A moscap-based modulator structure and its applications

Abstract

The application relates to a MOSCAP-based modulator structure and application thereof. The modulator structure at least comprises a ridge silicon waveguide, a lower electrode, an insulator layer, a gate electrode and a contact electrode. The gate electrode is located above the ridge silicon waveguide and is made of transparent conductive oxide. The ridge silicon waveguide is lightly doped. The lower electrode is connected with the flat plate area of the ridge silicon waveguide through a buffer area, and the lower electrode is heavily doped. The doping concentration of the buffer area is between the doping concentrations of the light doping and the heavy doping. The gate electrode and the lower electrode are respectively electrically connected with a contact electrode. The structure uses materials with large refractive index difference as the gate electrode material and the waveguide material respectively, so that the light field is concentrated in the waveguide for transmission. The change of the thickness and size of the gate electrode and the thickness of the insulator layer has little influence on the transmission loss of the overall device. The structure provides a new platform and idea for the electrode design of the modulator and the design of different functional modulators.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of modulator, in particular to a MOSCAP-based modulator structure and application thereof. BACKGROUND

[0002] With the advent of the era of big data, future information processing increasingly needs high-speed and large-bandwidth data processing capability, and the low-loss high-modulation rate and high-bandwidth capability of integrated optical modulators make them important components of the next generation of information and computing. At the same time, silicon devices based on CMOS technology can achieve large-size high-density production and are compatible with electrical driving and electrical chip design, thereby having the potential to realize mass production of Tb / s-level optical communication inter-rack and intra-rack interconnection.

[0003] Since the modulation efficiency of Pockels effect and Franz-Keldysh effect in silicon devices is low, silicon modulators are often realized through free carrier ion diffusion effect. Silicon modulators usually realize plasmonic diffusion effect through the following three ways: carrier injection, carrier depletion and carrier accumulation. The carrier injection modulator is realized by applying a forward voltage, but its modulation rate is limited by the carrier lifetime; the carrier depletion modulator can provide a larger modulation bandwidth by applying a reverse voltage, but has a smaller modulation efficiency; and the carrier accumulation modulator is constructed by embedding a MOSCAP structure into an optical waveguide, so that the free carriers accumulate on both sides of the dielectric layer, providing a larger modulation bandwidth while maintaining high modulation efficiency, thereby allowing smaller size and lower driving voltage. The MOSCAP modulator based on carrier accumulation is no longer limited by the carrier lifetime, but depends on the capacitance and resistance of the device structure, and has better advantages than the carrier injection modulator.

[0004] However, the existing optical modulator based on the MOSCAP structure still has various problems, and further research on it is of great significance to promote its development. SUMMARY

[0005] Therefore, the purpose of the present application is to provide a new MOSCAP-based modulator structure, which uses materials with large refractive index difference as gate electrode material and waveguide material respectively, so that the light field is concentrated in the transmission of the waveguide, and the change of the thickness and size of the gate electrode and the thickness of the insulating layer has little effect on the transmission loss of the overall device. This structure provides a new platform and idea for the design of modulator electrodes and the design of different functional modulators.

[0006] The application realizes the technical purpose through the following technical scheme: a modulator structure based on MOSCAP, at least comprising a ridge silicon waveguide, a lower electrode connected with the ridge silicon waveguide, an insulator layer deposited on the ridge silicon waveguide and the lower electrode, a gate electrode made on the insulator layer, and a contact electrode; the gate electrode is located above the ridge silicon waveguide and is made of transparent conductive oxide, the ridge silicon waveguide is lightly doped, the lower electrode is connected with the flat plate area of the ridge silicon waveguide through a buffer area, and the lower electrode is heavily doped, and the doping concentration of the buffer area is between the doping concentrations of the light doping and the heavy doping; the gate electrode and the lower electrode are respectively electrically connected with a contact electrode.

[0007] As a preferred embodiment, the ridge silicon waveguide is a top silicon layer on an SOI substrate.

[0008] As a preferred embodiment, the material of the gate electrode includes but is not limited to ZnO, ITO, FTO, ITiO.

[0009] As a preferred embodiment, the doping concentration of the light doping is 1x10 16 ~5x10 17 cm -3 , and the doping concentration of the heavy doping is 1x10 19 ~10 20 cm -3 .

[0010] As a preferred embodiment, the ion species doped on the ridge silicon waveguide includes but is not limited to B and P, and the doping mode includes but is not limited to ion implantation and thermal diffusion.

[0011] As a preferred embodiment, the ridge silicon waveguide is a MRR single-mode ridge silicon waveguide structure.

[0012] As a preferred embodiment, the material of the insulator layer is a high-K dielectric, including but not limited to silicon dioxide, hafnium dioxide, zirconium dioxide, hafnium silicate, zirconium silicate, nitrogenated hafnium silicate, and nitrogenated zirconium silicate.

[0013] As a preferred embodiment, the thickness of the insulator layer is 5-100 nm.

[0014] As a preferred embodiment, the total height of the ridge silicon waveguide is 220 nm, the height of the lower layer is 150 nm, the height of the upper layer is 70 nm, and the width of the upper layer is 470 nm.

[0015] As a preferred embodiment, the width of the buffer area along the direction of the lower electrode and the ridge silicon waveguide is 1-2 microns.

[0016] The application also provides application of the above-mentioned MOSCAP-based modulator structure in a Mach-Zehnder interference modulator and a micro-ring modulator.

[0017] The MOSCAP-based modulator structure provided by the application can change the thickness and size of the TCO material without affecting the transmission loss of the light in the whole device, because of the large refractive index difference between the transparent conductive oxide material (TCO) and the silicon material, and the light field is concentrated in the silicon waveguide transmission in the light transmission process, which can provide a new platform for the modulator electrode design, for example, a multi-pole gate electrode structure can be designed to reduce the series resistance and parasitic capacitance.

[0018] Because the transmission loss caused by the thickness change of the insulating layer can be ignored due to the fact that the light is mostly transmitted in the low-doped silicon waveguide, the thickness of the insulating layer can be used as an independent parameter for device design in the modulator design, without considering the mutual restriction of the optical loss and the modulation bandwidth, which provides a new platform and idea for the design of different functional modulators.

[0019] Because of the static capacitance difference caused by the MOSCAP structure characteristics, the modulator based on the structure can complete modulation at low power consumption, and by using the characteristics, the modulator device of the structure can be used not only for high-bandwidth high-speed modulators, but also for low-loss phase compensators, adjustable optical attenuators and optical filters. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A schematic diagram of a reverse PN junction MRR modulator structure reported in the prior art;

[0021] Figure 2 A schematic diagram of a horizontal MOSCAP structure reported in the prior art;

[0022] Figure 3 A schematic diagram of an optical modulator based on a MOSCAP provided by an embodiment of the application;

[0023] Figure 4 Influence of the thickness of the insulating layer on the capacitance value and voltage in the performance test of the embodiment of the application;

[0024] Figure 5 Influence of the thickness of the insulating layer on the modulation efficiency in the performance test of the embodiment of the application;

[0025] Figure 6 Influence of the thickness of the insulating layer on the phase change in the performance test of the embodiment of the application;

[0026] Figure 7 Influence of the thickness of the insulating layer on the optical loss in the performance test of the embodiment of the application;

[0027] Figure 8 The light field distribution map in the MOSCAP-based optical modulator provided by the embodiment of the present application.

[0028] In the figure:

[0029] 1 substrate, 2 ridge silicon waveguide, 3 insulator layer, 4 gate electrode, 5 contact electrode, 6 lower electrode, 7 buffer zone. DETAILED DESCRIPTION

[0030] In 2022, Haisheng Rong et al. reported a MRR modulator structure with a reverse PN junction, as shown in Figure 1 The MRR structure forms a micro-ring modulator with a reverse PN junction through P and N doping. After optimization of the waveguide ridge width, the width is 300 nm, and the micro-ring radius is 4 μm. The free spectral width of this design of micro-ring can reach 16.3 nm, the modulation efficiency reaches 0.42 V.cm, and the modulation rate can reach 240 Gb / s using PAM4 modulation. However, in order to increase the modulation efficiency, this modulator needs to increase the doping concentration, which leads to high transmission loss, and due to the use of a low-quality factor micro-ring structure, the extinction ratio is low.

[0031] In 2023, David J. Thomson et al. reported a horizontal MOSCAP structure, as shown in Figure 2 By doping the poly-silicon (Poly-Si) and single-crystal silicon (C-Si) with P and N respectively, and adding an oxide layer structure between the poly-silicon and single-crystal silicon, a horizontal MOSCAP structure is formed. The modulation efficiency of this design can reach 64 Gb / s under MRZ modulation. However, this structure has a high processing difficulty, and the thickness control process of the oxide layer is difficult, so the insulating layer thickness cannot be effectively adjusted to control the overall modulator capacitance. In addition, in order to improve the modulation efficiency of the MOSCAP, half of the optical field is distributed in the poly-silicon region, which leads to an increase in optical loss.

[0032] Based on the problems existing in the prior art, the purpose of the present application is to provide a new MOSCAP-based modulator structure, which uses materials with large refractive index difference as the gate electrode material and the waveguide material respectively, so that the light field is concentrated in the waveguide for transmission, and the changes in the thickness and size of the gate electrode and the thickness of the insulating layer have little effect on the transmission loss of the overall device. Specifically, the gate electrode uses transparent conductive oxide material, the ridge waveguide uses ridge silicon waveguide, the ridge silicon waveguide is lightly doped, the lower electrode is heavily doped, and the two are connected through a buffer zone with a doping concentration between the two, the insulating layer is realized by deposition, the thickness is highly controllable, the carrier concentration of the silicon waveguide is changed through the plasma dispersion effect, thereby changing the refractive index and absorption coefficient of the silicon waveguide in the transmission of the light in the silicon waveguide, achieving the purpose of modulating the phase and wavelength of the light wave, and maintaining high carrier concentration and low optical transmission loss, which can realize continuous modulation efficiency / loss adjustment, and is especially suitable for high-speed optical communication transmission.

[0033] The present application will be further described in detail below with reference to specific examples, so that those skilled in the art can more clearly understand the present application. The following examples are used to illustrate the present application, but are not used to limit the scope of the present application. Based on the specific examples in the present application, all other examples obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0034] As shown in Figure 3 The present application provides a MOSCAP-based optical modulator structure, which at least includes a ridge silicon waveguide 2, a lower electrode 6 connected to the ridge silicon waveguide 2, an insulator layer 3 deposited on the ridge silicon waveguide 2 and the lower electrode 6, a gate electrode 4 made on the insulator layer 3, and a contact electrode 5.

[0035] The gate electrode 4 is located above the ridge silicon waveguide 2 and is made of transparent conductive oxide, the ridge silicon waveguide 2 is lightly doped, the lower electrode 6 is connected to the flat plate area of the ridge silicon waveguide 2 through a buffer zone 7, the lower electrode 6 is heavily doped, and the doping concentration of the buffer zone 7 is between the doping concentrations of the light doping and the heavy doping.

[0036] The gate electrode 4 and the lower electrode 6 are respectively electrically connected to a contact electrode 5.

[0037] In the present application, the ridge silicon waveguide 2 is formed by light doping to form an ultra-shallow doped layer, and the light field distribution is concentrated in the low-doped ridge silicon waveguide region, the lower electrode connected with the flat plate area of the ridge silicon waveguide 2 is heavily doped, and the two are connected through a buffer zone 7 with a doping concentration between light doping and heavy doping. When a bias is applied to the lower electrode and the gate of the MOSCAP, the accumulation of charges on the lower electrode and the gate affects the effective refractive index of the material at the location through the plasma dispersion effect, and the optical field resonance mode is modulated according to the overlap of the light field and the carrier accumulation region. According to the Drude model, the size of the accumulated charge concentration and the change in the effective refractive index are positively correlated. The lower electrode in the structure is a heavily doped region, and the charges of the capacitor mainly accumulate on both sides of the insulating layer, so the accumulated charge concentration is high, the effective refractive index changes greatly, and the optical phase shift performance is good.

[0038] The insulating layer can be realized by deposition, and the thickness is controllable. Different modulator capacitance values can be obtained by designing insulating layers with different thicknesses, thereby changing the EO bandwidth, and further changing the modulation bandwidth and rate, so as to achieve high speed while maintaining high extinction ratio and low driving voltage. The structure can be used in Mach-Zehnder interferometer (MZI) modulator and micro-ring (MRR) modulator design.

[0039] It can be understood that the side of the ridge silicon waveguide protruding part is not limited to being perpendicular to the bottom surface of the ridge waveguide.

[0040] The thickness of the buffer zone 7 is the same as that of the lower electrode 6 and the flat plate area of the ridge silicon waveguide 2, and the width thereof in the direction of the lower electrode 6 and the ridge silicon waveguide 2 (i.e. the spacing between the lower electrode and the ridge silicon waveguide) is 1-2 μm.

[0041] In the present application, the ridge silicon waveguide 2 is a structure prepared on the substrate 1, and it can also be a top silicon layer structure on an SOI substrate directly. The general characteristics are: the total height is 220 nm, the lower layer height is 150 nm, the upper layer height is 70 nm, and the upper layer width is 470 nm.

[0042] In the present application, the material of the gate electrode 4 includes but is not limited to ZnO, ITO, FTO, ITiO.

[0043] The material of the insulator layer 3 is a high-K dielectric, including but not limited to silicon dioxide, hafnium dioxide, zirconium dioxide, hafnium silicate, zirconium silicate, nitrided hafnium silicate, and nitrided zirconium silicate. The thickness is usually 5-100 nm, and preferably 10-20 nm. The insulator layer 3 can be prepared by deposition, and the specific deposition methods include thermal oxidation and various thin film deposition processes, such as CVD, PVD, and ALD.

[0044] The material of the contact metal 5 is a metal material, including but not limited to Au, Cu, and Al.

[0045] The ion species doped in the ridge silicon waveguide 2 include, but are not limited to, B and P, the doping methods include, but are not limited to, ion implantation and thermal diffusion, and the preparation method is usually ion diffusion.

[0046] Further, the doping concentration of the light doping in the ridge silicon waveguide 2 is 1×10 16 ~5×10 17 cm -3 , the doping concentration of the heavy doping is 1×10 19 ~10 20 cm -3 , and the doping concentration of the buffer zone 7 is preferably 5×10 17 ~5×10 18 cm -3 .

[0047] Further, in order to further reduce the loss, the ridge silicon waveguide adopts an MRR single-mode ridge waveguide structure, which has excellent communication waveband light field confinement capability, and at the same time facilitates the concentration of the low-doped region in the light field distribution area.

[0048] The present application is highly controllable by using different doping concentrations and insulating layer thicknesses of different regions, and maintains high carrier concentration and low optical transmission loss, and can realize continuous modulation efficiency / loss adjustment, and is especially suitable for high-speed optical communication transmission, such as Mach-Zehnder interference modulators and micro-ring modulators.

[0049] The present application will be further described in detail below with specific cases, and the specific structure is as shown in Figure 1 The specific parameters are as follows: the ridge silicon waveguide 2 is a top silicon layer based on an SOI substrate, the total height of the ridge silicon waveguide is 220 nm, the lower layer height is 150 nm, the upper layer height is 70 nm, the upper layer width is 470 nm, the light doping and the heavy doping are B doped by ion implantation, wherein the doping concentration of the light doping is 1×10 17 , the doping concentration of the heavy doping is 5×10 19 , the width of the buffer zone is 2 μm, the doping concentration is 5×10 18 , the insulator layer 3 is deposited silicon dioxide material, the gate electrode 4 adopts ZnO material, the ridge silicon waveguide adopts an MRR single-mode ridge waveguide structure, and the change of the device performance with the change of the insulating layer thickness is studied under the above conditions, and the results are as follows:

[0050] (1) With the change of the insulating layer thickness, the change of the capacitance value and the voltage is as follows Figure 4The capacitance value decreases from 1.05 fF / μm to 0.21 fF / μm when a forward voltage of 2V is applied. According to the optical bandwidth formula, when the capacitance value drops to 0.6 fF / μm, an EO modulation bandwidth of 60 GHz can be supported (the system default resistance value is 60 ohm).

[0051] (2) The modulation efficiency is the device voltage application length that causes a Pi phase change in the optical carriers, defined as L and measured in volts*centimeter. The modulation efficiency from 20 nm to 100 nm (green 20 nm, purple 30 nm, pink 50 nm, black 100 nm) is shown in Figure 5 When the thickness of the insulating layer is increased to 100 nm, a high modulation capacity of 3.7 V*cm can still be maintained at Vpp=2V.

[0052] (3) The phase change is caused by a refractive index change in the optical carriers, which causes a phase change in the light transmission, measured in angle / millimeter. The phase change from 20 nm to 100 nm (green 20 nm, purple 30 nm, pink 50 nm, black 100 nm) is shown in Figure 6

[0053] (4) The relationship between optical loss and voltage is shown in Figure 7 From 20 nm to 100 nm (green 20 nm, purple 30 nm, pink 50 nm, black 100 nm), the overall loss is maintained at 0.12 dB / mm to 0.36 dB / mm, and as can be seen from the figure, the increase in the thickness layer can maintain low loss at high voltage, and the relative change in loss is small at a voltage of 0~2V, so it can be used as an optical characteristic that maintains low loss transmission within the voltage swing range. The low loss of the MOSCAP device structure is mainly due to the fact that the light field distribution is concentrated in the waveguide layer, and the MOSCAP structure characteristic causes the charges to be concentrated near the insulating layer, so the modulator based on the MOSCAP structure has lower transmission loss. Due to the low capacitance characteristic brought by the increase in the thickness of the insulating layer, the charge distribution of the entire device is more concentrated in the waveguide, so the waveguide loss increases with the increase in the thickness of the insulating layer at the resting voltage, but the carrier concentration of the waveguide gradually increases with the increase in the input of the forward bias voltage, resulting in a gradual increase in loss.

[0054] (5) The light field distribution is shown in Figure 8 ​As shown in the figure. Due to the light doping of the silicon waveguide, the influence of the doping on the light transmission in the waveguide is minimized. When the thickness of the insulating layer is increased, the transmission loss of the whole device does not increase significantly, and the low transmission loss characteristic is still maintained. Due to the large difference in refractive index between the TCO and the silicon, the light field is mostly bound to the silicon waveguide transmission, so the size change of the TCO electrode will not affect the transmission loss of the whole device. At the same time, due to the MOSCAP optical modulator structure, in order to avoid increasing the optical insertion loss, the metal contact of the gate region is often arranged far away from the waveguide region, but this in turn increases the series resistance and parasitic capacitance of the whole gate region, thereby limiting the modulation bandwidth. Due to the excellent conductive properties of the TCO material at low doping, the TCO material can replace the metal plate as the new gate material, reduce the parasitic capacitance caused by the metal connection away from the waveguide region, and at the same time ensure low insertion loss, so that the whole device has high modulation bandwidth.

[0055] The optical modulator structure provided by the present application has a change value of the capacitance value of only 4% between 2V and 4V as the voltage value increases, so that a high-voltage modulator with high-bandwidth linear change can be made using this performance; optimization of the doping concentration and the doping region can further reduce the transmission loss, reduce the thickness of the insulating layer at the present stage, and further reduce the Vpp value; selection of the insulating layer can further improve the efficiency of the change in the capacitance value, and selection of an insulating layer with a high K value can make a MOSCAP structure with an ultra-thin insulating layer, thereby improving the optical properties of the whole device, such as high extinction ratio, refractive index change, and the like, and providing a new design idea; due to the large refractive index difference between the TCO material and the silicon material, the light field is concentrated in the silicon waveguide transmission during light transmission, so that changing the thickness and size of the TCO material will not affect the transmission loss of the light in the whole device, thereby providing a new platform for modulator electrode design, for example, a multi-pole gate structure can be designed to reduce the series resistance and parasitic capacitance.

[0056] It should be pointed out here that the above embodiments are only for further description and illustration of the technical solutions of the present application, and are not further limitations on the technical solutions of the present application. The method of the present application is only a preferred embodiment, and is not used to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A MOSCAP based modulator structure, characterized by, At least comprising a ridge silicon waveguide (2), a lower electrode (6) connected with the ridge silicon waveguide (2), an insulator layer (3) deposited on the ridge silicon waveguide (2) and the lower electrode (6), a gate electrode (4) and a contact electrode (5) successively stacked on the insulator layer (3); The gate electrode (4) is located above the ridge silicon waveguide (2) and is made of transparent conductive oxide, the ridge silicon waveguide (2) is lightly doped, the lower electrode (6) is connected with the flat plate area of the ridge silicon waveguide (2) through a buffer area (7), and the lower electrode (6) is heavily doped, and the doping concentration of the buffer area (7) is between the doping concentrations of the light doping and the heavy doping. The gate electrode (4) and the lower electrode (6) are respectively connected with two contact electrodes (5), wherein the other contact electrode connected with the lower electrode (6) is provided through the insulator layer (3).

2. The MOSCAP-based modulator structure of claim 1, wherein, The ridge silicon waveguide (2) is a top silicon layer on an SOI substrate.

3. The MOSCAP-based modulator structure of claim 1, wherein, The material of the gate electrode (4) includes but is not limited to ZnO, ITO, FTO, ITiO.

4. The MOSCAP-based modulator structure according to any one of claims 1 to 3, characterized in that, The light doping has a doping concentration of 1 x 1015 16 cm 17 -3 -3 , and the heavy doping has a doping concentration of 1 x 1019 19 cm 20 -2 -3 .

5. The MOSCAP-based modulator structure according to any one of claims 1 to 3, wherein, The doped ion species includes but is not limited to B and P, and the doping method includes but is not limited to ion implantation and thermal diffusion.

6. The MOSCAP-based modulator structure according to any one of claims 1 to 3, wherein, The ridge silicon waveguide (2) is a MRR single-mode ridge silicon waveguide structure.

7. The MOSCAP-based modulator structure according to any one of claims 1 to 3, wherein, The material of the insulator layer (3) is a high-K dielectric, including but not limited to silicon dioxide, hafnium dioxide, zirconium dioxide, hafnium silicate, zirconium silicate, nitrogenated hafnium silicate and nitrogenated zirconium silicate.

8. The MOSCAP-based modulator structure according to any one of claims 1 to 3, wherein, The thickness of the insulator layer is 5-100 nm.

9. The MOSCAP-based modulator structure according to any one of claims 1 to 3, wherein, The width of the buffer area (7) along the direction of the lower electrode (6) and the ridge silicon waveguide (2) is 1-2 μm.

10. Application of the MOSCAP-based modulator structure according to any one of claims 1-9 in a Mach-Zehnder interference modulator and a micro-ring modulator.

Images