Optical semiconductor device and method for manufacturing the same

Abstract

To provide an optical semiconductor device that has improved novel constitution such that, for example, a power consumption for heat generation of a heater layer can be suppressed, and a method of manufacturing the optical semiconductor device.SOLUTION: An optical semiconductor device has, for example; a base which has a base surface crossing a first direction as a crystal azimuth [100]; a mesa which protrudes from the base surface in the first direction to extend along the base surface, and has an end face in the first direction; a waveguide layer which is provided, at a position of the mesa separate from the end face in the opposite direction from the first direction, to extend along the base surface; and a heater layer which is provided at a position of the mesa separate from the waveguide layer in the first direction to generate heat when supplied with electric power, where a cavity part including a groove which is recessed in the first direction and extends in a second direction crossing the first direction is provided at a position of the mesa apart from the waveguide layer in the opposite direction from the first direction, and the second direction crosses a crystal azimuth [011] of the base obliquely at a difference of 45° or less.SELECTED DRAWING: Figure 21

Description

[Technical field]

[0001] The present invention relates to an optical semiconductor device and a method for manufacturing the optical semiconductor device. [Background technology]

[0002] Conventionally, an optical semiconductor device having a heater layer on a mesa has been known (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2016-054168 A Summary of the Invention [Problem to be solved by the invention]

[0004] In this type of optical semiconductor device, it would be beneficial if the power consumption due to the heat generated by the heater layer could be suppressed.

[0005] Therefore, one object of the present invention is to provide an optical semiconductor device having an improved new configuration, which can suppress power consumption due to heat generation by a heater layer, and a method for manufacturing the optical semiconductor device. [Means for solving the problem]

[0006] An optical semiconductor device of the present invention may, for example, comprise: a base having a base surface intersecting a first direction which is a crystal orientation

[0100] direction; a mesa protruding from the base surface in the first direction, extending along the base surface and having an end face in the first direction; a waveguide layer provided on the mesa at a position away from the end face in the opposite direction to the first direction so as to extend along the base surface; and a heater layer provided on the mesa at a position away from the waveguide layer in the first direction and which generates heat when supplied with electric power, wherein a cavity including a groove recessed in the first direction and extending in a second direction intersecting the first direction is provided at a position of the mesa spaced from the waveguide layer in the opposite direction to the first direction, and the second direction intersects the crystal orientation

[0011] direction of the base at an angle of 45° or less.

[0007] In the optical semiconductor device, the absolute value of the angle difference may be in a direction not less than 10° and not more than 45°.

[0008] In the optical semiconductor device, the groove may be V-shaped and recessed in the first direction and extend in the second direction.

[0009] In the optical semiconductor device, the cavity may have a bottom surface that is located at an end of the cavity in a direction opposite to the first direction and extends in the second direction intersecting the first direction.

[0010] In the optical semiconductor device, the mesa may have a first portion whose extension direction along the base surface is substantially aligned with the second direction.

[0011] In the optical semiconductor device, the mesa may have a second portion where an extension direction of the mesa along the base surface intersects with the second direction.

[0012] In the optical semiconductor device, the mesa may be provided with a plurality of cavities arranged in a third direction intersecting the first direction and the second direction as the cavity.

[0013] In the optical semiconductor device, the mesa may have two side surfaces as an end surface in the third direction and an end surface in the opposite direction to the third direction, and the cavity may include a cavity portion open to the third direction in each of the two side surfaces.

[0014] In the optical semiconductor device, the cavity may include a closed cavity within the mesa.

[0015] In the optical semiconductor device, the cavity may include a cavity having a bottom surface located at an end of the cavity in a direction opposite to the first direction and extending in the second direction intersecting the first direction, and a coating layer covering the bottom surface.

[0016] The optical semiconductor device may have a thermal resistance layer having a lower thermal conductivity than an adjacent portion on the side of the mesa opposite the heater layer with respect to the waveguide layer and aligned with the cavity in the first direction.

[0017] The optical semiconductor device may have a thermal resistance layer having a lower thermal conductivity than adjacent portions on the side of the mesa opposite the heater layer with respect to the waveguide layer and aligned with the cavity in a third direction intersecting the first direction and the second direction.

[0018] In the optical semiconductor device, the thermal resistance layers may be located on both sides of the cavity in the third direction.

[0019] In the optical semiconductor device, the mesa may have a diffraction grating layer extending along the waveguide layer.

[0020] In the optical semiconductor device, the mesa may have a diffraction grating layer extending along the waveguide layer, and the cavity may be arranged so that both ends of the cavity in the extension direction overlap with both ends of the diffraction grating layer in the extension direction in the first direction, or may be arranged so as to protrude beyond the diffraction grating layer on both sides in the extension direction.

[0021] An optical semiconductor device of the present invention includes, for example, a base having a base surface intersecting a first direction, a mesa protruding from the base surface in the first direction, extending along the base surface, and having an end surface in the first direction, a waveguide layer provided on the mesa at a position away from the end surface in the opposite direction to the first direction so as to extend along an extension direction along the base surface of the mesa, and a heater layer provided on the mesa at a position away from the waveguide layer in the first direction and which generates heat when supplied with electric power, and a cavity portion including a groove that is V-shaped in the first direction and extends in a second direction intersecting the first direction is provided on the mesa at a position away from the waveguide layer in the opposite direction to the first direction.

[0022] An optical semiconductor device of the present invention includes, for example, a base having a base surface intersecting a first direction, a mesa protruding from the base surface in the first direction, extending along the base surface, and having an end surface in the first direction, a waveguide layer provided on the mesa at a position away from the end surface in the opposite direction to the first direction so as to extend along an extension direction along the base surface of the mesa, and a heater layer provided on the mesa at a position away from the waveguide layer in the first direction and which generates heat when supplied with electric power, and a cavity portion closed within the mesa is provided at a position of the mesa away from the waveguide layer in the opposite direction to the first direction.

[0023] In the optical semiconductor device, the cavity may extend in a direction parallel to the base surface of the mesa.

[0024] In the optical semiconductor device, the cavity may be V-shaped in the first direction and extend along the extension direction.

[0025] In the optical semiconductor device, the base and the mesa may be made of a III-V group semiconductor having a zinc blende structure.

[0026] A manufacturing method for an optical semiconductor device of the present invention includes, for example, the steps of providing a coating layer extending in a second direction intersecting with a first direction of a base on a base surface intersecting with the first direction, forming a laminate by epitaxial growth in which crystal grains grow in a first direction on the base surface, partially removing the laminate to form a mesa, and forming a heater layer that generates heat when supplied with electricity on an end face of the mesa in the first direction, wherein in the step of forming the laminate, the laminate is formed so as to have a cavity therein that includes a groove recessed in the first direction on the coating layer and extending in the second direction. Effect of the Invention

[0027] According to the present invention, for example, it is possible to obtain an optical semiconductor device having an improved novel configuration capable of suppressing power consumption due to heat generation by a heater layer, and a method for manufacturing the optical semiconductor device. [Brief description of the drawings]

[0028] [Figure 1] FIG. 1 is an illustrative schematic perspective view of an optical semiconductor device according to the first embodiment. [Diagram 2] FIG. 2 is a cross-sectional view taken along line II-II of FIG. [Diagram 3] FIG. 3 is a cross-sectional view taken along line III-III of FIG. [Figure 4] FIG. 4 is an exemplary schematic plan view of a wafer serving as a base for the optical semiconductor device of the first embodiment and a coating layer formed on the wafer. [Diagram 5] FIG. 5 is an exemplary schematic cross-sectional view showing a base of the optical semiconductor device of the first embodiment and a cladding layer grown on the base, in which the angle between the crystal orientation

[0011] and the extension direction of the coating layer is 0°. [Figure 6] FIG. 6 is an exemplary schematic cross-sectional view showing a base of the optical semiconductor device of the first embodiment and a cladding layer grown on the base, in which the angle between the crystal orientation

[0011] direction and the extension direction of the coating layer is greater than 0° and less than 10°. [Figure 7] FIG. 7 is an exemplary schematic cross-sectional view showing a base of the optical semiconductor device of the first embodiment and a cladding layer grown on the base, in which the angle between the crystal orientation

[0011] direction and the extension direction of the coating layer is 10° or more and 45° or less. [Figure 8] FIG. 8 is a graph showing the change in the ratio of the height of a protrusion appearing in a cladding layer grown on a base of an optical semiconductor device of the first embodiment to the thickness of the cladding layer depending on the angle difference between the crystal orientation

[0011] and the extension direction of the coating layer. [Figure 9] FIG. 9 is an exemplary schematic cross-sectional view showing a base of the optical semiconductor device of the first embodiment and a cladding layer grown on the base, illustrating a state in which a cavity of finite length is formed on the coating layer when the angle difference between the crystal orientation

[0011] direction and the extension direction of the coating layer is 10° or more and 45° or less. [Figure 10] FIG. 10 is an illustrative schematic cross-sectional view showing a step of the method for manufacturing the optical semiconductor device of the first embodiment. [Figure 11] FIG. 11 is an illustrative schematic cross-sectional view showing a step subsequent to that shown in FIG. 10 in the method for manufacturing the optical semiconductor device of the first embodiment. [Figure 12] FIG. 12 is an illustrative schematic cross-sectional view showing a step subsequent to that shown in FIG. 11 in the method for manufacturing the optical semiconductor device of the first embodiment. [Figure 13] FIG. 13 is an illustrative schematic cross-sectional view of the optical semiconductor device according to the second embodiment at the same position as in FIG. [Figure 14] FIG. 14 is an illustrative schematic cross-sectional view of the optical semiconductor device of the third embodiment at the same position as in FIG. [Figure 15] FIG. 15 is an illustrative schematic cross-sectional view of the optical semiconductor device of the fourth embodiment at the same position as in FIG. [Figure 16] FIG. 16 is an illustrative schematic cross-sectional view of the optical semiconductor device according to the fifth embodiment at the same position as in FIG. [Figure 17] FIG. 17 is an illustrative schematic cross-sectional view of the optical semiconductor device of the sixth embodiment at the same position as in FIG. [Figure 18] FIG. 18 is an exemplary schematic cross-sectional view of the optical semiconductor device of the sixth embodiment at the same position as in FIG. 2, showing a case where the position of the mesa and the position of the covering layer are misaligned in the width direction of the mesa. [Figure 19] FIG. 19 is an illustrative schematic cross-sectional view of the optical semiconductor device of the seventh embodiment at the same position as in FIG. [Figure 20] FIG. 20 is an illustrative schematic cross-sectional view of the optical semiconductor device according to the eighth embodiment. [Figure 21] FIG. 21 is an illustrative schematic plan view of the optical semiconductor device according to the ninth embodiment. [Figure 22] FIG. 22 is an illustrative schematic plan view of the optical semiconductor device according to the tenth embodiment. [Diagram 23] FIG. 23 is an illustrative schematic plan view of the optical semiconductor device according to the eleventh embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Exemplary embodiments of the present invention are disclosed below. The configurations of the embodiments shown below, and the actions and results (effects) brought about by the configurations are merely examples. The present invention can be realized by configurations other than those disclosed in the following embodiments. Furthermore, according to the present invention, it is possible to obtain at least one of the various effects (including derivative effects) obtained by the configurations.

[0030] The following embodiments have similar configurations. Therefore, according to the configurations of the respective embodiments, similar actions and effects based on the similar configurations can be obtained. In the following, the similar configurations are given the same reference numerals, and duplicated descriptions may be omitted.

[0031] In this specification, ordinal numbers are given for the sake of convenience to distinguish parts, directions, and the like, and do not indicate priority or order.

[0032] In each drawing, the X direction is represented by an arrow X, the Y direction is represented by an arrow Y, and the Z direction is represented by an arrow Z. The X direction, Y direction, and Z direction intersect with each other and are perpendicular to each other.

[0033] [First embodiment] Fig. 1 is a perspective view of an optical semiconductor device 10A (10) according to a first embodiment. As shown in Fig. 1, the optical semiconductor device 10A includes a base 11 and a mesa 12. In this embodiment, as an example, the mesa 12 extending linearly in the X direction in a plan view seen in the opposite direction to the Z direction will be described, but the mesa 12 may be bent or curved in the plan view. The mesa 12 may also extend in a direction intersecting the X direction.

[0034] The base 11 is a semiconductor substrate, and extends in the X and Y directions, intersecting and perpendicular to the Z direction. The base 11 has a base surface 11a. The base surface 11a intersects and perpendicular to the Z direction, and extends in the X and Y directions. The base 11 is made of a III-V group semiconductor having a zinc blende structure, such as n-type indium phosphide (InP). The base 11 may also be referred to as a substrate.

[0035] The mesa 12 protrudes in the Z direction from the base surface 11a. The mesa 12 extends in the X direction with a substantially constant height in the Z direction and a substantially constant width in the Y direction. That is, the mesa 12 has a wall-like shape that protrudes above the base surface 11a and extends along the base surface 11a. In this embodiment, the X direction is an example of the extension direction of the mesa 12, the Y direction is also referred to as the width direction of the mesa 12, and the Z direction can also be referred to as the height direction or protruding direction of the mesa 12. The Z direction is an example of a first direction.

[0036] The mesa 12 has two side surfaces 12a and a top surface 12b.

[0037] The two side surfaces 12a are end surfaces of the mesa 12 in the Y direction and the opposite direction to the Y direction, in other words, end surfaces on both sides in the width direction of the mesa 12. The side surfaces 12a intersect and are perpendicular to the Y direction, and extend in the X direction and the Z direction. The side surfaces 12a extend in the X direction at a substantially constant height in the Z direction. The two side surfaces 12a are also substantially parallel to each other.

[0038] The top surface 12b is located at an end of the mesa 12 in the Z direction. The top surface 12b intersects and is perpendicular to the Z direction, and extends in the X and Y directions. The top surface 12b extends in the X direction with a substantially constant width in the Y direction. The top surface 12b is also substantially parallel to the base surface 11a. The top surface 12b is an example of an end surface.

[0039] The mesa 12 has a waveguide layer 14 and cladding layers 13 and 15. In the mesa 12, the cladding layer 13, the waveguide layer 14, and the cladding layer 15 are stacked in this order in the Z direction. Note that the mesa 12 may have a different configuration from that shown in FIG. 1, for example, by having more layers.

[0040] The waveguide layer 14 is provided at a position away from the top surface 12b in the opposite direction to the Z direction, is located at the middle of the mesa 12 in the Z direction, and extends along the base surface 11a. The waveguide layer 14 intersects and is perpendicular to the Z direction, and extends in a strip shape in the X direction with a substantially constant thickness in the Z direction. The waveguide layer 14 also spans between the two side surfaces 12a of the mesa 12.

[0041] The cladding layer 13 is adjacent to the waveguide layer 14 on the opposite side in the Z direction, and the cladding layer 15 is adjacent to the waveguide layer 14 in the Z direction. That is, the waveguide layer 14 is located between the cladding layer 13 and the cladding layer 15 in the Z direction.

[0042] The waveguide layer 14 functions as a core for transmitting light, and the cladding layers 13 and 15 function as cladding for the waveguide layer 14. The materials of the waveguide layer 14 and the cladding layers 13 and 15 are set so that the refractive index of the cladding layers 13 and 15 is lower than the refractive index of the waveguide layer 14. As an example, the waveguide layer 14 is made of InGaAsP, and the cladding layers 13 and 15 are made of a III-V group semiconductor having a zinc blende structure, such as indium phosphide (InP).

[0043] The two side surfaces 12a and the top surface 12b of the mesa 12, as well as the base surface 11a, are covered with a dielectric layer (not shown).

[0044] A heater layer 16 is provided on the top surface 12b via a dielectric layer. The heater layer 16 extends in a strip shape in the X direction at a position away from the waveguide layer 14 in the Z direction with a substantially constant thickness in the Z direction. The heater layer 16 is made of an electric resistor that generates heat when supplied with power from electrodes and conductor wiring (not shown). The heater layer 16 is made of, for example, an alloy mainly composed of nickel (Ni) and chromium (Cr).

[0045] 1, a cavity 12c is provided in the mesa 12 on the opposite side of the heater layer 16 with respect to the waveguide layer 14, in other words, at a position away from the waveguide layer 14 in the opposite direction in the Z direction. The cavity 12c is provided in the base portion of the mesa 12. Note that the position of the cavity 12c in the Z direction may be between the base surface 11a and the waveguide layer 14, and is not limited to the base portion of the mesa 12.

[0046] The hollow portion 12c extends in the X direction. A cross section of the hollow portion 12c intersecting with the X direction has a triangular shape, more specifically, an isosceles triangle shape with a perpendicular line along the Z direction.

[0047] FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1, and FIG. 3 is a cross-sectional view taken along line III-III of FIG. 1. The cavity 12c is surrounded by a bottom surface 12c1 and two side surfaces 12c21. The bottom surface 12c1 is located at an end of the cavity 12c opposite to the Z direction, intersects with the Z direction, and extends in the X direction with a substantially constant width in the Y direction. The bottom surface 12c1 is, for example, a part of the base surface 11a. The two side surfaces 12c21 are inclined with respect to the Z direction so as to approach each other in the Y direction from both ends of the bottom surface 12c1 in the Y direction toward the Z direction. The cavity 12c having the cross-sectional shape shown in FIG. 2 extends in the X direction as shown in FIGS. 1 and 3. That is, the cavity 12c includes a V-shaped groove 12c2 recessed in the Z direction and extending in the X direction. The groove 12c2 may also be referred to as a V-groove. The recessed line 12c22 provided at the end of the groove 12c2 in the Z direction extends in the X direction. The X direction is an example of the second direction. The side surface 12c21 may also be referred to as an inclined surface. As shown in Figs. 1 and 3, in this embodiment, both ends of the cavity 12c (12c-1) in the extension direction, i.e., in the X direction, are open.

[0048] In this embodiment, the mesa 12 and the cavity 12c extend in the same direction, that is, in the X direction. The mesa 12 extends along the base surface 11a in the X direction, and is an example of a first portion.

[0049] A coating layer 17 is provided on the bottom surface 12c1 of the hollow portion 12c. The coating layer 17 has a substantially constant thickness in the Z direction and extends in a strip shape in the X direction. The function of the coating layer 17 will be described later. In this embodiment, the coating layer 17 exists in the hollow portion 12c, but the coating layer 17 may disappear during the manufacturing process of the mesa 12.

[0050] The heater layer 16 described above is provided to heat the waveguide layer 14. Heat generated in the heater layer 16 by the supply of power is transmitted in the cladding layer 15 in the opposite direction of the Z direction and reaches the waveguide layer 14. This heat is further transmitted from the waveguide layer 14 in the cladding layer 13 in the opposite direction of the Z direction and escapes to the base 11. If the amount of heat escaped from the waveguide layer 14 to the base 11 via the cladding layer 13 is large, the heating efficiency of the heater layer 16 decreases, which is one of the causes of increased power consumption.

[0051] In this regard, in the present embodiment, as described above, the cavity 12c is provided in the mesa 12, and therefore, as shown in Fig. 2, at the base portion of the mesa 12 close to the base 11, the width of the mesa 12 in the Y direction is narrowed on both sides of the cavity 12c in the Y direction. That is, the mesa 12 has a narrowed portion at the position where the cavity 12c is provided, the cross-sectional area intersecting the Z direction being narrower than other portions. Therefore, according to the present embodiment, it is difficult for heat to escape from the waveguide layer 14 to the base 11 via the cladding layer 13, i.e., the mesa 12, and therefore it is possible to suppress a decrease in the heating efficiency of the heater layer 16 and suppress power consumption.

[0052] [Cavity formation] 4 is a plan view showing the wafer WF that becomes the base 11 and the covering layer 17 provided on the wafer WF. In this embodiment, the portion that becomes the mesa 12, such as the cladding layer 13, is formed by epitaxial growth on the surface of the wafer WF that becomes the base surface 11a. In this case, the growth direction (stacking direction) of the portion that becomes the mesa 12 is the

[0100] direction of the crystal orientation of the wafer WF. The

[0100] direction is the Z direction. The Z direction can also be called the growth direction or the stacking direction. The orientation flat OF of the wafer WF is aligned with the [01-1] direction of the crystal orientation of the wafer WF.

[0053] In this embodiment, the covering layer 17 is made of a material on which the cladding layer 13 does not grow during epitaxial growth, specifically, for example, silicon dioxide (SiO2), silicon nitride (SiN x) or a conductor such as tungsten (W). Thus, in the epitaxial growth, the cladding layer 13 grows on the exposed areas not covered by the covering layer 17 when viewed in the opposite direction to the Z direction.

[0054] The inventors have found through extensive research that, when the cladding layer 13 grows by epitaxial growth on an exposed region outside the covering layer 17, if the covering layer 17 extends in a direction (X direction) that intersects with the crystal orientation

[0011] at an angle θ of 45° or less, as shown in Fig. 4, a cavity 12c adjacent to the covering layer 17 in the Z direction is formed in the growth layer, i.e., in the cladding layer 13. Note that the cavity 12c has a finite height in the Z direction and is closed in the Z direction.

[0055] 5 to 7 are cross-sectional views of the base 11 and the cladding layer 13, showing the difference in the growth state of the cladding layer 13 depending on the difference in the angle θ between the crystal orientation

[0011] direction and the X direction. Fig. 5 shows the case where the angle difference θ in Fig. 4 is 0°, Fig. 6 shows the case where the angle difference θ is greater than 0° and less than 10°, and Fig. 7 shows the case where the angle difference θ is greater than 10° and less than 45°.

[0056] 5, when the angle difference θ is 0°, in the epitaxial growth, two inclined surfaces 13a are formed in the cladding layer 13 that move away from each other in the Y direction as they move toward the Z direction from both ends of the coating layer 17 in the Y direction. Therefore, when the angle difference is 0°, it is difficult to form a cavity 12c having a finite height in the Z direction on the coating layer 17 in the cladding layer 13.

[0057] 6, when the angle difference θ is greater than 0° and smaller than 10°, in the epitaxial growth, two inclined surfaces approaching each other in the Y direction are formed in the cladding layer 13 from both ends of the coating layer 17 in the Y direction as they approach the Z direction, and then, from the middle, inclined surfaces approaching each other in the Y direction as they approach the Z direction are formed. In other words, the two cladding layers 13 adjacent to each other on both sides in the Y direction with respect to the coating layer 17 are formed with protruding portions 13b protruding in the Y direction at the middle position in the Z direction. In this case, by appropriately setting the width Wm of the coating layer 17 relative to the thickness (height) H of the cladding layer 13 in the Z direction, the peaks 13p of the two protruding portions 13b are configured to be in contact with or connected to each other, and thus a cavity 12c having a finite height in the Z direction can be formed on the coating layer 17.

[0058] Here, the acute angle (hereinafter referred to as the inward inclination angle) between the Y-direction end of coating layer 17 and base surface 11a of the inclined surface between peak 13p of protrusion 13b is α, the protruding height (width) in the Y direction of protrusion 13b from the Y-direction end of coating layer 17 is Wp, the Y-direction width of coating layer 17 is Wm, the Z-direction height from base surface 11a to peak 13p (hereinafter referred to as the peak height) is Hp, and the Z-direction thickness of cladding layer 13 is H, and the thickness of coating layer 17 is ignored, then if 2·Wp≧Wm, then the two peaks 13p will be in contact with or connected to each other. Here, since Hp=Wp·tan α, in this case, Wm≦2·Hp / tan α (1) The width Wm of the covering layer 17 may be set so as to satisfy the following: Note that, according to research by the inventors, it has been found that the inward inclination angle α is in the range of 36° to 45°, and varies depending on the conditions.

[0059] Furthermore, the inventors' research has revealed that the ratio Hp / H of the peak height Hp to the thickness H of the cladding layer 13 changes depending on the angle difference θ. FIG. 8 is a graph showing an experimental correlation between the ratio Hp / H and the angle difference θ. As shown in FIG. 8, it has been found that the ratio Hp / H gradually increases and approaches 1 as the angle difference θ increases in a range where the angle difference θ is greater than 0°, 1° or more, and 10° or less. Therefore, by determining the peak height Hp corresponding to the angle difference θ and the thickness H of the cladding layer 13 from the approximation function or map obtained from the correlation shown in FIG. 8 and determining the width Wm of the coating layer 17 so as to satisfy the above formula (1), the two peaks 13p are in contact with or connected to each other, and thus a cavity 12c having a finite height in the Z direction can be formed on the coating layer 17.

[0060] Moreover, the inventors' research has revealed that when the angle difference θ is 10° or more and 45° or less, the ratio Hp / H is approximately 1. In this case, in the epitaxial growth, as shown in FIG. 7, a protrusion 13b having two inclined surfaces approaching each other in the Y direction as it approaches the Z direction is formed in the cladding layer 13 from both ends of the coating layer 17 in the Y direction, and a peak 13p of the protrusion 13b appears at the end of the cladding layer 13 in the Z direction. In this case, the width Wm of the coating layer 17 can be determined so as to satisfy the above-mentioned formula (1). In other words, in order to prevent an opening OP from occurring between the protrusions 13b at the end of the cladding layer 13 in the Z direction, a sufficient thickness H of the cladding layer 13 in the Z direction corresponding to the width of the coating layer 17 in the Y direction may be secured, or the width of the coating layer 17 in the Y direction may be appropriately set within a range that is not too wide, corresponding to the thickness H of the cladding layer 13 in the Z direction.

[0061] 9 shows a state in which protruding portions 13b of the cladding layer 13 grown from both sides of the coating layer 17 in the Y direction are connected to each other at positions spaced apart from the coating layer 17 in the Z direction, forming a cavity 12c with a finite height in the Z direction on the coating layer 17. A groove 12c2 is formed in the cavity 12c, which is recessed in the Z direction and extends in the X direction. The groove 12c2 is a V-shaped groove that forms a recessed line 12c22 extending in the X direction.

[0062] When the angle difference θ is equal to or greater than 10° and equal to or less than 45°, a flat upper surface 13c of the cladding layer 13 is formed at a position away from the cavity 12c in the Z direction, as shown in Fig. 9. In this case, a step of processing the upper surface 13c into a flat surface after the epitaxial growth is not required, and therefore, for example, there is an advantage that the mesa 12 provided with the cavity 12c can be manufactured with less effort and at lower cost.

[0063] In addition, in the configuration of this embodiment, the covering layer 17 on the bottom surface 12c1 of the cavity 12c, the groove 12c2 recessed in the crystal orientation

[0100] direction (Z direction) of the cavity 12c, and the recessed line 12c22 as the Z-direction end of the groove 12c2 extend in the X direction (second direction) obliquely intersecting the crystal orientation

[0011] direction at an angle of 45° or less, which is evidence that the cavity 12c is formed in the mesa 12 by the above-mentioned epitaxial growth. In addition, as shown in FIG. 2, the cross section of the cavity 12c intersecting the X direction has a bottom surface 12c1 intersecting the Z direction and extending in the X direction, and two side surfaces 12c21 extending in the X direction and inclined so as to approach each other in the Y direction as they approach the Z direction, which is also evidence that the cavity 12c is formed in the mesa 12 by the above-mentioned epitaxial growth. Each crystal orientation can be determined by analyzing the material of the optical semiconductor device 10A (10).

[0064] [Mesa formation] 10 to 12 are cross-sectional views showing a process for forming the mesa 12. First, as shown in FIG. 10, in a state where the covering layer 17 having the above-mentioned angle difference θ is formed on the base surface 11a of the base 11, the laminate L of the cladding layer 13, the waveguide layer 14, and the cladding layer 15 is formed by epitaxial growth. At this time, as described above, the cavity 12c is formed on the covering layer 17. That is, in the step of forming the laminate L by epitaxial growth, the cavity 12c including the groove 12c2 is formed on the covering layer 17 inside the laminate L. In addition, a covering layer 18 (mask) made of a dielectric material or the like is formed on the top surface 12b located at the end of the cladding layer 15 in the Z direction.

[0065] 11, for example, by dry etching, exposed portions of the laminate L that are not covered with the covering layer 18 when viewed from the Z direction are removed, and the cladding layer 13, the waveguide layer 14, and the cladding layer 15 included in the mesa 12 are formed as portions of the laminate L aligned in the Z direction with the covering layer 18. The side surfaces 12a of the mesa 12 are aligned in the Z direction with both edges of the covering layer 18 in the Y direction.

[0066] Next, as shown in FIG. 12, after removing the covering layer 18 by, for example, wet etching, a dielectric layer 12f is formed to cover the side surface 12a, the top surface 12b, and the base surface 11a of the mesa 12, and a heater layer 16 is formed on the top surface 12b via the dielectric layer 12f.

[0067] Through these steps, an optical semiconductor device 10A (10) is obtained in which the mesa 12 protrudes above the base surface 11a and the cavity 12c is provided within the mesa 12.

[0068] According to this embodiment, the cavity 12c can be formed in the mesa 12 by the relatively simple process as described above, which has the advantage that, for example, heat is less likely to escape from the mesa 12 to the base 11, and the optical semiconductor device 10A having higher heating efficiency by the heater layer 16 can be manufactured with less effort and at lower cost.

[0069] [Second embodiment] Fig. 13 is a cross-sectional view of an optical semiconductor device 10B (10) of the second embodiment at the same position as in Fig. 3. In the optical semiconductor device 10A of the first embodiment, both ends in the X direction of the cavity 12c-1 (12c) are open, whereas in this embodiment, as shown in Fig. 13, an end 12c3 in the X direction of the cavity 12c-2 (12c) is located within the mesa 12. That is, the cavity 12c-2 is closed within the mesa 12.

[0070] Also, the X-direction end 12c3 of the cavity 12c is spaced apart in the X-direction from the X-direction end 16a of the heater layer 16, while the opposite X-direction end 12c3 of the cavity 12c is spaced apart in the opposite X-direction from the opposite X-direction end 16a of the heater layer 16. That is, the X-direction length Ls of the cavity 12c is longer than the X-direction length Lh of the heater layer 16. Thus, in this embodiment, the cavity 12c is provided so as to protrude on both sides of the heater layer 16 in the X-direction.

[0071] On the contrary, if the heater layer 16 were to protrude on both sides in the X direction relative to the cavity 12c, the heat from the heater layer 16 would be more likely to escape to the base 11 through the portions of the mesa 12 adjacent to both sides in the X direction relative to the cavity 12c. In this regard, in this embodiment, the cavity 12c is provided so as to protrude on both sides in the X direction relative to the heater layer 16, so that the heat from the heater layer 16 is less likely to escape to the base 11, which in turn suppresses a decrease in the heating efficiency of the heater layer 16 and reduces power consumption. Note that the ends 16a on both sides in the X direction of the heater layer 16 and the ends 12c3 on both sides in the X direction of the cavity 12c may be aligned in the Z direction.

[0072] In the optical semiconductor device 10B of this embodiment, the portion 12d adjacent to the cavity 12c in the Z direction is supported at both ends by the portions 12e adjacent to the portion 12d on both sides in the X direction. This allows the desired shape and posture of the portion 12d adjacent to the cavity 12c in the Z direction to be maintained even when the cavity 12c is provided. With this configuration, the portion 12d can be supported by the portion 12e, which is a part of the mesa 12, so that the optical semiconductor device 10B can be made smaller and the manufacturing effort and cost can be reduced compared to a configuration in which a support or the like for supporting the portion 12d is provided separately from the mesa 12. In this embodiment, the portions on both sides in the Y direction of the cavity 12c also support the portion 12d, so that the reduction in rigidity and strength of the mesa 12 due to the cavity 12c can be suppressed. The portion 12d can also be called a bridge portion, and the portion 12e can also be called a support portion. In this case, the cavity 12c may be partially open in the Y direction (width direction) on at least one of the side surfaces 12a.

[0073] [Third embodiment] 14 is a cross-sectional view of an optical semiconductor device 10C (10) of the third embodiment at the same position as in FIG. 2. As shown in FIG. 14, in the optical semiconductor device 10C of the third embodiment, a thermal resistance layer 19 having a lower thermal conductivity than the cladding layer 13 adjacent to the thermal resistance layer 19 is provided between the cavity 12c and the waveguide layer 14 of the cladding layer 13, i.e., on the opposite side of the heater layer 16 with respect to the waveguide layer 14. In this embodiment, the thermal resistance layer 19 is aligned with the cavity 12c in the Z direction. The thermal resistance layer 19 is made of, for example, InGaAs, InGaAsP, AlInAs, or the like. The cladding layer 13 is an example of an adjacent portion.

[0074] According to this embodiment, the thermal resistance layer 19 makes it even more difficult for heat to escape to the base 11, which has the advantage that, for example, the heating efficiency of the heater layer 16 can be further prevented from decreasing, and power consumption can be further reduced.

[0075] [Fourth embodiment] Fig. 15 is a cross-sectional view of an optical semiconductor device 10D (10) of the fourth embodiment at the same position as in Fig. 2. As shown in Fig. 15, the optical semiconductor device 10D of the fourth embodiment also has a thermal resistance layer 19 on the opposite side of the heater layer 16 with respect to the waveguide layer 14, so as to be aligned with the cavity 12c in the Z direction. However, in this embodiment, the thermal resistance layer 19 is located in the opposite direction from the cavity 12c in the Z direction.

[0076] In this embodiment as well, the thermal resistance layer 19 makes it even more difficult for heat to escape to the base 11, thereby providing the advantage that, for example, the heating efficiency of the heater layer 16 can be further prevented from decreasing, and power consumption can be further reduced.

[0077] [Fifth embodiment] Fig. 16 is a cross-sectional view of an optical semiconductor device 10E (10) of the fifth embodiment at the same position as in Fig. 2. As shown in Fig. 16, the optical semiconductor device 10E of the fifth embodiment also has a thermal resistance layer 19 on the opposite side of the heater layer 16 with respect to the waveguide layer 14. However, in this embodiment, the thermal resistance layer 19 is aligned in the Y direction with respect to the cavity 12c or the covering layer 17.

[0078] In this embodiment as well, the thermal resistance layer 19 makes it even more difficult for heat to escape to the base 11, thereby providing the advantage that, for example, the heating efficiency of the heater layer 16 can be further prevented from decreasing, and power consumption can be further reduced.

[0079] [Sixth embodiment] Fig. 17 is a cross-sectional view of an optical semiconductor device 10F(10) according to a sixth embodiment at the same position as in Fig. 2. As shown in Fig. 17, in the optical semiconductor device 10F according to the sixth embodiment, a plurality of cavities 12c aligned in the Y direction are provided in the mesa 12. The Y direction is an example of a third direction.

[0080] In this embodiment, since the hollow portion 12c is provided, heat is less likely to escape from the mesa 12 to the base 11, which has the advantage that, for example, a decrease in the heating efficiency of the heater layer 16 can be prevented, and power consumption can be reduced.

[0081] However, in this embodiment, of the three cavities 12c provided in the mesa 12, the cavity 12c located in the center in the Y direction is closed in the Y direction, while the other two cavities 12c are open on the side surface 12a in the Y direction or in the opposite direction to the Y direction.

[0082] Such a configuration can be obtained, for example, by the following manufacturing process. That is, first, a plurality of covering layers 17 extending in the X direction are arranged on the base surface 11a in a substantially parallel manner with an interval in the Y direction, and a laminate L is formed on the covering layer 17. As a result, a laminate L including a plurality of substantially parallel cavities 12c is obtained. Next, the covering layer 18 is formed on the top surface 12b of the laminate L so that the edges extending in the X direction on both sides of the Y direction of the covering layer 18 overlap with different cavities 12c in the Z direction. Next, the exposed portion of the laminate L that is not covered by the covering layer 18 when viewed in the opposite direction to the Z direction is removed by dry etching. In this embodiment, the side surface 12a of the mesa 12 is also aligned with the edges of both sides of the covering layer 18 in the Y direction in the Z direction. Therefore, this process results in a mesa 12 as shown in FIG. 17, in which different cavities 12c are opened on the side surfaces 12a located on both sides of the Y direction. Thereafter, a dielectric layer (not shown) and a heater layer 16 are formed in the same manner as in the first embodiment.

[0083] In the case where the mesa 12 has two open cavities 12c on each side 12a thereof as in this embodiment, as is clear from FIG. 17, the width of the narrowed portion in the mesa 12 is a value corresponding to the width d between adjacent cavities 12c in the Y direction (2d in this embodiment, as an example), and is determined by the arrangement of the multiple cavities 12c, i.e., the arrangement of the coating layer 17.

[0084] Fig. 18 shows a case where the position of the cavity 12c and the positions of the two side surfaces 12a are shifted in the Y direction from the configuration of Fig. 17 in the configuration of this embodiment. As is clear from a comparison of Fig. 17 and Fig. 18, in this embodiment, even if the position of the cavity 12c and the positions of the two side surfaces 12a are shifted in the Y direction, the size of the width (2d) of the narrowed portion does not change. Furthermore, although not shown, even if the width between the two side surfaces 12a changes, according to this embodiment, the size of the width (2d) of the narrowed portion does not change.

[0085] That is, in the case where the mesa 12 has the open cavity 12c on each of the two side surfaces 12a as in this embodiment, the width of the narrowed portion in the mesa 12 does not depend on the position of the side surface 12a of the mesa 12, i.e., the positions of both edges in the Y direction of the coating layer 18. Therefore, according to this configuration, compared to the case where the width of the narrowed portion depends on both the width d and the position of the side surface 12a, for example, it is possible to suppress the individual difference (variation) in the width of the narrowed portion, and therefore it is possible to obtain an advantage that the individual difference (variation) in the amount of heat transfer escaping from the narrowed portion, and therefore the heating performance of the heater layer 16, can be suppressed.

[0086] [Seventh embodiment] Fig. 19 is a cross-sectional view of an optical semiconductor device 10G (10) of the seventh embodiment at the same position as in Fig. 3. As shown in Fig. 19, in the optical semiconductor device 10G of the seventh embodiment, the mesa 12 extends in the Y direction. That is, the extension direction of the mesa 12, the waveguide layer 14, and the heater layer 16 is the Y direction. A plurality of cavities 12c aligned in the Y direction are provided in the mesa 12. The extension direction of each of the cavities 12c and each of the grooves 12c2 is the X direction.

[0087] That is, in this embodiment, the cavity 12c extends in the X direction, whereas the mesa 12 extends in the Y direction. The extension direction (Y direction) of the mesa 12 along the base surface 11a intersects with the X direction, and the mesa 12 is an example of a second portion.

[0088] In this embodiment, since the hollow portion 12c is provided, heat is less likely to escape from the mesa 12 to the base 11, and therefore the heating efficiency of the heater layer 16 is prevented from decreasing, and power consumption can be reduced.

[0089] [Eighth embodiment] 20 is a cross-sectional view of an optical semiconductor device 10H(10) of the eighth embodiment. The optical semiconductor device 10H of this embodiment is an example of a wavelength tunable laser device having a wavelength tunable laser resonator including a DBR section 20 (DBR: distributed Bragg reflector), a phase adjustment section 30, and a gain section 40. The DBR section 20, the phase adjustment section 30, and the gain section 40 are integrally formed and have a mesa 12 that extends linearly in the X direction.

[0090] The DBR section 20 has a laminated structure of a waveguide layer 14 and a diffraction grating layer 21 as a feedback layer. The DBR section 20 has reflection spectrum characteristics having a peak wavelength determined by the spatial period of the diffraction grating layer 21, and constitutes one of the reflecting sections of the laser resonator.

[0091] The DBR section 20 also has a heater layer 16. By heating the heater layer 16, the refractive index of the waveguide layer 14 can be changed, and thus the reflection peak can be shifted in the frequency axis direction.

[0092] The phase adjustment unit 30 has a heater layer 16. By heating the heater layer 16, the refractive index of the waveguide layer 14 is changed, and the optical length of the laser resonator can be adjusted. By adjusting the optical length of the laser resonator, the frequency of the resonator mode (cavity mode) can be shifted in the frequency axis direction while being finely adjusted. Fine adjustment of the resonator mode allows the selection of the resonator mode in laser oscillation, and allows the frequency to be changed in a small range.

[0093] The gain section 40 has an active layer 14A optically connected to the waveguide layer 14. When a current is applied to the active layer 14A, it generates optical gain, which results in laser oscillation. An end face 14a of the active layer 14A constitutes the other reflecting section of the laser resonator.

[0094] In this embodiment, a cavity 12c corresponding to the heater layer 16 is provided in the mesa 12 of the DBR section 20 and the phase adjustment section 30. The cavity 12c extends in the X direction and is closed within the mesa 12.

[0095] In the DBR section 20, the X-direction end 12c3 of the cavity 12c is separated in the X-direction from the X-direction end 21a of the diffraction grating layer 21, while the opposite X-direction end 12c3 of the cavity 12c is separated in the opposite X-direction from the opposite X-direction end 21a of the diffraction grating layer 21. That is, the X-direction length Ls of the cavity 12c is longer than the X-direction length Lg of the diffraction grating layer 21. Thus, in this embodiment, the cavity 12c is provided so as to protrude on both sides in the X-direction of the diffraction grating layer 21.

[0096] Conversely, if the diffraction grating layer 21 were to protrude on both sides in the X direction relative to the cavity 12c, heat from the heater layer 16 would be more likely to escape to the base 11 through portions of the mesa 12 adjacent to both sides in the X direction relative to the cavity 12c. In this case, a temperature difference (temperature distribution) would occur in the X direction in the diffraction grating layer 21, making it difficult to obtain the desired reflection characteristics. In this regard, in this embodiment, the cavity 12c is provided so as to protrude on both sides in the X direction relative to the diffraction grating layer 21, making it difficult for heat from the heater layer 16 to escape to the base 11, and thus it is possible to suppress the occurrence of a temperature difference (temperature distribution) in the X direction in the diffraction grating layer 21, making it difficult to obtain the desired reflection characteristics.

[0097] Moreover, in both the DBR section 20 and the phase adjustment section 30, similarly to the second embodiment, both ends in the X direction of the cavity 12c are provided to overlap both ends in the Z direction of the X direction of the heater layer 16, or are provided to protrude further on both sides in the X direction than the heater layer 16. Thus, according to the present embodiment, it becomes difficult for heat to escape from the heater layer 16 to the base 11, which in turn prevents a decrease in the heating efficiency of the heater layer 16 and reduces power consumption.

[0098] [Ninth embodiment] 21 is a plan view of an optical semiconductor device 10I (10) of the ninth embodiment. The optical semiconductor device 10I of this embodiment includes a connection section 50 and a ring resonator 60 in addition to an SG-DBR section 20S (SG-DBR: sampled-grating DBR), a phase adjustment section 30, and a gain section 40. The optical semiconductor device 10I is an example of a wavelength tunable laser device having a wavelength tunable laser resonator that utilizes the Vernier effect.

[0099] The connection section 50 is branched at a branching section, such as a 1×2 MMI coupler, optically connected to the gain section 40, and includes two mesas 12 that are bent in a plan view in opposite directions in the Z direction. The waveguide layer 14 of each mesa 12 is optically connected at a coupling section C to the annular waveguide layer 14 of the ring resonator 60 by a 2×2 MMI coupler or the like.

[0100] The phase adjustment section 30 is provided in a part of the connection section 50 .

[0101] The ring resonator 60 has a reflection spectrum characteristic having a comb-shaped peak with a different period from that of the SG-DBR section 20S, and constitutes the other reflecting section of the laser resonator.

[0102] The ring resonator 60 also includes a heater layer 16. By heating the heater layer 16, the refractive index of the waveguide layer 14 is changed, and this makes it possible to shift the comb-shaped reflection peak in the frequency axis direction.

[0103] In the mesa 12 of the SG-DBR portion 20S, the mesa 12, the cavity 12c-2 with both ends in the extension direction closed, and the recessed line 12c22 of the cavity 12c-2 all extend in the X20 direction (X direction). In the SG-DBR portion 20S, the mesa 12 also extends in the X20 direction. Therefore, the mesa 12 of the SG-DBR portion 20S is an example of a first portion.

[0104] In the mesa 12 of the phase adjustment unit 30, the mesa 12, the cavity 12c-2 having both closed ends in the extension direction, and the recessed line 12c22 of the cavity 12c-2 all extend in the X30 direction (X direction). In the phase adjustment unit 30, the mesa 12 also extends in the X30 direction. Thus, the mesa 12 of the phase adjustment unit 30 is an example of a first portion.

[0105] In addition, the annular portion of the mesa 12 of the ring resonator 60 is provided with a plurality of cavities 12c-1 with both ends in the extension direction being open. In addition, the coupling portion C is provided with a cavities 12c-2 that extend along the extension direction of the mesa 12 of the connection portion 50 and have both ends in the extension direction being closed. The cavities 12c-1, 12c-2 and their recessed lines 12c22 all extend in the X60 direction. In contrast, the mesa 12 of the ring resonator 60 extends in an annular shape, intersecting with the X60 direction. Thus, the mesa 12 of the ring resonator 60 is an example of the second portion. The extension direction at each position of the curved mesa 12 is defined as the tangent direction of the extension direction of the mesa 12 at the center position of the mesa 12 in the width direction in a plan view seen in the opposite direction to the Z direction.

[0106] As shown in FIG. 21, in this embodiment, the X20 direction and the X60 direction are parallel (the same direction), and the X30 direction is a direction that intersects (is a different direction from) the X20 direction and the X60 direction.

[0107] The X20, X30, and X60 directions all intersect obliquely with the crystal orientation

[0011] at angles of 45° or less. Therefore, according to this embodiment, the SG-DBR section 20S, the phase adjustment section 30, and the heater layer 16 of the ring resonator 60 have the same effects as those of the above embodiment due to the provision of the cavities 12c-1 and 12c-2.

[0108] [Tenth embodiment] Fig. 22 is a plan view of an optical semiconductor device 10J(10) of the tenth embodiment. As shown in Fig. 22, in this embodiment, the mesa 12 of the ring resonator 60 is provided with only a cavity 12c-2 whose both ends in the extension direction are closed. Except for this point, the optical semiconductor device 10J has a similar configuration to the optical semiconductor device 10I of the ninth embodiment. Although the shape of the cavity 12c-2 is different, this embodiment also provides the same effects as the ninth embodiment.

[0109] [Eleventh embodiment] Fig. 23 is a plan view of an optical semiconductor device 10K(10) of the eleventh embodiment. As shown in Fig. 23, the optical semiconductor device 10K of the present embodiment has a similar configuration to the optical semiconductor device 10I of the ninth embodiment, except that the shape of the connection portion 50 and the position and direction of the phase adjustment portion 30 provided in the connection portion 50 are different.

[0110] As shown in Fig. 23, in this embodiment, the X20 direction, the X30 direction, and the X60 direction are all the same direction and are obliquely intersecting with the crystal orientation

[0011] direction at 45° or less. Therefore, this embodiment also provides the same effects as the ninth embodiment. In addition, according to this embodiment, since the multiple coating layers 17 extend in parallel in the same direction, for example, there is an advantage that the process of forming the coating layers 17 can be performed more easily or more quickly.

[0111] Although the embodiment and the modified examples of the present invention have been illustrated above, the above-mentioned embodiment and the modified examples are merely examples and are not intended to limit the scope of the invention. The above-mentioned embodiment and the modified examples can be implemented in various other forms, and various omissions, substitutions, combinations, and modifications can be made without departing from the gist of the invention. In addition, the specifications of each configuration, shape, and the like (structure, type, direction, model, size, length, width, thickness, height, number, arrangement, position, material, etc.) can be appropriately changed and implemented. [Explanation of symbols]

[0112] 10, 10A~10K...Optical semiconductor device 11...Bass 11a...Base surface 12...Mesa (first part, second part) 12a...side 12b…Top surface 12c,12c-1,12c-2…Cavity part 12c1…Bottom surface 12c2…Groove 12c21…side 12c22…concave line 12c3...end 12d...part 12e…part 12f...Dielectric layer 13…Clad layer 13a…Slope surface 13b...Protruding part 13c…Top surface 13p…Peak 14...Waveguide layer 14A…Active layer 14a...end face 15…Clad layer 16…Heater layer 16a...end 17…Covering layer 18…Covering layer 19...Heat resistance layer 20...DBR section 20S…SG-DBR section 21... Diffraction grating layer 21a...end 30...Phase adjustment section 40…Gain section 50…Connection 60…Ring resonator C…Connection part d…(Width of the constricted part) L…Laminated body Lg, Lh, Ls...Length H…(cladding layer) thickness Hp…Peak height OF…Orientation Flat OP…Opening WF: Wafer Wm…(Coating layer) width Wp…(protruding part) protruding height X,X20,X30,X60…direction (second direction) Y…direction (third direction) Z…direction (first direction) α…Inward tilt angle θ…Angle difference

Claims

1. A base having a base surface intersecting a first direction which is a crystal orientation [100] direction; a mesa protruding from the base surface in the first direction, extending along the base surface, having an end face in the first direction, a cladding layer, and a waveguide layer, and having a cavity partially provided therein; a heater layer provided on the end surface and generating heat when supplied with electric power; a coating layer covering the base surface, the coating layer being made of a material on which the cladding layer does not epitaxially grow; having the waveguide layer is provided to extend along the base surface at a position away from the end surface of the mesa in a direction opposite to the first direction, the cladding layer is provided between the base surface and the waveguide layer and between the waveguide layer and the end surface, the cavity portion includes a groove that faces a side of the coating layer opposite to the base surface, is V-shaped in the first direction, and extends in a second direction intersecting the first direction; The second direction obliquely intersects with a [011] crystal orientation of the base at an angle of 45° or less.

2. The optical semiconductor device according to claim 1 , wherein the absolute value of the angular difference is in a direction of 10° or more and 45° or less.

3. The optical semiconductor device according to claim 1 , wherein the cavity is located at an end of the cavity in a direction opposite to the first direction, and has a bottom surface that extends in the second direction intersecting the first direction.

4. 4. The optical semiconductor device according to claim 1, wherein the mesa has a first section extending in the second direction and in which the cavity is provided.

5. 5. The optical semiconductor device according to claim 1, wherein the mesa has a second section extending along the base surface in a direction intersecting the second direction and in which the cavity is provided.

6. 6. The optical semiconductor device according to claim 1, wherein the mesa is provided with a plurality of cavities arranged in a third direction intersecting the first direction and the second direction as the cavity.

7. the mesa has two side surfaces as an end surface in the third direction and an end surface in a direction opposite to the third direction; The optical semiconductor device according to claim 6 , wherein the cavity includes a cavity that is open in the third direction on each of the two side surfaces.

8. 8. The optical semiconductor device according to claim 1, wherein the cavity includes a closed cavity within the mesa.

9. 9. The optical semiconductor device according to claim 1, further comprising a thermal resistance layer having a lower thermal conductivity than an adjacent portion, the thermal resistance layer being located on the opposite side of the waveguide layer from the heater layer and aligned with the cavity in the first direction.

10. 10. An optical semiconductor device according to claim 1, further comprising a thermal resistance layer having a lower thermal conductivity than adjacent portions at a position on the opposite side of the mesa from the heater layer with respect to the waveguide layer and aligned with the cavity in a third direction intersecting the first direction and the second direction.

11. The optical semiconductor device according to claim 10 , wherein the thermal resistance layers are located on both sides of the cavity in the third direction.

12. 12. The optical semiconductor device according to claim 1, wherein the mesa has a diffraction grating layer extending along the waveguide layer.

13. the mesa has a grating layer extending along the waveguide layer; 5. The optical semiconductor device according to claim 4, wherein the cavity is provided such that both ends of the cavity in the second direction overlap both ends of the diffraction grating layer in the second direction in the first direction, or such that the cavity protrudes beyond the diffraction grating layer on both sides in the second direction.

14. a base having a base surface intersecting the first direction; a mesa protruding from the base surface in the first direction, extending along the base surface, having an end face in the first direction, a cladding layer, and a waveguide layer, and having a cavity partially provided therein; a heater layer provided on the end surface and generating heat when supplied with electric power; a coating layer covering the base surface, the coating layer being made of a material on which the cladding layer does not epitaxially grow; having the waveguide layer is provided to extend along the base surface at a position away from the end surface of the mesa in a direction opposite to the first direction, the cladding layer is provided between the base surface and the waveguide layer and between the waveguide layer and the end surface, The cavity portion faces the side of the covering layer opposite the base surface, and includes a groove that is V-shaped in the first direction and extends in a second direction intersecting the first direction.

15. 15. The optical semiconductor device according to claim 1, wherein the base and the mesa are made of a III-V group semiconductor having a zinc blende structure.

16. A method for manufacturing an optical semiconductor device according to any one of claims 1 to 15, comprising the steps of: providing the coating layer on the base surface; forming a laminate by epitaxial growth of crystal grains on the base surface in the first direction; removing a portion of the laminate to form the mesa; forming the heater layer on the end surface of the mesa; having The method for manufacturing an optical semiconductor device, wherein in the step of forming the laminate, the cavity is formed inside the laminate.

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