Optical modulator
The optical modulator with a three- or four-layer pn junction structure enhances ion implantation control, addressing limitations in conventional Si optical modulators by achieving low-voltage, high-efficiency modulation for improved optical communication systems.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- NIPPON TELEGRAPH & TELEPHONE CORP
- Filing Date
- 2022-06-10
- Publication Date
- 2026-06-24
AI Technical Summary
Conventional Si optical modulators face limitations in power consumption, speed, and functionality due to constraints in ion implantation patterns, which affect the modulation efficiency and optical loss.
The optical modulator incorporates a semiconductor layer with a pn junction structure comprising low-concentration p-type and n-type semiconductor layers, along with medium-concentration layers added to the slab regions, forming a three- or four-layer optical waveguide core to enhance ion implantation control and reduce electrical resistivity.
This configuration allows for low-voltage, high-efficiency optical modulation, improving the modulation efficiency and enabling high-speed, low-power optical communication.
Smart Images
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Abstract
Description
Technical Field
[0001] The present invention relates to an optical modulator, and more particularly to an optical modulator used in an optical communication system, an optical information processing system, etc., which can perform a high-speed optical modulation operation, has excellent frequency characteristics and waveform quality, and enables long-distance optical communication.
Background Art
[0002] Due to the spread of high-definition video distribution services and mobile communications, the amount of traffic flowing through the network has become enormous and continues to increase year by year. In order to build a high-speed and large-capacity optical network that can meet such traffic demands, the development of basic devices capable of high-speed operation used in each node has been vigorously carried out. An optical modulator that directly modulates an optical signal with a broadband baseband signal is one of the important devices.
[0003] A Mach-Zehnder (MZ) type optical modulator has a structure in which light incident on an optical waveguide is branched into two optical waveguides with a 1:1 intensity, the branched light is propagated for a certain length, and then recombined. In an MZ type optical modulator, the phases of the two branched lights are changed by phase modulation units provided in the two branched optical waveguides respectively. By changing the optical interference conditions when the two lights that have received the two phase changes are recombined, the intensity and phase of the light can be modulated.
[0004] As materials for constituting the optical waveguide of the MZ type optical modulator, dielectrics such as LiNbO3, and semiconductors such as InP, GaAs, and Si are used. A modulation electric signal is input to an electrode arranged near the optical waveguide constituted by these materials, and a modulation voltage is applied to the optical waveguide, thereby changing the phase of the light propagating through the optical waveguide.
[0005] In MZ-type optical modulators, the mechanism for changing the phase of light utilizes the Pockels effect when the material is LiNbO3. When the material is InP or GaAs, the Pockels effect and the Quantum Confined Stark Effect (QCSE) are used, and when the material is Si, the carrier plasma effect is primarily used.
[0006] To achieve high-speed, low-power optical communication, an optical modulator with a high modulation speed and low drive voltage is required. Specifically, it is necessary to perform optical modulation at a high speed of 10 Gbps or more and with an amplitude voltage of a few volts. To achieve this, a traveling wave electrode is required that matches the speed of high-speed electrical signals with the speed of light propagating through the phase modulator, and allows the two to interact while the light and electrical signals propagate. As an optical modulator using a traveling wave electrode, for example, optical modulators with electrode lengths ranging from a few millimeters to tens of millimeters have been put into practical use, as disclosed in Patent Document 1.
[0007] In optical modulators using traveling wave electrodes, low-loss, low-reflection electrode and optical waveguide structures are required to propagate electrical signals without reducing their amplitude or the intensity of light propagating through the waveguide. Specifically, for electrical signals, an electrode structure with low reflection and propagation loss over a wide frequency band is necessary, and for light, a waveguide structure that minimizes reflection, efficiently confines light, and allows it to propagate without loss is required.
[0008] Among MZ-type optical modulators, the Si optical modulator, in which the optical waveguide is constructed from Si, is a promising option from the perspective of substrate material and fabrication process. The Si optical modulator is fabricated from an SOI (Silicon on Insulator) substrate, in which a thin film of Si is attached to a buried oxide (BOX) layer obtained by thermally oxidizing the surface of a Si substrate. The optical waveguide is fabricated by processing the Si thin film into a fine wire so that light can guide through the SOI layer, and then implanting impurities to form p-type and n-type semiconductors. Finally, SiO2, which will serve as the optical cladding layer, is deposited, and electrodes are formed, etc., to complete the fabrication.
[0009] In this case, the optical waveguide must be designed and fabricated to minimize optical loss. Specifically, the p-type and n-type impurity doping and electrode fabrication must be designed and fabricated in a way that minimizes not only optical loss but also reflection loss and propagation loss of high-speed electrical signals.
[0010] Figure 1 shows the cross-sectional structure of an optical waveguide that forms the basis of a conventional Si optical modulator. Figure 1 shows a cross-section (xz plane) of an optical waveguide 200 constructed on an SOI substrate, cut perpendicular to the direction of light propagation (y-axis). Light propagates in the direction perpendicular to the plane of the paper (y-axis direction). The optical waveguide 200 of the Si optical modulator consists of a Si layer 2 sandwiched between upper and lower SiO2 cladding layers 1 and 3. The Si nanowire formed in the center of Figure 1 for confining light is a structure called a rib waveguide, which has a difference in thickness. That is, as shown in Figure 1, the rib waveguide consists of a thick Si layer 201 in the center and thin slab regions 202a and 202b on both sides. The thick Si layer 201 in the center of the Si layer 2 is used as the optical waveguide core, and the optical waveguide 200 that confines light propagating in the direction perpendicular to the plane of the paper is constructed by utilizing the difference in refractive index with the surrounding SiO2 cladding layers 1 and 3.
[0011] Thin slab regions 202a and 202b on either side of the thick Si layer 201 optical waveguide core (hereinafter referred to as the Si optical waveguide core 201) are provided with a high-density p-type semiconductor layer 211 and a high-density n-type semiconductor layer 214, respectively. Furthermore, a pn junction structure is formed in and near the Si optical waveguide core 201 by a low-density p-type semiconductor layer 212 and a low-density n-type semiconductor layer 213. As will be described later, a modulated electrical signal and a bias voltage are applied from both the left and right ends of the Si layer 2 in Figure 1 via electrodes (not shown). Instead of the pn junction in the center of the core, a pin structure may be used, in which an undoped i-type (intrinsic) semiconductor is sandwiched between the pn junction structure of the low-density p-type semiconductor layer 212 and the low-density n-type semiconductor layer 213.
[0012] The phase modulation operation in the optical waveguide 200 of the Si optical modulator can be explained as follows. Although not shown in Figure 1, two metal electrodes are provided in contact with the high-density p-type semiconductor layer 211 and the high-density n-type semiconductor layer 214 at both ends of the Si layer 2, respectively. A reverse bias voltage is applied to the pn junction in the center of the core via the two metal electrodes, along with a high-frequency (RF) modulating electrical signal. That is, a voltage is applied from the right end to the left end (x-axis direction) of the optical waveguide 200, with the high-density n-type semiconductor layer 214 side being positive and the high-density p-type semiconductor layer 211 side being negative. The reverse bias voltage and the modulating electrical signal change the carrier density inside the thick Si optical waveguide core 201. By changing the carrier density, the refractive index of the Si optical waveguide core 201 is changed by the carrier plasma effect, thereby modulating the phase of the light propagating through the optical waveguide.
[0013] The dimensions of the optical waveguide in a Si optical modulator depend on the refractive index of each material that forms the core and cladding. An example of a ribbed silicon waveguide structure with a thick Si optical waveguide core 201 and slab regions 202a and 202b on either side, as shown in Figure 1, is listed below. The width (x-axis direction) of the Si optical waveguide core 201 is 400-600 nm, the height of the core (z-axis direction) is 150-300 nm, the thickness of the slab region is 50-200 nm, and the length of the optical waveguide (y-axis direction) is several mm.
[0014] One of the outstanding features of Si optical modulators is that the large refractive index difference between the Si core through which light propagates and the SiO2 cladding layer allows for the construction of a compact optical modulator. Because of this large refractive index difference, light can be confined to a small area, and the bending radius of the optical waveguide can be made very small, around 10 μm. Therefore, the optical multiplexing and demultiplexing circuit portion in the Si optical modulator, which will be described next, can be made smaller.
[0015] (Conventional dual-electrode Mach-Zehnder type optical modulator) Figure 2 shows a Si optical modulator constituting a conventional dual-electrode Mach-Zehnder optical modulator. It is a planar structure viewed from above through the surface (xy plane) of a Si(SOI) substrate. Optical input from the left end of the optical modulator is branched into two optical waveguides 7a and 7b, modulated, and then recombined and output as modulated light from the right end of the optical modulator. As the input light propagates in the y-axis direction through the two branched optical waveguides 7a and 7b, it is phase-modulated by modulating electrical signals (RF signals) applied to the RF electrodes 15a and 15b, respectively. The optical modulator has a coplanar waveguide (CPW) with respect to optical waveguide 7a consisting of two ground electrodes 16a and 17 flanking the RF electrode 15a. Similarly, it has a CPW with respect to optical waveguide 7b consisting of two ground electrodes 16b and 17 flanking the RF electrode 15b.
[0016] This configuration, which has two RF signal inputs in a single Mach-Zehnder (MZ) optical modulator, is called a dual-electrode structure. The MZ optical modulator shown in Figure 2 has a symmetrical structure with respect to a center line parallel to the y-axis passing through the center of the ground electrode 17.
[0017] Figure 3 shows the cross-sectional structure of III-III' in Figure 2, and shows only the phase modulation section including the optical waveguide 7a that is modulated and the corresponding CPW. One phase modulation section is an optical waveguide having a cross-sectional structure similar to the optical waveguide 200 shown in Figure 1. It includes an RF electrode 15a, which is a high-frequency line that receives one of a pair of differential modulated electrical signals (RF signals), and two ground electrodes 16a and 17 provided on either side of the RF electrode 15a. An optical waveguide core 7a is provided between the RF electrode 15a and the ground electrode 16a, and a pn junction structure is formed within the optical waveguide 7a by a low-density p-type semiconductor layer 212 and a low-density n-type semiconductor layer 213. The RF electrode 15a is in contact with a high-density n-type semiconductor layer 214 via a via 19b. The ground electrode 16a is in contact with a high-density p-type semiconductor layer 211 via a via 19a.
[0018] Although the ground electrode 17 is not in contact with any semiconductor layer, together with the ground electrode 16a, it forms a GSG (Ground Signal Ground) high-frequency transmission line (CPW) with respect to the RF electrode 15a. This transmission line structure allows for adjustment of the characteristic impedance of the RF electrode as a transmission line, thereby improving transmission characteristics. Furthermore, because the signal line of the RF electrode 15a is surrounded by the two ground electrodes 16a and 17, signal leakage is reduced, making it possible to form an optical modulator with less crosstalk and propagation loss.
[0019] Figure 3 shows a phase modulation section including an RF electrode 15a, which is a high-frequency line that receives one of the modulated electrical signals (RF signals) in a differential configuration. The phase modulation section including the other RF electrode 15b has the same configuration as in Figure 3, except that the arrangement order of the multiple semiconductor regions in the x-axis direction is reversed with respect to the z-axis as the axis of symmetry.
[0020] The characteristic impedance of the RF electrodes 15a and 15b of the Si optical modulator as a high-frequency transmission line is greatly influenced by the capacitance of the pn junctions of the Si layer optical waveguide cores 7a and 7b. However, since the capacitance between the RF electrode and the ground electrode also has an influence, in a dual-electrode structure Si modulator, the characteristic impedance can be adjusted relatively easily by adjusting the capacitance between the RF electrode 15a and the ground electrode 17. It is possible to set the characteristic impedance to about 50Ω in a single-ended drive configuration and about 100Ω in a differential drive configuration.
[0021] Here, we have described an example configuration in which the RF electrode 15a is in contact with the high-density n-type semiconductor layer 214 and the ground electrode 16a is in contact with the high-density p-type semiconductor layer 211. Alternatively, the orientation of the pn junction may be reversed, with the RF electrode 15a in contact with the high-density p-type semiconductor layer and the ground electrode 16a in contact with the high-density n-type semiconductor layer. In this case, the pn junction can be reverse-biased by applying a negative voltage to the ground electrode 16a as a bias voltage superimposed on the RF signal and applied to the RF electrode 15a.
[0022] (Conventional vertical pn junction optical modulator) Figure 4 shows the cross-sectional structure of the optical waveguide of a conventional Si optical modulator having a vertical pn junction. Figure 4 shows a cross-section (xz plane) of the optical waveguide 200 constructed on an SOI substrate, cut perpendicular to the direction of light propagation (y axis). Light propagates in the direction perpendicular to the plane of the paper (y axis direction). The method of constructing the optical waveguide of the Si optical modulator, the classification of impurity doping, the configuration of electrodes, and the operating principle are the same as those of the conventional Si optical modulator shown in Figure 1. The difference from the conventional structure is that the pn junction structure, consisting of a low-concentration p-type semiconductor layer 212 and a low-concentration n-type semiconductor layer 213, is arranged vertically (top and bottom in the figure). That is, the pn junction surface is arranged horizontally (left and right in the figure). Such a structure is called a vertical pn junction (see, for example, Non-Patent Literature 1).
[0023] The Si optical waveguide core 201 has a width (x-axis direction) of 400-600 nm, a core height (z-axis direction) of 150-300 nm, and a slab thickness of 50-200 nm. As a result, light propagating within the Si optical waveguide core 201 takes on a flattened shape in the width (x-axis direction). Therefore, in a vertical pn junction optical waveguide with a pn junction surface in the horizontal direction, the overlap between the pn junction portion, where the carrier density changes with the application of an electric field, and the light propagation modes becomes larger. That is, as shown in Figure 1, in a vertical pn junction optical waveguide, the overlap between the pn junction portion and the light propagation modes is larger than in an optical waveguide where the pn junction structure is arranged horizontally (left and right in the figure) and the pn junction surface is in the vertical direction. The change in carrier density causes a change in the refractive index of the Si optical waveguide core 201, which modulates the phase of the light propagating through the core of the optical waveguide. Therefore, a vertical pn junction can modulate light even at low voltages, enabling the realization of an optical modulator with high modulation efficiency.
[0024] Impurity doping of Si optical modulators is performed using a method called ion implantation, commonly known as "implantation." Ion implantation involves implanting ions with velocity into a target material, thereby altering the properties of the implanted material. Implanting boron (B) into Si results in a p-type semiconductor, while implanting phosphorus (P) and arsenic (As) results in an n-type semiconductor. The concentration and depth of ion implantation are controlled by factors such as the dose (number of ions per unit area), the acceleration voltage (energy used to accelerate the ions), and the inclination of the wafer relative to the direction of the ion beam.
[0025] When fabricating a vertical pn junction optical waveguide, a thick Si layer 201 and a thin slab region 202 are formed on an SOI substrate by etching. Then, impurities such as P (phosphorus) and As (arsenic) are implanted by ion implantation to form a low-concentration n-type semiconductor layer 213. Next, to form a low-concentration p-type semiconductor layer 212, impurities such as B (boron) are implanted to a depth close to the surface of the Si layer. This is a commonly used method. In the Si optical waveguide core 201, ion implantation is performed so that the impurities that generate the n-type semiconductor layer and the impurities that form the p-type semiconductor layer overlap, but the ion dose and acceleration voltage are adjusted so that the upper part of the optical waveguide becomes a low-concentration p-type semiconductor layer.
[0026] Alternatively, the p-type and n-type semiconductor layers in the structure shown in Figure 4 may be reversed, so that the upper part of the Si optical waveguide core 201 is a low-concentration n-type semiconductor layer and the lower part is a low-concentration p-type semiconductor layer. However, ion implantation into the Si layer is generally more difficult for impurities to create a p-type semiconductor layer, and ion implantation deep from the substrate surface can damage the Si crystal. For this reason, it is more common to have a p-type semiconductor layer on top and an n-type semiconductor layer on the bottom.
[0027] (Conventional interleaved pn junction optical modulator) Referring to FIGS. 5-7, the optical waveguide of a conventional Si optical modulator having an interleaved pn junction will be described. FIG. 5 is a top view of a part of the optical waveguide 200 formed on an SOI substrate, seen from a direction perpendicular to the substrate surface (x-y plane). FIG. 6 is a cross-sectional view (x-z plane) of the optical waveguide 200 formed on the SOI substrate, cut perpendicular to the light propagation direction (y-axis), and is the cross-sectional view of VI-VI' in FIG. 5. FIG. 7 is the cross-sectional view of VII-VII' in FIG. 5, and the light propagates in the direction perpendicular to the paper surface (y-axis direction). The method of constructing the optical waveguide of the Si optical modulator, the classification of impurity doping, the configuration of the electrodes, the operating principle, etc. are the same as those of the conventional Si optical modulator shown in FIG. 1. The difference from the conventional structure is that along the light propagation direction (y-axis direction), the doping regions that occupy most of the Si optical waveguide core 201 are arranged such that the low-concentration p-type semiconductor layer and the low-concentration n-type semiconductor layer are alternately arranged. Such a structure is called an interleaved pn junction (for example, see Non-Patent Document 2).
[0028] When the interval at which the low-concentration p-type semiconductor layer 212 and the low-concentration n-type semiconductor layer 213 are interchanged is shortened, the overlapping portion between the light propagating in the Si optical waveguide core 201 and the pn junction portion becomes larger. That is, as shown in FIG. 1, the overlapping portion between the pn junction portion and the light propagation mode is larger than that of the optical waveguide in which the pn junction surfaces of the low-concentration p-type semiconductor layer 212 and the low-concentration n-type semiconductor layer 213 are arranged near the center of the Si optical waveguide core 201. The effect of the change in the refractive index of the Si optical waveguide core 201 due to the change in the carrier density can be more greatly received. Therefore, the interleaved pn junction can modulate light even at a low voltage, and an optical modulator with good modulation efficiency can be realized.
[0029] Note that although the low-concentration p-type semiconductor layer 212 and the low-concentration n-type semiconductor layer 213 shown in FIG. 5 extend to the sidewalls of the thick Si layer 201, the low-concentration p-type semiconductor layer 212 and the low-concentration n-type semiconductor layer 213 may be in contact inside the Si optical waveguide core 201. Further, the low-concentration p-type semiconductor layer 212 and the low-concentration n-type semiconductor layer 213 may extend beyond the thick Si layer 201, and the low-concentration p-type semiconductor layer 212 and the low-concentration n-type semiconductor layer 213 may be in contact in the slab regions 202a and 202b.
[0030] As described above, in the ion implantation pattern in the Si optical waveguide core 201, in addition to the structure in which the pn junction structure of the low-concentration p-type semiconductor layer and the low-concentration n-type semiconductor layer is arranged in the horizontal direction (left and right in the figure) and has a pn junction surface in the vertical direction, there are vertical pn junctions and interleaved pn junctions. These structures can perform the phase change of light at a lower voltage and can achieve highly efficient optical modulation. By performing optical modulation with high efficiency, power consumption reduction, high speed, and high functionality of an optical communication system can be realized.
[0031] However, in the conventional ion implantation pattern, there were limitations in both the power consumption, speed, and functionality of the Si optical modulator.
Prior Art Documents
Patent Documents
[0032]
Patent Document 1
Non-Patent Documents
[0033]
Non-Patent Document 1
[0034] The object of the present invention is to provide an optical modulator that can improve optical modulation efficiency by having a structure that allows control of the ion implantation pattern with a wider range of options.
[0035] To achieve this objective, one embodiment of the present invention provides an optical modulator that includes a semiconductor layer having a pn junction in an optical waveguide core, and modulates an optical signal by applying a bias voltage to the semiconductor layer together with a high-frequency (RF) signal, wherein the optical modulator comprises a low-concentration p-type semiconductor layer and a low-concentration n-type semiconductor layer forming the pn junction, and a medium-concentration p-type semiconductor layer added to the low-concentration p-type semiconductor layer or a medium-concentration n-type semiconductor layer added to the low-concentration n-type semiconductor layer, and the optical waveguide core is composed of three layers: the low-concentration p-type semiconductor layer, the medium-concentration p-type semiconductor layer and the low-concentration n-type semiconductor layer, or the low-concentration p-type semiconductor layer, the medium-concentration n-type semiconductor layer and the low-concentration n-type semiconductor layer. [Brief explanation of the drawing]
[0036] [Figure 1] Figure 1 shows the cross-sectional structure of an optical waveguide, which is the basic component of a conventional Si optical modulator. [Figure 2] Figure 2 is a plan view showing a Si optical modulator that constitutes a conventional dual-electrode Mach-Zehnder type optical modulator. [Figure 3] Figure 3 is a cross-sectional view showing a Si optical modulator that constitutes a conventional dual-electrode Mach-Zehnder type optical modulator. [Figure 4] Figure 4 shows the cross-sectional structure of the optical waveguide of a conventional Si optical modulator having a vertical pn junction. [Figure 5] Figure 5 is a plan view showing the optical waveguide of a conventional Si optical modulator having an interleaved pn junction. [Figure 6] Figure 6 is a cross-sectional view showing the optical waveguide of a conventional Si optical modulator having an interleaved pn junction. [Figure 7] Figure 7 is a cross-sectional view showing the optical waveguide of a conventional Si optical modulator having an interleaved pn junction. [Figure 8] Figure 8 shows the cross-sectional structure of the optical waveguide of the Si optical modulator according to Embodiment 1 of the present invention. [Figure 9] Figure 9 is a plan view showing the optical waveguide of a Si optical modulator according to Embodiment 2 of the present invention. [Figure 10]Figure 10 shows the cross-sectional structure of the optical waveguide of the Si optical modulator in Example 2. [Figure 11] Figure 11 shows the cross-sectional structure of the optical waveguide of the Si optical modulator in Example 2. [Modes for carrying out the invention]
[0037] Embodiments of the present invention will be described in detail below with reference to the drawings. [Examples]
[0038] Figure 8 shows the cross-sectional structure of the optical waveguide of the Si optical modulator according to Embodiment 1 of the present invention. Figure 8 shows a cross-section (xz plane) of the optical waveguide 200 constructed on an SOI substrate, cut perpendicular to the direction of light propagation (y axis). Light propagates in the direction perpendicular to the plane of the paper (y axis direction). The Si optical modulator of Embodiment 1 is a Si optical modulator having a vertical pn junction, and the method of constructing the optical waveguide of the Si optical modulator, the classification of impurity doping, the configuration of electrodes, and the operating principle are the same as those of a conventional Si optical modulator. The optical waveguide core 201 is a rib waveguide consisting of a Si layer sandwiched between SiO2 cladding layers on a Si substrate, and is the thick Si layer in the center. The difference from the conventional structure is that medium-concentration p-type semiconductor layers 215a, 215b and a medium-concentration n-type semiconductor layer 216 have been added.
[0039] The medium-density p-type semiconductor layer 215a is ion-implanted in the slab region 202a, which is thinner than the Si optical waveguide core 201, overlapping with the ion implantation pattern for forming the low-density p-type semiconductor layer 211. Similarly, the medium-density n-type semiconductor layer 216 is ion-implanted in the slab region 202b, which is thinner than the Si optical waveguide core 201, overlapping with the ion implantation pattern for forming the low-density n-type semiconductor layer 213. The medium-density p-type semiconductor layer 215a and the medium-density n-type semiconductor layer 216 are also used to reduce the electrical resistivity of the slab regions 202a and 202b. Reducing the electrical resistivity of the slab region 202 reduces the loss of high-frequency electrical signals applied to the optical modulator, which is effective for the high-speed operation of the optical modulator.
[0040] The medium-concentration p-type semiconductor layer 215a and the medium-concentration n-type semiconductor layer 216 are intended for ion implantation into thin slab regions 202a and 202b, and the acceleration voltage during ion implantation is set lower compared to when ions are implanted into the low-concentration p-type semiconductor layer 212 and the low-concentration n-type semiconductor layer 213. When implanting ions into the low-concentration p-type semiconductor layer 212 and the low-concentration n-type semiconductor layer 213, if multiple ion implantations are performed in shallow regions close to the substrate surface and deep regions extending beyond the substrate surface, the acceleration voltage is set to be approximately the same as the acceleration voltage used for ion implantation in the shallow regions.
[0041] Here, we classify ions into three types: high concentration, medium concentration, and low concentration. However, ion implantation into the medium concentration semiconductor layer is performed by overlapping it with the pattern of low concentration ion implantation. Therefore, the dose amount, which indicates the number of ions per unit area, can be smaller than that for ion implantation into the low concentration semiconductor layer. This is because the gap between the high concentration p-type semiconductor layer 211 and the high concentration n-type semiconductor layer 214 is only a few microns, which is small relative to the fabrication accuracy of the ion implantation mask. Therefore, high and medium concentration ion implantation is generally performed by overlapping it with the pattern of the lower concentration.
[0042] In the Si optical modulator of Example 1, the medium-concentration p-type semiconductor layer 215b extends not only to the slab region 202a but also to the region of the Si optical waveguide core 201 close to the substrate surface. The mask used when forming the medium-concentration p-type semiconductor layer is extended to the portion overlapping with the Si optical waveguide core 201, and ion implantation is performed in the shallow region close to the substrate surface. In this way, the Si optical waveguide core 201 is composed of three layers: a low-concentration p-type semiconductor layer 212, a low-concentration n-type semiconductor layer 213, and the medium-concentration p-type semiconductor layer 215b.
[0043] In the Si optical waveguide core 201, a pn junction structure consisting of a low-density p-type semiconductor layer 212 and a low-density n-type semiconductor layer 213 is arranged horizontally (left and right in the figure), and a pn junction structure consisting of a medium-density p-type semiconductor layer 215 and a low-density n-type semiconductor layer 213 is arranged vertically (top and bottom in the figure). A modulated electrical signal and a bias voltage are applied via electrodes in contact with the high-density p-type semiconductor layer 211 and the high-density n-type semiconductor layer 214 at both ends of the Si layer 2. The carrier density of the Si optical waveguide core 201 is changed by the modulated electrical signal and the reverse bias voltage. By changing the carrier density, the refractive index of the Si optical waveguide core 201 is changed by the carrier plasma effect, thereby modulating the phase of light propagating through the optical waveguide core.
[0044] The Si optical waveguide core 201 has a width (x-axis direction) of 400-600 nm, a core height (z-axis direction) of 150-300 nm, and a slab thickness of 50-200 nm. As a result, the light propagating within the Si optical waveguide core 201 has a flattened shape in the width (x-axis direction). Therefore, the horizontal pn junction surface formed on the Si optical waveguide core 201 increases the overlap between the pn junction portion, where the carrier density changes with the application of an electric field, and the light propagation mode. The change in carrier density changes the refractive index of the Si optical waveguide core 201, thereby modulating the phase of the light propagating through the core of the optical waveguide. Consequently, the Si optical modulator of Example 1 can modulate light even at low voltages, realizing an optical modulator with good modulation efficiency.
[0045] Furthermore, in order to position the pn junction in the center of the Si optical waveguide core 201, where the distribution density of propagating light is high, that is, to make the pn junction surface larger in the horizontal direction, the boundary between the low-density p-type semiconductor layer 212 and the low-density n-type semiconductor layer 213 is shifted toward the slab region 202a. With this structure, an optical modulator with higher modulation efficiency can be realized.
[0046] Conventional vertical pn junctions consist of two layers: a low-density p-type semiconductor layer 212 and a low-density n-type semiconductor layer 213. Therefore, at the boundary between the optical waveguide core 201 and the slab region 202, the ion-implanted semiconductor layer in the shallow region close to the substrate surface may not connect, resulting in electrical disconnection or increased resistance. To prevent this, it was necessary to set the pn junction surface in a deeper region further from the substrate surface in order to connect the ion-implanted semiconductor layer in the shallow region close to the substrate surface.
[0047] In the Si optical modulator of Example 1, a low-density p-type semiconductor layer 212 is located between the medium-density p-type semiconductor layer 215b formed in the shallow region of the optical waveguide core 201 and the medium-density p-type semiconductor layer 215a formed in the slab region 202. Therefore, there is no concern about the medium-density p-type semiconductor layers 215a and 215b being disconnected, and the pn junction surface can be set at any position.
[0048] In Figure 8, the medium-density p-type semiconductor layer 215b is ion-implanted in a shallow region close to the substrate surface of the optical waveguide core 201. However, a structure in which the medium-density n-type semiconductor layer 216 is ion-implanted in a shallow region close to the substrate surface of the optical waveguide core 201 is also possible. In this case, the pn junction is positioned in the center of the optical waveguide core 201 where the distribution density of propagating light is high. For this reason, the boundary between the low-density p-type semiconductor layer 212 and the low-density n-type semiconductor layer 213 should not be located in the center of the optical waveguide core 201, but rather shifted towards the slab region 202b. This structure makes it possible to realize an optical modulator with better modulation efficiency. [Examples]
[0049] Figure 9 shows the optical waveguide of a Si optical modulator according to Embodiment 2 of the present invention. Figure 9 is a top view of a part of the optical waveguide 200 constructed on an SOI substrate, viewed from a direction perpendicular to the substrate surface (xy plane). Figure 10 is a plane (xz plane) of the optical waveguide 200 constructed on an SOI substrate, cut perpendicular to the direction of light propagation (y axis), and is a cross-sectional view of X-X' in Figure 9. Figure 11 is a cross-sectional view of XI-XI' in Figure 9, where light propagates in the direction perpendicular to the plane of the paper (y axis direction).
[0050] The Si optical modulator of Example 2 is a Si optical modulator having an interleaved pn junction. The configuration method of the optical waveguide of the Si optical modulator, the classification of impurity doping, the electrode configuration, and the operating principle are the same as those of the conventional Si optical modulators shown in Figures 1 and 5. The difference from the conventional structure is the addition of medium-concentration p-type semiconductor layers 215a, 215b and medium-concentration n-type semiconductor layers 216a, 216b. The ion implantation method for forming the medium-concentration semiconductor layers is the same as that of the Si optical modulator of Example 1.
[0051] The difference from the Si optical modulator of Example 1 is that the doping region distributed in the area close to the surface of the Si optical waveguide core 201 along the direction of light propagation (y-axis direction) alternates between the medium-concentration p-type semiconductor layer 215b and the medium-concentration n-type semiconductor layer 216b. Furthermore, the boundary between the low-concentration p-type semiconductor layer 212 and the low-concentration n-type semiconductor layer 213 is not located in the center of the optical waveguide core 201, but alternates between being closer to the slab region 202a and closer to the slab region 202b. In this way, the interleaved pn junction is realized as a vertical pn junction.
[0052] In the Si optical modulator of Example 2, waveguides are alternately arranged such that medium-density n-type semiconductor layers 216a and 216b extend not only to the slab region 202b but also to the region close to the substrate surface of the optical waveguide core 201, and waveguides are alternately arranged such that medium-density p-type semiconductor layers 215a and 215b extend not only to the slab region 202a but also to the region close to the substrate surface of the optical waveguide core 201. This can be achieved by dividing the process into two parts: one where the mask used when forming the medium-density n-type semiconductor layer is extended to the part overlapping with the optical waveguide core 201 and ion implantation is performed in a shallow region close to the substrate surface, and another where the mask used when forming the medium-density p-type semiconductor layer is extended to the part overlapping with the optical waveguide core 201 and ion implantation is performed in a shallow region close to the substrate surface. Thus, in the Si optical modulator of Example 2, the optical waveguide core 201 is composed of four layers: a low-density p-type semiconductor layer 212, a low-density n-type semiconductor layer 213, a medium-density p-type semiconductor layer 215b, and a medium-density n-type semiconductor layer 216b.
[0053] The optical waveguide core 201 of the Si optical modulator has a pn junction structure formed by a medium-density p-type semiconductor layer 215b and a low-density p-type semiconductor layer 212, and a medium-density n-type semiconductor layer 216b and a low-density n-type semiconductor layer 213. A modulation electrical signal and a bias voltage are applied from both ends of the Si layer 2 via electrodes (not shown). The carrier density of the optical waveguide core 201 is changed by the carrier plasma effect by which the refractive index of the optical waveguide core 201 is changed, thereby modulating the phase of the light propagating through the optical waveguide core.
[0054] The Si optical waveguide core 201 has a width (x-axis direction) of 400-600 nm, a core height (z-axis direction) of 150-300 nm, and a slab thickness of 50-200 nm. As a result, the light propagating within the Si optical waveguide core 201 has a flattened shape in the width (x-axis direction). Therefore, the horizontal pn junction surface formed on the Si optical waveguide core 201 increases the overlap between the pn junction portion, where the carrier density changes with the application of an electric field, and the light propagation mode. The change in carrier density changes the refractive index of the Si optical waveguide core 201, thereby modulating the phase of the light propagating through the core of the optical waveguide. Consequently, even in the Si optical modulator of Example 2, light can be modulated even at low voltages, and an optical modulator with good modulation efficiency can be realized.
[0055] Furthermore, in the Si optical modulator of Example 2, the boundary between the medium-density p-type semiconductor layer 215b and the medium-density n-type semiconductor layer 216b, formed in the shallow region of the optical waveguide core 201, also becomes a pn junction. Therefore, the overlap between the light propagating through the optical waveguide core 201 and the pn junction can be made larger than in the conventional vertical pn junction waveguide structure of the Si optical modulator in Example 1. Consequently, the effect of changes in the refractive index of the optical waveguide core 201 due to changes in carrier density can be more significantly utilized. According to the Si optical modulator of Example 2, In this way, along the direction of light propagation (y-axis direction), light can be modulated even at lower voltages compared to conventional vertical pn junction and interleaved pn junction optical modulators, resulting in an optical modulator with high modulation efficiency. [Industrial applicability]
[0056] The present invention can be used in optical communication systems in general. In particular, it can be applied to optical modulators in optical transmitters of optical communication systems.
Claims
1. In an optical modulator that includes a semiconductor layer having a pn junction in an optical waveguide core, and modulates an optical signal by applying a bias voltage to the semiconductor layer together with a high-frequency signal, A first cladding layer provided on the upper surface of the substrate, A low-concentration p-type semiconductor layer and a low-concentration n-type semiconductor layer are provided on the upper surface of the first cladding layer to form the pn junction, A first medium-concentration p-type semiconductor layer added to the low-concentration p-type semiconductor layer or a first medium-concentration n-type semiconductor layer added to the low-concentration n-type semiconductor layer, The second cladding layer is provided so as to sandwich each of the semiconductor layers between the first cladding layer and the second cladding layer, and has an electrode on its upper surface to which the high-frequency signal or the bias voltage is applied. The optical waveguide core is composed of a three-layer structure consisting of the low-concentration p-type semiconductor layer, the first medium-concentration p-type semiconductor layer, and the low-concentration n-type semiconductor layer, or a three-layer structure consisting of the low-concentration p-type semiconductor layer, the first medium-concentration n-type semiconductor layer, and the low-concentration n-type semiconductor layer. The pn junction is formed at the interface where the sides of the low-concentration p-type semiconductor layer and the low-concentration n-type semiconductor layer are in contact with each other. The side of the low-concentration p-type semiconductor layer opposite to the side forming the interface is in contact with the side of a second medium-concentration p-type semiconductor layer provided in a first slab region on the upper surface of the first cladding layer that is thinner than the optical waveguide core, or the side of the low-concentration n-type semiconductor layer opposite to the side forming the interface is in contact with the side of a second medium-concentration n-type semiconductor layer provided in a second slab region on the upper surface of the first cladding layer that is thinner than the optical waveguide core. An optical modulator characterized in that, in the three-layer structure, a first medium-concentration p-type semiconductor layer or a first medium-concentration n-type semiconductor layer is provided in contact with the upper surface of each of the low-concentration p-type semiconductor layer and the low-concentration n-type semiconductor layer, on the surface opposite to the surface in contact with the first cladding layer.
2. In an optical modulator that includes a semiconductor layer having a pn junction in an optical waveguide core, and modulates an optical signal by applying a bias voltage to the semiconductor layer together with a high-frequency signal, A first cladding layer provided on the upper surface of the substrate, A low-concentration p-type semiconductor layer and a low-concentration n-type semiconductor layer are provided on the upper surface of the first cladding layer to form the pn junction, A first medium-concentration p-type semiconductor layer added to the low-concentration p-type semiconductor layer or a first medium-concentration n-type semiconductor layer added to the low-concentration n-type semiconductor layer, The second cladding layer is provided so as to sandwich each of the semiconductor layers between the first cladding layer and the second cladding layer, and has an electrode on its upper surface to which the high-frequency signal or the bias voltage is applied. The optical waveguide core has a configuration in which the three-layer structure of the low-concentration p-type semiconductor layer, the first medium-concentration p-type semiconductor layer, and the low-concentration n-type semiconductor layer, and the three-layer structure of the low-concentration p-type semiconductor layer, the first medium-concentration n-type semiconductor layer, and the low-concentration n-type semiconductor layer are arranged alternately along the direction of light propagation. The pn junction is formed at the interface where the sides of the low-concentration p-type semiconductor layer and the low-concentration n-type semiconductor layer are in contact with each other. In each of the three-layer structures, the side of the low-concentration p-type semiconductor layer opposite to the side forming the interface is in contact with the side of the second medium-concentration p-type semiconductor layer provided in a first slab region less thick than the optical waveguide core on the upper surface of the first cladding layer, and the side of the low-concentration n-type semiconductor layer opposite to the side forming the interface is in contact with the side of the second medium-concentration n-type semiconductor layer provided in a second slab region less thick than the optical waveguide core on the upper surface of the first cladding layer. An optical modulator characterized in that, in each of the three layers, a first medium-concentration p-type semiconductor layer or a first medium-concentration n-type semiconductor layer is provided in contact with the upper surface of the low-concentration p-type semiconductor layer and the low-concentration n-type semiconductor layer, on the surface opposite to the surface in contact with the first cladding layer.
3. The substrate is a Si substrate, and the first cladding layer and the second cladding layer are each made of SiO 2 The cladding layer is formed, and the optical waveguide core is formed on the Si substrate, with the SiO 2 An optical modulator according to claim 1 or 2, comprising a rib waveguide with a Si layer sandwiched between cladding layers, characterized in that the central Si layer is thicker.