life of the semiconductor laser apparatus  31 .  
      Although the efficiency of dissipating heat from the incomplete adherent layer  51  to the mount  72  through the solder layer  71  is slightly decreased due to the concavity  91 , the efficiency of heat dissipation in the complete adherent region  69  is high enough to provide a semiconductor laser apparatus having a long service life, in which the operating current during operation at a high temperature can be prevented from increasing and of which operating reliability at a high temperature is enhanced, as in the case of the semiconductor laser apparatus  31  according to the preceding embodiment.  
       FIG. 10  is a plan view of a semiconductor laser device  132  provided in the semiconductor laser apparatus according to still another embodiment of the invention when seen from a side to be mounted onto the mount  72 . The semiconductor laser apparatus according to the present embodiment and the above-described semiconductor laser apparatus  31  shown in FIGS.  1  to  4  have basically the same structure except only for regions for forming the complete adherent layer  53  and the incomplete adherent layer  51  located on the outermost surface of semiconductor laser device to which the solder layer  71  is applied. Accordingly, the constituent components that play the same or corresponding roles as in the semiconductor laser apparatus  31  will be denoted by the same reference symbols, and descriptions thereof will be omitted while only different parts will be described.  
      In the semiconductor laser device  132 , the incomplete adherent layer  51  and the complete adherent layer  53  of the outermost surface  55  of the semiconductor laser device  132  are provided alternately along the longitudinal direction X of the semiconductor laser device  132  in an area  60  ranging outward by the predetermined second distance L 3  on either side from a virtual plane which passes through the Y direction-wise center of the light-emitting region  40  and which is perpendicular to the width direction Y.  
      Assume that in the order from the light-emitting end face  132 A to the light-reflecting end face  132 B, the incomplete adherent layers  51  distanced away from each other along the longitudinal direction X are referred to as first to n-th (symbol n is an integer of 2 or more) incomplete adherent layers T 1 , T 2 , . . . , (Tn- 1 ), Tn, and X direction-wise lengths of the first to n-th incomplete adherent layers T 1 , T 2 , . . . , (Tn- 1 ), Tn are represented respectively by N 1 , N 2 , . . . , (Nn- 1 ), Nn. In this case, the sum of X direction-wise lengths N 1 , N 2 , . . . , (Nn- 1 ), Nn of the first to n-th incomplete adherent layers T 1 , T 2 , . . . , (Tn- 1 ), Tn, that is to say, a value obtained by the calculation N(N=N 1 +N 2 +. . . +Nn− 1 +Nn) is selected so as to satisfy the following relation: 
 
0.2 ×L 6 ≦N ≦0.8 ×L 6  . . . (2) 
 
 wherein L 6  represents the X direction-wise length of the light-emitting region  40 . 
 
      The first incomplete adherent layer T 1  is formed in an area which is located at a distance N 1  from the light emitting end face  132 A. The X direction-wise lengths N 1 , N 2 , . . . , (Nn- 1 ), Nn of the first to n-th incomplete adherent layers T 1 , T 2 , . . . , (Tn- 1 ), Tn are respectively selected to be 100 μm, so as to be 50 μm or more and less than 300 μm, for example. The lengths N 1  to Nn are adjusted so as to equalize the internal stress of the semiconductor laser device, thereby equalizing a distribution of internal temperature.  
      In the present embodiment, the first incomplete adherent layer T 1  and the second adherent layer T 2  are formed. In the outermost surface of semiconductor laser device  132  to which the solder layer  71  is applied, the first incomplete adherent layer T 1  is formed in an area ranging from the light-emitting end face  132 A along the longitudinal direction X by the length N 1 , and the second incomplete adherent layer T 2  is formed in an area ranging from a position at a predetermined distance L 7  from an end of the first incomplete adherent layer T 1  on the light-reflecting end face  132 B-side along the longitudinal direction X by the length N 2 . An end of the second incomplete adherent layer T 2  on the light-reflecting end face  132 B-side and the light-reflecting end face  132 B are distanced away from each other by a predetermined distance L 8 . Accordingly, the first incomplete adherent layer T 1  and the second incomplete adherent layer T 2  are formed in the range  60  so that a calculation L 6 =N 1 +L 7 +N 2 +L 8  is satisfied.  
      The values N 1 , L 7 , N 2 , and L 8  are preferably selected to be substantially the same for the sake of equalization of the internal stress of the semiconductor laser device and equalization of the distribution of internal temperature.  
      Upon mounting the above-described semiconductor laser device  132  onto the mount  72  by use of the AuSn-made solder material under the above-stated heating conditions, the complete adherent layer  53  and solder material of the outermost surface are alloyed with each other to from an alloyed layer while the first and second incomplete adherent layers T 1  and T 2  and the solder material are not alloyed with each other. Accordingly, a part where the complete adherent layer  53  is formed turns out to be a complete adherent region, and a part where the incomplete adherent layer  51  is formed turns out to be an incomplete adherent region. As a result, the internal stress of the semiconductor laser device  132  can be dispersed from the area  60  in the longitudinal direction X of the light-emitting region  40  and furthermore, the heat transmission paths of high heat conduction can be dispersed in the longitudinal direction of the light-emitting region  40 . Accordingly, the internal stress imparted to the light-emitting region  40  is equalized as much as possible when viewed in the longitudinal direction X so that the distortions arising in the radiation pattern can be reduced. Further, the heat conduction from the light-emitting region  40  to the mount  72  is equalized as much as possible when viewed in the longitudinal direction X so that a temperature of the light-emitting region  40  can be made as uniform as possible. As a result, it is possible to further prevent the operating current at a high temperature from increasing.  
      Although the incomplete adherent layer and the complete adherent layer are formed in the outermost surface to which the solder layer is applied, of the semiconductor laser device having the ridge structure in the above-described embodiments, the incomplete adherent layer and the complete adherent layer may be formed in the outermost surface to which the solder layer is applied, of the semiconductor laser device having a rib structure. Also in a semiconductor laser apparatus provided with the semiconductor laser device having a rib structure, it is possible to obtain effects which are the same as those obtained in the semiconductor laser apparatus provided with the semiconductor laser device having the ridge structure.  
      The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.