Patent Publication Number: US-8124431-B2

Title: Nitride semiconductor laser device and method of producing the same

Description:
PRIORITY STATEMENT 
     This application is a divisional under 35 U.S.C. §121 of U.S. application Ser. No. 11/715,443, filed Mar. 8, 2007, now U.S. Pat. No. 7,804,878 which claims priority under 35 U.S.C. §119 to Japanese Application No. 2006-069350, filed Mar. 14, 2006, the entire contents of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to improvement in a nitride semiconductor laser device and a method of producing the same. 
     2. Description of the Related Art 
     From the past, prototypes of nitride semiconductor laser devices for emitting light in a wavelength range from blue to ultraviolet have been fabricated using nitride-based semiconductor materials such as GaN, InN, AlN, and a mixed crystal semiconductor thereof. 
       FIG. 9  is a schematic cross-sectional view exemplarily showing a main part of a conventional nitride semiconductor laser device for lasing at a wavelength of 405 nm. This semiconductor laser device includes an n-type GaN layer (thickness: 3 μm)  902 ; an n-type In 0.05 Ga 0.95 N buffer layer  903 ; an n-type Al 0.05 Ga 0.95 N clad layer (thickness: 2.0 μm)  904 ; an n-type GaN optical waveguide layer (thickness: 0.1 μm)  905 ; an In 0.2 Ga 0.8 N/n-type In 0.05 Ga 0.95 N (respective thicknesses: 4 nm/8 nm) triple-quantum-well (3MQW) active layer  906 ; a p-type Al 0.2 Ga 0.8 N carrier stop layer (thickness: 20 nm)  907 ; a p-type GaN optical waveguide layer (thickness: 0.1 μm)  908 ; a p-type Al 0.05 Ga 0.95 N clad layer (thickness: 0.5 μm)  909 ; and a p-type GaN contact layer (thickness: 0.2 μm)  910  stacked in this order on an n-type GaN substrate  901 . 
     A ridge-like stripe  902  of 2 μm width is formed by partially etching p-type GaN contact layer  910  through to a partial depth of p-type AlGaN clad layer  909 . Accordingly, the laser device of  FIG. 9  has an optical confinement waveguide structure in which active layer  906  and optical waveguide layers  905  and  908  are sandwiched between clad layers  904  and  909 , and then light generated in active layer  906  is confined in this waveguide structure and causes lasing action. It should be noted that  FIG. 9  does not show the entire width of the laser device chip but only a part thereof in the vicinity of the ridge-like stripe. 
     A p-side contact electrode  911  is formed on p-type GaN contact layer  910  left after the partial etching, and insulating films  913  are formed on the partially etched areas. A p-side electrode pad  914  is formed in contact with p-side contact electrode  911 . On the other hand, an n-side electrode  915  is formed on a rear surface of substrate  901 . 
     The laser device of  FIG. 9  is obtained by dividing a wafer including a large number of laser device structures into chips. To facilitate the chip division, therefore, substrate  901  is polished to have a thickness of 50 to 200 μm before n-side electrode  915  is formed thereon. Then, the nitride semiconductor laser device of  FIG. 9  with a width of 300 to 400 μm is obtained by dividing the wafer into chips. 
       FIG. 10  is a schematic plan view of the nitride semiconductor laser device of  FIG. 9  seen from above. Ridge-like stripe  912  is formed on the upper surface of the nitride semiconductor laser device, and the p-side contact electrode (not shown) is formed over the entire upper surface of the ridge-like stripe. Further, p-side electrode pad  914  is formed to cover almost the entire upper surface of the nitride semiconductor laser device. 
     The nitride semiconductor laser device of  FIG. 10  is mounted on a stem, and thereafter wire bonding is performed in an area not including ridge-like stripe  912  on p-side electrode pad  914 . Therefore, p-side electrode pad  914  is generally formed such that a circular region of more than 80 μm diameter is securely obtained for a wire bonding area. 
     Generally, in order to reduce the cost for a nitride semiconductor laser device, it is desirable to increase the yield rate of the laser devices obtained from one wafer by reducing the width of the laser device to 50 to 250 μm. However, when a nitride semiconductor laser device has a reduced width, problems arise in a laser device fabricated by a technique described below. 
     In Japanese Patent Laying-Open No. 2004-356454, trenched regions each having a stripe-like groove are formed at an interval of several hundred micrometers on a nitride semiconductor substrate in order to suppress occurrence of cracks during crystal growth of nitride semiconductor layers. Accordingly, a hill portion is naturally formed between adjacent trenched regions. By growing nitride semiconductor layers and form a layered structure thereof on a substrate subjected to such working as above (hereinafter referred to as a worked substrate), occurrence of cracks in the layered structure can be prevented, and surface flatness of the layered structure over the hill portion can be improved to some extent. 
     When the nitride semiconductor layers are grown to form the layered structure on the worked substrate, however, a swelling with a height of several micrometers is caused adjacent to a trenched region in an upper surface of the layered structure. Taking account of the photolithographic process in fabricating a nitride semiconductor laser device, therefore, the ridge-like stripe structure should be formed more than 90 μm away and preferably more than 110 μm away from the trenched region. 
       FIG. 11  is a schematic plan view showing an example of an upper surface of a bar obtained after formation of resonator end faces of each nitride semiconductor laser device (i.e., after a wafer including a large number of laser device structures is divided into bars) and just before the bar (hereinafter referred to as a laser bar) is divided into individual chips. In  FIG. 11 , trenched regions  1101  are formed at an interval of 800 μm for example, and then ridge-like stripes  1102  and p-side electrode pads  1103  are formed between trenched regions  1101 . Every dotted line in  FIG. 11  indicates a chip division plane, and the chip width is set to 200 μm for example. 
     As described above, in  FIG. 11 , a distance (L) between ridge-like stripe  1102  and trenched region  1101  should desirably be set to more than 110 μm. In order to set the chip width to 200 μm, therefore, ridge-like stripe  1102  in each of certain particular chips is set at a position different from that in the other chips, with respect to the width direction of the chip. More specifically, in  FIG. 11 , the position of ridge-like stripe  1102  in a laser device chip (B) is different from that in a laser device chip (A), with respect to the width direction of the chip. 
     The laser bar in such a state as above may cause problems as described below during the subsequent process of producing laser devices. 
     Firstly, after the laser bar is divided into individual laser device chips, it is necessary to inspect all the chips. In the case of using an automatic chip inspection apparatus, if the apparatus is set to determine the position of the ridge-like stripe in chip (A) as normal, then there is a problem that it determines all of chips (B) as defective. 
     Further, when a chip is mounted on a stem, the light-emitting point in the chip should be made coincident with the center of the stem. However, since the light-emitting point in chip (B) is different from that in chip (A) with respect to the width direction, it is necessary to mount chip (B) at a chip position changed relatively on the stem. 
     SUMMARY 
     The present invention has been made to solve the aforementioned problems, and makes it possible that, when nitride semiconductor laser devices of a small width are handled in an automatic chip inspection apparatus or an automatic mounting apparatus, only some particular chips can be selected readily and automatically by image recognition. 
     In the specification of the present application, the automatic image recognition includes processes of measuring a size of an area having a light intensity of more than a preset threshold value in a light intensity distribution obtained by imaging a chip from above with a camera, comparing the measured size with a preset size, and determining whether it falls within a preset tolerance. 
     A method of producing a nitride semiconductor laser device according to the present invention includes the steps of: forming a wafer including a nitride semiconductor layer of a first conductivity type, an active layer of a nitride semiconductor, a nitride semiconductor layer of a second conductivity type, and an electrode pad for the second conductivity type stacked in this order on a main surface of a conductive substrate and also including a plurality of stripe-like waveguide structures parallel to the active layer; cutting the wafer to obtain a first type of nitride semiconductor laser device chip and a second type of nitride semiconductor laser device chip; and distinguishing between the first type and the second type of laser device chips by automatic image recognition, wherein the first type and the second type of laser device chips are different from each other in position of the stripe-like waveguide structure with respect to a width direction of each chip and also in area ratio of the electrode pad for the second conductivity type to the main surface of the substrate. 
     It is desirable that the area ratio of the electrode pad for the second conductivity type in the first type of laser device chip is less than 90% of that of the electrode pad for the second conductivity type in the second type of laser device chip. In each of the first type and the second type of laser device chips, a metal layer of less than 0.1 μm thickness may be formed in an area not having the electrode pad for the second conductivity type on an upper surface of the semiconductor layer of the second conductivity type, or the metal layer may be omitted. 
     It is desirable that, in each of the first type and the second type of laser device chips, the electrode pad for the second conductivity type includes a circular area of 80 μm diameter. Preferably, the circular area is spaced by a distance of more than 10 μm from the stripe-like waveguide structure. 
     Preferably, in each of the first type and the second type of laser device chips, the main surface of the substrate is a rectangle having two sides parallel to and the other two sides perpendicular to the stripe-like waveguide structure, and each of the other two sides has a length of more than 50 μm and less than 250 μm. 
     It is desirable that, in each of the first type and the second type of laser device chips, an area not having the electrode pad for the second conductivity type on an upper surface of the semiconductor layer of the second conductivity type is lower in reflectance by more than 10% with respect to almost the entire wavelength range of incident illumination light vertical to the chip as compared to the other area having the electrode pad for the second conductivity type during the automatic image recognition. 
     For that purpose, it is preferable that the area not having the electrode pad for the second conductivity type on the upper surface of the semiconductor layer of the second conductivity type has a layer of more than 10 nm thickness made of an absorbing material having an absorption coefficient of more than 10000 cm −1  with respect to almost the entire wavelength range of incident illumination light vertical to the chip during the automatic image recognition. Preferably, the absorbing material is insulator or semiconductor, and can include silicon, germanium, or TiO 2 . 
     Instead of having such a light absorbing layer, the area not having the electrode pad for the second conductivity type on the upper surface of the semiconductor layer of the second conductivity type may have minute surface unevenness, and a root mean square roughness over a length of 5 μm in a direction parallel to the uneven surface is preferably more than 1 nm and less than 200 nm. 
     A nitride semiconductor laser device according to the present invention includes a nitride semiconductor layer of a first conductivity type, an active layer of a nitride semiconductor, a nitride semiconductor layer of a second conductivity type, and an electrode pad for the second conductivity type stacked in this order on a main surface of a conductive substrate, wherein a stripe-like waveguide structure parallel to the active layer is formed, and an area of the electrode pad for the second conductivity type is more than 2% and less than 90% of an area of the main surface of the substrate. 
     In an area not having the electrode pad for the second conductivity type on an upper surface of the semiconductor layer of the second conductivity type, a metal layer of less than 0.1 μm thickness may be formed, or may be omitted. 
     Preferably, the electrode pad for the second conductivity type includes a circular area of 80 μm diameter, and the circular area is spaced by a distance of more than 10 μm from the stripe-like waveguide structure. 
     Preferably, the main surface of the substrate is a rectangle having two sides parallel to and the other two sides perpendicular to the stripe-like waveguide structure, and each of the other two sides has a length of more than 50 μm and less than 250 μm. 
     It is desirable that an area not having the electrode pad for the second conductivity type on an upper surface of the semiconductor layer of the second conductivity type is lower by more than 10% in reflectance with respect to almost the entire wavelength range of incident illumination light vertical to the chip as compared to the other area having the electrode pad for the second conductivity type during the automatic image recognition. 
     For that purpose, it is preferable that the area not having the electrode pad for the second conductivity type on the upper surface of the semiconductor layer of the second conductivity type has a layer of more than 10 nm thickness made of an absorbing material having an absorption coefficient of more than 10000 cm −1  with respect to almost the entire wavelength range of incident illumination light vertical to the chip during the automatic image recognition. Preferably, the absorbing material is insulator or semiconductor, and can include silicon, germanium, or TiO 2 . 
     Instead of having such a light absorbing layer, the area not having the electrode pad for the second conductivity type on the upper surface of the semiconductor layer of the second conductivity type may have minute surface unevenness, and a root mean square roughness over a length of 5 μm in a direction parallel to the uneven surface is preferably more than 1 nm and less than 200 nm. 
     The nitride semiconductor laser device according to the present invention can preferably be included as a light source in an optical information reproducing apparatus. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view showing nitride semiconductor laser devices in a state of a laser bar in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic plan view illustrating a shape of a p-side electrode pad of a chip (B) in  FIG. 1  in more detail. 
         FIG. 3  is a schematic plan view illustrating a shape of a p-side electrode pad of a chip (A) in  FIG. 1  in more detail. 
         FIG. 4  is a conceptual block diagram showing a general constitution of an automatic chip inspection apparatus. 
         FIG. 5  is a schematic plan view illustrating a shape of a p-side electrode pad of a chip regarding a nitride semiconductor laser device according to another embodiment of the present invention. 
         FIG. 6  is a schematic cross-sectional view showing a main part of a layered structure in a nitride semiconductor laser device according to still another embodiment of the present invention. 
         FIG. 7  is a schematic block diagram illustrating an optical system and a propagation path of light for reproduction in an optical information reproducing apparatus according to still another embodiment of the present invention. 
         FIG. 8  is a schematic cross-sectional view showing a main part of a layered structure in the nitride semiconductor laser device shown in  FIG. 1 . 
         FIG. 9  is a schematic cross-sectional view exemplarily showing a main part of a conventional nitride semiconductor laser device. 
         FIG. 10  is a schematic plan view of the nitride semiconductor laser device of  FIG. 9  seen from above. 
         FIG. 11  is a schematic plan view exemplarily showing an upper surface of a laser bar according to the prior art just before division into individual chips. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
       FIG. 1  is a schematic plan view showing nitride semiconductor laser devices in a state of a laser bar according to a first embodiment of the present invention. In a laser bar  101  of this drawing, there are formed a plurality of trenched regions  102 . Between trenched regions  102 , a plurality of ridge-like stripes  103  are formed, and a plurality of p-side electrode pads  104  are formed to cover ridge-like stripes  103 . Every dotted line in  FIG. 1  indicates a chip division plane. 
     In the first embodiment, an area ratio of p-side electrode pad  104  to the upper surface area (hereinafter referred to as a chip area) of a nitride semiconductor laser device (B) is less than 90% differently from that of a nitride semiconductor laser device (A). 
     In an automatic chip inspection apparatus that uses an automatic image recognition process, it is preset to determine whether a chip is acceptable or not based on the shape of p-side electrode pad  104  and preset to determine nitride semiconductor laser device (A) as acceptable, for example. When the nitride semiconductor laser devices of the first embodiment are introduced into the automatic chip inspection apparatus, only chips (A) are extracted for chip inspection, and chips (B) are determined as having an unacceptable shape and left in a chip supplying area without being inspected. Therefore, it is possible to eliminate a problem that all of the laser devices are subjected to chip inspection and rather many of them are determined as defective in the case of chip inspection for the conventional nitride semiconductor laser devices. 
       FIG. 2  is a schematic plan view illustrating a shape of the p-side electrode pad of chip (B) in more detail. In this drawing, a nitride semiconductor laser device  201  is set to have a chip width  202  of 50 to 250 μm (e.g., 200 μm) and a length  203  (corresponding to a resonator length) of 300 to 1500 μm (e.g., 600 μm). A trenched region  204  of a worked substrate exists along the right side of laser device  201 . A ridge-like stripe  205  is formed at a position spaced by a distance  208  of 120 μm from the right side. 
     A p-side electrode pad  206  includes a first area covering ridge-like stripe  205  and a second area for wire bonding. The first area has a width of 5 to 50 μm (e.g., 20 μm) and a length retracted by 5 to 50 μm (e.g., 20 μm) from each end of the resonator. The second area of electrode pad  206  is formed as an area (e.g., as a square with a side of 80 μm) including a circle  207  of 80 μm diameter. 
     In chip (B), the area ratio of the p-side electrode pad to the chip area is set to less than 90% of that in chip (A). For example, when the chip has a width of 200 μm and a length of 600 μm, and the p-side electrode pad includes the first area of 20 μm width and 560 μm length and the second area of a square with a side of 80 μm, the chip area is 120000 μm 2  and the area of the p-side electrode pad is 17600 μm 2 . Thus, the area of the p-side electrode pad accounts for about 15% of the chip area. 
       FIG. 3  is a schematic plan view illustrating a shape of the p-side electrode pad of chip (A) in more detail. In this drawing, a nitride semiconductor laser device  301  is set to have a chip width  302  of 50 to 250 μm (e.g. 200 μm) and a length  303  of 300 to 1500 μm (e.g., 600 μm). A ridge-like stripe  304  is formed at a position spaced by a distance  306  of 120 μm from the left side of the chip. A p-side electrode pad  305  is formed being retracted by a distance of 10 to 50 μM (e.g., 20 μm) from the chip division planes and the resonator end faces. For example, when the chip has a width of 200 μm and a length of 600 μm, and the p-side electrode pad is formed to have a width of 160 μm and a length of 560 μm, the chip area is 120000 μm 2  and the area of the p-side electrode pad is 89600 μm 2 . Thus, the area of the p-side electrode pad accounts for about 75% of the chip area. 
     In the first embodiment, therefore, the area ratio of the p-side electrode pad to the chip area in chip (B) accounts for 20% (=15÷75×100%) of that in chip (A). 
       FIG. 4  is a conceptual block diagram showing a general constitution of an automatic chip inspection apparatus. A chip to be introduced into the automatic chip inspection apparatus is set in a chip supplying area  401 . Here, the automatic chip inspection apparatus recognizes the chip by automatic image recognition, confirms its shape and carry out alignment thereof, and then transfers the chip to a chip inspection area  402 . After measurement of properties such as I-L and I-V properties of the semiconductor laser in the chip inspection area, the automatic chip inspection apparatus transfers the measured chip to a chip storage area  403 . 
     In the image recognition in chip supplying area  401 , an image of a chip is taken from its upper surface with a camera, and the shape of the p-side electrode pad of the laser device can be recognized based on brightness distribution in the image. When a recognized size of the p-side electrode pad is compared with a size of a recognition image input beforehand into the automatic chip inspection apparatus and matches the size of the recognition image within a tolerance input beforehand, the chip is transferred to the subsequent chip inspection area. On the other hand, if the recognized size of the p-side electrode pad falls out of the tolerance, the chip is determined as unacceptable and disregarded, and the automatic chip inspection apparatus shifts to image recognition for a subsequent chip. 
     If the tolerance is set for example to 30% on this occasion, all of chips (A) can be transported to chip inspection area  402 . However, chip (B) is determined as unacceptable because the area of its p-side electrode pad falls out of the tolerance, differently from chip (A). Therefore, chip (B) is left in chip supplying area  401  without being transported to chip inspection area  402 . Consequently, chips (B) left in chip supplying area  401  can be handled after chips (A) set in chip supplying area  401  are all inspected and transferred to chip storage area  403 . That is, chip (A) and chip (B) can readily be separated based on the shapes of their p-side electrode pads. 
     As described above, since chip (A) and chip (B) can be separated and inspected by the automatic chip inspection apparatus, it is possible to set inspection conditions optimized for each position of their ridge-like stripes. Further, since chip (A) and chip (B) can be separately introduced into an automatic mounting apparatus, it is possible to set process conditions optimized for each position of their light-emitting points. 
     In order to separate both the types of chips in a more stable manner, it is preferable to set the area ratio of the p-side electrode pad to the chip area in chip (B) to a value obtained by subtracting a tolerance from the area ratio in chip (A) or further subtracting more than 5% (preferably more than 10%). For example, when the area ratio of the p-side electrode pad to the chip area in chip (A) is 100% and the aforementioned tolerance is 0%, both the types of chips can be separated in a stable manner if the area ratio of the p-side electrode pad to the chip area in chip (B) is set to 90%. In other words, in order to obtain the effect of the present invention, the upper limit of the area ratio of the p-side electrode pad to the chip area is calculated as 90%, based on the above calculation. 
     In order to separate both the types of chips in a more stable manner, however, the tolerance is preferably set to about ±1% to 50%, more preferably at 10% to 40%. This is because, even when chips are fabricated with the same design, some chips are liable to be determined as unacceptable due to contamination on the chip, peeling of the p-side electrode pad caused during the process, or scratches on the chip, even if these defects cause no problems in the properties of the laser device. Consequently, when the aforementioned tolerance is set to 30% for example, the area ratio of the p-side electrode pad to the chip area in chip (B) is desirably set to less than 60% and preferably less than 40% of the area ratio in chip (A). 
       FIG. 8  is a schematic cross-sectional view showing a main part of a layered structure in the nitride semiconductor laser device of the first embodiment. This laser device includes an n-type GaN layer (thickness: 3 μm)  802 ; an n-type In 0.05 Ga 0.95 N buffer layer  803 ; an n-type Al 0.05 Ga 0.95 N clad layer  804  (thickness: 2.0 μm); an n-type GaN optical waveguide layer  805  (thickness: 0.1 μm); an In 0.2 Ga 0.8 N/n-type In 0.05 Ga 0.95 N (respective thicknesses: 4 nm/8 nm) triple-quantum-well (3MQW) active layer  806 ; a p-type Al 0.2 Ga 0.8 N carrier stop layer  807  (thickness: 20 nm); a p-type GaN optical waveguide layer  808  (thickness: 0.1 μm); a p-type Al 0.05 Ga 0.95 N clad layer  809  (thickness: 0.5 μm); and a p-type GaN contact layer  810  (thickness: 0.2 μm) stacked in this order on an n-type GaN substrate  801 . 
     A ridge-like stripe  812  of 2 μm width is formed by partially etching p-type GaN contact layer  810  through to a partial depth of p-type AlGaN clad layer  809 . The laser device includes an optical confinement waveguide structure in which active layer  806  and optical waveguide layers  805  and  808  are sandwiched between clad layers  804  and  809 , and light generated in the active layer is confined in this waveguide structure and causes lasing. It should be noted that  FIG. 8  does not show the entire width of the laser device chip but only a part thereof in the vicinity of the ridge-like stripe. 
     A p-side contact electrode  811  is formed on p-type GaN contact layer  810  left after the partial etching, while an insulating film  813  and a metal layer  816  of less than 0.1 μm thickness are formed on the partially etched areas. A p-side electrode pad  814  is formed to cover p-side contact electrode  811 . In the first embodiment, metal layer  816  is formed to improve adhesiveness between insulating film  813  and p-side electrode pad  814 ; by preferably using Mo or the like, though there arises no problem in laser device properties even if it is omitted. 
     On the other hand, an n-side electrode  815  is formed on a rear surface of substrate  801 . The laser device of  FIG. 8  is obtained by dividing a wafer including a large number of device structures into chips. To facilitate the chip division, therefore, substrate  801  is polished to have a thickness of 50 to 200 μm before n-side electrode  815  is formed thereon. Then, the wafer is divided into chips to obtain the nitride semiconductor laser device of  FIG. 8  with a width of 300 to 400 μm. 
     As described above, it is necessary in the present invention to recognize the shape of the p-side electrode pad in the nitride semiconductor laser device by automatic image recognition. On the upper surface of the chip, therefore, as compared to the area having the p-side electrode pad, the other remaining area should have a lower reflectance with respect to almost the entire wavelength range of incident illumination light vertical to the chip during the automatic image recognition. Here, if metal layer  816  has a thickness of more than 0.1 μm, there is only a small difference in reflectance between the area having p-side electrode pad  814  and the other remaining area. In contrast, if metal layer  816  has a thickness of less than 0.1 μm, the normal incident light transmits through metal layer  816 , and thus it is possible to cause a difference in reflectance between the area having p-side electrode pad  814  and the other remaining area. That is, although the effect of the present invention can be obtained even without metal layer  816 , if metal layer  816  is provided to improve adhesiveness of p-side electrode pad  814  to insulating film  813 , it is desirable that metal layer  816  has a thickness of less than 0.1 μm. 
     Since the problems to be solved by the present invention are caused when the chip has a relatively narrow width such as 50 to 250 μm as described above, the present invention is particularly effective when there is a need to reduce the chip width. Although the ridge-like stripe is formed at a position 120 μm away from the right or left side of the chip in the first embodiment, it should be noted that it may be formed at another position as long as it is spaced from the trenched region of the substrate by more than 90 μm, preferably by more than 110 μm. Further, although the chip division plane is provided along the trenched region in the first embodiment, this is not necessary. 
     Second Embodiment 
       FIG. 5  is a schematic plan view illustrating a shape of a p-side electrode pad of a chip (B) as a nitride semiconductor laser device according to a second embodiment of the present invention. In this drawing, a nitride semiconductor laser device  501  is set to have a width  502  of 50 to 250 μm (e.g., 200 μm) and a length (corresponding to a resonator length)  503  of 300 to 1500 μm (e.g., 600 μm). A trenched region  504  of a worked substrate exists along the right side of chip  501 . A ridge-like stripe  505  is formed at a position spaced by a distance  508  of 120 μm from the right side of the chip. A p-side electrode pad  506  is formed to include its partial area placed to cover ridge-like stripe  505  and another partial area (e.g., a square with a side of 80 μm) including a circle  507  of 80 μm diameter for wire bonding. P-side electrode pad  506  is formed with its area falling in a range of more than 2% and less than 90% of the chip area. 
     Circular area  507  for wire bonding is placed preferably more than 5 μm and more preferably more than 10 μm (e.g., 20 μm) away from ridge-like stripe  505 . This structure can eliminate a risk that ultrasonic power applied during the wire bonding may reach a semiconductor part near the ridge-like stripe and cause defects such as cracks. 
     In the second embodiment, for example, when chip  501  has a width of 250 μm and a length of 1500 μm, and p-side electrode pad  506  includes its partial area with a width extending by 20 μm to each of the right and left sides from the center of ridge-like stripe  505  and its another partial area of a square with a side of 80 μm including circle  207  of 80 μm diameter for wire bonding, the chip area is 375000 μm 2  and the area of the p-side electrode pad is 9600 μm 2 . Thus, the area of the p-side electrode pad accounts for about 2.5% of the chip area. In order to obtain the effect of the present invention, the lower limit of the area ratio of the p-side electrode pad to the chip area is 2.5% as calculated by the above calculation. 
     Third Embodiment 
       FIG. 6  is a schematic cross-sectional view showing a main part of a layered structure in a nitride semiconductor laser device according to a third embodiment of the present invention. The cross-sectional structure of  FIG. 6  is different from that of  FIG. 8  only in that metal layer  816  of less than 0.1 μm thickness is replaced by a layer  616  of a light absorbing material of more than 10 nm thickness. That is, other layers  601  to  615  in  FIG. 6  correspond to layers  801  to  815  in  FIG. 8 , respectively. Further, the p-side electrode pad in the nitride semiconductor laser device of the third embodiment can have a shape identical to that of the first or second embodiment. 
     Since light absorbing layer  616  is formed on insulating film  613  in the nitride semiconductor laser device of the third embodiment, an area having p-side electrode pad  614  has a higher reflectance with respect to visible light, as compared to the other remaining area not having p-side electrode pad  614 . Owing to this, when image recognition is carried out in the automatic chip inspection apparatus, the high contrast between the p-side electrode pad and the other part makes it possible to obtain an effect that recognition errors during the image recognition can sufficiently be prevented. 
     To make this effect significant, it is desirable during the automatic image recognition that, as compared to the area having the p-side electrode pad, the area not having the p-side electrode pad is lower by more than 10% (preferably, by more than 20%) in reflectance with respect to almost the entire wavelength range of incident illumination light vertical to the chip. This is achieved if light absorbing layer  616  has an absorption coefficient of more than 10000 cm −1 . As a material satisfying this condition, Si, Ge, TiO 2 , or the like can be used for example. Desirably, light absorbing layer  616  also serves as an insulating layer, and for this purpose, light absorbing layer  616  is preferably an insulator such as TiO 2  or a semiconductor such as Si or Ge. 
     Fourth Embodiment 
     A nitride semiconductor laser device according to a fourth embodiment of the present invention is different from those of the first and second embodiments only in that it has minute surface unevenness on bottom surfaces of areas partially etched and removed to form the ridge-like stripe. With such surface unevenness, as compared to the area having the p-side electrode pad, the area not having the p-side electrode pad can be lower by more than 10% in reflectance with respect to almost the entire wavelength range of incident illumination light vertical to the chip during the automatic image recognition. The minute surface unevenness can realize the above reflectance ratio less by more than 10% when the RMS (root mean square) roughness is more than 1 nm over a length of 5 μm in a direction parallel to the etched bottom surface. The minute surface unevenness can be formed with adjustment of etching conditions as appropriate. 
     Fifth Embodiment 
     In a fifth embodiment of the present invention, one of the nitride semiconductor laser devices disclosed in the aforementioned embodiments is used as a light source for reproduction in an optical information reproducing apparatus. The optical information reproducing apparatus can include known components as components other than the light source. 
       FIG. 7  is a schematic block diagram illustrating an optical system and a propagation path of light for reproduction in an optical information reproducing apparatus according to the fifth embodiment of the present invention. This optical information reproducing apparatus includes a nitride semiconductor laser device  701  of the present invention, beam emission control means (not shown), a collimator lens  702 , a beam shaping prism  703 , a beam splitter  704 , an objective lens (focusing means)  705 , an optical disk (optical recording medium)  706 , focus position control means (not shown), and a light detecting system (light detecting means)  707  for detecting light. For clarity and simplicity of the drawing, components (means) not important for illustrating the features of the present invention are omitted in  FIG. 7 . Needless to say, on the other hand, optical disk (optical recording medium)  706  shown in  FIG. 7  is not specific to the optical information reproducing apparatus and is accepted when information is recorded or reproduced. 
     In the optical information reproducing apparatus of the fifth embodiment, nitride semiconductor laser device  701  can serve as a light source for both recording and reproduction. In recording operation and erasing operation, laser light emitted from nitride semiconductor laser device  701  is converted to parallel light or nearly parallel light by collimator lens  702 , passes through beam splitter  704 , and is focused by objective lens  705  onto an information recording surface of optical disk  706 . Then, bit-information is written on the recording surface of optical disk  706  by magnetic modulation or refractive index modulation. In reproducing operation, laser light emitted from nitride semiconductor laser device  701  is focused onto the record surface of optical disk  706  recorded by unevenness, magnetic modulation, or refractive index modulation. The focused laser light is reflected by the information record surface, passes through objective lens  705  and beam splitter  704 , and then enters light detecting system  707  in which an optically detected signal is converted to an electrical signal to read recorded information. 
     Since the optical information reproducing apparatus of the fifth embodiment uses a nitride semiconductor laser device that is inexpensive and has a small chip width in a range of 50 to 250 μm, its production cost can be suppressed. 
     As has been described, the present invention can provide a nitride semiconductor laser device of a small width at a low cost, and also contribute to cost reduction of an optical information reproducing apparatus by using the laser device. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.