Abstract:
A laser capable of improving surge withstand voltage by preventing damage to a read end surface, and a method of manufacturing the same are provided. A laser diode includes a laser resonator between a first end surface as a main emmission end surface and a second end surface facing the first end surface, and the laser diode includes a light absorption inhibition region on the second end surface side of the laser resonator.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Japanese Patent Application JP 2006-045312 filed in the Japanese Patent Office on Feb. 22, 2006, the entire contents of which being incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a laser diode suitable in the case where a very low operating current is desired, and a method of manufacturing the same. 
         [0004]    2. Description of the Related Art 
         [0005]    In a laser diode, in the case where a very low operation current (very low current consumption) is desired, in general, a design focusing on the efficiency of a guided wave in a resonator and featuring very high optical density is adopted. Thereby, while a reduction in the operating current is achieved, a disadvantage such as a large decline in surge withstand voltage becomes apparent, so it is necessary to handle the laser with extra care. 
         [0006]    In particular, an InAlGaP-based red laser diode mounted in a portable game console or the like using a DVD (Digital Versatile Disk) as a medium has a structure in which current consumption is reduced to a minimum. Therefore, compared to a typical laser for replaying DVDs, low surge withstand voltage as one characteristic of the laser diode appears prominently. In some cases, the surge withstand voltage is reduced to approximately 10 V. 
         [0007]    A decline in the surge withstand voltage depends on the material of a mirror film arranged on an end surface. As widely used materials of the mirror film, an Al 2 O 3  single layer for a front end surface (a main emission end surface) and an Al 2 O 3 /a-Si multilayer film for a rear end surface are used. However, in the case where Al 2 O 3 /a-Si is used for the rear end surface in a red laser diode, a-Si slightly absorbs red light, so when a surge current or an overcurrent enters, the rear end surface becomes susceptible to damage. This causes an impediment to improvement in the surge withstand voltage. 
         [0008]    Therefore, to improve the surge withstand voltage, it can be considered to change the material of the mirror film on the rear end surface. More specifically, a method of using a material such as TiO x  instead of a-Si can be considered (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-164978). 
       SUMMARY OF THE INVENTION 
       [0009]    However, such an alternative material does not have as high a refractive index as a-Si, so there are issues that a necessity to increase the number of layers of the mirror film, difficulty in using the most simple electron beam evaporation system with short tact, and a necessity to strictly control a composition ratio to obtain a stable refractive index, and compared to a-Si, it is more difficult to handle the alternative material, and its cost tends to be higher. Therefore, the material is not usually used for a laser for replay of which costs are necessary to be reduced. 
         [0010]    On the other hand, as a method of reinforcing end surfaces for the purpose of higher output, in related arts, a method of having a so-called window structure in the front and rear end surfaces is widely used (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-82863). However, when the window structure is applied to the laser for replay, a FFP (Far Field Pattern) is reduced, so there is an issue such as deterioration of replay characteristics or conditions of application to an optical system. 
         [0011]    Moreover, in general, variations in the FFP tend to be wider by the application of the window structure, and specifically in the laser for replay, a radiation angle is relatively large, so variations in the FFP becomes wider accordingly, thereby variations in optical coupling becomes wider. 
         [0012]    Further, in a step of forming a laser coupler, for example, by bonding a laser to a photodetection IC (Integrated Circuit) together with a prism, in some cases, the laser emits light at a threshold value or less to recognize the image of a luminous point. At this time, when the window structure is provided to the front end surface, a luminous portion becomes blurred to have wider variations, thereby the recognition accuracy is reduced. Luminous point recognition accuracy is an extremely important parameter in terms of high accuracy mounting. In particular, in a process for achieving very high accuracy mounting by passive alignment, a decline in the luminous point recognition accuracy may be a critical issue. 
         [0013]    In view of the foregoing, it is desirable to provide a laser diode capable of improving a surge withstand voltage by preventing damage to a rear end surface, and a method of manufacturing the laser diode. 
         [0014]    According to an embodiment of the invention, there is provided a laser diode including a laser resonator between a first end surface as a main emission end surface and a second end surface facing the first end surface, the laser diode including: a light absorption inhibition region on the second end surface side of the laser resonator. 
         [0015]    In the laser diode according to the embodiment of the invention, the light absorption inhibition region is arranged on the second end surface side facing the first end surface where light is emitted, so even in the case where a mirror film on the second end surface is made of a material which absorbs light generated in the laser resonator, the effect of light absorption is mitigated. Therefore, even if a surge current or an overcurrent is applied, damage to the second end surface is prevented, and a surge withstand voltage is improved. 
         [0016]    According to an embodiment of the invention, there is provided a method of manufacturing a laser diode, the laser diode including a laser resonator between a first end surface as a main emission end surface and a second end surface facing the first end surface, the method including the steps of: forming a semiconductor layer including a plurality of planned laser resonator regions; forming a light absorption inhibition region in the semiconductor layer along a position where the second end surface is planned to be formed; and forming the first end surface and the second end surface so that the light absorption inhibition region is disposed on the second end surface side of the laser resonator. 
         [0017]    In the laser diode according to the embodiment of the invention, the light absorption inhibition region is arranged on the second end surface side facing the first end surface as the main emission end surface, so even in the case where a mirror film on the second end surface is made of a material which absorbs light generated in the laser resonator, the effect of light absorption can be mitigated, and damage to the second end surface due to the application of a surge current or an overcurrent can be prevented, and a surge withstand voltage can be improved. In particular, the laser diode is suitably used in the case where a very low operating current and very low current consumption are strongly desired, for example, as a laser for replaying DVDs or the like mounted in a portable game console. 
         [0018]    In the method of manufacturing a laser diode according to the embodiment of the invention, after the light absorption inhibition region is formed in the semiconductor layer, the first end surface and the second end surface are formed so that the light absorption inhibition region is disposed on the second end surface side of the laser resonator, so the light absorption inhibition region can be easily arranged on the second end surface side by adjusting the positions of the first end surface and the second end surface. 
         [0019]    Other and further objects, features and advantages of the invention will appear more fully from the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a sectional view of a laser diode according to an embodiment of the invention; 
           [0021]      FIG. 2  is a plan view of the laser diode shown in  FIG. 1  when viewed from a stripe-shaped portion side; 
           [0022]      FIG. 3  is a plan view showing a modification of the laser diode shown in  FIG. 2 ; 
           [0023]      FIG. 4  is a plan view for describing a method of manufacturing the laser diode shown in  FIG. 1 ; 
           [0024]      FIG. 5  is a plan view for showing a modification of  FIG. 4 ; 
           [0025]      FIG. 6  is a plan view for describing a manufacturing step following a step shown in  FIG. 4 ; 
           [0026]      FIG. 7  is a plan view for describing a manufacturing step following the step shown in  FIG. 6 ; and 
           [0027]      FIG. 8  is a plan view for showing a modification of  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    A preferred embodiment will be described in detail below referring to the accompanying drawings. 
         [0029]      FIG. 1  shows an example of a sectional structure of a laser diode according to an embodiment of the invention. The laser diode is used, for example, as a laser for replaying DVDs of a portable game console or the like, and has an oscillation wavelength of approximately 660 nm and an output of approximately 4 mW to 5 mW. The laser diode has a structure in which an n-type cladding layer  12 , an active layer  13 , a p-type cladding layer  14  and a p-side contact layer  15  are laminated in this order on the substrate  11 . 
         [0030]    The substrate  11  is made of n-type GaAs doped with silicon as an n-type impurity. The n-type cladding layer  12  is made of, for example, n-type AlGaInP mixed crystal doped with silicon as an n-type impurity. The active layer  13  has a multiquantum well structure including a well layer and a barrier layer which are made of, for example, Al x Ga y In 1-x-y P (x≧0 and y≧0) mixed crystal with different compositions. The p-type cladding layer  14  is made of, for example, p-type AlGaInP mixed crystal doped with zinc as a p-type impurity. The p-side contact layer  15  is made of, for example, p-type GaAs doped with zinc as a p-type impurity. A part of the p-type cladding layer  14  and the p-side contact layer  15  form a thin stripe-shaped portion  16  extending in a resonator direction for current confinement, and a region corresponding to the p-side contact layer  15  of the active layer  13  is a light-emitting region (a current injection region). The width of the stripe-shaped portion  16  is extremely thin, thereby the optical density in the light-emitting region is increased, and the threshold value is reduced to approximately ten-odd mA so that the laser diode is allowed to operate at a very low operating current. Moreover, a current block region  17  is arranged on both sides of the stripe-shaped portion  16 . 
         [0031]    A p-side electrode  21  is arranged on the p-side contact layer  15 . The p-side electrode  21  is formed, for example, by laminating titanium (Ti), platinum (Pt) and gold (Au) on the p-side contact layer  15  in this order, and is electrically connected to the p-side contact layer  15 . On the other hand, an n-side electrode  22  is formed on the back surface of the substrate  11 . The n-side electrode  22  is formed, for example, by laminating an alloy of gold (Au) and germanium (Ge), nickel (Ni) and gold (Au) on the substrate  11  in order, and then alloying them by heat treatment. 
         [0032]      FIG. 2  shows a plan view of the laser diode when viewed from the stripe-shaped portion  16 . The p-side electrode  21  is not shown in  FIG. 2 . The laser diode has a pair of facing resonator end surfaces in the extending direction of the stripe-shaped portion  16 , that is, a first (front) end surface  10 F and a second (rear) end surface  10 R, and a laser resonator  10  is formed between the first end surface  10 F and the second end surface  10 R. The first end surface  10 F is a so-called main emission end surface, and light generated in the active layer  13  is mainly emitted from the first end surface  10 F. On the first end surface  10 F, a first mirror film  30 F made of, for example, an Al 2 O 3  single layer is formed. On the other hand, on the second end surface  10 R, a second mirror film  30 R made of, for example, an Al 2 O 3 /a-Si multilayer film is formed. 
         [0033]    Moreover, in the laser diode, a light absorption inhibition region  40  is arranged on the second end surface  10 R side of the laser resonator  10 . The light absorption inhibition region  40  inhibits light generated in the active layer  13  from being absorbed by a-Si included in the second mirror film  30 R, and is preferably an impurity-doped region including an impurity such as boron (B), silicon (Si) or zinc (Zn). Among them, boron (B) is preferable. Thereby, in the laser diode, even in the case where the second mirror film  30 R is made of a-Si which absorbs light generated in the active layer  13 , damage to the second end surface  10 R can be prevented by mitigating the effect of light absorption. 
         [0034]    The end of the light absorption inhibition region  40  does not necessarily match the second end surface  10 R, and, for example, as shown in  FIG. 3 , a gap  50  may be arranged between the second end surface  10 R and the light absorption inhibition region  40 . As will be described later in manufacturing steps, it is because in the case where the second end surface  10 R is formed by cleavage after forming the light absorption inhibition region  40 , it is difficult to form the second end surface  10 R so as to match the end of the light absorption inhibition region  40 , and the laser diode can be easily manufactured by arranging the gap  50  with an appropriate width between them. 
         [0035]    The laser diode can be manufactured by the following steps. 
         [0036]    At first, for example, the substrate  11  made of n-type GaAs is prepared, and the n-type cladding layer  12 , the active layer  13 , the p-type cladding layer  14  and the p-side contact layer  15  made of the above-described materials are grown in order on a surface of the substrate  11  by, for example, a MOCVD (Metal Organic Chemical Vapor Deposition) method. As shown in  FIG. 4 , a plurality of planned laser resonator regions  10 A are included in the substrate  11 , the n-type cladding layer  12 , the active layer  13 , the p-type cladding layer  14  and to the p-side contact layer  15  formed in the above manner, and they are separated by a plurality of vertical separation lines  10 B and a plurality of horizontal separation lines  10 C. Referring to  FIG. 4 , the vertical separation lines  10 B and horizontal separation lines  10 C are shown by dotted lines, and one planned laser resonator region  10 A is diagonally shaded, and a planned first end surface position  10 FA and a planned second end surface position  10 RA are shown in the diagonally shaded planned laser resonator region  10 A. 
         [0037]    Next, as shown in  FIG. 4 , an impurity is diffused or injected from the p-side contact layer  15  so as to form the light absorption inhibition region  40  inside the vertical separation line  10 B along with the planned second end surface position  10 RA. 
         [0038]    More specifically, an insulating film (not shown) made of, for example, SiO 2  is formed on the p-side contact layer  15  by, for example, a CVD (Chemical Vapor Deposition) method, and an aperture is formed by photolithography and etching in a position corresponding to a position where the light absorption inhibition region  40  is planned to be formed in the insulating film. Next, for example, boron (B), silicon (Si) or zinc (Zn) as the impurity, preferably boron (B) is diffused into semiconductor crystal via the aperture by impurity diffusion or impurity implantation. Thereby, the impurity reaches the active layer  13 , and the active layer  13  is disordered so that the light absorption inhibition region  40  is formed. After that, the insulating film is removed. 
         [0039]    At this time, as shown in  FIG. 5 , the light absorption inhibition region  40  is formed along every other vertical separation line  10 B, and the vertical separation line  10 B may be positioned at the center of the light absorption inhibition region  40 . 
         [0040]    After forming the light absorption inhibition region  40 , an insulating film (not shown) made of, for example, SiO 2  is formed on the p-side contact layer  15  again by, for example, the CVD method, and the insulating film is subjected to, for example, photolithography and etching to form a mask for forming the stripe-shaped portion  16 . Next, a part of the p-side contact layer  15  and a part of the p-type cladding layer  14  are removed by etching using the mask so as to form the stripe-shaped portion  16  as shown in  FIG. 6 . After that, the current block region  17  is formed on both sides of the stripe-shaped portion  16  by selective epitaxial growth using the mask. After that, the mask is removed. 
         [0041]    After forming the stripe-shaped portion  16  and the current block region  17 , the p-side electrode  21  made of the above-described material is formed on the p-side contact layer  15 , and the n-side electrode  22  made of the above-described material is formed on the back surface of the substrate  11  (refer to  FIG. 1 ). 
         [0042]    After forming the p-side electrode  21  and the n-side electrode  22 , as shown in  FIG. 7 , the first end surface  10 F and the second end surface  10 R is formed by, for example, cleavage along the vertical separation lines  10 B so that the light absorption inhibition region  40  is disposed on the second end surface  10 R side of the laser resonator  10 . At this time, as shown in  FIG. 8 , the gap  50  may be arranged between the second end surface  10 R and the light absorption inhibition region  40 . After that, the planned laser resonator regions  10 A are further separated by the horizontal separation lines  10 C to form separate laser resonators  10 . Finally, the first mirror film  30 F and the second mirror film  30 R are formed on the first end surface  10 F and the second end surface  10 R, respectively. Thus, the laser diode shown in  FIGS. 1 and 2  is completed. 
         [0043]    In the laser diode, when a voltage is applied between the p-side electrode  21  and the n-side electrode  22 , a current is injected into the active layer  13  to cause electron-hole recombination, thereby light is emitted. The light is reflected from the first mirror film  30 F and the second mirror film  30 R, and travels between them to cause laser oscillation, and the light is emitted from the first end surface  10 F to outside as a laser beam. In this case, the light absorption inhibition region  40  is disposed on the second end surface  10 R side opposite to the first end surface  10 F where the light is emitted, so even in the case where the second mirror film  30 R is made of a-Si which absorbs light generated in the laser resonator  10 , the effect of light absorption is mitigated. Therefore, even if a surge current or an overcurrent is applied, damage to the second end surface  10 R is prevented, and the surge withstand voltage is improved. 
         [0044]    Thus, in the embodiment, the light absorption inhibition region  40  is arranged on the second end surface  10 R side opposite to the first end surface  10 F where the light is emitted, so even if the second mirror film  30 R is made of a-Si which absorbs light generated in the laser resonator  10 , the effect of light absorption can be mitigated, and damage to the second end surface  10 R due to the application of a surge current or an overcurrent can be prevented, and the surge withstand voltage can be improved. In particular, the laser diode according to the embodiment is suitably used in the case where a very low operating current and very low current consumption are strongly desired, for example, as a laser for replaying DVDs or the like mounted in a portable game console. 
         [0045]    Moreover, a-Si is a material which is low-cost and easy to handle, so the method according to the embodiment is superior in cost and process stability, compared to a method using another material such as TiO x . 
         [0046]    Further, the light absorption inhibition region  40  is arranged on a side opposite to the first end surface  10 F where light is emitted, so the FFP is not narrowed, and a wide radiation angle θ// can be maintained, and high replay characteristics can be obtained specifically in a laser for replay. Variations in the FFP is not increased, and conditions of application to an optical system are advantageous, and when the laser diode according to the embodiment is used in an OP (Optical Pickup), advantages can be obtained in stability in jitter or view characteristics. Further, variations in optical coupling can be reduced, and yields can be increased, and stable characteristics can be achieved. 
         [0047]    In addition, the recognition accuracy at the time of recognizing the image of a luminous point can be improved, and variations in performance in the case where an optical integrated device or a hybrid device is formed with high accuracy mounting can be prevented, and yields can be improved. 
         [0048]    Further, in the embodiment, after the light absorption inhibition region  40 , the first end surface  10 F and the second end surface  10 R are formed so that the light absorption inhibition region  40  is disposed on the second end surface  10 R side of the laser resonator  10 , so the light absorption inhibition region  40  can be easily arranged by adjusting the positions of the first end surface  10 F and the second end surface  10 R. The position of the light absorption inhibition region  40  can be controlled only by the adjustment of the cutting positions of the first end surface  10 F and the second end surface  10 R, and a related-art process or wafer design can be used. 
         [0049]    Although the present invention is described referring to the embodiment, the invention is not necessarily limited to the embodiment, and can be variously modified. For example, in the embodiment, the case where an impurity-doped region is formed as the light absorption inhibition region  40  is described; however, the light absorption inhibition region  40  may be formed by removing the active layer  13  and burying the p-type cladding layer  14 . Moreover, in the case where the active layer  13  has a quantum well structure, the quantum well thickness of the light absorption inhibition region  40  may be reduced. 
         [0050]    Moreover, in the embodiment, the case where after the first end surface  10 F and the second end surface  10 R are formed, the planned laser resonator regions  10 A are further separated by the horizontal separation lines  10 C to form separate laser resonators  10  is described; however, the invention is applicable to a laser bar which is not separated by the horizontal separation lines  10 C. 
         [0051]    Further, the invention is not necessarily limited to the material, the thickness, the forming method and forming conditions and the like of each layer described in the embodiment, and any other material and any other thickness may be used, or any other forming method and any other forming conditions may be used. For example, in the embodiment, the case where each layer made of an AlGaInP-based compound is formed by the MOCVD method is described; however, the layer may be formed by any other vapor deposition method such as a MBE (Molecular Beam Epitaxy) method or a hydride vapor phase epitaxy method. The hydride vapor phase epitaxy method is a vapor deposition method in which halogen contributes to transport or reaction. 
         [0052]    In addition, each layer may be made of any other Group III-V compound semiconductor including at least indium (In) selected from Group  3 B elements in the short form of the periodic table of the elements and at least phosphorus (P) selected from Group  5 B elements in the short form of the periodic table of the elements. Further, the invention is applicable to a laser diode made of any other material such as a GaN-based material or a GaAs-based material. 
         [0053]    Further, in the embodiment, the laser diode is described referring to a specific structure as an example; however, the invention is applicable to a laser diode with any other structure in the same manner. For example, in the embodiment, a ridge waveguide type laser diode which is a combination of a gain waveguide type and a refractive index waveguide type is described as an example; however, the invention is applicable to a gain waveguide type laser diode and a refractive index waveguide type laser diode in the same manner. 
         [0054]    In addition, in the embodiment, the structure of the laser diode is described in detail; however, all layers are not necessarily included, and any other layer may be added. For example, a light guide layer may be arranged between the active layer and the n-type cladding layer or the p-type cladding layer. 
         [0055]    Further, the invention is not limited to a low-output, low-threshold-value and low-operating-current laser, and is applicable to high-output laser or the like. 
         [0056]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.