Optical modulator

Abstract

To provide an optical modulator that reduces capacitance in an optical waveguide structure. [Solution] The optical modulator comprises an optical waveguide structure extending along a first direction. The optical waveguide structure comprises a first III-V compound semiconductor layer of a first conductivity type, a second III-V compound semiconductor layer of a second conductivity type, a core layer disposed between the first III-V compound semiconductor layer and the second III-V compound semiconductor layer, and a silicon-containing layer. The first III-V compound semiconductor layer is disposed between the silicon-containing layer and the core layer. The second III-V compound semiconductor layer has a width smaller than the width of the core layer.

Description

Technical Field

[0001] The present disclosure relates to an optical modulator.

Background Art

[0002] Patent Document 1 discloses a Mach-Zehnder type optical modulator. This optical modulator includes a substrate formed of indium phosphide (InP). Two arm waveguides are formed on the substrate. In each arm waveguide, a lower contact layer, a lower cladding layer, a core layer, an upper cladding layer, and an upper contact layer are stacked in this order. The lower contact layer and the lower cladding layer are formed of n-type InP doped with silicon. The core layer has a multiple quantum well structure. The upper cladding layer is formed of p-type InP doped with zinc. The upper contact layer is formed of p-type indium gallium arsenide (InGaAs) doped with zinc.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present disclosure provides an optical modulator that reduces capacitance in an optical waveguide structure.

Means for Solving the Problems

[0005] An optical modulator according to one aspect of the present disclosure comprises an optical waveguide structure extending along a first direction, the optical waveguide structure comprising a first III-V compound semiconductor layer of a first conductivity type, a second III-V compound semiconductor layer of a second conductivity type, a core layer disposed between the first III-V compound semiconductor layer and the second III-V compound semiconductor layer, and a silicon-containing layer, wherein the first III-V compound semiconductor layer is disposed between the silicon-containing layer and the core layer, and the second III-V compound semiconductor layer has a width smaller than the width of the core layer. [Effects of the Invention]

[0006] According to this disclosure, an optical modulator is provided that reduces capacitance in an optical waveguide structure. [Brief explanation of the drawing]

[0007] [Figure 1] Figure 1 is a schematic plan view showing an optical modulator according to one embodiment. [Figure 2] Figure 2 is a schematic plan view showing a part of the optical modulator shown in Figure 1. [Figure 3] Figure 3 is a cross-sectional view along the line III-III in Figure 2. [Figure 4] Figure 4 is a cross-sectional view along the line IV-IV in Figure 2. [Figure 5] Figure 5 is a cross-sectional view along the VV line in Figure 2. [Figure 6] Figure 6 is a cross-sectional view showing an example of the configuration of a first III-V compound semiconductor layer, a second III-V compound semiconductor layer, a core layer, and a silicon-containing layer. [Figure 7] Figure 7 is a cross-sectional view showing another example of the configuration of the first III-V compound semiconductor layer, the second III-V compound semiconductor layer, the core layer, and the silicon-containing layer. [Modes for carrying out the invention]

[0008] [Description of Embodiments in this Disclosure] First, the embodiments of this disclosure will be listed and explained.

[0009] (1) An optical modulator according to one embodiment includes an optical waveguide structure extending along a first direction, the optical waveguide structure comprising a first III-V compound semiconductor layer of a first conductivity type, a second III-V compound semiconductor layer of a second conductivity type, a core layer disposed between the first III-V compound semiconductor layer and the second III-V compound semiconductor layer, and a silicon-containing layer, wherein the first III-V compound semiconductor layer is disposed between the silicon-containing layer and the core layer, and the second III-V compound semiconductor layer has a width smaller than the width of the core layer.

[0010] According to the above optical modulator, the capacitance in the optical waveguide structure can be reduced.

[0011] (2) According to (1) above, the second III-V compound semiconductor layer includes a lower region adjacent to the core layer, and in the lower region, the width of the second III-V compound semiconductor layer may decrease as it approaches the core layer.

[0012] In this case, the capacitance in the optical waveguide structure can be further reduced.

[0013] (3) In (1) or (2) above, the first direction may be the same as the [0-11] direction of the second III-V compound semiconductor layer.

[0014] (4) In any one of (1) to (3) above, the second III-V compound semiconductor layer may have a thickness greater than the thickness of the core layer.

[0015] (5) In any one of (1) to (4) above, the first III-V compound semiconductor layer may have a thickness less than or equal to the thickness of the core layer.

[0016] [Details of the embodiments of this disclosure] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same or equivalent elements are denoted by the same reference numerals, and duplicate descriptions are omitted. In the drawings, the X-axis direction, the Y-axis direction, and the Z-axis direction that intersect each other are shown as necessary. The X-axis direction, the Y-axis direction, and the Z-axis direction are orthogonal to each other, for example.

[0017] FIG. 1 is a plan view schematically showing an optical modulator according to the present embodiment. The optical modulator 1 can modulate the intensity or phase of light and generate a modulation signal, for example, in optical communication. The optical modulator 1 may include an input port P1, a plurality of Mach-Zehnder modulator sections MZ1, MZ2, MZ3, MZ4, MZ5, an optical filter F1, a plurality of optical couplers C1, C2, C3, C4, C5, C6, C7, and a plurality of output ports P2. The input port P1, the plurality of Mach-Zehnder modulator sections MZ1 to MZ5, the optical filter F1, the plurality of optical couplers C1 to C7, and the plurality of output ports P2 may be provided on the substrate 11.

[0018] The substrate 11 extends along the X-axis direction and the Y-axis direction. The main surface of the substrate 11 may have a substantially rectangular shape. The main surface of the substrate 11 includes an edge 11a extending in the Y-axis direction and an edge 11b extending in the Y-axis direction. The edge 11b is located on the opposite side of the substrate 11 from the edge 11a in the X-axis direction. The input port P1 and the plurality of output ports P2 may be provided on the edge 11a. The input port P1 is located in the middle of the plurality of output ports P2.

[0019] The input port P1 is optically coupled to the optical filter F1 by an optical waveguide. The optical filter F1 is, for example, a 1-input 1-output optical component. The optical filter F1 is optically coupled to the optical coupler C1 by an optical waveguide. The optical coupler C1 is, for example, a 1-input 2-output MMI (Multi-Mode Interface) coupler. The optical coupler C1 is optically coupled to a plurality (e.g., two) of optical couplers C2 respectively by a plurality (e.g., two) of optical waveguides. Each optical coupler C2 is, for example, a 1-input 2-output MMI coupler. Each optical coupler C2 is optically coupled to the optical couplers C3 and C4 respectively by a plurality (e.g., two) of optical waveguides. Each of the optical couplers C3 and C4 is, for example, a 1-input 2-output MMI coupler. The optical waveguide may be formed in a silicon-containing layer 110 (see FIG. 3) described later. When the optical waveguide is formed in the silicon-containing layer 110, the core of the optical waveguide contains silicon. The core of the optical waveguide has, for example, a width of 0.5 μm. The core of the optical waveguide has, for example, a height of 0.2 μm. The optical couplers C1 to C7 and the optical filter F1 may be formed in the silicon-containing layer 110. The optical couplers C1 to C7 and the optical filter F1 may have a width larger than the width of the optical waveguide. A III-V compound semiconductor is not provided on the upper part of the optical waveguide. The upper surfaces of the optical waveguide, the optical couplers C1 to C7, and the optical filter F1 may be covered with an insulating film.

[0020] The optical coupler C3 is optically coupled to the first arm waveguide and the second arm waveguide of the Mach-Zehnder modulator MZ1 respectively by a plurality (e.g., two) of optical waveguides. A modulation electrode part E1a is provided on the first arm waveguide of the Mach-Zehnder modulator MZ1. A modulation electrode part E1b is provided on the second arm waveguide of the Mach-Zehnder modulator MZ1.

[0021] The first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ1 are optically coupled to the first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ3, respectively, by a plurality of (e.g., two) optical waveguides. A heater H3a is provided on the first arm waveguide of the Mach-Zehnder modulation unit MZ3. A heater H3b is provided on the second arm waveguide of the Mach-Zehnder modulation unit MZ3.

[0022] The first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ3 are optically coupled to an optical coupler C5 by a plurality (e.g., two) of optical waveguides. The optical coupler C5 is, for example, a 2-input 1-output MMI coupler. The optical coupler C5 is optically coupled to the first arm waveguide of the Mach-Zehnder modulation unit MZ5 by an optical waveguide. A heater H5a is provided on the first arm waveguide of the Mach-Zehnder modulation unit MZ5. One optical coupler C3, one Mach-Zehnder modulation unit MZ1, one Mach-Zehnder modulation unit MZ3, and one optical coupler C5 constitute one sub-Mach-Zehnder modulator.

[0023] The optical coupler C4 is optically coupled to the first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ2 by multiple (e.g., two) optical waveguides. A modulation electrode section E2a is provided on the first arm waveguide of the Mach-Zehnder modulation unit MZ2. A modulation electrode section E2b is provided on the second arm waveguide of the Mach-Zehnder modulation unit MZ2.

[0024] The first and second arm waveguides of the Mach-Zehnder modulation unit MZ2 are optically coupled to the first and second arm waveguides of the Mach-Zehnder modulation unit MZ4, respectively, by a plurality of (e.g., two) optical waveguides. A heater H4a is provided on the first arm waveguide of the Mach-Zehnder modulation unit MZ4. A heater H4b is provided on the second arm waveguide of the Mach-Zehnder modulation unit MZ4. One optical coupler C4, one Mach-Zehnder modulation unit MZ2, one Mach-Zehnder modulation unit MZ4, and one optical coupler C6 constitute one sub-Mach-Zehnder modulator.

[0025] The first and second arm waveguides of the Mach-Zehnder modulation unit MZ4 are optically coupled to an optical coupler C6 by a plurality (e.g., two) of optical waveguides. The optical coupler C6 is, for example, a 2-input, 1-output MMI coupler. The optical coupler C6 is optically coupled to the second arm waveguide of the Mach-Zehnder modulation unit MZ5 by an optical waveguide. A heater H5b is provided on the second arm waveguide of the Mach-Zehnder modulation unit MZ5.

[0026] The first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ5 are optically coupled to an optical coupler C7 by multiple (e.g., two) optical waveguides. The optical coupler C7 is, for example, a 2-input, 2-output MMI coupler. The optical coupler C7 is optically coupled to multiple (e.g., two) output ports P2 by multiple (e.g., two) optical waveguides.

[0027] Optical modulator 1 includes four sub-Mach-Zehnder modulators. After being input to input port P1, light is output from output port P2 via Mach-Zehnder modulation section MZ1 and MZ5. The light is converted into four different signal lights by the four sub-Mach-Zehnder modulators. The four signal lights are combined by two optical couplers C7. The four signal lights are combined and output from two of the four output ports P2. Mach-Zehnder modulation sections MZ1 and MZ2 may include arm waveguides containing III-V compound semiconductors. Each arm waveguide of MZ1 and MZ2 may include a waveguide section containing a III-V compound semiconductor and a waveguide section not containing a III-V compound semiconductor. For example, the waveguide section containing a III-V compound semiconductor corresponds to the optical waveguide structure WS described later. In waveguide sections that do not contain III-V compound semiconductors, each arm waveguide has a silicon-containing core. In each arm waveguide, the waveguide section containing the III-V compound semiconductor and the waveguide section that does not contain the III-V compound semiconductor are optically coupled. In the arm waveguide, the silicon-containing core and the core layer 130 (see Figure 3) contained in the III-V compound semiconductor are optically coupled. The refractive index of the arm waveguide can be changed by applying a high-frequency voltage between the electrode sections E1a, E1b, E2a, E2b and the ground electrode. On the other hand, the Mach-Zehnder modulation sections MZ3 to MZ5 are equipped with silicon-containing arm waveguides. The first arm waveguides and second arm waveguides of the Mach-Zehnder modulation sections MZ3 to MZ5 do not necessarily have to contain III-V compound semiconductors. The refractive index of the arm waveguide can be changed by heating the arm waveguide with heaters H3a, H3b, H4a, H4b, H5a, H5b. Heaters H3a, H3b, H4a, H4b, H5a, and H5b may also be conductive patterns for resistance heating.

[0028] Figure 2 is a schematic plan view showing a part of the optical modulator in Figure 1. Figure 2 shows the portion of the first arm waveguide where electrode portion E1a is provided and the portion of the second arm waveguide where electrode portion E1b is provided in the Mach-Zehnder modulation section MZ1. As shown in Figure 2, the first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation section MZ1 are provided with an optical waveguide structure WS that extends along the X-axis direction (first direction). Electrode portions E1a and E1b are provided on the optical waveguide structure WS. The first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation section MZ2 may each be provided with an optical waveguide structure WS.

[0029] As shown in Figure 2, the optical waveguide structure WS comprises a plurality of first regions R1 arranged spaced apart from each other along the X-axis direction, and second regions R2 located between the plurality of first regions R1. The plurality of first regions R1 and each second region R2 are electrically isolated.

[0030] The electrode section E1a comprises an electrode E1a1, a wiring EWa1, and a wiring EWa2. In this embodiment, the electrode E1a1 is located on each first region R1 of the optical waveguide structure WS provided by the first arm waveguide of the Mach-Zehnder modulation section MZ1. Each electrode E1a1 is connected to the wiring EWa2 via the wiring EWa1. Each electrode E1a1 extends along the X-axis. Each wiring EWa1 extends along the Y-axis. The wiring EWa2 extends in the X-axis direction across a plurality of first regions R1. The electrode E1a1, wiring EWa1, and wiring EWa2 are located on the substrate 11. The electrode E1a1, wiring EWa1, and wiring EWa2 may contain metal.

[0031] The electrode section E1b comprises an electrode E1b1, wiring EWb1, and wiring EWb2. In this embodiment, the electrode E1b1 is located on each first region R1 of the optical waveguide structure WS provided by the second arm waveguide of the Mach-Zehnder modulation section MZ1. Each electrode E1b1 is connected to wiring EWb2 via wiring EWb1. Each electrode E1b1 extends along the X-axis. Each wiring EWb1 extends along the Y-axis. Wiring EWb2 extends in the X-axis direction across a plurality of first regions R1. The electrode E1b1, wiring EWb1, and wiring EWb2 are located on the substrate 11. The electrode E1b1, wiring EWb1, and wiring EWb2 may contain metal.

[0032] Figure 3 is a cross-sectional view along line III-III in Figure 2. Figure 3 shows a cross-section of the first region R1 at the locations where electrode E1a1, wiring EWa1, electrode E1b1, and wiring EWb1 are located. As shown in Figure 3, each optical waveguide structure WS comprises a silicon-containing layer 110, a first III-V compound semiconductor layer 120 of a first conductivity type (e.g., n-type), a core layer 130, and a second III-V compound semiconductor layer 140 of a second conductivity type (e.g., p-type). The second conductivity type is the opposite conductivity type of the first conductivity type. The silicon-containing layer 110 may be located between the substrate 11 and the first III-V compound semiconductor layer 120. The first III-V compound semiconductor layer 120 may be located between the silicon-containing layer 110 and the core layer 130. The core layer 130 may be located between the first III-V compound semiconductor layer 120 and the second III-V compound semiconductor layer 140.

[0033] The substrate 11 may comprise a silicon substrate 100 and a silicon oxide layer 102 on the silicon substrate 100. The silicon oxide layer 102 is disposed between the silicon substrate 100 and the silicon-containing layer 110. The thickness of the silicon substrate 100 may be 100 μm or more. The thickness of the silicon oxide layer 102 may be 2 μm or more. In one embodiment, the thickness of the silicon substrate 100 is 500 μm, and the thickness of the silicon oxide layer 102 is 2 μm.

[0034] The silicon-containing layer 110 may be disposed between the silicon oxide layer 102 and the first III-V compound semiconductor layer 120. The silicon-containing layer 110 may be formed by filling with silicon. The silicon-containing layer 110 may be a silicon layer. The silicon-containing layer 110 may have a thickness T110 that is smaller than the thickness T120 of the first III-V compound semiconductor layer 120. The thickness T110 of the silicon-containing layer 110 may be 0.1 to 0.5 μm. In one embodiment, the thickness T110 is 0.3 μm.

[0035] The first III-V compound semiconductor layer 120 may contain at least one of indium phosphide (InP), indium gallium arsenide phosphide (InGaAsP), aluminum indium gallium arsenide (AlInGaAs), gallium arsenide (GaAs), or aluminum gallium arsenide (AlGaAs). An example of an n-type dopant is silicon (Si). The first III-V compound semiconductor layer 120 may have a thickness T120 that is less than or equal to the thickness T130 of the core layer 130. The thickness T120 of the first III-V compound semiconductor layer 120 may be 0.1 to 1.0 μm. In one embodiment, the thickness T120 is 0.6 μm.

[0036] When the thickness T120 of the first III-V compound semiconductor layer 120 is 1.0 μm or less, it is possible to prevent a decrease in the optical transition efficiency when light transitions from the optical waveguide containing the silicon layer 110 to the core layer 130 containing the III-V compound semiconductor. In the arm waveguide, it is possible to prevent a weakening of the optical coupling between the optical waveguide section having a silicon-containing core and the waveguide section containing the III-V compound semiconductor. It is possible to prevent insufficient modulation of light performed by the core layer 130.

[0037] When the thickness T120 of the first III-V compound semiconductor layer 120 is 0.1 μm or more, it is possible to prevent the electrical resistance of the first III-V compound semiconductor layer 120 from increasing. It is possible to prevent a decrease in the efficiency of the electric field applied to the core layer 130 via the first III-V compound semiconductor layer 120. It is possible to prevent a decrease in modulation efficiency in the high-frequency band.

[0038] The core layer 130 comprises an undoped III-V compound semiconductor. The core layer 130 may have a multiple quantum well structure or it may be a bulk layer. Examples of III-V compound semiconductors include InGaAsP and AlInGaAs. The core layer 130 has a thickness T130. The thickness T130 may be 0.1 to 0.9 μm. In one embodiment, the thickness T130 is 0.3 μm.

[0039] The second III-V compound semiconductor layer 140 may contain at least one of InP, InGaAsP, AlInGaAs, GaAs, or AlGaAs. An example of a p-type dopant is zinc (Zn). The second III-V compound semiconductor layer 140 may have a thickness T140 that is greater than the thickness T130. Thickness T140 may be greater than thickness T120 and greater than thickness T110. Thickness T140 may be greater than the sum of thicknesses T120 and T110. The thickness T140 of the second III-V compound semiconductor layer 140 may be 1.5 to 2.2 μm. In one embodiment, the thickness T140 is 1.9 μm. The X-axis direction, which is the direction in which the optical waveguide structure WS extends, may be the same as the [0-11] direction of the second III-V compound semiconductor layer 140. Alternatively, the X-axis direction may be the same as the [0-1-1] direction of the second III-V compound semiconductor layer 140.

[0040] The first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation section MZ1 may further comprise an insulating film 150. The insulating film 150 may be a silicon oxide film or a benzocyclobutene (BCB) film. Each optical waveguide structure WS may be covered by the insulating film 150. Therefore, the silicon-containing layer 110, the first III-V compound semiconductor layer 120, the core layer 130, and the second III-V compound semiconductor layer 140 may be covered by the insulating film 150. In this embodiment, as shown in Figure 3, wiring EWa1 and wiring EWa2 are provided on the insulating film 150. Similarly, wiring EWb1 and wiring EWb2 are also provided on the insulating film 150. Note that in Figure 2, the insulating film 150 is omitted from the illustration in order to clearly show each part.

[0041] The insulating film 150 has an opening H1 on the second III-V compound semiconductor layer 140 of the first arm waveguide. The opening H1 exposes the upper surface of the second III-V compound semiconductor layer 140 of the first arm waveguide. An electrode E1a1 is provided within the opening H1. As a result, the electrode E1a1 is connected to the second III-V compound semiconductor layer 140 of the second arm waveguide.

[0042] Wiring EWa1 extends along the side surface of the insulating film 150 in the Z-axis direction. Furthermore, wiring EWa1 may extend from the core layer 130 to the first III-V compound semiconductor layer 120 in the Y-axis direction. Wiring EWa2 may extend outside the first III-V compound semiconductor layer 120 in the Y-axis direction.

[0043] The distance d1 between electrode E1a1 and the side surface of insulating film 150 may be 300 to 600 nm. Distance d1 is the shortest distance between electrode E1a1 and the side surface of insulating film 150 in the Y-axis direction. The distance d2 between wiring EWa1 and the upper surface of core layer 130 may be 600 to 900 nm. Distance d2 is the thickness of insulating film 150 on core layer 130 in the Z-axis direction.

[0044] The insulating film 150 has an opening H2 on the second III-V compound semiconductor layer 140 of the second arm waveguide. The opening H2 exposes the upper surface of the second III-V compound semiconductor layer 140 of the second arm waveguide. An electrode E1b1 is provided within the opening H2. As a result, the electrode E1b1 is connected to the second III-V compound semiconductor layer 140 of the second arm waveguide.

[0045] Wiring EWb1 extends along the side surface of the insulating film 150 in the Z-axis direction. Furthermore, wiring EWb1 may extend from the core layer 130 to the first III-V compound semiconductor layer 120 in the Y-axis direction. Wiring EWb2 may extend outside the first III-V compound semiconductor layer 120 in the Y-axis direction. The distance between electrode E1b1 and the side surface of the insulating film 150 may be the same as distance d1. The distance between wiring EWb1 and the top surface of the core layer 130 may be the same as distance d2.

[0046] As shown in Figure 3, the distance d3 between the optical waveguide structure WS of the first arm waveguide and the optical waveguide structure WS of the second arm waveguide may be, for example, 5 to 20 μm. The distance d3 is the distance in the Y-axis direction between the center of the second III-V compound semiconductor layer 140 of the first arm waveguide and the center of the second III-V compound semiconductor layer 140 of the second arm waveguide.

[0047] Figure 4 is a cross-sectional view along line IV-IV in Figure 2. Figure 4 shows a cross-section of the first region R1 at the location where only electrodes E1a1 and E1b1 are located. As shown in Figure 4, even in the first region R1 at the location where only electrodes E1a1 and E1b1 are located, the optical waveguide structure WS comprises a silicon-containing layer 110, a first III-V compound semiconductor layer 120, a core layer 130, and a second III-V compound semiconductor layer 140. In the first region R1 at the location where only electrodes E1a1 and E1b1 are located, wirings EWa1 and EWb1 are not provided on the insulating film 150 covering the core layer 130 and the first III-V compound semiconductor layer 120. Therefore, in the first region R1 at the location where only electrodes E1a1 and E1b1 are located, the surface of the insulating film 150 covering the core layer 130 and the first III-V compound semiconductor layer 120 is exposed.

[0048] Figure 5 is a cross-sectional view along the VV line in Figure 2. Figure 5 shows a cross-section of the second region R2. As shown in Figure 5, in the second region R2 as well, the optical waveguide structure WS comprises a silicon-containing layer 110, a first III-V compound semiconductor layer 120, a core layer 130, and a second III-V compound semiconductor layer 140. In the second region R2, no openings H1 and H2 are formed in the insulating film 150. That is, in the second region R2, the insulating film 150 covers the upper surface of the second III-V compound semiconductor layer 140. In the second region R2, no electrode E1a1 is provided on the insulating film 150 that covers the second III-V compound semiconductor layer 140. Therefore, in the second region R2, the surface of the insulating film 150 that covers the second III-V compound semiconductor layer 140 is exposed. In the second region R2, wiring EWa1 and wiring EWb1 are not provided on the insulating film 150 that covers the core layer 130 and the first III-V compound semiconductor layer 120. Therefore, in the first region R1, the surface of the insulating film 150 that covers the core layer 130 and the first III-V compound semiconductor layer 120 is exposed.

[0049] A ground electrode is connected to the first III-V compound semiconductor layer 120. A voltage is applied between electrode E1a1 and the ground electrode. For example, a DC reverse bias voltage and an AC voltage are superimposed between electrode E1a1 and the ground electrode. A voltage is applied between electrode E1b1 and the ground electrode. For example, a DC reverse bias voltage and an AC voltage are superimposed between electrode E1b1 and the ground electrode. As a result, an electrical signal flows between electrode E1a1 and the ground electrode and between electrode E1b1 and the ground electrode. This electrical signal changes the refractive index of the first III-V compound semiconductor layer 120, the core layer 130, and the second III-V compound semiconductor layer 140. The change in refractive index modulates the phase of light propagating through the optical waveguide structure WS.

[0050] Light propagating through the core layer 130 may also propagate through the silicon-containing layer 110, the first III-V compound semiconductor layer 120, and the second III-V compound semiconductor layer 140.

[0051] Next, with reference to Figure 6, the relationship between the silicon-containing layer 110, the first III-V compound semiconductor layer 120, the core layer 130, and the second III-V compound semiconductor layer 140 will be explained in more detail. Figure 6 is a cross-sectional view showing the configuration of the first III-V compound semiconductor layer, the second III-V compound semiconductor layer, the core layer, and the silicon-containing layer. For the sake of explanation, the substrate 11 is omitted from the illustration in Figure 6.

[0052] As shown in Figure 6, the second III-V compound semiconductor layer 140 has a width W1 smaller than the width W2 of the core layer 130. The width W1 is the length of the second III-V compound semiconductor layer 140 in the Y-axis direction. The width W1 may also be the width at the bottom surface of the second III-V compound semiconductor layer 140. The bottom surface of the second III-V compound semiconductor layer 140 may be in contact with the top surface of the core layer 130. The width W2 is the length of the core layer 130 in the Y-axis direction. The width W2 may also be the width at the top surface of the core layer 130. The width W1 may be between 1.2 and 1.9 μm. The width W2 may be between 5 and 12 μm. The ratio of width W1 to width W2 may be between 10 and 20%.

[0053] The width W1 may be smaller than the width of the first III-V compound semiconductor layer 120. That is, the second III-V compound semiconductor layer 140 may have a width W1 smaller than the width of the first III-V compound semiconductor layer 120. The width of the first III-V compound semiconductor layer 120 is the length of the first III-V compound semiconductor layer 120 in the Y-axis direction. The width of the first III-V compound semiconductor layer 120 may be 20 to 40 μm. The ratio of the width W1 to the width of the first III-V compound semiconductor layer 120 may be 3 to 10%.

[0054] The width W2 may be smaller than the width of the first III-V compound semiconductor layer 120. That is, the core layer 130 may have a width W2 smaller than the width of the first III-V compound semiconductor layer 120. The ratio of the width W2 to the width of the first III-V compound semiconductor layer 120 may be 13 to 40%.

[0055] Optical modulator 1 can reduce the capacitance in the optical waveguide structure WS. For example, compared to an optical modulator where the width of the first III-V compound semiconductor layer and the width of the second III-V compound semiconductor layer are the same, optical modulator 1 can reduce the capacitance in the optical waveguide structure WS. When the width W1 on the lower surface of the second III-V compound semiconductor layer 140 is reduced, the capacitance in the optical waveguide structure WS is reduced.

[0056] The optical waveguide structure WS can be fabricated as follows. First, a second III-V compound semiconductor layer 140, a core layer 130, and a first III-V compound semiconductor layer 120 are sequentially stacked on the surface of a III-V compound semiconductor substrate. The III-V compound semiconductor substrate is, for example, an InP substrate or a GaAs substrate. Each layer is formed, for example, by MOCVD (Metal Organic Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy).

[0057] Next, multiple small pieces are formed by cutting the III-V compound semiconductor substrate along the scribe lines formed on its surface.

[0058] On the other hand, the optical waveguide and silicon-containing layer 110 are formed by patterning the silicon layer located on the surface of the SOI (Silicon On Insulator) substrate.

[0059] Next, small pieces are bonded to the SOI substrate so that the first III-V compound semiconductor layer 120 is bonded to the silicon-containing layer 110. Bonding is performed, for example, by a surface activation bonding method using nitrogen plasma. After exposing the surface of the SOI substrate and the surface of the small piece to nitrogen plasma, the surface of the small piece is brought into contact with the surface of the SOI substrate. Then, the SOI substrate and the small piece are heated while a load is applied.

[0060] Next, small pieces of III-V compound semiconductor substrate are removed, for example, by wet etching. Alternatively, before bonding, ions such as hydrogen ions may be injected into the surface of the III-V compound semiconductor substrate to form a release layer, and after bonding, the release layer may be heated and removed. Once the release layer is removed, the III-V compound semiconductor substrate is peeled off.

[0061] Next, the second III-V compound semiconductor layer 140, the core layer 130, and the first III-V compound semiconductor layer 120 are processed by photolithography and etching. For example, the second III-V compound semiconductor layer 140, the core layer 130, and the first III-V compound semiconductor layer 120 are processed by dry etching or wet etching. The processing is carried out so that the width W1 of the second III-V compound semiconductor layer 140 is smaller than the width W2 of the core layer 130, and the width W2 of the core layer 130 is smaller than the width of the first III-V compound semiconductor layer 120. This forms an optical waveguide structure WS. Subsequently, an insulating film 150 is formed to cover the optical waveguide structure WS. Then, apertures H1 and H2 are formed in the insulating film 150 by photolithography and etching. Subsequently, an electrode E1a1 is formed in aperture H1 and an electrode E1b1 is formed in aperture H2 by lift-off.

[0062] Next, a modified example of the optical waveguide structure WS will be described with reference to Figure 7. Figure 7 is a cross-sectional view showing another example of the configuration of the first III-V compound semiconductor layer, the second III-V compound semiconductor layer, the core layer, and the silicon-containing layer. In this modified example, the structure of the second III-V compound semiconductor layer 140 differs from the embodiment described above. The differences between the embodiment described above and this modified example will be mainly described below. Note that in Figure 7, the substrate 11 is omitted from the illustration for ease of explanation.

[0063] In this modified example, the second III-V compound semiconductor layer 140 includes a lower region 140a adjacent to the core layer and an upper region 140b located opposite to the lower region 140a. In the second III-V compound semiconductor layer 140, the lower region 140a and the upper region 140b are formed integrally, for example.

[0064] The height of the lower region 140a may be 0.1 to 0.3 μm. The ratio of the height of the lower region 140a to the thickness T140 of the second III-V compound semiconductor layer 140 may be 5 to 15%.

[0065] In this modified example, the second III-V compound semiconductor layer 140 has a width W1a in the lower region 140a. The second III-V compound semiconductor layer 140 has a width W1b in the upper region 140b. The width W1a is the length of the lower region 140a in the Y-axis direction. The width W1b is the length of the upper region 140b in the Y-axis direction. The width W1a may be between 1.1 and 1.8 μm. The width W1b may be between 1.2 and 1.9 μm.

[0066] In this modified example, the width W1a decreases as it approaches the core layer 130. The width W1a is minimum at the bottom surface of the second III-V compound semiconductor layer 140 and maximum at the boundary with the upper region 140b. The width W1a at the bottom surface of the second III-V compound semiconductor layer 140 may be between 1.1 and 1.8 μm. The width W1a may decrease continuously or stepwise as it approaches the core layer 130.

[0067] In the upper region 140b, the width W1b may be constant. The width W1b may be greater than the width W1a. When the width W1b is greater than the width W1a, it is easier to secure a sufficient width to provide electrodes E1a1 and E1b1. The ratio of the width W1a to the width W1b may be 85 to 95%. The widths W1a and W1b are smaller than the width W2. Therefore, in this modified example as well, the second III-V compound semiconductor layer 140 has widths W1a and W1b that are smaller than the width W2 of the core layer 130.

[0068] According to this modified example, the capacitance in the optical waveguide structure WS can be further reduced.

[0069] In this modified example, the second III-V compound semiconductor layer 140 is processed as follows. First, dry etching is performed to process the second III-V compound semiconductor layer 140 so that the width W1b of the second III-V compound semiconductor layer 140 is smaller than the width W2 of the core layer 130. Next, the region that will become the lower region 140a of the second III-V compound semiconductor layer 140 is processed by wet etching. As a result, a lower region 140a with a width W1a is formed in the second III-V compound semiconductor layer 140. If the X-axis direction is the same as the [0-11] direction of the second III-V compound semiconductor layer 140, the width W1a of the lower region 140a decreases as it approaches the core layer 130.

[0070] While preferred embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above embodiments.

[0071] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims, not in the sense described above, and all modifications within the sense and scope equivalent to the claims are intended. [Explanation of Symbols]

[0072] 1…Optical modulator 11… Circuit board 11a... Edge 11b...Rest 100...Silicon substrate 102...Silicon oxide layer 110...Silicon-containing layer 120…First III-V compound semiconductor layer 130... Core Layer 140…Second III-V compound semiconductor layer 140a…Lower area 140b…upper area 150… Insulating film C1... Optical coupler C2... Optical coupler C3... Optical coupler C4... Optical coupler C5... Optical coupler C6... Optical coupler C7... Optical coupler d1…Distance d2…distance d3...interval E1a…electrode part E1a1…electrode E1b…electrode part E1b1…electrode E2a…electrode part E2b…electrode part EWa1...Wiring EWa2...Wiring EWb1...Wiring EWb2...Wiring F1... Light filter H1…Aperture H2…Aperture H3a... Heater H3b... Heater H4a... Heater H4b... Heater H5a... Heater H5b... Heater MZ1...Mach-Zehnder modulation section MZ2...Mach Zehnder Modulation Unit MZ3...Mach-Zehnder modulation section MZ4...Mach-Zehnder modulation section MZ5...Mach Zehnder Modulation Unit P1...Input port P2…Output port R1…first area R2…Second area W1…Width W1a…width W1b... width W2...Width WS…Optical waveguide structure

Claims

1. It comprises an optical waveguide structure extending along the first direction, The optical waveguide structure is, A first-type conductivity first-group III-V compound semiconductor layer, A second-conductivity type 2III-V compound semiconductor layer, A core layer disposed between the first III-V compound semiconductor layer and the second III-V compound semiconductor layer, A silicon-containing layer, The first III-V compound semiconductor layer is disposed between the silicon-containing layer and the core layer. The second III-V compound semiconductor layer has a width smaller than the width of the core layer, and is an optical modulator.

2. The second III-V compound semiconductor layer includes a lower region adjacent to the core layer, The optical modulator according to claim 1, wherein in the lower region, the width of the second III-V compound semiconductor layer decreases as it approaches the core layer.

3. The optical modulator according to claim 1 or claim 2, wherein the first direction is the same as the [0-11] direction of the second III-V compound semiconductor layer.

4. The optical modulator according to claim 1 or 2, wherein the second III-V compound semiconductor layer has a thickness greater than the thickness of the core layer.

5. The optical modulator according to claim 1 or claim 2, wherein the first III-V compound semiconductor layer has a thickness less than or equal to the thickness of the core layer.

Images