Phase modulator

Abstract

To provide a phase modulator that can suppress absorption of propagation light by an electrode.SOLUTION: The phase modulator includes: a semiconductor substrate including an optical waveguide; and an electrode in contact with the semiconductor substrate. The electrode includes a first electrode in contact with the semiconductor substrate and a second electrode in contact with the semiconductor substrate, the first electrode and the second electrode being separate from each other. The optical waveguide is formed to change the phase of propagating propagation light when a voltage is applied between the first electrode and the second electrode. At least a part of the electrode includes a transparent electrode.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The technology disclosed in this specification relates to a phase modulator used in an Optical Phased Array (OPA).

Background Art

[0002] Patent Documents 1 to 3 disclose an example of a phase modulator used in an optical phased array. This type of phase modulator includes a semiconductor substrate including an optical waveguide and an electrode in contact with the semiconductor substrate. The electrode has a first electrode in contact with the semiconductor substrate and a second electrode in contact with the semiconductor substrate and disposed apart from the first electrode. The optical waveguide is configured such that when a voltage is applied between the first electrode and the second electrode, its refractive index or light absorption rate changes. When the refractive index or light absorption rate changes, the phase of the propagating light propagating through the optical waveguide changes.

[0003] An optical phased array is configured to generate a plurality of propagating lights having a phase difference with each other using such a phase modulator and emit each of the plurality of propagating lights from a corresponding transmission antenna. The plurality of propagating lights emitted from the transmission antenna become an optical beam having a specific deflection angle. The deflection angle of the optical beam depends on the phase difference of the plurality of propagating lights. Therefore, an optical phased array including a phase modulator can scan the optical beam emitted by controlling the voltage applied to the semiconductor substrate. As described above, an optical phased array including a phase modulator has a feature that it can change the deflection angle of the optical beam without using, for example, a mechanical mirror structure. Such an optical phased array can be mounted, for example, on a LiDAR (Light Detection and Ranging) device.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] To widen the scan angle of the light beam emitted from the transmitting antenna, the pitch distance between adjacent optical waveguides must be reduced. However, our research has shown that reducing the pitch distance between adjacent optical waveguides leads to a problem where a portion of the propagating light is absorbed by the electrodes, resulting in increased optical loss. This specification provides a technique for suppressing the absorption of propagating light by electrodes in a phase modulator. [Means for solving the problem]

[0006] A phase modulator disclosed herein may include a semiconductor substrate including an optical waveguide and electrodes in contact with the semiconductor substrate. The electrodes may include a first electrode in contact with the semiconductor substrate and a second electrode in contact with the semiconductor substrate but positioned away from the first electrode. The optical waveguide may be configured such that its refractive index changes when a voltage is applied between the first electrode and the second electrode, and its light absorption rate changes. In either case, the optical waveguide is configured such that the phase of the propagating light changes when a voltage is applied between the first electrode and the second electrode. At least a portion of the electrodes includes a transparent electrode. The transparent electrode is provided on at least one of the first electrode and the second electrode. The transparent electrode may be provided on each of the first electrode and the second electrode. With this phase modulator, since at least a portion of the electrodes includes the transparent electrode, absorption of propagating light by the electrodes can be suppressed.

[0007] The transparent electrode may be in contact with the semiconductor substrate. The portion of the electrode in contact with the semiconductor substrate will be located near the optical waveguide and may be within the distribution range of the propagating light moving through the optical waveguide. In this phase modulator, by providing the transparent electrode at a position in contact with the semiconductor substrate, absorption of propagating light by the electrode can be suppressed.

[0008] The transparent electrode may include a low-concentration transparent electrode and a high-concentration transparent electrode in contact with the low-concentration transparent electrode, which has a higher carrier concentration than the low-concentration transparent electrode. The high-concentration transparent electrode is in contact with the semiconductor substrate. This phase modulator makes it possible to suppress both the absorption of propagating light and the increase in contact resistance.

[0009] The semiconductor substrate may have a first semiconductor region of a first conductivity type in contact with the first electrode and a second semiconductor region of a second conductivity type in contact with the second electrode. The optical waveguide may be arranged between the first semiconductor region and the second semiconductor region.

[0010] The electrode may include a wiring electrode electrically connected to the semiconductor substrate via the transparent electrode. The wiring electrode may have a first wiring electrode and a second wiring electrode positioned between the first wiring electrode and the transparent electrode, and in contact with each of the first wiring electrode and the transparent electrode. The work function of the second wiring electrode may be greater than or equal to the work function of the transparent electrode and less than the work function of the first wiring electrode. By interposing the second wiring electrode between the first wiring electrode and the transparent electrode, the contact resistance between the first wiring electrode and the transparent electrode can be reduced.

[0011] The optical waveguide may include a protrusion projecting from one of the main surfaces of the semiconductor substrate. The transparent electrode may be positioned in a plane parallel to the main surface of the semiconductor substrate so as to face the protrusion. The portion of the electrode located opposite the protrusion of the optical waveguide will be located close to the optical waveguide and may be within the distribution range of the propagating light moving through the optical waveguide. In this phase modulator, by providing the transparent electrode at a position opposite the protrusion of the optical waveguide, absorption of the propagating light by the electrode can be suppressed.

[0012] The distance between the optical waveguide and the transparent electrode may be between 175 and 2000 nm. The portion of the electrode within this distance range will be located close to the optical waveguide and may be within the distribution range of the propagating light moving through the optical waveguide. In this phase modulator, by providing the transparent electrode within this distance range, absorption of the propagating light by the electrode can be suppressed. [Brief explanation of the drawing]

[0013] [Figure 1] This diagram schematically shows a perspective view of the main components of an optical phased array. [Figure 2] This diagram schematically shows a cross-sectional view of the main components of a phase modulator. [Figure 3] This diagram schematically shows a cross-sectional view of the main part of a phase modulator, illustrating the distribution of propagating light as it moves through the optical waveguide. [Figure 4] This diagram schematically shows a cross-sectional view of the main parts of a modified phase modulator. [Figure 5] This diagram schematically shows a cross-sectional view of the main parts of a modified phase modulator. [Figure 6] This diagram schematically shows a cross-sectional view of the main parts of a modified phase modulator. [Figure 7] This diagram schematically shows a cross-sectional view of the main parts of a modified phase modulator. [Modes for carrying out the invention]

[0014] (Configuration of Optical Phased Array) As shown in FIG. 1, the optical phased array 1 includes an optical waveguide 4, a distributor 6, a phase modulator 8, and a transmission antenna 9. The optical waveguide 4, the distributor 6, the phase modulator 8, and the transmission antenna 9 are configured as an optical integrated circuit on a semiconductor substrate 2.

[0015] The distributor 6 is configured such that the optical waveguide 4 is branched into a plurality of paths, and divides the propagating light input to the optical waveguide 4 into a plurality of propagating lights having the same phase. Each of the plurality of divided propagating lights is phase-modulated by the phase modulator 8. The plurality of phase-modulated propagating lights are input to the transmission antenna 9. The transmission antenna 9 includes diffraction gratings provided corresponding to each of the plurality of optical waveguides 4, and emits the plurality of phase-modulated propagating lights as optical beams outward. The phase modulator 8 of the optical phased array 1 can control the phase difference of the plurality of propagating lights and adjust the deflection angle of the emitted optical beam. Such an optical phased array 1 is not particularly limited, but is mounted on, for example, a LiDAR device.

[0016] (Structure of Phase Modulator) Hereinafter, the phase modulator 8 will be described with reference to the drawings. As shown in FIGS. 1 and 2, the phase modulator 8 includes a plurality of optical waveguides 4 formed on the semiconductor substrate 2. The plurality of optical waveguides 4 are arranged adjacent to each other in the x direction. Each of the plurality of optical waveguides 4 extends between the distributor 6 and the transmission antenna 9 along the y direction. The x direction indicates a direction parallel to the main surface 2S of the semiconductor substrate 2, the y direction indicates a direction parallel to the main surface 2S of the semiconductor substrate 2 and orthogonal to the x direction, and the z direction indicates a direction orthogonal to the main surface 2S of the semiconductor substrate 2 (orthogonal to each of the x direction and the y direction). As shown in FIG. 2, the portion surrounded by the broken line is the unit unit 10 that constitutes the phase modulator 8. The distance between adjacent optical waveguides 4 is referred to as the pitch distance 10D. Hereinafter, the unit unit 10 will be described by referring to it as the phase modulator 8.

[0017] The phase modulator 8 includes a semiconductor substrate 2, a first electrode 40, and a second electrode 50. The material of the semiconductor substrate 2 of the phase modulator 8 is not particularly limited, and for example, it may be silicon. The semiconductor substrate 2 has an optical waveguide 4, a p-type semiconductor region 20, and an n-type semiconductor region 30.

[0018] The optical waveguide 4 is provided at a position exposed on one main surface 2S of the semiconductor substrate 2. In this example, the optical waveguide 4 is a non-doped region formed as a part of the semiconductor substrate 2. Instead of this example, the optical waveguide 4 may be formed of a semiconductor material different from the semiconductor substrate 2, for example, silicon nitride. The optical waveguide 4 is disposed between the first electrode 40 and the second electrode 50 when viewed from the z-axis direction (hereinafter referred to as "when the semiconductor substrate 2 is viewed in plan view"). Also, the optical waveguide 4 is provided between the p-type semiconductor region 20 and the n-type semiconductor region 30 when the semiconductor substrate 2 is viewed in plan view. The optical waveguide 4 has a convex portion 4A protruding from the main surface 2S of the semiconductor substrate 2. On the upper and lower main surfaces of the semiconductor substrate 2, a cladding layer (not shown) made of a material having a refractive index smaller than that of the optical waveguide 4 is provided so as to sandwich at least the optical waveguide 4. The material of the cladding layer is not particularly limited, and for example, it may be silicon oxide (SiO2). When the convex portion 4A is provided on the optical waveguide 4, the convex portion 4A is covered with the cladding layer (not shown), and the propagating light is effectively confined within the optical waveguide 4. The convex portion 4A may not be formed if necessary.

[0019] The p-type semiconductor region 20 has a high-concentration p-type semiconductor region 22 and a low-concentration p-type semiconductor region 24. The high-concentration p-type semiconductor region 22 is provided at a position exposed on one main surface 2S of the semiconductor substrate 2 and is a region containing a higher concentration of p-type impurities than the low-concentration p-type semiconductor region 24. The high-concentration p-type semiconductor region 22 is provided between the first electrode 40 and the low-concentration p-type semiconductor region 24 and is in contact with each of the first electrode 40 and the low-concentration p-type semiconductor region 24.

[0020] The low-concentration p-type semiconductor region 24 is located in a position exposed on one of the main surfaces 2S of the semiconductor substrate 2, and is a region containing a lower concentration of p-type impurities than the high-concentration p-type semiconductor region 22. The low-concentration p-type semiconductor region 24 is located between the high-concentration p-type semiconductor region 22 and the optical waveguide 4, and is in contact with both the high-concentration p-type semiconductor region 22 and the optical waveguide 4.

[0021] The n-type semiconductor region 30 comprises a high-concentration n-type semiconductor region 32 and a low-concentration n-type semiconductor region 34. The high-concentration n-type semiconductor region 32 is located in a position exposed on one of the main surfaces 2S of the semiconductor substrate 2 and contains a higher concentration of n-type impurities than the low-concentration n-type semiconductor region 34. The high-concentration n-type semiconductor region 32 is located between the second electrode 50 and the low-concentration n-type semiconductor region 34 and is in contact with both the second electrode 50 and the low-concentration n-type semiconductor region 34.

[0022] The low-concentration n-type semiconductor region 34 is located in a position exposed on one of the main surfaces 2S of the semiconductor substrate 2, and is a region containing a lower concentration of n-type impurities than the high-concentration n-type semiconductor region 32. The low-concentration n-type semiconductor region 34 is located between the high-concentration n-type semiconductor region 32 and the optical waveguide 4, and is in contact with both the high-concentration n-type semiconductor region 32 and the optical waveguide 4.

[0023] The first electrode 40 and the second electrode 50 are positioned apart in the x-axis direction with the optical waveguide 4 in between. The first electrode 40 is provided on the main surface 2S of the semiconductor substrate 2 and extends parallel to the optical waveguide 4, that is, along the y-axis direction. Similarly, the second electrode 50 is provided on the main surface 2S of the semiconductor substrate 2 and extends parallel to the optical waveguide 4, that is, along the y-axis direction.

[0024] The first electrode 40 has a transparent electrode 42 and a wiring electrode 44. In the first electrode 40, the transparent electrode 42 is in contact with the main surface 2S of the semiconductor substrate 2, and the wiring electrode 44 is laminated on the transparent electrode 42. The second electrode 50 also has a transparent electrode 52 and a wiring electrode 54. In the second electrode 50 as well, the transparent electrode 52 is in contact with the main surface 2S of the semiconductor substrate 2, and the wiring electrode 54 is laminated on the transparent electrode 52. Alternatively, either the first electrode 40 or the second electrode 50 may have a transparent electrode.

[0025] The transparent electrode 42 of the first electrode 40 and the transparent electrode 52 of the second electrode 50 are made of a material that is transparent to propagating light that propagates through the optical waveguide 4. The material of the transparent electrode 42 of the first electrode 40 and the transparent electrode 52 of the second electrode 50 is not particularly limited, but may be an oxide semiconductor such as indium oxide (InO), tin oxide (SnO), or zinc oxide (ZnO). The impurities doped into these oxide semiconductors may include at least one selected from the group consisting of, for example, indium (In), gallium (Ga), aluminum (Al), antimony (Sb), fluorine (F), tin (Sn), yttrium (Y), cadmium (Cd), arsenic (As), niobium (Nb), tantalum (Ta), germanium (Ge), molybdenum (Mo), titanium (Ti), zirconium (Zr), hafnium (Hf), tungsten (W), tellurium (Te), boron (B), scandium (Sc), vanadium (V), and silicon (Si).

[0026] The transparent electrode 42 of the first electrode 40 is provided between the semiconductor substrate 2 and the wiring electrode 44, and is in contact with both the semiconductor substrate 2 and the wiring electrode 44. The transparent electrode 42 is positioned in a plane parallel to the main surface 2S of the semiconductor substrate 2, facing the protrusion 4A of the optical waveguide 4. In other words, the transparent electrode 42 has a portion that is at the same height as the protrusion 4A of the optical waveguide 4.

[0027] The wiring electrode 44 of the first electrode 40 is connected to wiring for electrically connecting the transparent electrode 42 to a control terminal (not shown) provided on the optical phased array 1. The material of the wiring electrode 44 of the first electrode 40 is not particularly limited, but may be copper (Cu), for example. The wiring electrode 44 is not positioned in a plane parallel to the main surface 2S of the semiconductor substrate 2 so as to face the protrusion 4A of the optical waveguide 4. In other words, the junction surface of the transparent electrode 42 and the wiring electrode 44 is located away from the top surface of the protrusion 4A of the optical waveguide 4 in the stacking direction (z-axis direction).

[0028] The transparent electrode 52 of the second electrode 50 is provided between the semiconductor substrate 2 and the wiring electrode 54, and is in contact with both the semiconductor substrate 2 and the wiring electrode 54. The transparent electrode 52 is positioned in a plane parallel to the main surface 2S of the semiconductor substrate 2, facing the protrusion 4A of the optical waveguide 4. In other words, the transparent electrode 52 has a portion that is positioned at the same height as the protrusion 4A of the optical waveguide 4.

[0029] The wiring electrode 54 of the second electrode 50 is connected to wiring for electrically connecting the transparent electrode 52 to a control terminal (not shown) provided on the optical phased array 1. The material of the wiring electrode 54 of the second electrode 50 is not particularly limited, but may be copper (Cu), for example. The wiring electrode 54 is not positioned in a plane parallel to the main surface 2S of the semiconductor substrate 2 so as to face the protrusion 4A of the optical waveguide 4. In other words, the junction surface of the transparent electrode 52 and the wiring electrode 54 is located away from the top surface of the protrusion 4A of the optical waveguide 4 in the stacking direction (z-axis direction).

[0030] (Operation of a phase modulator) When a voltage more positive than that applied to the first electrode 40 is applied to the second electrode 50, current flows from the first electrode 40 to the second electrode 50 via the p-type semiconductor region 20, the optical waveguide 4, and the n-type semiconductor region 30. This increases the carrier concentration in the optical waveguide 4, changing the phase of the propagating light moving through the optical waveguide 4. The phase modulator 8 adjusts the phase of the propagating light moving through the optical waveguide 4 and outputs the phase-modulated propagating light to the transmitting antenna 9 (see Figure 1).

[0031] As shown in Figure 3, the distribution 4B of the propagating light propagating through the optical waveguide 4 includes the area surrounding the optical waveguide 4. As shown in distribution 4B, a portion of the propagating light propagating through the optical waveguide 4 comes into contact with the transparent electrode 42 of the first electrode 40 and the transparent electrode 52 of the second electrode 50. For example, if these contacting portions were opaque metal electrodes, a portion of the propagating light would be absorbed by those metal electrodes, increasing optical loss. On the other hand, in the phase modulator 8, these contacting portions are composed of transparent electrodes 42 and 52, so such optical loss is suppressed. Therefore, the phase modulator 8 can output the propagating light to the transmitting antenna 9 (see Figure 1) while maintaining the intensity of the propagating light. As a result, when the optical phased array 1 (see Figure 1) is mounted on a LiDAR device, the LiDAR device becomes capable of measuring long distances.

[0032] Furthermore, when the optical phased array 1 (see Figure 1) is mounted on a LiDAR device, the pitch distance 10D between adjacent optical waveguides 4 must be reduced in order to widen the scan angle of the light beam emitted from the transmitting antenna 9. For example, in order to emit a light beam with a wavelength of 1550 nm at a scan angle of 60°, the pitch distance 10D between adjacent optical waveguides 4 must be 1.55 μm or less. However, reducing the pitch distance 10D between optical waveguides 4 leads to the problem of increased optical loss, as some of the propagating light is absorbed by the metal electrodes as described above. The phase modulator 8, which has transparent electrodes 42 and 52, can suppress this increase in optical loss. As a result, a LiDAR device equipped with the optical phased array 1 including the phase modulator 8 can widen the scan angle by reducing the pitch distance 10D between adjacent optical waveguides 4 while suppressing the increase in optical loss. Furthermore, since the pitch distance 10D of the optical waveguide 4 can be reduced, the optical phased array 1 can also be miniaturized.

[0033] In the phase modulator 8, the distance between the optical waveguide 4 and the transparent electrode 42 of the first electrode 40 is 175 to 2000 nm. Similarly, the distance between the optical waveguide 4 and the transparent electrode 52 of the second electrode 50 is 175 to 2000 nm. Here, the distance between them is the distance measured between the geometric center of the optical waveguide 4 and the geometric centers of the transparent electrodes 42 and 52 in a cross section parallel to the xz plane. When the optical waveguide 4 and the transparent electrodes 42 and 52 are arranged in this distance relationship, the pitch distance 10D between adjacent optical waveguides 4 can be reduced while suppressing the increase in optical loss, thereby widening the scan angle. In addition, since the pitch distance 10D between the optical waveguides 4 can be reduced, the optical phased array 1 can also be miniaturized.

[0034] (A modified example of a phase modulator) The following describes some modified versions of the phase modulator 8. Components common to the phase modulator 8 shown in Figures 2 and 3 are denoted by the same reference numerals, and their descriptions are omitted.

[0035] In the modified version of the phase modulator 8 shown in Figure 4, the transparent electrode 42 of the first electrode 40 has a first high-concentration transparent electrode 41, a low-concentration transparent electrode 43, and a second high-concentration transparent electrode 45. The first high-concentration transparent electrode 41 and the second high-concentration transparent electrode 45 are adjusted to have a higher concentration of impurities to be doped and a higher carrier concentration than the low-concentration transparent electrode 43. In the modified version of the phase modulator 8 shown in Figure 4, the transparent electrode 52 of the second electrode 50 also has a first high-concentration transparent electrode 51, a low-concentration transparent electrode 53, and a second high-concentration transparent electrode 55. The configuration of the transparent electrode 52 of the second electrode 50 is the same as that of the transparent electrode 42 of the first electrode 40. Below, only the transparent electrode 42 of the first electrode 40 will be described, but the transparent electrode 52 of the second electrode 50 is similar. Note that the configuration of the transparent electrode consisting of multiple parts with different carrier concentrations may be formed in at least one of the first electrode 40 and the second electrode 50.

[0036] The first high-density transparent electrode 41, the low-density transparent electrode 43, and the second high-density transparent electrode 45 are stacked in this order. The first high-density transparent electrode 41 is located between the semiconductor substrate 2 and the low-density transparent electrode 43 and is in contact with both the semiconductor substrate 2 and the low-density transparent electrode 43. The low-density transparent electrode 43 is located between the first high-density transparent electrode 41 and the second high-density transparent electrode 45 and is in contact with both the first high-density transparent electrode 41 and the second high-density transparent electrode 45. The second high-density transparent electrode 45 is located between the low-density transparent electrode 43 and the wiring electrode 44 and is in contact with both the low-density transparent electrode 43 and the wiring electrode 44. The thickness of the low-density transparent electrode 43 in the z-axis direction is greater than the thickness of the first high-density transparent electrode 41 and the second high-density transparent electrode 45 in the z-axis direction.

[0037] When the carrier concentration of the transparent electrode 42 is low, the absorption of propagating light is suppressed. On the other hand, when the carrier concentration of the transparent electrode 42 is low, the contact resistance to the semiconductor substrate 2 and the wiring electrode 44 increases. In the modified phase modulator 8 shown in Figure 4, a first high-concentration transparent electrode 41 with a high carrier concentration is provided, so the contact resistance between the high-concentration p-type semiconductor region 22 of the semiconductor substrate 2 and the transparent electrode 42 decreases. Furthermore, since a second high-concentration transparent electrode 45 with a high carrier concentration is provided, the contact resistance between the transparent electrode 42 and the wiring electrode 44 decreases. Note that the first high-concentration transparent electrode 41 and the second high-concentration transparent electrode 45 contain 1 × 10⁻¹⁶ gallium (Ga) as an impurity. 21 cm -3 It may be included at the above concentrations. On the other hand, since the carrier concentration of the low-concentration transparent electrode 43 is low, the absorption of propagating light in the low-concentration transparent electrode 43 is suppressed. For example, in the low-concentration transparent electrode 43, the absorption coefficient of propagating light at a wavelength of 1550 nm is 8100 cm². -1 The carrier concentration is adjusted to the following extent. Thus, in the modified phase modulator 8 shown in Figure 4, it is possible to suppress both the absorption of propagating light and the increase in contact resistance.

[0038] In the modified phase modulator 8 shown in Figure 4, the junction surface of the first high-density transparent electrode 41 and the low-density transparent electrode 43 is formed to be parallel to the main surface 2S of the semiconductor substrate 2, but this shape of the junction surface is just one example. The first high-density transparent electrode 41 only needs to be interposed between the semiconductor substrate 2 and the low-density transparent electrode 43, and the junction surface of the first high-density transparent electrode 41 and the low-density transparent electrode 43 can be any other shape. The same applies to the junction surface of the second high-density transparent electrode 45 and the low-density transparent electrode 43. The second high-density transparent electrode 45 only needs to be interposed between the low-density transparent electrode 43 and the wiring electrode 44, and the junction surface of the second high-density transparent electrode 45 and the low-density transparent electrode 43 can be any other shape.

[0039] In the modified version of the phase modulator 8 shown in Figure 5, the wiring electrode 44 has a first wiring electrode 46 and a second wiring electrode 48. The work function of the second wiring electrode 48 is greater than the work function of the transparent electrode 42 and less than the work function of the first wiring electrode 46. In the modified version of the phase modulator 8 shown in Figure 5, the wiring electrode 54 of the second electrode 50 also has a first wiring electrode 56 and a second wiring electrode 58. The configuration of the wiring electrode 54 of the second electrode 50 is the same as that of the wiring electrode 44 of the first electrode 40. Below, only the wiring electrode 44 of the first electrode 40 will be described, but the wiring electrode 54 of the second electrode 50 is similar. Note that the configuration of the wiring electrode consisting of multiple parts with different work functions may be formed on at least one of the first electrode 40 and the second electrode 50.

[0040] The second wiring electrode 48 is positioned between the first wiring electrode 46 and the transparent electrode 42, and is in contact with both the first wiring electrode 46 and the transparent electrode 42. By interposing the second wiring electrode 48 between the first wiring electrode 46 and the transparent electrode 42, the potential barrier generated at the junction between the transparent electrode 42 and the wiring electrode 44 can be reduced.

[0041] In the modified phase modulator 8 shown in Figure 6, the low-density p-type semiconductor region 24 of the p-type semiconductor region 20 and the low-density n-type semiconductor region 34 of the n-type semiconductor region 30 are in direct contact, and an optical waveguide 14 is formed between the low-density p-type semiconductor region 24 and the low-density n-type semiconductor region 34. In this example, the pn junction of the low-density p-type semiconductor region 24 and the low-density n-type semiconductor region 34 extends parallel to the yz plane. The geometric center of the optical waveguide 14 in this example is defined as the center in the z direction of the pn junction of the low-density p-type semiconductor region 24 and the low-density n-type semiconductor region 34. The example in which the pn junction extends parallel to the yz plane is just one example; the optical waveguide 14 is formed between the low-density p-type semiconductor region 24 and the low-density n-type semiconductor region 34 in various configurations in which the low-density p-type semiconductor region 24 and the low-density n-type semiconductor region 34 are in direct contact. Although not particularly limited, for example, the optical waveguide 14 may be formed by arranging a portion of the low-concentration p-type semiconductor region 24 and a portion of the low-concentration n-type semiconductor region 34 so as to alternately repeat along the y-axis direction, in which case the pn junction may extend parallel to the xz plane. Alternatively, the optical waveguide 14 may be formed by stacking a portion of the low-concentration p-type semiconductor region 24 and a portion of the low-concentration n-type semiconductor region 34 in the z-axis direction, in which case the pn junction may extend parallel to the xy plane.

[0042] In the modified phase modulator 8 shown in Figure 7, the protrusion 4A of the optical waveguide 4 is provided on one main surface of the semiconductor substrate 2, and the first electrode 40 and the second electrode 50 are provided on the other main surface of the semiconductor substrate 2. If the pitch distance 10D between adjacent optical waveguides 4 becomes narrow, absorption of propagating light may become a problem even with this arrangement. In the modified phase modulator 8 shown in Figure 7, as described above, the first electrode 40 and the second electrode 50 have transparent electrodes, so absorption of propagating light can be suppressed.

[0043] Although specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples illustrated above. Furthermore, the technical elements described in this specification or drawings exhibit technical usefulness individually or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technologies illustrated in this specification or drawings can achieve multiple objectives simultaneously, and achieving even one of these objectives itself constitutes technical usefulness. [Explanation of Symbols]

[0044] 2: Semiconductor substrate, 4: Optical waveguide, 8: Phase modulator, 20: P-type semiconductor region, 30: N-type semiconductor region, 40: First electrode, 42: Transparent electrode, 44: Wiring electrode, 50: Second electrode, 52: Transparent electrode, 54: Wiring electrode

Claims

1. A phase modulator, A semiconductor substrate including an optical waveguide, It comprises an electrode in contact with the semiconductor substrate, The electrode is The first electrode in contact with the semiconductor substrate, It has a second electrode that is in contact with the semiconductor substrate and is positioned away from the first electrode, The optical waveguide is configured such that when a voltage is applied between the first electrode and the second electrode, the phase of the propagating light changes. The electrode includes a transparent electrode and a wiring electrode that is laminated on the transparent electrode and electrically connected to the semiconductor substrate via the transparent electrode. The aforementioned semiconductor substrate is A first semiconductor region of a first conductivity type in contact with the first electrode, It has a second semiconductor region of a second conductivity type that is in contact with the second electrode, The optical waveguide is positioned between the first semiconductor region and the second semiconductor region. The optical waveguide includes a protrusion that extends from one main surface of the semiconductor substrate. In a plane parallel to the main surface of the semiconductor substrate, the transparent electrode is arranged so as to face the protrusion. In a plane parallel to the main surface of the semiconductor substrate, the wiring electrodes are not arranged to face the protrusions. A phase modulator in which the bonding surface of the wiring electrode and the transparent electrode is located away from the top surface of the protrusion of the optical waveguide in the stacking direction.

2. The phase modulator according to claim 1, wherein the transparent electrode is in contact with the semiconductor substrate.

3. The transparent electrode is Low-concentration transparent electrode, The system has a high-concentration transparent electrode that is in contact with the low-concentration transparent electrode and has a higher carrier concentration than the low-concentration transparent electrode. The phase modulator according to claim 2, wherein the high-concentration transparent electrode is in contact with the semiconductor substrate.

4. The wiring electrode is First wiring electrode and It has a second wiring electrode positioned between the first wiring electrode and the transparent electrode, and in contact with each of the first wiring electrode and the transparent electrode, The phase modulator according to any one of claims 1 to 3, wherein the work function of the second wiring electrode is greater than or equal to the work function of the transparent electrode and less than the work function of the first wiring electrode.

5. The phase modulator according to any one of claims 1 to 4, wherein the distance between the optical waveguide and the transparent electrode is 175 to 2000 nm.

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