Patent Publication Number: US-2023155347-A1

Title: Optical semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Japanese patent applications JP2022-020198 filed on Feb. 14, 2022 and JP2021-185154 filed on Nov. 12, 2021, the contents of which are hereby incorporated by reference into this application. 
     TECHNICAL FIELD 
     The present disclosure relates generally to an optical semiconductor device. 
     BACKGROUND 
     An optical semiconductor device that is to be used for optical communication can include an optical functional layer that converts electricity into light or light into electricity. For example, a laser and an external modulator can include an optical functional layer that uses a multiple quantum well layer. In addition, in a light-receiving element, an optical functional layer can be formed of a semiconductor absorption layer. In many cases, the optical semiconductor device can include an electrode made of a metal in order to apply a voltage to the optical functional layer, wherein a part of the electrode is electrically and physically connected to a semiconductor layer. In addition, an optical semiconductor device can include an insulating film that is arranged on, for the purpose of protecting, a surface of the semiconductor layer on which a metal is not arranged. 
     SUMMARY 
     When an optical semiconductor device is driven, an optical functional layer and other semiconductor layers, electrodes, and the like of the optical semiconductor device generate heat. The generated heat degrades characteristics of the optical semiconductor device. For example, a continuous wave laser (CW laser) that outputs continuous light has optical output characteristics as its main characteristics. It is desired that an optical output be large, and under a same drive current, the optical output to increase as a temperature of the optical semiconductor device decreases. In an environment with a constant outside temperature, when heat generated by the optical semiconductor device is released to the outside in a large amount, an effective temperature of the optical semiconductor device is decreased, and the optical output is improved. In other optical semiconductor devices as well as the CW laser, it is important that heat generated by the optical semiconductor devices be released to the outside in a large amount. 
     As described above, an optical semiconductor device may include a metal electrode and an insulating film (protective film). The electrode may be made of a metal, and hence may have high thermal conductivity and may provide excellent heat dissipation. The insulating film may be an oxide film or a silicon nitride film. Compared to a semiconductor and a metal, those materials have low thermal conductivity, which may inhibit release of generated heat to the outside. 
     In addition, a connection region between the electrode and the semiconductor layer may be limited to a narrow region. For example, in an optical semiconductor device having a stripe structure, a contact point between the electrode and the semiconductor layer is limited to only the upper surface of the stripe structure. However, from the viewpoint of heat dissipation, the electrode extends to a region wider than the width of the stripe structure. In this case, in order to achieve insulation between the electrode arranged on a region other than the region on the stripe structure and the semiconductor layer, the above-mentioned insulating film is arranged. 
     Accordingly, the insulating film is widely arranged on the surface of the semiconductor layer except for a small partial region, and the electrode is arranged on the insulating film. Because of this, the wide insulating film is positioned in a path through which heat generated in the semiconductor layer is released to the outside. As a result, a heat dissipation amount is limited, which serves as a factor for degrading characteristics of the optical semiconductor device. 
     Some implementations disclosed herein solve the above-mentioned problem, and provide an optical semiconductor device that provides improved heat dissipation. 
     In some implementations, an optical semiconductor device includes: a substrate; a semiconductor multilayer which is formed on the substrate, and includes an optical functional layer; an insulating film formed on the semiconductor multilayer; and an electrode formed on a part of the insulating film, wherein the insulating film covers the semiconductor multilayer except for a region in which the semiconductor multilayer and the electrode are electrically connected to each other, and wherein at least a part of a region of the insulating film that is overlapped with the electrode is thinner than a region of the insulating film that is not overlapped with the electrode. 
     In some implementations, the optical semiconductor device provides excellent heat dissipation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a top view of an example of an optical semiconductor device according to a first example implementation of the present invention. 
         FIG.  2    is a schematic sectional view taken along the line A-A′ of the optical semiconductor device illustrated in  FIG.  1   . 
         FIG.  3    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device according to the first example implementation of the present invention. 
         FIG.  4    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of an optical semiconductor device according to a second example implementation of the present invention. 
         FIG.  5    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device according to the second example implementation of the present invention. 
         FIG.  6    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of an optical semiconductor device according to a third example implementation of the present invention. 
         FIG.  7    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device according to the third example implementation of the present invention. 
         FIG.  8    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of an optical semiconductor device according to a fourth example implementation of the present invention. 
         FIG.  9    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device according to the fourth example implementation of the present invention. 
         FIG.  10    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 2 of the optical semiconductor device according to the fourth example implementation of the present invention. 
         FIG.  11    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of an optical semiconductor device according to a fifth example implementation of the present invention. 
         FIG.  12    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device according to the fifth example implementation of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Some implementations are specifically described in detail in the following with reference to drawings. In the drawings, the same members are denoted by the same reference numerals and have the same or equivalent functions, and a repetitive description thereof may be omitted for the sake of simplicity. Note that, the drawings referred to in the following are only for illustrating the example implementations, and are not necessarily drawn to scale. 
       FIG.  1    is a top view of an optical semiconductor device  1  according to a first example implementation of the present invention.  FIG.  2    is a schematic sectional view taken along the line A-A′ of  FIG.  1   . Here, the optical semiconductor device  1  is an edge-emitting CW laser. The optical semiconductor device  1  may include a stripe structure  3 . A top electrode  20  may be arranged on the surface of the optical semiconductor device  1 . The top electrode  20  may be an electrode formed on a part of an insulating film  26 , and is, for example, a metal film containing Au. The metal film may be formed of a plurality of materials. In addition, on the upper surface of the optical semiconductor device  1 , the insulating film  26  may be arranged in a region other than the top electrode  20 . The insulating film  26  may be arranged also under the top electrode  20  as illustrated in  FIG.  2   . The insulating film  26  is, for example, a silicon oxide film, a silicon nitride film, or an aluminum oxide film. The detail of the insulating film  26  may be described later. A low-reflection end face coating film  11  may be arranged on an end face on the left side of  FIG.  1   , and a high-reflection end face coating film  12  may be arranged on an end face on the right side of  FIG.  1   . The coating films may be merely examples, and the low-reflection end face coating films may be arranged on both the end faces. 
     As illustrated in  FIG.  2   , in the optical semiconductor device  1 , the stripe structure  3  may be formed on a first conductivity type substrate  21 . A buried layer  30  may be arranged on each side of the stripe structure  3 . The buried layer  30  may be a semi-insulating semiconductor layer or a semiconductor layer in which a plurality of p-type and n-type semiconductor layers may be combined. The insulating film  26  may be arranged on the upper surface of the buried layer  30 . The stripe structure  3  may be formed on a part of the substrate  21  so as to include a plurality of semiconductor layers. The plurality of semiconductor layers may be formed so as to include, from the bottom, a first conductivity type optical confinement layer  22 , an active layer  23  (an optical functional layer) formed of a multiple quantum well layer, a second conductivity type optical confinement layer  24 , a second conductivity type cladding layer  25 , and a contact layer  35 . A diffraction grating layer  33  may be formed in the middle of the second conductivity type cladding layer  25 . The stripe structure  3  may or may not include a part of the substrate  21 . The layers from the first conductivity type optical confinement layer  22  to the contact layer  35  may be hereinafter referred to as “semiconductor multilayer.” In addition, in the first example implementation, the semiconductor multilayer may include the stripe structure  3  and the buried layer  30  formed on each side of the stripe structure  3 . A rear electrode  31  may be arranged on the rear side of the substrate  21 . In the first example implementation, the semiconductor multilayer may be a CW laser corresponding to a wavelength band of 1.3 micrometers. However, the implementations described herein may not be limited thereto, and the wavelength band of laser light output by the semiconductor multilayer may be another wavelength band. In addition, an insulating substrate may be used as the substrate  21 . In this case, it may be required to arrange a first conductivity type semiconductor layer between the substrate  21 , which may be an insulating substrate, and the stripe structure  3 . 
     In some implementations, the insulating film  26  may cover the semiconductor multilayer except for a region in which the semiconductor multilayer and the top electrode  20  may be electrically connected to each other, and at least a part of a region of the insulating film  26  that is overlapped with the top electrode  20  may be thinner than a region of the insulating film  26  that is not overlapped with the top electrode  20 . In the first example implementation, as illustrated in  FIG.  2   , the thickness of the insulating film  26  may be different between the region in which the top electrode  20  and the buried layer  30  are overlapped with each other and the region in which the top electrode  20  and the buried layer  30  are not overlapped with each other. The region of the insulating film  26  that is overlapped with the top electrode  20  may be thinner than the region of the insulating film  26  that is not overlapped with the top electrode  20 . 
     In the optical semiconductor device  1 , the active layer  23  may emit light when a voltage is applied (a current is injected) between the top electrode  20  and the rear electrode  31 . The active layer  23  may also generate heat in addition to light emission. In addition, other semiconductor layers may also generate heat due to the flow of the current. The heat generated in the semiconductor multilayer may be released to the outside through the substrate  21  and the buried layer  30 . In the optical semiconductor device  1  of the first example implementation, the stripe structure  3  may be formed on a top surface side of the substrate  21 . For example, the distance from the top electrode  20  to the active layer  23  may be several micrometers, whereas the distance from the active layer  23  to the rear electrode  31  may be as thick as from tens of micrometers to 100 micrometers. Because of this, the generated heat may be released to an external environment in a larger amount on the top electrode  20  side as compared to the rear electrode  31  side. In addition, the heat generation amount becomes larger at a position closer to the active layer  23 , and hence most of the generated heat may be released from the top electrode  20  side. A part of the heat may pass through the stripe structure  3  and may be released directly from the top electrode  20 . However, besides the foregoing, as the heat dissipation path, there may be also a path leading to the top electrode  20  through the buried layer  30 . 
     The insulating film  26  may be arranged between the top electrode  20  and the buried layer  30 . The insulating film  26  may be a silicon oxide film, a silicon nitride film, an aluminum oxide film, or the like in which thermal conductivity may be smaller than that of the semiconductor multilayer as described above. Because of this, the insulating film  26  arranged between the top electrode  20  and the buried layer  30  may hinder heat dissipation. However, in the first example implementation, the insulating film  26  in the region that is overlapped with the top electrode  20  may be formed to be thin. The insulating film  26  in the region is, for example, may be 100 nanometers or less. Accordingly, heat may be released to the outside without significantly reducing heat dissipation. 
     Meanwhile, the insulating film  26  in the region that is not overlapped with the top electrode  20  may be set to hundreds of nanometers. When the thickness of the insulating film  26  in the region that is not overlapped with the top electrode  20  is set to be the same as the thickness of the region that is overlapped with the top electrode  20 , there may be a risk in that the function of the insulating film  26  as a protective film in the region that is not overlapped with the top electrode  20  may not be sufficiently obtained, and there is a risk in that the reliability of the optical semiconductor device  1  may be influenced. In addition, in the region in which the top electrode  20  is arranged, the top electrode  20  functions as a protective film, and hence the reliability may not be substantially influenced even when the insulating film  26  is thinned. 
     With this configuration, an optical semiconductor device having excellent optical characteristics because of excellent heat dissipation may be provided while the reliability is ensured. 
       FIG.  3    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device  1 . The difference from the foregoing is the shape of the insulating film  26 . In Modification Example 1, the thickness of the insulating film  26  in the vicinity of each end portion of the top electrode  20  is larger than the thickness in the vicinity of the stripe structure  3 . That is, a part of the thick region of the insulating film  26  is overlapped with a part of the end portion of the top electrode  20 . This structure may be excellent from the viewpoint of manufacturability. In the manufacturing procedure of the optical semiconductor device  1 , after the buried layer  30  and the semiconductor multilayer are formed and the insulating film  26  is arranged, the top electrode  20  may be arranged on the insulating film  26 . In the foregoing, the end portion of the top electrode  20  and the boundary in which the thickness of the insulating film  26  is changed are matched with each other, but such matching may not be obtained due to manufacturing variations. In this case, for example, the top electrode  20  may not be overlapped with the thin region of the insulating film  26 . As described above, when the insulating film  26  is thin, there may be a concern in that the reliability may be decreased. In order to avoid a structure in which the thin region of the insulating film  26  is not covered with the top electrode  20  due to the manufacturing variations, in Modification Example 1, the thickness of the insulating film  26  in the vicinity of the end portion of the top electrode  20  may be intentionally set to be the same as the thickness of the region that is not overlapped with the top electrode  20 . With this configuration, the risk in that the thin region of the insulating film  26  may not be overlapped with the top electrode  20  may be reduced. Although Modification Example 1 may provide less heat dissipation as compared to the first example implementation, the influence may be small because the boundary in which the thickness is changed is the region away from the stripe structure  3 . The position of the boundary in which the thickness of the insulating film  26  is changed may be determined in consideration of the manufacturing variations. Specifically, it may be desired that the region in which the thin insulating film  26  and the top electrode  20  are overlapped with each other be set to at least 50% or more of the area of the top electrode  20 . In addition, it may be desired that the length in the A-A′ cross-section of the region in which the thin insulating film  26  and the top electrode  20  are overlapped with each other be secured to be 10 micrometers or more on one side from the stripe structure. The length in the A-A′ cross-section of the region in which the top electrode  20  and the thick insulating film  26  are overlapped with each other may be, for example, 3 micrometers. 
       FIG.  4    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of an optical semiconductor device  201  according to a second example implementation of the present invention. The difference from the first example implementation lies in that the insulating film in the first example implementation is integrally formed of a single material, whereas in the second example implementation, the insulating film  26  may include a first insulating layer formed in the thin region of the insulating film  26  and a second insulating layer formed in the thick region of the insulating film  26  with a material different from that for the first insulating layer. As illustrated in  FIG.  4   , the optical semiconductor device  201  according to the second example implementation may include a first insulating layer  27  that may be overlapped with the top electrode  20  and a second insulating layer  28  arranged in a region that is not overlapped with the top electrode  20 . Here, the first insulating layer  27  and the second insulating layer  28  may be made of different materials from each other. For example, the first insulating layer  27  may be a silicon nitride film, and the second insulating layer  28  may be a silicon oxide film. Alternatively, the first insulating layer  27  may be a silicon oxide film, and the second insulating layer  28  may be a silicon nitride film. Still alternatively, any one of the first insulating layer  27  and the second insulating layer  28  may be made of aluminum oxide. 
     In the first example implementation, it may be desired to form two regions having different thicknesses in the insulating film  26  made of one material. There may be several production methods for forming two regions having different thicknesses. For example, there may be a method involving thinning only the region of the insulating film  26  formed to be thick, which may be overlapped with the top electrode  20 , by etching. In the case of this production method, the etching amount depends on the etching time, and hence there may be a concern in that stable film thickness control may not be performed. Meanwhile, in the second example implementation, the region of the insulating film  26  that is overlapped with the top electrode  20  and the region of the insulating film  26  that is not overlapped therewith may be made of different materials. Because of this, the first insulating layer  27  and the second insulating layer  28  may be formed individually, and hence each of the insulating layers may be formed to a desired thickness. As a result, stable film thickness control may be performed. Needless to say, the effects described in the first example implementation may be obtained also in the second example implementation. In particular, the silicon nitride film may provide greater thermal conductivity than the silicon oxide film, and hence an optical semiconductor device that may provide greater heat dissipation may be provided by using the silicon nitride film as the first insulating layer  27  and the silicon oxide film as the second insulating layer  28 . Similarly, the aluminum oxide film may provide greater thermal conductivity than the silicon oxide film, and hence the first insulating layer  27  may be made of aluminum oxide. 
       FIG.  5    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device  201 . The difference from the foregoing lies in that a part of the second insulating layer  28  may be overlapped with the end portion of the top electrode  20 . As described with reference to  FIG.  3   , it is may not be desired from the viewpoint of reliability that the thin first insulating layer  27  that is not overlapped with the top electrode  20  be exposed. According to Modification Example 1, the optical semiconductor device  201  excellence in manufacturability may be provided. 
       FIG.  6    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of an optical semiconductor device  301  according to a third example implementation of the present invention. The difference from the second example implementation lies in that the first insulating layer  27  may be arranged up to a region that is not overlapped with the top electrode  20 . As illustrated in  FIG.  6   , the first insulating layer  27  arranged in the region that is not overlapped with the top electrode  20  may be arranged under the second insulating layer  28 . The second insulating layer  28  may be arranged in the region that is not overlapped with the top electrode  20 . In the second example implementation, the position of the boundary between the first insulating layer  27  and the second insulating layer  28  may be influenced by manufacturing variations. In a manufacturing procedure of the structure illustrated in  FIG.  5   , for example, after the first insulating layer  27  is formed in a desired region, the region in which the first insulating layer  27  may be formed may be masked. Then, the second insulating layer  28  may be formed in a region that may not be masked. However, due to an alignment accuracy of the mask, the boundary of the region to be masked and the position of the end portion of the first insulating layer  27  may be displaced from each other. In the case of displacement, there may be a risk in that the second insulating layer  28  may not be formed, and the semiconductor layer (buried layer  30  in this case) may remain exposed. However, in this structure, the surface of the optical semiconductor device  301  may be covered with the first insulating layer  27 , and hence the semiconductor layer may not be exposed even when the formation position of the second insulating layer  28  is displaced. Accordingly, an optical semiconductor device excellent in reliability may be provided. In addition, as a method involving removing only the second insulating layer  28  after continuously forming the first insulating layer  27  and the second insulating layer  28 , the difference in rate of wet etching may be utilized. When an etchant having a high etching rate only with respect to the second insulating layer  28  is used, only the second insulating layer  28  under the top electrode  20  may be removed through use of a mask having a portion corresponding to the top electrode  20  opened. That is, as a mask for determining each shape of the second insulating layer  28  and the top electrode  20 , the same mask may be used, which may be desired in manufacturability. 
       FIG.  7    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device  301  of the third example implementation. The difference from the foregoing lies in that a part of the second insulating layer  28  may be overlapped with the end portion of the top electrode  20 . Also in the above-mentioned structure, there may be a concern in that a region in which the thin first insulating layer  27  may not be overlapped with any of the top electrode  20  and the second insulating layer  28  may occur due to the manufacturing variations. In Modification Example 1, in the same manner as in the above-mentioned effects, the thick second insulating layer  28  may be arranged in the region that is not covered with the top electrode  20 , and as a result, an optical semiconductor device with excellence in reliability may be provided. 
       FIG.  8    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of an optical semiconductor device  401  according to a fourth example implementation of the present invention. The difference from the third example implementation lies in that the first insulating layer  27  may be formed on the second insulating layer  28  in the region in which the insulating film  26  is not overlapped with the top electrode  20 . Specifically, as illustrated in  FIG.  8   , the first insulating layer  27  arranged in the region that is not overlapped with the top electrode  20  may be arranged on the second insulating layer  28 . According to this structure, a region in which the semiconductor layer (buried layer  30  in the fourth example implementation) is not covered with the insulating film  26  may be prevented from being formed in the same manner as in the third example implementation. 
       FIG.  9    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device  401  of the fourth example implementation. The difference from  FIG.  8    lies in that a part of the second insulating layer  28  may be overlapped with the end portion of the top electrode  20 . Also in the structure in the fourth example implementation, there may be a concern in that a region in which the thin first insulating layer  27  is not overlapped with any of the top electrode  20  and the second insulating layer  28  may occur due to manufacturing variations. In Modification Example 1, in the same manner as in the above-mentioned effects, the thick second insulating layer  28  may be arranged in the region in which the thin first insulating layer  27  is not covered with the top electrode  20 , and as a result, an optical semiconductor device with excellence in reliability may be provided. 
       FIG.  10    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 2 of the optical semiconductor device  401  according to the fourth example implementation. The difference from  FIG.  9    lies in that only the second insulating layer  28  may be formed in the region in which the insulating film  26  and the top electrode  20  are not overlapped with each other. That is, only the first insulating layer  27  may be formed under the top electrode  20  except for the vicinity of the end portion. Meanwhile, in the end portion of the top electrode  20 , both the first insulating layer  27  and the second insulating layer  28  may be formed, and the first insulating layer  27  may be arranged on the second insulating layer  28 . Only the second insulating layer  28  may be formed in the region that is not overlapped with the top electrode  20 . There may be two merits of this configuration. One of the merits is advantageous from the viewpoint of stress. The insulating film  26  may serve as a stress factor with respect to the semiconductor layer. In general, stress may be larger when the thickness of a film is larger. In Modification Example 2, the thickness of the insulating film  26  in the region that is not overlapped with the top electrode  20  may be thinner than the thicknesses in  FIG.  6    to  FIG.  9   . Accordingly, the generation of stress may be suppressed to the extent possible while the merit of providing a structure in which the surface of the semiconductor layer is securely covered with the insulating film  26  may be kept. 
     The second merit is stabilization of formation of the shape of the top electrode  20 . As one of production methods for the top electrode  20 , there may be a method involving forming an electrode on an entire surface and then removing an unnecessary region so as to have a desired shape. The manufacturing procedure of Modification Example 2 is as described below. First, each layer up to the semiconductor layer (that is, each layer up to the buried layer  30  and the contact layer  35 ) may be formed. Next, the second insulating layer  28  may be formed in a desired region. Then, the first insulating layer  27  may be formed on an entire surface. At this point, the first insulating layer  27  may be formed on the second insulating layer  28  also in a region that is not overlapped with the top electrode  20  later (same as the state without the top electrode  20  shown in  FIG.  9   ). Next, an electrode may be formed on the entire surface of the first insulating layer  27 . The method of forming the electrode may be, for example, a deposition method. Next, a region that is to be the top electrode  20  may be masked, and the electrode in a region that is not masked is may be removed. A milling method or the like may be used for removing the electrode. In this case, only the electrode may be removed so as to leave the first insulating layer  27 , but there may be a risk in that a region in which the electrode cannot be sufficiently removed may occur due to variations in a wafer surface. As a result, there may be a risk in that the shape of the top electrode  20  may not be stable when viewed as a whole wafer. In view of the foregoing, the risk in that the electrode may remain may be securely eliminated by removing a larger amount to the extent that the first insulating layer  27  is also removed simultaneously with the removal of the electrode. In this case, a part of the second insulating layer  28  may also be removed, but there may be no problem as long as the second insulating layer  28  is also formed to be thick so that the thickness which finally functions as a protective film remains. Then, the second insulating layer  28  may be a region away from the stripe structure  3 , and hence heat dissipation may be less influenced even when the second insulating layer  28  is somewhat thick. Thus, according to the structure of Modification Example 2, the following advantages may be obtained. First, when the insulating film  26  is formed so that a large part thereof in the region that is overlapped with the top electrode  20  includes only the thin first insulating layer  27 , the heat dissipation may be improved, and the characteristics of the optical semiconductor device  401  may be improved. Further, when the insulating film  26  is formed so that the insulating film  26  in the region that is not overlapped with the top electrode  20  includes only the second insulating layer  28  that is thicker than the first insulating layer  27 , the reliability may be improved. In addition, the first insulating layer  27  and the second insulating layer  28  may be overlapped with each other in the end portion of the second insulating layer  28 , and hence a region in which the semiconductor layer (here, the buried layer  30 ) is not covered with the insulating film  26  due to the influence of the manufacturing variations may be prevented from being formed. Further, the formation of the shape of the top electrode  20  may be stabilized. 
       FIG.  11    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of an optical semiconductor device  501  according to a fifth example implementation of the present invention. The difference of the fifth example implementation lies in that the first insulating layer  27  may be arranged on each side surface of the stripe structure  3 . The optical semiconductor device  501  may be a ridge optical semiconductor device. The stripe structure  3  may be formed of the second conductivity type cladding layer  25  including the diffraction grating layer  33  and the contact layer  35 . In addition, a similar semiconductor multilayer may be arranged on each side of the stripe structure  3 . The first conductivity type optical confinement layer  22 , the active layer  23 , and the second conductivity type optical confinement layer  24  may be widely arranged on the substrate  21 . In the same manner as in the other embodiments, in the vicinity of the stripe structure  3 , the insulating film  26  overlapped with the top electrode  20  may include only the first insulating layer  27 . In addition, the side surface of the stripe structure  3  may be also covered with the first insulating layer  27 . In the region that is not overlapped with the top electrode  20 , the insulating film  26  may include only the second insulating layer  28 . In the vicinity of the end portion of the top electrode  20 , the insulating film  26  may include the first insulating layer  27  and the second insulating layer  28 . In the region close to the stripe structure  3 , the insulating film  26  may include only the thin first insulating layer  27 , and thus the optical semiconductor device  501  may provide excellence in heat dissipation. In addition, the structures of the above-mentioned other embodiments and modification examples may be applied to the configurations of the insulating film  26  in the end portion of the top electrode  20  and the region that is not covered with the top electrode  20 . 
       FIG.  12    is a schematic sectional view taken along the line A-A′ of  FIG.  1    of Modification Example 1 of the optical semiconductor device  501  according to the fifth example implementation. The difference from  FIG.  11    lies in that the second insulating layer  28  may be arranged also in a part of the side surface of the stripe structure  3 . The feature of Modification Example 1 lies in that the second insulating layer  28  may be arranged between the side surface of the stripe structure  3  and the first insulating layer  27  in a lower portion of the side surface of the stripe structure  3 . In the case of a related-art ridge optical semiconductor device, the insulating film  26  covering each side surface of the stripe structure  3  may have the same thickness between the region that is overlapped with the top electrode  20  and the region that is not overlapped therewith. Because of this, the insulating film  26  on the side surface of the stripe structure  3  may have a thickness enough to function as a protective layer. Accordingly, when the loss of a waveguide mode is taken into consideration, the seeping of the waveguide mode into the insulating film  26  may be sufficiently small in the boundary portion between the insulating film  26  and the top electrode  20 . However, in the fifth example implementation, the insulating film  26  may be formed as a layer thinner than the protective layer in order to improve heat dissipation. As a result, the seeping of the waveguide mode into the top electrode  20  portion becomes large, and there may be a risk in that the loss of the waveguide mode may be increased. In view of the foregoing, in Modification Example 1, the insulating film  26  that covers the stripe structure  3  may be formed to be thick only on the active layer  23  side, which may be to be the center of light. Specifically, the side surface of the stripe structure  3  may have a structure in which a lower portion is covered with both the first insulating layer  27  and the second insulating layer  28 , and an upper portion may be covered only with the first insulating layer  27 . The second insulating layer  28  may have a thickness that functions as a protective layer, and hence the seeping of the waveguide mode into the top electrode  20  portion may be suppressed. Although the structure illustrated in  FIG.  11    is superior from the viewpoint of heat dissipation, when the optical characteristics are also taken into consideration, Modification Example 1 may be superior. It may be only required to select any one of those structures in accordance with the operating temperature and the required characteristics. The width covered with the second insulating layer  28  on the side surface of the stripe structure  3  may be determined in accordance with the required characteristics. For example, when a half or more of the height of the stripe structure  3  is covered, the loss of the waveguide mode may be reduced. In addition, the entire side surface of the stripe structure  3  may be covered with the second insulating layer  28 . Even with this structure, the region slightly distant from the stripe structure  3  may be covered only with the first insulating layer  27 , and hence the effect of improving heat dissipation may be obtained. 
     The present invention is not limited to the embodiments described above, and various modifications may be made thereto. For example, the optical semiconductor device is not limited to the above-mentioned examples, and may be an electro-absorption modulator, an MZ modulator, an amplifier, or a light-receiving element. In the case of those optical semiconductor devices, the optical functional layer functions as an absorption layer. 
     The foregoing disclosure provides illustration and description, but is not intended to be exhaustive or to limit the implementations to the precise forms disclosed. Modifications and variations may be made in light of the above disclosure or may be acquired from practice of the implementations. Furthermore, any of the implementations described herein may be combined unless the foregoing disclosure expressly provides a reason that one or more implementations may not be combined. 
     Even though particular combinations of features are recited in the claims and/or disclosed in the specification, these combinations are not intended to limit the disclosure of various implementations. In fact, many of these features may be combined in ways not specifically recited in the claims and/or disclosed in the specification. Although each dependent claim listed below may directly depend on only one claim, the disclosure of various implementations includes each dependent claim in combination with every other claim in the claim set. As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover a, b, c, a-b, a-c, b-c, and a-b-c, as well as any combination with multiple of the same item. 
     No element, act, or instruction used herein should be construed as critical or essential unless explicitly described as such. Also, as used herein, the articles “a” and “an” are intended to include one or more items, and may be used interchangeably with “one or more.” Further, as used herein, the article “the” is intended to include one or more items referenced in connection with the article “the” and may be used interchangeably with “the one or more.” Furthermore, as used herein, the term “set” is intended to include one or more items (e.g., related items, unrelated items, or a combination of related and unrelated items), and may be used interchangeably with “one or more.” Where only one item is intended, the phrase “only one” or similar language is used. Also, as used herein, the terms “has,” “have,” “having,” or the like are intended to be open-ended terms. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise. Also, as used herein, the term “or” is intended to be inclusive when used in a series and may be used interchangeably with “and/or,” unless explicitly stated otherwise (e.g., if used in combination with “either” or “only one of”). Further, spatially relative terms, such as “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the apparatus, device, and/or element in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated  90  degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.