Optical semiconductor device

Abstract

An optical semiconductor device includes a substrate, a semiconductor multilayer formed on the substrate and including an optical functional layer, an insulating film formed on the semiconductor multilayer, and an electrode formed on a part of the insulating film. The insulating film covers the semiconductor multilayer except for a region where the semiconductor multilayer and the electrode are electrically connected to each other. At least a part of a region of the insulating film overlapping the electrode is thinner than a region of the insulating film not overlapping the electrode.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to Japanese Patent Application JP2022-020198, filed February 14, 2022, and JP2021-185154, filed November 12, 2021, the contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure generally relates to optical semiconductor devices. Background Technology

[0004] Optical semiconductor devices for optical communication may include optical functional layers that convert electricity into light or light into electricity. For example, lasers and external modulators may include optical functional layers using multiple quantum well layers. Furthermore, in light-receiving elements, the optical functional layer may be formed of a semiconductor absorption layer. In many cases, the optical semiconductor device may include electrodes made of metal to apply a voltage to the optical functional layer, wherein a portion of the electrodes is electrically and physically connected to the semiconductor layer. Additionally, the optical semiconductor device may include an insulating film disposed on the surface of the semiconductor layer where no metal is disposed, for protective purposes. Summary of the Invention

[0005] When driving optical semiconductor devices, heat is generated in the optical functional layers, other semiconductor layers, electrodes, etc. This heat degrades the characteristics of the optical semiconductor device. For example, continuous-wave lasers (CW lasers) that output continuous light have optical output as their primary characteristic. High optical output is desirable, and at the same driving current, the optical output increases as the temperature of the optical semiconductor device decreases. In an environment with a constant external temperature, when a significant amount of heat generated by the optical semiconductor device is released to the outside, the effective temperature of the optical semiconductor device decreases, and the optical output increases. In other optical semiconductor devices, as well as CW lasers, it is important to release a large amount of the heat generated by the optical semiconductor device to the outside.

[0006] As described above, optical semiconductor devices may include metal electrodes and insulating films (protective films). The electrodes can be made of metal, thus possessing high thermal conductivity and providing excellent heat dissipation. The insulating film can be an oxide film or a silicon nitride film. Compared to semiconductors and metals, these materials have lower thermal conductivity, which may suppress the release of generated heat to the outside.

[0007] Further, the connecting region between the electrode and the semiconductor layer can be limited to a narrow region. For example, in an optical semiconductor device having a stripe structure, the contact point between the electrode and the semiconductor layer is limited to 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 and the semiconductor layer provided on a region other than the stripe structure, the above-described insulating film is provided.

[0008] Therefore, the insulating film is widely disposed on the surface of the semiconductor layer except for a small partial region, and the electrode is disposed on the insulating film. Thus, a wide insulating film is located in the path of releasing heat generated in the semiconductor layer to the outside. As a result, the amount of heat dissipation is limited, which becomes a factor of lowering the characteristics of the optical semiconductor device.

[0009] Some embodiments disclosed herein solve the above-described problems and provide an optical semiconductor device that provides improved heat dissipation.

[0010] In some embodiments, an optical semiconductor device includes: a substrate; a semiconductor multilayer formed on the substrate and including an optical functional layer; an insulating film formed on the semiconductor multilayer; and an electrode formed on a portion of the insulating film, wherein the insulating film covers the semiconductor multilayer except for a region where the semiconductor multilayer and the electrode are electrically connected to each other, and wherein at least a portion of a region of the insulating film overlapping with the electrode is thinner than a region of the insulating film not overlapping with the electrode.

[0011] In some embodiments, an optical semiconductor device provides excellent heat dissipation. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 is a top view of an example of an optical semiconductor device according to a first example embodiment of the present application.

[0013] Figure 2 is a schematic cross-sectional view taken along line A-A' of the optical semiconductor device shown in Figure 1

[0014] Figure 3 is a schematic cross-sectional view taken along line A-A' of the optical semiconductor device according to Modified Example 1 of the first example embodiment of the present application. Figure 1

[0015] Figure 4 is a schematic cross-sectional view taken along line A-A' of the optical semiconductor device according to the second example embodiment of the present application. Figure 1

[0016] Figure 5 ​​​This is a modified example 1 of the optical semiconductor device according to the second exemplary embodiment of the present invention. Figure 1 A schematic cross-sectional view of the line AA′.

[0017] Figure 6 It is an optical semiconductor device according to a third exemplary embodiment of the present invention. Figure 1 A schematic cross-sectional view of the line AA′.

[0018] Figure 7 This is a modified example 1 of the optical semiconductor device according to the third exemplary embodiment of the present invention. Figure 1 A schematic cross-sectional view of the line AA′.

[0019] Figure 8 It is an optical semiconductor device according to a fourth exemplary embodiment of the present invention. Figure 1 A schematic cross-sectional view of the line AA′.

[0020] Figure 9 This is a modified example 1 of the optical semiconductor device according to the fourth exemplary embodiment of the present invention. Figure 1 A schematic cross-sectional view of the line AA′.

[0021] Figure 10 This is a modified example 2 of the optical semiconductor device according to the fourth exemplary embodiment of the present invention. Figure 1 A schematic cross-sectional view of the line AA′.

[0022] Figure 11 It is an optical semiconductor device according to a fifth exemplary embodiment of the present invention. Figure 1 A schematic cross-sectional view of the line AA′.

[0023] Figure 12 This is a modified example 1 of the optical semiconductor device according to the fifth exemplary embodiment of the present invention. Figure 1 A schematic cross-sectional view of the line AA′. Detailed Implementation

[0024] Some embodiments are described in detail below with reference to the accompanying drawings. In the drawings, the same components are represented by the same reference numerals and have the same or equivalent functions; for simplicity, repeated descriptions of them may be omitted. Note that the drawings mentioned below are for illustrative purposes only and are not necessarily drawn to scale.

[0025] Figure 1 This is a top view of an optical semiconductor device 1 according to a first exemplary embodiment of the present invention. Figure 2 It is along Figure 1A-A' line taken in the schematic cross-sectional view. Here, the optical semiconductor device 1 is an edge emitting CW laser. The optical semiconductor device 1 can include a bar structure 3. A top electrode 20 can be arranged on a surface of the optical semiconductor device 1. The top electrode 20 can be an electrode formed on a portion of an insulating film 26, and is a metal film containing Au, for example. The metal film can be formed of a variety of materials. In addition, on the upper surface of the optical semiconductor device 1, the insulating film 26 can be arranged in a region other than the top electrode 20. As shown in Figure 2 , the insulating film 26 can also be arranged under the top electrode 20. The insulating film 26 is a silicon oxide film, a silicon nitride film, or an aluminum oxide film, for example. Details of the insulating film 26 will be described later. A low-reflectance end face coating film 11 can be arranged on the end face on the left side, and a high-reflectance end face coating film 12 can be arranged on the end face on the right side. Figure 1 Figure 1 The coating films can be merely examples, and a low-reflectance end face coating film can be arranged on both end faces.

[0026] As shown in Figure 2 , in the optical semiconductor device 1, the bar structure 3 can be formed on the first-conductivity-type substrate 21. A buried layer 30 can be arranged on each side of the bar structure 3. The buried layer 30 can be a semi-insulating semiconductor layer or a semiconductor layer in which a plurality of p-type and n-type semiconductor layers can be combined. The insulating film 26 can be arranged on the upper surface of the buried layer 30. The bar structure 3 can be formed on a portion of the substrate 21 so as to include a plurality of semiconductor layers. The plurality of semiconductor layers can be formed to include, from the bottom, a first-conductivity-type optical confinement layer 22, an active layer 23 (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 can be formed in the middle of the second-conductivity-type cladding layer 25. The bar structure 3 can or can not include a portion of the substrate 21. The layers from the first-conductivity-type optical confinement layer 22 to the contact layer 35 can be referred to as a "semiconductor multilayer" hereinafter. Furthermore, in the first example embodiment, the semiconductor multilayer can include the bar structure 3 and the buried layer 30 formed on each side of the bar structure 3. A back electrode 31 can be arranged on the back side of the substrate 21. In the first example embodiment, the semiconductor multilayer can be a CW laser corresponding to a 1.3-micrometer wavelength band. However, the embodiments described herein can not be limited to this, and the wavelength band of laser light output by the semiconductor multilayer can be another wavelength band. Furthermore, an insulating substrate can be used as the substrate 21. In this case, it can be necessary to arrange a first-conductivity-type semiconductor layer between the substrate 21 and the bar structure 3, and the substrate 21 can be an insulating substrate.

[0027] ​In some embodiments, the insulating film 26 can cover the semiconductor multilayer except for the region where the semiconductor multilayer and the top electrode 20 can be electrically connected to each other, and at least a part of the region of the insulating film 26 overlapping the top electrode 20 can be thinner than the region of the insulating film 26 not overlapping the top electrode 20. In the first example embodiment, as shown in FIG. 1, the thickness of the insulating film 26 can be different between the region where the top electrode 20 and the buried layer 30 overlap each other and the region where the top electrode 20 and the buried layer 30 do not overlap each other. The region of the insulating film 26 overlapping the top electrode 20 can be thinner than the region of the insulating film 26 not overlapping the top electrode 20. Figure 2

[0028] In the optical semiconductor device 1, when a voltage is applied between the top electrode 20 and the back electrode 31 (current is injected), the active layer 23 can emit light. In addition to the light emission, the active layer 23 can also generate heat. Furthermore, other semiconductor layers can also generate heat due to the flow of current. The heat generated in the semiconductor multilayer can be released to the outside through the substrate 21 and the buried layer 30. In the optical semiconductor device 1 of the first example embodiment, the stripe structure 3 can be formed on the top surface side of the substrate 21. For example, the distance from the top electrode 20 to the active layer 23 can be several micrometers, while the distance from the active layer 23 to the back electrode 31 can be as thick as several tens of micrometers to 100 micrometers. Therefore, the generated heat can be released to the outside environment in a larger amount on the top electrode 20 side than on the back electrode 31 side. Furthermore, the amount of heat generation becomes larger at a position closer to the active layer 23, and thus a large portion of the generated heat can be released from the top electrode 20 side. A part of the heat can pass through the stripe structure 3, and can be released directly from the top electrode 20. However, in addition to the foregoing, there can also be a path to the top electrode 20 through the buried layer 30 as a heat dissipation path.

[0029] The insulating film 26 can be disposed between the top electrode 20 and the buried layer 30. The insulating film 26 can be a silicon oxide film, a silicon nitride film, an aluminum oxide film, or the like, in which the thermal conductivity can be smaller than that of the semiconductor multilayer as described above. Therefore, the insulating film 26 disposed between the top electrode 20 and the buried layer 30 hinders heat dissipation. However, in the first example embodiment, the insulating film 26 in the region overlapping the top electrode 20 can be formed thin. The insulating film 26 in this region can be, for example, 100 nanometers or less. Therefore, heat can be released to the outside without significantly reducing heat dissipation.

[0030] ​Meanwhile, the insulating film 26 in the region not overlapping the top electrode 20 can be provided to be several hundred nanometers. When the thickness of the insulating film 26 in the region not overlapping the top electrode 20 is provided to be the same as that of the region overlapping the top electrode 20, there can be a risk that the function of the insulating film 26 in the region not overlapping the top electrode 20 as a protective film cannot be sufficiently obtained, and there can be a risk of affecting the reliability of the optical semiconductor device 1. Further, in the region where the top electrode 20 is disposed, the top electrode 20 functions as a protective film, so even if the insulating film 26 is thinned, the reliability is not significantly affected.

[0031] With this configuration, it is possible to provide an optical semiconductor device having excellent optical characteristics due to excellent heat dissipation while ensuring reliability.

[0032] Figure 3 is along the optical semiconductor device 1 of Modification Example 1 Figure 1A schematic cross-sectional view taken by line AA′. The difference from the previous example lies in the shape of the insulating film 26. In modified example 1, the thickness of the insulating film 26 near each end of the top electrode 20 is greater than the thickness near the strip structure 3. That is, a portion of the thicker region of the insulating film 26 overlaps with a portion of the end of the top electrode 20. From a manufacturability perspective, this structure is likely excellent. During the fabrication of the optical semiconductor device 1, after forming the buried layer 30 and the semiconductor multilayer and arranging the insulating film 26, the top electrode 20 can be arranged on the insulating film 26. In the above, the ends of the top electrode 20 and the boundaries of the thickness variation of the insulating film 26 match each other, but due to manufacturing variations, this matching may not be possible. In this case, for example, the top electrode 20 may not overlap with the thinner region of the insulating film 26. As mentioned above, when the insulating film 26 is thin, there may be concerns about potentially reduced reliability. To avoid a structure where thin areas of the insulating film 26 are not covered by the top electrode 20 due to manufacturing variations, in Modified Example 1, the thickness of the insulating film 26 near the end of the top electrode 20 can be intentionally set to be the same as the thickness of the area that does not overlap with the top electrode 20. This configuration reduces the risk that thin areas of the insulating film 26 may not overlap with the top electrode 20. Although Modified Example 1 may provide less heat dissipation compared to the first exemplary embodiment, the impact is likely small because the boundary of the thickness change is away from the strip structure 3. The location of the boundary of the thickness change of the insulating film 26 can be determined considering the manufacturing variations. Specifically, it may be desirable to set the area where the thin insulating film 26 and the top electrode 20 overlap to at least 50% or more of the area of ​​the top electrode 20. Furthermore, it may be desirable to ensure that the length of the AA′ cross-section of the area where the thin insulating film 26 and the top electrode 20 overlap is 10 micrometers or greater on one side of the strip structure. The length of the AA′ cross-section of the area where the top electrode 20 and the thick insulating film 26 overlap may be, for example, 3 micrometers.

[0033] Figure 4 It is along the optical semiconductor device 201 according to the second exemplary embodiment of the present invention. Figure 1 A schematic cross-sectional view taken by line AA′. The difference from the first exemplary embodiment is that, in the first exemplary embodiment, the insulating film is integrally formed from a single material, while in the second exemplary embodiment, the insulating film 26 may include a first insulating layer formed in a thin region of the insulating film 26 and a second insulating layer formed in a thick region of the insulating film 26, the second insulating layer having a different material than the first insulating layer. For example... Figure 4As shown, the optical semiconductor device 201 according to the second example embodiment can include a first insulating layer 27 which can overlap the top electrode 20 and a second insulating layer 28 which is arranged in a region which does not overlap the top electrode 20. Here, the first insulating layer 27 and the second insulating layer 28 can be made of different materials from each other. For example, the first insulating layer 27 can be a silicon nitride film and the second insulating layer 28 can be a silicon oxide film. Alternatively, the first insulating layer 27 can be a silicon oxide film and the second insulating layer 28 can be a silicon nitride film. Still alternatively, either one of the first insulating layer 27 and the second insulating layer 28 can be made of aluminum oxide.

[0034] In the first example embodiment, it can be desirable to form two regions having different thicknesses in the insulating film 26 made of one kind of material. There can be several production methods to form two regions having different thicknesses. For example, there can be a method which includes thinning only the region of the insulating film 26 which is formed thick, which can overlap the top electrode 20, by etching. In the case of this production method, the etching amount depends on the etching time, and thus there can be a concern that stable film thickness control cannot be performed. Meanwhile, in the second example embodiment, the region of the insulating film 26 which overlaps the top electrode 20 and the region of the insulating film 26 which does not overlap the same can be made of different materials. Thus, the first insulating layer 27 and the second insulating layer 28 can be formed separately, and thus each insulating layer can be formed to a desired thickness. As a result, stable film thickness control can be performed. Needless to say, the effects described in the first example embodiment can also be obtained in the second example embodiment. In particular, a silicon nitride film can provide greater thermal conductivity than a silicon oxide film, and thus an optical semiconductor device which can provide greater heat dissipation can be provided by using a silicon nitride film as the first insulating layer 27 and a silicon oxide film as the second insulating layer 28. Similarly, an aluminum oxide film can provide greater thermal conductivity than a silicon oxide film, and thus the first insulating layer 27 can be made of aluminum oxide.

[0035] Figure 5 is a schematic cross-sectional view taken along line A-A' of the modified example 1 of the optical semiconductor device 201. Figure 1 of the optical semiconductor device 201 according to the second example embodiment. Unlike the foregoing, a portion of the second insulating layer 28 can overlap the end portion of the top electrode 20. As described with reference to Figure 3 from the viewpoint of reliability. According to the modified example 1, an optical semiconductor device 201 which is excellent in manufacturability can be provided.

[0036] Figure 6 is an optical semiconductor device 301 according to a third example embodiment of the present application, along with Figure 1A-A' of the optical semiconductor device 301 of the third exemplary embodiment. The difference from the second exemplary embodiment is that the first insulating layer 27 can be arranged to a region that does not overlap with the top electrode 20. As shown in Figure 6 the first insulating layer 27 arranged in the region that does not overlap with the top electrode 20 can be arranged under the second insulating layer 28. The second insulating layer 28 can be arranged in the region that does not overlap with the top electrode 20. In the second exemplary embodiment, the boundary position between the first insulating layer 27 and the second insulating layer 28 is affected by manufacturing variation. In Figure 5 the manufacturing process of the structure shown, for example, after the first insulating layer 27 is formed in the desired region, the region in which the first insulating layer 27 can be formed can be masked. Then, the second insulating layer 28 can be formed in the region that can not be masked. However, due to the 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 can be displaced from each other. In the case of displacement, there can be a risk that the second insulating layer 28 cannot be formed, and the semiconductor layer (in this case, the buried layer 30) can remain exposed. However, in this structure, the surface of the optical semiconductor device 301 can be covered with the first insulating layer 27, so even when the formation position of the second insulating layer 28 is displaced, the semiconductor layer is not exposed. Therefore, an optical semiconductor device excellent in reliability can be provided. Further, as a method involving removing only the second insulating layer 28 after the first insulating layer 27 and the second insulating layer 28 are formed in succession, a difference in wet etching rate can be utilized. When an etchant having a high etching rate only with respect to the second insulating layer 28 is used, by using a mask having an opening portion corresponding to the top electrode 20, only the second insulating layer 28 under the top electrode 20 can be removed. That is, as a mask for determining each shape of the second insulating layer 28 and the top electrode 20, the same mask can be used, which can be desirable in terms of manufacturability.

[0037] Figure 7 is a schematic cross-sectional view taken along line A-A' of a modified example 1 of the optical semiconductor device 301 of the third exemplary embodiment. The difference from the foregoing is that a portion of the second insulating layer 28 can overlap with the end portion of the top electrode 20. Further, in the above-described structure, due to manufacturing variation, there can be a concern that a region in which a thin first insulating layer 27 can not overlap with any of the top electrode 20 and the second insulating layer 28 can occur. In the modified example 1, in the same manner as the above-described effect, a thick second insulating layer 28 can be arranged in the region that is not covered with the top electrode 20, as a result, an optical semiconductor device excellent in reliability can be provided. Figure 1

[0038] Figure 8 is a schematic cross-sectional view taken along line A-A' of a modified example 1 of the optical semiconductor device 301 of the third exemplary embodiment. The difference from the foregoing is that a portion of the second insulating layer 28 can overlap with the end portion of the top electrode 20. Further, in the above-described structure, due to manufacturing variation, there can be a concern that a region in which a thin first insulating layer 27 can not overlap with any of the top electrode 20 and the second insulating layer 28 can occur. In the modified example 1, in the same manner as the above-described effect, a thick second insulating layer 28 can be arranged in the region that is not covered with the top electrode 20, as a result, an optical semiconductor device excellent in reliability can be provided.​Figure 1 is a schematic cross-sectional view taken along line A-A' of the optical semiconductor device 401 of the fourth exemplary embodiment. The difference from Figure 8 is that the first insulating layer 27 arranged in the region not overlapping with the top electrode 20 can be arranged on the second insulating layer 28. Specifically, as shown, the first insulating layer 27 arranged in the region not overlapping with the top electrode 20 can be arranged on the second insulating layer 28. According to this structure, it is possible to prevent the region where the semiconductor layer (the buried layer 30 in the fourth exemplary embodiment) formed in the same manner as in the third exemplary embodiment is not covered with the insulating film 26.

[0039] Figure 9 is a schematic cross-sectional view taken along line A-A' of the optical semiconductor device 401 of the fourth exemplary embodiment. The difference from Figure 1 is that a portion of the second insulating layer 28 can overlap with the end portion of the top electrode 20. Further, in the structure of the fourth exemplary embodiment, due to manufacturing variation, there can be a concern that a region where the thin first insulating layer 27 does not overlap with either of the top electrode 20 and the second insulating layer 28 can occur. In the modified example 1, in the same manner as the above-described effect, the thick second insulating layer 28 can be arranged in the region where the thin first insulating layer 27 is not covered with the top electrode 20, as a result, it is possible to provide the optical semiconductor device having excellent reliability. Figure 8

[0040] Figure 10 is a schematic cross-sectional view taken along line A-A' of the optical semiconductor device 401 of the fourth exemplary embodiment. The difference from Figure 1 is that only the second insulating layer 28 can be formed in the region where the insulating film 26 and the top electrode 20 do not overlap with each other. That is, only the first insulating layer 27 can be formed under the top electrode 20 except in the vicinity of the end portion. Meanwhile, at the end portion of the top electrode 20, the first insulating layer 27 and the second insulating layer 28 can be formed, and the first insulating layer 27 can be arranged on the second insulating layer 28. Only the second insulating layer 28 can be formed in the region not overlapping with the top electrode 20. This configuration can have two advantages. One of the advantages is advantageous from the viewpoint of stress. The insulating film 26 can serve as a stress factor with respect to the semiconductor layer. Generally, when the thickness of the film is large, the stress can be large. In the modified example 2, the thickness of the insulating film 26 in the region not overlapping with the top electrode 20 can be thinner than that in Figure 9 . Therefore, it is possible to suppress generation of stress as much as possible while the advantage of providing the structure where the surface of the semiconductor layer is firmly covered with the insulating film 26 can be maintained. Figures 6 to 9

[0041] ​​The second advantage is stability of the shape of the top electrode 20. As one of the production methods of the top electrode 20, there can be a method including forming an electrode on the entire surface, and then removing unnecessary areas so as to have a desired shape. The manufacturing process of the modification example 2 is described as follows. First, each layer up to the semiconductor layer (i.e., each layer up to the buried layer 30 and the contact layer 35) can be formed. Next, the second insulating layer 28 can be formed in a desired area. Then, the first insulating layer 27 can be formed on the entire surface. At this point, the first insulating layer 27 can also be formed in an area on the second insulating layer 28 which does not overlap with the top electrode 20 later (the same as the state shown in Figure 9 Next, an electrode can be formed on the entire surface of the first insulating layer 27. The method of forming the electrode can be, for example, a deposition method. Next, the area to be the top electrode 20 can be masked, and the electrode in the area which is not masked can be removed. A milling method or the like can be used for removing the electrode. In this case, only the electrode can be removed to leave the first insulating layer 27, but due to variations in the wafer surface, there can be a risk that an area where the electrode cannot be sufficiently removed will occur. As a result, there can be a risk that the shape of the top electrode 20 is not stable when viewed as the entire wafer. In view of the foregoing, by removing a larger amount of the electrode to the extent that the first insulating layer 27 is also removed while the electrode is removed, the risk that the electrode can remain can be reliably eliminated. In this case, a part of the second insulating layer 28 can also be removed, but as long as the second insulating layer 28 is also formed to be relatively thick so that the thickness finally used as a protective film is maintained, there is no problem. Then, the second insulating layer 28 can be an area away from the stripe-shaped structure 3, so even when the second insulating layer 28 is somewhat thick, the heat dissipation is less affected. Thus, according to the structure of the modification example 2, the following advantages can be obtained. First, when the insulating film 26 is formed so that a large part of the area overlapping with the top electrode 20 includes only the thin first insulating layer 27, the heat dissipation can be improved, and the characteristics of the optical semiconductor device 401 can be improved. Further, when the insulating film 26 is formed so that the insulating film 26 in the area not overlapping with the top electrode 20 includes only the second insulating layer 28 which is thicker than the first insulating layer 27, the reliability can be improved. Further, the first insulating layer 27 and the second insulating layer 28 can overlap with each other at the end portion of the second insulating layer 28, so it is possible to prevent an area where the semiconductor layer (here, the buried layer 30) is not covered with the insulating film 26 from being formed due to the influence of manufacturing variations. Further, the formation of the shape of the top electrode 20 can be stabilized.

[0042] Figure 11 is a cross-sectional view of the optical semiconductor device 501 along the Figure 1A-A' taken along the line A-A' of the optical semiconductor device 501 according to the fifth example embodiment. The fifth example embodiment is different in that the first insulating layer 27 can be arranged on each side surface of the bar-shaped structure 3. The optical semiconductor device 501 can be a ridge optical semiconductor device. The bar-shaped structure 3 can be formed of the second conductive type cladding layer 25 including the diffraction grating layer 33 and the contact layer 35. Further, a similar semiconductor multilayer can be arranged on each side of the bar-shaped structure 3. The first conductive type optical confinement layer 22, the active layer 23, and the second conductive type optical confinement layer 24 can be widely arranged on the substrate 21. In the same manner as in the other embodiments, in the vicinity of the bar-shaped structure 3, the insulating film 26 overlapping with the top electrode 20 can include only the first insulating layer 27. Further, the side surface of the bar-shaped structure 3 can also be covered with the first insulating layer 27. In the region not overlapping with the top electrode 20, the insulating film 26 can include only the second insulating layer 28. In the vicinity of the end portion of the top electrode 20, the insulating film 26 can include the first insulating layer 27 and the second insulating layer 28. In the region close to the bar-shaped structure 3, the insulating film 26 can include only a thin first insulating layer 27, and thus the optical semiconductor device 501 can provide excellent heat dissipation. Further, the configuration of the insulating film 26 in the end portion of the top electrode 20 and the region not covered with the top electrode 20 can apply the structures of the other embodiments and the modified examples described above.

[0043] Figure 12 is a schematic cross-sectional view taken along the line A-A' of a modified example 1 of the optical semiconductor device 501 according to the fifth example embodiment. Figure 1 A-A' taken along the line A-A' of the optical semiconductor device 501 according to the fifth example embodiment. The fifth example embodiment is different in that the first insulating layer 27 can be arranged on each side surface of the bar-shaped structure 3. The optical semiconductor device 501 can be a ridge optical semiconductor device. The bar-shaped structure 3 can be formed of the second conductive type cladding layer 25 including the diffraction grating layer 33 and the contact layer 35. Further, a similar semiconductor multilayer can be arranged on each side of the bar-shaped structure 3. The first conductive type optical confinement layer 22, the active layer 23, and the second conductive type optical confinement layer 24 can be widely arranged on the substrate 21. In the same manner as in the other embodiments, in the vicinity of the bar-shaped structure 3, the insulating film 26 overlapping with the top electrode 20 can include only the first insulating layer 27. Further, the side surface of the bar-shaped structure 3 can also be covered with the first insulating layer 27. In the region not overlapping with the top electrode 20, the insulating film 26 can include only the second insulating layer 28. In the vicinity of the end portion of the top electrode 20, the insulating film 26 can include the first insulating layer 27 and the second insulating layer 28. In the region close to the bar-shaped structure 3, the insulating film 26 can include only a thin first insulating layer 27, and thus the optical semiconductor device 501 can provide excellent heat dissipation. Further, the configuration of the insulating film 26 in the end portion of the top electrode 20 and the region not covered with the top electrode 20 can apply the structures of the other embodiments and the modified examples described above. Figure 11The difference is that the second insulating layer 28 can also be arranged in a part of the side surface of the bar-shaped structure 3. The modified example 1 is characterized in that the second insulating layer 28 can be arranged between the first insulating layer 27 in the side surface of the bar-shaped structure 3 and the lower part of the side surface of the bar-shaped structure 3. In the case of the ridge-shaped optical semiconductor device of the related art, the insulating film 26 covering each side surface of the bar-shaped structure 3 can have the same thickness between the region overlapping with the top electrode 20 and the region not overlapping therewith. Therefore, the insulating film 26 on the side surface of the bar-shaped structure 3 can have a thickness enough to serve as a protective layer. As a result, the penetration of the waveguide mode into the boundary part between the insulating film 26 and the top electrode 20 can be small enough when considering the loss of the waveguide mode. However, in the fifth example embodiment, the insulating film 26 can be formed as a layer thinner than the protective layer in order to improve heat dissipation. As a result, the penetration of the waveguide mode into the top electrode 20 part becomes large, and there can be a risk that the loss of the waveguide mode can increase. In view of the above, in the modified example 1, the insulating film 26 covering the bar-shaped structure 3 can be formed to be thick only on the active layer 23 side, which can be the center of light. Specifically, the side surface of the bar-shaped structure 3 can have a structure in which the lower part is covered with the first insulating layer 27 and the second insulating layer 28, and the upper part can be covered with only the first insulating layer 27. The second insulating layer 28 can have a thickness enough to serve as a protective layer, and thus the penetration of the waveguide mode into the top electrode 20 part can be suppressed. Although it is better from the viewpoint of heat dissipation that the structure shown in the fifth example embodiment is adopted, Figure 11 the modified example 1 can be better when also considering the optical characteristics. It can be necessary to select any one of these structures according to the operating temperature and the required characteristics. The width of the side surface of the bar-shaped structure 3 covered with the second insulating layer 28 can be determined according to the required characteristics. For example, when half or more of the height of the bar-shaped structure 3 is covered, the loss of the waveguide mode can be reduced. Furthermore, the entire side surface of the bar-shaped structure 3 can be covered with the second insulating layer 28. Even with this structure, the region slightly away from the bar-shaped structure 3 can be covered with only the first insulating layer 27, and thus the effect of improving heat dissipation can be obtained.

[0044] The present application is not limited to the above-described embodiments, and various modifications can be made thereto. For example, the optical semiconductor device is not limited to the above-described examples, and can be an electro-absorption modulator, an MZ modulator, an amplifier, or a light-receiving element. In the case of these optical semiconductor devices, the optical functional layer serves as an absorption layer.

[0045] The foregoing disclosure provides illustration and description, but is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Modifications and variations can be possible in light of the above disclosure or can be acquired from practice of the embodiments. Additionally, any of the embodiments described herein can be combined, unless the foregoing disclosure expressly provides a reason that one or more implementations cannot be combined.

[0046] Even if a particular combination is recited in the claims and / or disclosed in the specification, these combinations are not intended to limit the disclosure of various embodiments. Indeed, many of the features can be combined in ways not specifically recited in the claims and / or disclosed in the specification. Although each dependent claim listed below can directly depend on only one claim, the disclosure of various embodiments includes each dependent claim in combination with every other claim in the claim set. As used herein, the phrase“at least one of’ a series 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 of multiples of the items.

[0047] 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 can be used interchangeably with“one or more.” Furthermore, as used herein, the article“the” is intended to include one or more items referenced, and can be used interchangeably with“the one or more.” Also, as used herein, the term“set” is intended to include one or more items (for example, related items, unrelated items, or a combination of related and unrelated items), and can be used interchangeably with“one or more.” If 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 of items (for example,“a, b, or c” or“a, b, and c”) unless explicitly stated otherwise (for example,“only one of a, b, and c” or“at least one of a, b, and c”). Moreover, as used herein, spatially relative terms, such as“beneath,”“below,”“lower,”“above,”“upper,” and the like, can be used to describe an element’s or feature’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 device, apparatus, and / or element in use or in operation, depending on the specific context in which the spatially relative terms are used. The device can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

Claims

1. An optical semiconductor device comprising: a substrate; a semiconductor multilayer formed on the substrate and including an optical functional layer; an insulating film formed on the semiconductor multilayer; and an electrode formed on a portion of the insulating film, wherein the insulating film covers the semiconductor multilayer except for a region where the semiconductor multilayer and the electrode are electrically connected to each other, wherein at least a portion of a region of the insulating film located above the semiconductor multilayer and overlapping the electrode is thinner than a region of the insulating film located above the semiconductor multilayer and not overlapping the electrode, and wherein at least the portion of the region of the insulating film overlapping the electrode is adjacent to and located above the semiconductor multilayer.

2. The optical semiconductor device according to claim 1, wherein a first region of the insulating film overlaps a portion of an end portion of the electrode.

3. The optical semiconductor device according to claim 1, wherein the insulating film is integrally formed of a single material.

4. The optical semiconductor device according to claim 1, wherein the insulating film includes a first insulating layer formed in the at least the portion of the region and a second insulating layer formed in another region, a material of the second insulating layer being different from a material used for the first insulating layer.

5. The optical semiconductor device according to claim 4, wherein the first insulating layer is provided in the region of the insulating film located above the semiconductor multilayer and not overlapping the electrode.

6. The optical semiconductor device according to claim 5, wherein the first insulating layer provided in the region of the insulating film located above the semiconductor multilayer and not overlapping the electrode is provided under the second insulating layer.

7. The optical semiconductor device according to claim 5, wherein the first insulating layer is provided on the second insulating layer.

8. The optical semiconductor device according to claim 7, wherein the first insulating layer is provided on the second insulating layer in the end portion of the electrode.

9. The optical semiconductor device according to claim 1, wherein the semiconductor multilayer includes a bar-shaped structure and a buried layer formed on each side of the bar-shaped structure.

10. The optical semiconductor device according to claim 4, wherein the semiconductor multilayer includes a bar-shaped structure, and wherein the first insulating layer is provided on each side surface of the bar-shaped structure.

11. The optical semiconductor device according to claim 10, wherein the second insulating layer is provided between the side surface of the bar-shaped structure and the first insulating layer in a portion of the side surface of the bar-shaped structure.

12. The optical semiconductor device according to claim 1, wherein the at least the portion of the region includes one of a silicon nitride film or an aluminum oxide film.

13. The optical semiconductor device according to claim 1, wherein the thick region of the insulating film includes a silicon oxide film.

14. A method of forming an optical semiconductor device, comprising: forming a semiconductor multilayer on a substrate, the semiconductor multilayer including an optical functional layer; forming an insulating film on the semiconductor multilayer; and forming an electrode on a portion of the insulating film. ​ ​ wherein the insulating film covers the semiconductor multilayer except for a region where the semiconductor multilayer and the electrode are electrically connected to each other, wherein at least a part of a region of the insulating film overlapping with the electrode is thinner than another region of the insulating film not overlapping with the electrode, and wherein at least a part of a region of the insulating film overlapping with the electrode is adjacent to and located above the semiconductor multilayer.

15. The method according to claim 14, wherein the another region of the insulating film overlaps with a part of an end portion of the electrode.

16. The method according to claim 14, wherein the insulating film is integrally formed of a single material.

17. The method according to claim 14, wherein forming the insulating film comprises: forming a first insulating layer in at least a part of a region; and forming a second insulating layer in another region outside the at least a part of the region, a material of the second insulating layer being different from a material used for the first insulating layer.

18. The method according to claim 17, wherein forming the first insulating layer comprises: forming the first insulating layer in the another region.

19. The method of claim 18, wherein forming the second insulating layer comprises: forming the second insulating layer on the first insulating layer.

20. The method according to claim 18, wherein forming the first insulating layer comprises: forming the first insulating layer on the second insulating layer.

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