Light Modulation Element

Abstract

The optical modulation element (100) has a rib-type optical waveguide, the rib-type optical waveguide having: a rib (112) having a PN junction; a P-type plate-shaped region (114) continuous with the P-type region of the rib (112); and an N-type plate-shaped region (116) continuous with the N-type region of the rib (112). A first thin film (130) is formed on the P-type plate-shaped region 114 and is made of a material having an electron affinity different from that of the material of the P-type plate-shaped region (114).

Description

Technical Field

[0001] The present invention relates to a light modulation element. Background Art

[0002] Optical networks are used for inter-block and inter-data center communications that require bandwidth in large-capacity optical networks for data centers, 5G backhaul, connected cars, and high-speed optical information processing for autonomous driving.

[0003] Optical network transceivers often use optical modulators to convert electrical signals into optical signals, and increasing the speed and reducing the cost of optical modulators have become important issues.

[0004] Figure 1 (a) is a cross-sectional view of a rib-type optical waveguide 900 used in a conventional optical modulator. The rib-type optical waveguide 900 comprises a core 910, an upper cladding 920 sandwiching the core 910, and a lower cladding 922. A rib-type PN junction 912 is formed in the center of the core 910, and slab regions 914 and 916 are connected to both sides of the rib-type PN junction 912. Electrodes are drawn from the slab regions 914 and 916. A DC reverse bias voltage and an AC voltage overlapping with the DC reverse bias voltage are applied between the electrodes. By controlling the AC voltage, the refractive index of the core 910 can be changed, thereby functioning as a phase shifter (optical modulator).

[0005] Figure 1 (b) is Figure 1 (a) shows the equivalent circuit diagram of rib-type optical waveguide 900. The equivalent circuit of rib-type optical waveguide 900 is a series circuit consisting of capacitor C and resistor R, with a time constant τ given by equation (1). C is the capacitance of PN junction 912, and R is the resistance component between the metal electrode and PN junction 912.

[0006] τ=2RC…(1)

[0007] Therefore, in order to increase the speed of the rib-type optical waveguide 900, it is necessary to reduce R and shorten the response time constant τ determined by the product RC of R and PN junction capacitance C.

[0008] Patent Document 1 discloses a technique for increasing the doping density of plate-shaped regions 914 and 916 on both sides of a rib-shaped PN junction 912 to increase the carrier (free electron or hole) density and thereby reduce the series resistance R. According to Non-Patent Document 1, this technique enables optical modulation in a frequency band of 54 GHz.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-126728

[0012] Patent Document 2: Japanese Patent Application Publication No. 2019-049612

[0013] Patent Document 3: Japanese Patent Application Laid-Open No. 2017-072808

[0014] Non-patent literature

[0015] Non-patent document 1: J.Zhou et al., "Silicon Photonics Carrier DepletionModulators Capable of 85Gbaud 16QAM and 64Gbaud 64QAM", Optical FiberCommunication Conference 2019, Page-Tu2H.2

[0016] Non-patent document 2: JH.Han, F.Boeuf, J.Fujikata, S.Takahashi, S Takagi, andM.Takenaka, "Efficient low-loss InGaAsP / Si hybrid MOS optical modulator," Nature Photonics Vol.11, pp.486-490 (2017) Summary of the Invention

[0017] Problems to be solved by the invention

[0018] In the technology of Patent Document 1, the doping concentration (carrier density) of the slab region is increased by approximately four times. As a result, free-carrier light absorption increases by approximately 30%. In other words, in the technology of Patent Document 1, there is a trade-off between increased speed and optical loss. To compensate for the increased optical loss, it is necessary to increase the intensity of the incident laser or perform optical amplification, which increases power consumption.

[0019] The present invention has been made under such circumstances, and one exemplary object of one embodiment thereof is to provide an optical modulation element that suppresses an increase in optical loss and increases speed.

[0020] Means for solving problems

[0021] One embodiment of the present invention relates to an optical modulator. The optical modulator includes a rib-type optical waveguide having a rib portion having a PN junction; a P-type slab region continuous with the P-type region of the rib; and an N-type slab region continuous with the N-type region of the rib; and a first thin film formed on the P-type slab region, having different electron affinities for holes and the material of the P-type slab region.

[0022] According to this method, the dopant is locally present in the P-type slab region, forming a two-dimensional hole system, or thin hole layer, in the first thin film in contact therewith. The mobility of the two-dimensional hole system is higher than that of electrons and holes in conventional optical modulators, significantly reducing series resistance and achieving higher speeds. Furthermore, the high density of free carriers in the first thin film suppresses any increase in light loss in the P-type slab region.

[0023] The first thin film may be formed with a gap between it and the side wall of the rib portion. Providing the gap can suppress the light guided in the rib portion from leaking into the first thin film and the P-type slab region.

[0024] The optical modulator further includes a second thin film formed on the N-type slab region, with an electron affinity different from that of the material of the N-type slab region. This method allows the dopant to be localized in the second thin film, forming a two-dimensional electron system in the N-type slab region in contact with it. The mobility of the two-dimensional electron system is higher than that of electrons and holes in conventional optical modulators, significantly reducing series resistance and enabling higher speeds. Furthermore, while free carriers exist in the N-type slab region, the carrier concentration remains at the same level as in conventional optical modulators, resulting in the same level of optical loss.

[0025] The second thin film may be formed with a gap between it and the side wall of the rib. By providing the gap, it is possible to suppress the light guided in the rib from leaking into the second thin film and the N-type slab region.

[0026] The PN junction of the rib may also have a horizontal structure. The PN junction of the rib may also have a vertical structure.

[0027] The rib-type optical waveguide may contain Si, and the first thin film (and the second thin film) may contain Ge. This allows the optical modulation element to be easily integrated on the platform of the CMOS substrate.

[0028] Effects of the Invention

[0029] According to one aspect of the present invention, it is possible to achieve higher speed while suppressing an increase in loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 (a) is a cross-sectional view of a rib-type optical waveguide used in a conventional optical modulation element. Figure 1 (b) is Figure 1 (a) Equivalent circuit diagram of the rib-type optical waveguide.

[0031] Figure 2 It is a cross-sectional view of a light modulation element according to an embodiment.

[0032] Figure 3 This is the energy band diagram of the Type II heterojunction of Si and Ge.

[0033] Figure 4 (a) and (b) are schematic representations of Figure 2 A diagram showing the distribution of carriers and dopants in the P-type and N-type regions of the optical modulation element.

[0034] Figure 5 This is a graph showing the relationship between the mobility and impurity concentration of Si, Ge, and GaAs.

[0035] Figure 6 It is a diagram showing simulation results of the electric field intensity of the light modulation element according to the embodiment.

[0036] Figure 7 Graphs showing simulation results of a Mach-Zehnder optical modulator using the optical modulation element according to the embodiment.

[0037] Figure 8 This is a cross-sectional view of a light modulation element according to Modification 2.

[0038] Figure 9 This is a cross-sectional view of a light modulation element according to Modification 3. DETAILED DESCRIPTION

[0039] The present invention is described below with reference to the accompanying drawings based on preferred embodiments. Identical or equivalent components, parts, and processes shown in the various drawings are denoted by the same reference numerals, and duplicate descriptions are omitted as appropriate. The embodiments are illustrative rather than limiting of the invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

[0040] Figure 2 1 is a cross-sectional view of an optical modulation element 100 according to an embodiment. The optical modulation element 100 includes a rib waveguide having a rib-shaped core 110 and an upper cladding layer 120 and a lower cladding layer 122 sandwiching the core 110 .

[0041] Core 110 includes rib 112 having a lateral PN junction, P-type plate region 114 continuous with the P-type region of rib 112 , and N-type plate region 116 continuous with the N-type region of the rib. Core 110 further includes first thin film 130 and second thin film 132 .

[0042] The first thin film 130 is formed on the P-type slab region 114. The first thin film 130 can be made of a semiconductor material having different electron affinities for the material of the P-type slab region 114 and holes. The electron affinity of a semiconductor refers to the energy obtained when an electron moves from the external vacuum level to the bottom of the semiconductor's conduction band.

[0043] The second thin film 132 is formed on the N-type plate-shaped region 116. The second thin film 132 can be made of a semiconductor material having an electron affinity different from that of the material of the N-type plate-shaped region 116.

[0044] For example, in the case of a silicon waveguide, Si is selected for the core 110 and SiO 2 is selected for the cladding. In this case, Ge is preferably used as the first thin film 130 and the second thin film 132 .

[0045] The first thin film 130 may be formed with a gap g between it and the P-side sidewall of the rib 112 . Similarly, the second thin film 132 may be formed with a gap g between it and the N-side sidewall of the rib 112 .

[0046] Exemplary dimensions of the optical modulator 100 are described. The lateral width of the rib 112 can be 450 to 500 nm, and the height can be approximately 220 to 400 nm. The thickness of the P-type slab region 114 and the N-type slab region 116 is preferably 100 nm or less. The thickness of the first thin film 130 and the second thin film 132 is preferably 30 to 50 nm. Furthermore, the lateral gap between the first thin film 130 and the rib 112, and the lateral gap between the second thin film 132 and the rib 112, can be approximately 100 to 150 nm.

[0047] The above is the structure of the optical modulator 100. Next, its operation will be described. A heterojunction of Si and Ge is formed at the interface between the first thin film 130 and the P-type slab region 114. Similarly, a heterojunction of Si and Ge is formed at the interface between the second thin film 132 and the N-type slab region 116. Figure 3 This is the energy band diagram of the Type II heterojunction of Si and Ge. Figure 3 The energy band configuration of semiconductor heterojunctions is due to the difference in electron affinities.

[0048] Figure 4 (a) and (b) are schematic representations of Figure 2 Graph showing the distribution of carriers and dopants in the P-type region and the N-type region of the light modulation element 100 .

[0049] Reference Figure 4 (a) of FIG. 1 illustrates the situation of the P-type region. In the junction between the first thin film 130 and the P-type plate-shaped region 114, dopant ions are locally present in the shaded region on the side of the P-type plate-shaped region 114. Moreover, a two-dimensional hole gas (2DHG) is formed in the first thin film 130 isolated from the dopant ions. The mobility of the two-dimensional hole gas is more than twice that of Si with a high concentration and doped with an acceptor. Therefore, the conductivity of the first thin film 130 can be improved, and the resistance value Rp between the electrode and the PN junction can be reduced to less than 1 / 2 compared to the past.

[0050] Reference Figure 4 (b) illustrates the situation in the N-type region. At the junction between the second thin film 132 and the N-type slab region 116, dopants are locally present in the shaded area of ​​the second thin film 132, forming a two-dimensional electron gas (2DEG) in the isolated N-type slab region 116. The mobility of the 2DEG is more than twice that of high-concentration, donor-doped Si. Consequently, the conductivity of the N-type slab region 116 is improved, reducing the resistance Rn between the electrode and the PN junction to less than 50% compared to conventional methods.

[0051] Here, the time constant of the light modulation element 100 is expressed by equation (2).

[0052] τ=C×(Rp+Rn)…(2)

[0053] C is the junction capacitance of the PN junction, Rp is the resistance value on the P side, and Rn is the resistance value on the N side. As described above, since the resistance values ​​Rp and Rn can be reduced, high-speed operation can be achieved.

[0054] The light loss of the light modulator 100 will be described. Figure 4 (a) , there is no high-density free carrier in the P-type slab region 114, so the increase in light loss in the P-type slab region 114 can be suppressed. Figure 4 In (b), electrons as free carriers exist in the N-type plate region 116, but the carrier concentration is similar to that of the conventional N-type plate region 116, and therefore the light loss is also similar. Therefore, the increase in light loss in the N-type region is also suppressed.

[0055] As described above, according to the light modulation element 100 of the embodiment, it is possible to suppress an increase in light loss and achieve a higher speed.

[0056] The light modulation element 100 according to the embodiment is compared with the technology described in Patent Document 1. The resistance in a semiconductor material is inversely proportional to the product of the mobility μ and the carrier concentration n.

[0057] R∝1 / (μ·n)…(3)

[0058] In conventional technology, if the doping density is increased in order to increase the carrier concentration n, the scattering of ionized impurities increases, which tends to reduce the mobility μ. Figure 5 This is a graph showing the relationship between the mobility and impurity concentration of Si, Ge, and GaAs. For example, to achieve a frequency band of 54 GHz, 10 18 cm -3 The doping density is different from that of the undoped case (10 14 cm -3Compared with (below), the mobility μ is reduced to less than half.

[0059] In contrast, in this embodiment, electrons and holes, which serve as free carriers, are generated in a region isolated from ionized impurities (dopants). Therefore, they are not subject to scattering caused by ionized impurities, maintaining the mobility of the undoped state. Consequently, this embodiment can further reduce resistance compared to conventional technologies, and can be expected to increase the frequency band to 100 GHz, twice the current level.

[0060] Figure 6 : is a graph showing the simulation results of the electric field intensity of the optical modulation element 100 according to the embodiment. Figure 6 The lower part of shows the dimensions of each part used in the simulation. According to the light modulation element 100, the light beam can be confined in the rib 112 of the core 110 and its vicinity, and a good beam profile can be obtained.

[0061] The use of the light modulator 100 is not particularly limited, but two light modulators may be arranged on two arms of a Mach-Zehnder interferometer to form a light modulator. Figure 7 1 is a graph showing simulation results (solid line) of the frequency characteristics of a Mach-Zehnder optical modulator using the optical modulation element 100. For comparison, a dashed line shows simulation results of the frequency characteristics of a conventional modulator of the same size.

[0062] The Mach-Zehnder optical modulator is configured using the rib waveguide of the optical modulation element 100 as a phase modulation unit. The simulation parameters are as follows.

[0063] Dimensions of the waveguide in the phase modulation section

[0064] Rib width 500nm, rib height 340nm, waveguide length 3mm

[0065] The optical loss in the waveguide of the phase modulation unit is assumed to be 0.5 dB / mm. In the conventional technique, the optical loss is 1.0 dB / mm.

[0066] In the simulation, the electrical characteristics of a circuit consisting of a series resistor R and a capacitor C were numerically analyzed. The resulting electrical characteristics were input as the frequency characteristics of the phase modulator to derive the frequency response of the optical interference intensity. The following values ​​were used for resistor R and capacitor C.

[0067] Series resistance R: 1.7Ω for implementation, 3.5Ω for prior art

[0068] Electrostatic capacitance implementation method and existing technology are both 1.2pF (DC reverse bias 3V)

[0069] The simulation results show that optical loss has been improved by 5dB compared to the conventional -10dB, reaching -5dB in this embodiment. Furthermore, a comparison of the frequency band (-3dB) shows that the frequency band has been expanded to 105GHz in this embodiment, compared to the conventional 52GHz, approximately doubling the bandwidth, enabling high-speed operation.

[0070] The present invention has been described above based on the embodiments. Those skilled in the art will appreciate that these embodiments are merely illustrative and that various variations of the combinations of components and processing steps are possible, and that such variations are also within the scope of the present invention. These variations are described below.

[0071] (Variation 1)

[0072] In the embodiment, the case where both the first thin film 130 and the second thin film 132 are formed is described, but the present invention is not limited thereto. Only the first thin film 130 or only the second thin film 132 may be formed.

[0073] (Variation 2)

[0074] Figure 8 1 is a cross-sectional view of a light modulator 100A according to Modification 2. In this modification, the shoulder portions 140 and 142 of the rib 112 are doped with impurities at a low concentration (compensating doping).

[0075] (Variation 3)

[0076] Figure 9 FIG. 1 is a cross-sectional view of a light modulator 100B according to a third modification. In the embodiment, a PN junction of a horizontal structure is described. Figure 9 As shown, the present invention can also be applied to a vertical PN junction.

[0077] (Variation 4)

[0078] In the embodiment, Si waveguide is described, but the application of the present invention is not limited thereto and can also be applied to waveguides based on other semiconductor materials. For example, compound semiconductor GaAs / Al 1-x Ga x As combination. Al can also be used 1-x Ga xAs mixed crystals are used as the material for the ribs 112, the P-type plate-shaped regions 114, and the N-type plate-shaped regions 116, and GaAs crystals are used in the first and second thin films 130 and 132. In this material system, due to the energy band configuration of a Type I heterojunction, 2DHG and 2DEG are formed in the first and second thin films 130 and 132, respectively. The composition ratio x can be set to 0.7, for example. The composition ratio x is not limited to this value and can be adjusted based on the desired 2DHG and 2DEG density, post-processing oxidation resistance, and other requirements.

[0079] Industrial applicability

[0080] The present invention relates to a light modulation element.

[0081] Label Description

[0082] 100 Light Modulation Element

[0083] 110 cores

[0084] 112 ribs

[0085] 114 P-type plate region

[0086] 116 N type plate area

[0087] 130 1st film

[0088] 132 2nd film

[0089] 120 upper cladding

[0090] 122 lower cladding

Claims

1. A light modulation element, characterized in that: The light modulation element comprises a rib-type optical waveguide and a first thin film. The rib-type optical waveguide comprises: a rib portion having a PN junction; a P-type plate-shaped region which is continuous with the P-type region of the rib portion; and an N-type plate-shaped region continuous with the N-type region of the rib; The first thin film is formed on the P-type plate-shaped region and isolated from the dopant, and a two-dimensional hole gas, namely 2DHG, is formed inside the first thin film. The electron affinity of the first thin film is different from that of the material of the P-type plate-shaped region.

2. The light modulation element according to claim 1, wherein: The first thin film is formed with a gap between it and the side wall of the rib.

3. The light modulation element according to claim 1, wherein: The light modulation element further includes a second thin film formed on the N-type slab region and having an electron affinity different from that of a material of the N-type slab region.

4. The light modulation element according to claim 3, wherein: The second thin film is formed with a gap between it and the side wall of the rib.

5. The light modulation element according to any one of claims 1 to 4, characterized in that: The rib-type optical waveguide includes Si, and the first thin film includes Ge.

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