A modulator based on suspended graphene to regulate optical properties of phase change material

Abstract

This invention discloses a modulator for controlling the optical properties of phase change materials based on suspended graphene. The modulator includes: a substrate with a channel array on its upper surface; a suspended graphene microheater array, comprising graphene functional strips spanning each channel in the channel array, with the middle portion of each graphene functional strip suspended above the channel array to form a suspended region, and the two ends of each graphene functional strip disposed on the upper surface of the substrate on both sides of the channel; a source / drain electrode array, comprising multiple sets of source / drain electrodes, each set of source / drain electrodes contacting the two ends of the corresponding graphene functional strip; and a phase change material layer disposed on the upper surface of the suspended region of the graphene functional strip, which undergoes a phase transformation under thermal drive to change its optical properties. This invention employs a suspended graphene structure, creating an air gap between the graphene and the substrate, thereby fundamentally cutting off the solid-state heat transfer path of out-of-plane heat conduction from Joule heat to the substrate.

Description

Technical Field

[0001] This invention relates to the field of integrated photonics technology, and in particular, to a modulator based on suspended graphene to control the optical properties of phase change materials. Background Technology

[0002] Phase change material (PCM) optical modulators, as core devices in silicon photonics integrated circuits, directly determine the transmission capacity and energy efficiency of optical interconnect systems, and have urgent application needs in high-speed data processing scenarios such as artificial intelligence and 5G communication. Among various methods of controlling PCM, using Joule heating generated by microheaters to drive the crystal phase transition of PCM has become the mainstream technology route due to its electrical driving characteristics and compatibility with CMOS processes. Currently, graphene is considered a highly promising microheater material. Its atomic-level thickness results in extremely low heat capacity and ultra-fast thermal response speed, while its extremely high in-plane thermal conductivity facilitates uniform heat distribution within the plane, theoretically enabling uniform phase transition of PCM. These characteristics make it superior to traditional metal or doped silicon heaters in terms of response speed and heating uniformity.

[0003] However, graphene microheaters face an unavoidable dilemma in practical applications. Common graphene microheaters require support and integration on a silicon substrate, but the direct contact between graphene and the substrate creates out-of-plane heat dissipation channels, allowing Joule heat to dissipate rapidly through the substrate. This means that even with extremely high in-plane thermal conductivity, graphene cannot raise its temperature to the crystallinity required for phase change materials while maintaining out-of-plane heat dissipation channels. Researchers might consider suspending the graphene region to cut off the heat dissipation channels, but existing fabrication methods for suspended graphene arrays employ a bottom-up process, typically relying on wet etching for sacrificial layer removal. This process has inherent drawbacks: the etching solution directly contacts and wets the entire surface of atomically thin graphene, and during the subsequent drying process, the surface tension of the liquid exerts capillary forces on the graphene film strong enough to cause it to break and curl. Simultaneously, impurity ions in the etching solution are adsorbed onto the graphene surface, causing contamination and doping.

[0004] Given the aforementioned problems with wet etching, researchers often consider dry etching as an alternative. However, dry etching typically involves high-energy ion bombardment, and this physical force has a significant damaging effect on graphene with atomic-level thickness. Furthermore, the gaseous products from dry etching are difficult to completely escape from the channel structure with its large aspect ratio. Another conventional approach is to use supercritical drying technology to circumvent the surface tension problem of liquids. However, supercritical drying still requires contact between graphene and the liquid medium, making chemical contamination unavoidable. Moreover, this process is costly and complex, making it difficult to meet the needs of large-scale wafer-level fabrication. Therefore, in the specific scenario of fabricating suspended graphene microheater arrays, all of the above-mentioned existing technological approaches have their own technical obstacles and cannot be directly adopted.

[0005] More notably, the aforementioned difficulties do not exist in the fabrication of macroscopic heating devices. Macroscopic heater films are typically hundreds of nanometers to micrometers thick, possessing sufficient mechanical strength to resist the surface tension of etching solutions, and their surface contaminants can be effectively removed through cleaning processes. However, for atomically thin graphene, the mechanical strength and surface cleanliness have a far greater impact on device performance than macroscopic heaters: any tiny wrinkles or damage will significantly alter the carrier mobility and thermal conductivity of graphene, and any surface-adsorbed impurities will introduce additional scattering centers. In other words, while there is some experience in fabricating suspended thin-film heaters in the existing technology, this experience is based on macroscopic films. Applying this directly to atomically thin graphene fundamentally changes the technical problem. The process deviations that are tolerable in macroscopic heater fabrication are precisely the key factors determining success or failure in graphene devices. Therefore, how to achieve the release of large-scale suspended graphene microheater arrays without damaging the graphene, and how to enable them to efficiently drive phase change materials to complete crystal phase transitions, has become a technical bottleneck restricting the practical application of such devices. Summary of the Invention

[0006] One of the objectives of this invention is to provide a modulator that modulates the optical properties of phase change materials based on suspended graphene.

[0007] To achieve the above-mentioned objectives, the specific technical solution is as follows: A modulator for controlling the optical properties of phase change materials based on suspended graphene, comprising: A substrate, wherein the upper surface of the substrate is provided with a channel array; A suspended graphene microheater array includes graphene functional strips spanning each channel in the channel array. The middle part of the graphene functional strip is suspended above the channel array to form a suspended region, and the two ends of the graphene functional strip are respectively disposed on the upper surface of the substrate on both sides of the channel. The source / drain electrode array includes multiple sets of source / drain electrodes, each set of source / drain electrodes being in contact with both ends of the corresponding graphene functional strip; A phase change material layer is disposed on the upper surface of the suspended region of the graphene functional strip. The phase change material layer undergoes a crystal phase transformation under thermal drive to change its optical properties.

[0008] According to embodiments of the present invention, one of the technical solutions has at least one of the following advantages or beneficial effects: Due to the high thermal conductivity of silicon substrates, approximately Joule heat is largely lost to the substrate through out-of-plane heat conduction, limiting the tunable temperature range of graphene and making it difficult to meet the modulation requirements of high crystallization temperature phase change materials. This also results in low electrothermal conversion efficiency, hindering the achievement of ultra-low power non-volatile modulation. Furthermore, in traditional heating schemes, uneven temperature distribution leads to non-uniform phase transitions in the phase change material, causing a decrease in optical modulation accuracy. Traditional optical modulators rely on complex optical systems with low integration, making them unsuitable for the demands of large-scale silicon photonic integrated circuits.

[0009] To overcome the aforementioned technical difficulties, this invention employs a structure where the central portion of a graphene functional strip is suspended above the channel, creating an air gap between the graphene and the substrate. This fundamentally cuts off the solid-state heat transfer path from Joule heat to the substrate via out-of-plane heat conduction. When a voltage is applied to the suspended graphene through the source-drain electrodes, the generated Joule heat cannot dissipate through the substrate; the heat is confined within the graphene surface, allowing the graphene to reach a higher lattice temperature under the same electrical power input. Experiments have shown that the single-layer graphene using this suspended structure can reach temperatures exceeding 1200K at a voltage of 0.536V / μm, far exceeding the adjustable temperature range of traditional graphene microheaters on substrates.

[0010] Meanwhile, after the suspended structure eliminates heat dissipation from the substrate, the Joule heat in graphene is mainly conducted along the in-plane direction. Graphene itself has extremely high in-plane thermal conductivity. This anisotropic thermal property allows heat to be transferred quickly and uniformly laterally in the suspended region. After the phase change material layer is placed on the upper surface of the suspended region, heat is uniformly conducted from graphene to the phase change material layer, enabling the phase change material to achieve a uniform crystal phase transformation during heating. This avoids problems such as incomplete phase transformation or local overheating and burn-out caused by uneven heat distribution in traditional heaters.

[0011] Furthermore, graphene, as a heating element with atomically thin thickness, has extremely small heat capacity and extremely fast electronic heating and cooling speeds, completing heating and cooling cycles within microseconds, enabling high-speed reversible control of phase change materials. Combined with a suspended graphene microheater array structure, this solution is compatible with CMOS processes, allowing for large-scale arrayed optical modulator integration on the same chip. This invention, through the structural combination of a substrate channel array, suspended graphene functional stripes, source / drain electrodes, and a phase change material layer, achieves substrate-free heat dissipation, ultra-low power heating, uniform in-plane temperature distribution, and rapid thermal response, providing an effective solution for the application of non-volatile phase change materials based on suspended graphene microheaters in large-scale silicon photonic integrated circuits such as non-volatile optical modulators and photonic storage.

[0012] According to one embodiment of the present invention, a modulator based on suspended graphene to control the optical properties of phase change materials is prepared by the following steps: S1 etches a channel array on the upper surface of the substrate, fills the channel array with a sacrificial layer, and performs planarization on the substrate surface so that the sacrificial layer is flush with the surface of the substrate to form a continuous plane. Graphene is transferred onto the continuous plane and etched to form graphene functional strips spanning the channel array. The two ends of the graphene functional strips overlap the substrate surfaces on both sides of the channel array, and the middle region is attached to the surface of the sacrificial layer. Source and drain electrodes are fabricated at both ends of the graphene functional strips. The sacrificial layer in the channel array is removed by reacting it with high-temperature gas, so that the part of the graphene functional strip located above the channel array is suspended, thus obtaining a suspended graphene micro heater array. S2 performs Raman spectroscopy tests on the suspended region of the prepared suspended graphene microheater array. By measuring the peak position shift of the graphene Raman characteristic peaks under different ambient temperatures and different applied voltages, the heating capacity of the suspended graphene microheater array is calibrated, and a quantitative correspondence between the applied voltage and the temperature of the suspended graphene lattice is established. S3 is the deposition of a phase change material layer on the upper surface of the graphene corresponding to the suspended region of the suspended graphene microheater array. Based on the quantitative correspondence established in S2, S4 applies a set voltage to the suspended graphene microheater array that matches the crystallization temperature of the phase change material layer, and uses graphene Joule heating to drive the phase change material layer to undergo a crystal phase transition, thereby changing its optical properties and achieving light modulation.

[0013] According to embodiments of the present invention, one of the technical solutions has at least one of the following advantages or beneficial effects: The high-temperature gas reaction method (S1), the precise in-situ temperature measurement and calibration of Raman spectroscopy (S2), and the integrated timing protection (S3) of this invention provide solutions to three interrelated technical problems in the prior art, and the three also have an inseparable timing synergy relationship.

[0014] Specifically: The S1 high-temperature gas reaction method completely removes the sacrificial layer without contact with any liquid medium, thereby obtaining a suspended graphene film with a complete structure and a clean surface. This method differs fundamentally from existing wet etching, which relies on the chemical dissolution at the liquid-solid interface, and its destructiveness stems from the surface tension of the liquid phase. This invention uses a gas-solid phase chemical conversion reaction, with no liquid phase involved throughout the process, fundamentally avoiding the existence of surface tension.

[0015] The purpose of S2's precise in-situ Raman spectroscopy temperature calibration is to address the fact that the temperature rise of suspended graphene is affected by multiple factors, including graphene mass, suspension span, contact resistance, and radiative heat dissipation. Theoretical simulations cannot accurately predict the actual lattice temperature, while thermocouples and infrared thermal imagers are unsuitable for temperature measurement in micrometer-scale suspended regions due to their large physical size and insufficient spatial resolution, respectively. This differs fundamentally from conventional Raman spectroscopy, which uses lasers as a heat source to measure thermal conductivity and falls under the category of physical property measurement. Its temperature response characteristics cannot be directly applied to engineering calibration in electrojoule heating scenarios. This invention provides a systematic calibration process for electrojoule heating-driven temperature rise scenarios, with engineering applications as its goal. The purpose of the S3 timing protection integration is to enable the phase change material to be integrated with suspended graphene without undergoing a high-temperature gas reaction, ensuring its phase purity and tunable optical properties. Because the high-temperature gas reaction temperature of 350℃~400℃ is much higher than the crystallization temperature of most phase change materials (approximately 150℃~200℃), if the phase change material is deposited before release, it will prematurely crystallize during the device fabrication stage, locking its optical properties and making subsequent electrical actuation impossible to effectively control. This differs from the fabrication sequence of conventional thin-film devices, which typically involve deposition followed by release. Given the high-temperature gas reaction method used in this invention, this conventional approach would lead to complete loss of device function.

[0016] According to one embodiment of the present invention, if wet etching is used in S1, the etching solution is in direct contact with the atomically thick graphene. During the drying process, the surface tension of the liquid generates a capillary force on the graphene film sufficient to cause it to break and curl. At the same time, impurity ions in the etching solution are adsorbed on the graphene surface, causing contamination and doping, and the device fails. If dry etching is used, high-energy ion bombardment also damages the atomically thick graphene lattice, and the gas products are difficult to be discharged from the channels with a large depth-to-width ratio.

[0017] According to one embodiment of the present invention, if a thermocouple is used for temperature measurement in S2, its physical size is much larger than the suspended graphene region (micrometer scale) and contact will destroy the suspended structure; if an infrared thermal imager is used for temperature measurement, its spatial resolution of tens of micrometers is usually insufficient to distinguish a single suspended graphene heating unit.

[0018] According to one embodiment of the present invention, the high-temperature gas in S1 is hydrogen chloride gas, and the sacrificial layer is a vanadium dioxide layer.

[0019] The reaction temperature of hydrogen chloride with vanadium dioxide is lower than the thermal damage threshold of graphene and lower than the thermal diffusion temperature of the source and drain electrodes, enabling low-temperature non-destructive release.

[0020] If HF gas is used to react with the SiO2 sacrificial layer, HF will corrode the substrate and graphene, causing damage to the device structure. If Cl2 gas is used to react directly with the metal sacrificial layer, the reaction temperature will be too high and cause thermal damage to the graphene. In addition, the melting point of metal chlorides is high, making it difficult to completely remove them in gaseous form.

[0021] According to one embodiment of the present invention, the hydrogen chloride gas is generated by the decomposition of ammonium chloride at 300°C to 350°C, and the reaction temperature between the high-temperature gas and the sacrificial layer is 350°C to 400°C.

[0022] Although the temperature of the high-temperature gas reaction (350℃~400℃) is higher than the crystallization temperature of some phase change materials, since S3 of the present invention is arranged after the high-temperature gas reaction, the phase change material will not experience high temperature throughout the process, thereby avoiding thermal damage.

[0023] Furthermore, if HCl gas is introduced directly from the outside instead of being decomposed in situ with ammonium chloride, additional corrosive HCl cylinders and a precision gas delivery system are required, which increases the complexity of the process and the safety risks.

[0024] According to one embodiment of the present invention, the etching depth of the channel array in S1 is 0.1 μm to 1 μm, and the width of the channel in the channel array is 1 μm to 10 μm.

[0025] The channel width range of 1μm to 10μm and the depth range of 0.1μm to 1μm ensures unobstructed convection and diffusion paths for high-temperature reaction gases, making the sacrificial layer removal rate consistent in each channel and ensuring the performance uniformity of each heating unit in the array.

[0026] If the channel width is greater than 10 μm, the suspended graphene film will sag at high temperatures due to the excessive span, and adhere and break after contacting the bottom of the channel. If the channel width is less than 1 μm, the suspended area is too small, resulting in insufficient optical modulation of the phase change material layer, and the diffusion of the reactant gas in the narrow channel is hindered. If the etching depth is less than 0.1 μm, the phase change material layer may protrude beyond the channel, affecting the flatness of the device and subsequent optical path coupling. If the etching depth is greater than 1 μm, the substrate etching process becomes significantly more difficult, and the channel sidewall area increases, making it difficult to guarantee the uniformity of sacrificial layer filling and gas reaction removal.

[0027] According to one embodiment of the present invention, in S1, chemical mechanical polishing is used to perform the planarization treatment, the polishing depth is greater than or equal to the thickness of the sacrificial layer on the upper surface of the substrate, and the roughness of the upper surface of the substrate after polishing does not exceed 2 nm.

[0028] The low-roughness and flat surface ensure a uniform distribution of van der Waals forces between graphene and the substrate during the transfer process. The suspended graphene, free from stress concentration, can withstand repeated loading of thermal stress without fatigue fracture during subsequent high-temperature gas heating.

[0029] According to one embodiment of the present invention, in S1, the length of the graphene functional strip is greater than the width of the channel in the channel array, the two ends of the graphene functional strip are respectively located on both sides of the same channel, and the width of the graphene functional strip does not exceed the channel length.

[0030] The strip length is greater than the channel width, and both ends are located on either side of the channel. This ensures that the graphene functional strip completely covers the channel and overlaps with the substrate on both sides of the channel, forming the basis for mechanical support and electrical contact. The graphene functional strip completely covers the top of the channel. During high-temperature gas reactions, the continuous graphene functional strip can confine the reactant gas inside the channel, improving the contact efficiency and reaction rate between the reactant gas and the sacrificial layer. Simultaneously, the geometric structure of the strip spanning the channel clearly separates the suspended and non-suspended regions spatially. Raman spectroscopy can be precisely focused on the suspended regions for testing, unaffected by signal interference from the substrate support region.

[0031] According to one embodiment of the present invention, each group of source and drain electrodes in the source and drain electrode array in S1 is in contact with both ends of the graphene functional strip.

[0032] This invention defines the electrical contact position relationship between the source / drain electrodes and the graphene functional strip, ensuring that the current flows along the length direction of the graphene functional strip (i.e., the direction across the channel), so that Joule heating is mainly generated in the suspended region, thereby achieving spatial overlap between the heating region and the suspended region and maximizing heating efficiency.

[0033] According to one embodiment of the present invention, the process of calibrating the heating capacity of the suspended graphene micro-heater array in S2 includes: S21 sets the initial ambient temperature and measures the Raman spectral peak position of the graphene suspended region at the current ambient temperature; S22 Updates the current ambient temperature to a value greater than the initial ambient temperature, and measures the Raman spectral peak positions of the graphene suspended region under different ambient temperatures; S23 calculates the shift of the characteristic peak position of graphene Raman spectrum as a function of ambient temperature, and performs a linear fit on the relationship between the shift and ambient temperature change to obtain the ambient temperature coefficient of graphene. , The unit is ; S24 measures the Raman spectral peak positions of the graphene suspended region under a set ambient temperature without applied voltage; S25 applies a set voltage to the suspended graphene microheater and measures the Raman spectral peak position of the suspended graphene region under the set voltage. S26 Calculates the shift of the characteristic peak position of graphene Raman spectrum under a set voltage compared to when no voltage is applied. According to the formula The lattice temperature of the graphene suspended region under a set voltage was calculated, where, It is the shift in the peak position of the characteristic peak in the graphene Raman spectrum under a set voltage compared to when no voltage is applied. It is the ambient temperature coefficient of graphene. This refers to the ambient temperature of graphene when no electricity is applied. and All figures are in Kelvin.

[0034] The calibration method of this invention establishes a quantitative correspondence between applied voltage, Raman peak shift, and lattice temperature, enabling the set voltage applied in S4 to accurately correspond to the crystallization temperature required for the phase change material. Without this calibration method, it is impossible to predict the applied voltage required to bring suspended graphene to the crystallization temperature required for the phase change material, because the temperature rise of suspended graphene is affected by various factors (such as graphene mass, suspension span, ambient air pressure, contact resistance, etc.), and theoretical simulations cannot accurately predict this.

[0035] According to one embodiment of the present invention, the Raman spectral characteristic peaks in S23 are G peaks and / or 2D peaks.

[0036] According to one embodiment of the present invention, the phase change material layer in S3 is an antimony selenide layer, and the thickness of the phase change material layer is 15nm~30nm.

[0037] Antimony selenide has a crystallization temperature much lower than the maximum achievable temperature of suspended graphene, leaving ample thermal margin to ensure phase transition can be achieved at a relatively low driving voltage. A thickness of 15nm to 30nm corresponds to the optimal path length for the interaction between light and the phase transition material in the 1550nm communication band, because if it is too thin, the modulation depth is insufficient, and if it is too thick, the driving power consumption increases and the upper material may not be able to complete the phase transition.

[0038] According to one embodiment of the present invention, in S3, a mask-assisted vapor deposition technique is used to vapor deposit the phase change material layer on the upper surface of the suspended region.

[0039] The high-temperature gas reaction of this invention ensures that the surface of the suspended graphene is clean and free of contaminants, and the mask-assisted evaporation ensures that the phase change material is deposited only in the clean suspended area without introducing additional photoresist and etching solution contamination. Together, they ensure that a high-quality van der Waals heterojunction interface is formed between the phase change material and the suspended graphene.

[0040] If a full-area evaporation deposition method is used, followed by photolithography and etching to define the phase change material region, the etching process will introduce additional photoresist contamination and etching solution damage. Furthermore, the etching solution may come into contact with the suspended graphene, causing it to break or become contaminated. If a maskless full-area evaporation deposition method is used, the phase change material will cover the source and drain electrode regions, resulting in poor electrical contact between the electrodes and the probes.

[0041] According to one embodiment of the present invention, the set voltage in S4 is the calibration voltage of the suspended graphene microheater array corresponding to the crystallization temperature of the phase change material layer.

[0042] According to one embodiment of the present invention, the set voltage applied to the suspended graphene microheater array in S4 is a pulse voltage.

[0043] If DC voltage is used instead of pulsed voltage, the phase change material will remain in a crystalline state after reaching its crystallization temperature. Without additional cooling, reversible crystalline-to-amorphous switching cannot be achieved, and the device can only be programmed once and cannot be repeatedly erased and rewritten, making it unsuitable for optical modulation applications requiring multiple reconfigurable operations. If continuous-wave laser heating is used instead of electrical pulse heating, a large laser device and a complex optical path system are required, making chip-level integration impossible.

[0044] According to one embodiment of the present invention, the thickness of the source and drain electrodes is 50 nm to 100 nm.

[0045] According to one embodiment of the present invention, the phase change material layer is disposed on the upper surface of the suspended region of the suspended graphene film, and there is no intermediate buffer layer between the phase change material layer and the suspended graphene film.

[0046] In this invention, the phase change material layer is directly bonded to the upper surface of the suspended graphene, with no intermediate layer between them. Heat is directly transferred to the phase change material via van der Waals interfaces through phonon-phonon coupling. The absence of an intermediate layer means no additional interface phonon scattering, minimizing interface thermal resistance.

[0047] The second objective of this invention is to provide an application of the modulator based on suspended graphene to regulate the optical properties of phase change materials.

[0048] The specific technical solution is as follows: The application of the modulator based on the optical properties of phase change materials regulated by suspended graphene in silicon photonic integrated circuits.

[0049] In addition to the objectives, features and advantages described above, the present invention has other objectives, features and advantages. Attached Figure Description

[0050] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is a flowchart of the preparation method of the modulator based on suspended graphene to regulate the optical properties of phase change materials in Example 1.

[0051] Figure 2 The graph shows the modulation speed test results of the suspended graphene heater in Example 1.

[0052] Figure 3 The graph shows the lattice temperature test results of the monolayer suspended graphene under different applied voltages in Example 2. Detailed Implementation

[0053] The terms "preferred," "more preferred," etc., used in this invention refer to embodiments of the invention that provide certain beneficial effects under certain circumstances. However, other embodiments may also be preferred under the same or other circumstances. Furthermore, the description of one or more preferred embodiments does not imply that other embodiments are unavailable, nor is it intended to exclude other embodiments from the scope of this invention.

[0054] When a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0055] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of the present invention.

[0056] Unless otherwise specified, the reagents, methods and equipment used in this invention are all conventional reagents, methods and equipment in this technical field.

[0057] Example 1 A modulator for controlling the optical properties of phase change materials based on suspended graphene, comprising: The substrate has a channel array on its upper surface; The suspended graphene microheater array includes graphene functional strips that span each channel in the channel array. The middle part of the graphene functional strips is suspended above the channel array to form a suspended region. The two ends of the graphene functional strips are respectively disposed on the upper surface of the substrate on both sides of the channel. The source-drain electrode array includes multiple sets of source-drain electrodes, each set of which is in contact with both ends of the corresponding graphene functional strip. A phase change material layer is disposed on the upper surface of the suspended region of the graphene functional strip. The phase change material layer undergoes a crystal phase transformation under thermal drive to change its optical properties.

[0058] A flowchart of a method for fabricating a modulator based on suspended graphene to control the optical properties of phase change materials is shown below. Figure 1 As shown, specifically, it involves the following steps: S1 uses a highly doped silicon wafer with a 300nm silicon oxide insulating layer as the substrate. Photoresist AZ5214 is spin-coated onto the substrate surface, and ultraviolet lithography is used to expose and develop the channel array pattern and alignment marks. Subsequently, inductively coupled plasma etching is used to etch the silicon oxide insulating layer, and then acetone is used to remove the photoresist, yielding the channel array and alignment marks. The channel width is 3μm, and the channel depth is 1μm. Vanadium dioxide with a thickness of approximately 300nm is deposited as a sacrificial layer on the etched substrate surface using electron beam evaporation, ensuring the sacrificial layer completely fills the channel array. Chemical mechanical polishing (CMP) is used to planarize the sample surface to a depth of approximately 300nm, making the vanadium dioxide surface in the channels flush with the silicon oxide surface outside the channels. After polishing, the surface roughness of the substrate does not exceed 2nm. A large-area chemical vapor deposition (CVD) grown three-layer graphene was transferred to the polished sample surface using a dry transfer method. Specifically, polydimethylsiloxane (PDMS) was used as a support layer, and the large-area CVD graphene on a copper substrate was transferred to a SiO2 / Si substrate using a surface energy gradient distribution, covering all channel arrays. Photoresist AZ5241 was spin-coated onto the graphene-coated sample surface, and a graphene functional strip array pattern spanning the channels was developed using UV lithography. The width of the graphene functional strips was 3 μm. The graphene regions outside the functional strips were etched using oxygen plasma, and the remaining photoresist was removed with acetone, forming a graphene functional strip array spanning the channels. The length of the graphene functional strips was greater than the channel width, with the two ends of the strips located on opposite sides of the channels, and the strip width not exceeding the channel length. AZ5214 photoresist was spin-coated onto the sample surface, and the source / drain electrode array pattern was developed using ultraviolet lithography, ensuring that each group of source / drain electrodes was in contact with both ends of the graphene functional strips. 5nm chromium and 50nm gold were sequentially deposited onto the sample surface using electron beam evaporation, and the photoresist and metal outside the source / drain electrodes were removed using acetone via a lift-off process, forming the source / drain electrode array. The source / drain electrode thickness was 56nm. Ammonium chloride and the aforementioned sample were placed upstream and downstream of a vacuum tube furnace, respectively. Argon gas at a flow rate of 100 sccm was introduced into the furnace as a carrier gas and to provide a protective atmosphere. The upstream ammonium chloride decomposed at 300°C, releasing hydrogen chloride gas, which was carried to the surface of the downstream sample by the carrier gas. The downstream sample underwent a gas-solid reaction with the vanadium dioxide sacrificial layer at 400°C. The gaseous products generated by the reaction were discharged with the carrier gas, completely removing vanadium dioxide from the channel and releasing graphene above the channel, thus fabricating a suspended graphene microheater array. S2 uses Raman spectroscopy to test the suspended region of the prepared suspended graphene microheater array to calibrate its heating capacity. S3 employs magnetron sputtering deposition technology, using a mask to deposit a 30nm thick layer of antimony selenide as a phase change material on the sample surface, positioning the antimony selenide on the upper surface of the suspended region of the graphene microheater. There is no intermediate buffer layer between the antimony selenide and the suspended graphene; they directly contact each other to form a van der Waals heterojunction interface. During the deposition process, the mask blocks the electrode area and the outer region of the channel, ensuring that the phase change material is deposited only on the upper surface of the suspended region of the graphene. In step S4, the sample is placed in the optical path system, and a voltage is applied to the suspended graphene microheater unit through contact between the probe and the source / drain electrodes. Based on the quantitative correspondence between the applied voltage and the lattice temperature of the suspended graphene calibrated in step S2, a set voltage corresponding to the crystallization temperature of antimony selenide is applied. Under Joule heating, the temperature of the suspended graphene rises above the crystallization temperature of antimony selenide, driving antimony selenide to transform from an amorphous state to a crystalline state, resulting in a significant change in its optical refractive index. A reversible phase transition of antimony selenide can be achieved by applying pulse voltages with amplitudes corresponding to the crystallization temperature (200℃) and the amorphization temperature (600℃) calibrated in step S2. Specifically, the pulse width for crystallizing antimony selenide is set to 10 μs, and the pulse width for amorphizing antimony selenide is set to 1 μs. By applying different pulses to the suspended graphene microheater unit, the transformation of antimony selenide from crystalline to amorphous and back to crystalline is achieved. The difference in optical properties between the amorphous and crystalline states of antimony selenide is utilized to modulate the amplitude, phase, or polarization of the transmitted light.

[0059] The process for calibrating the heating capacity of the suspended graphene microheater array described in S2 includes: S21 sets the initial ambient temperature to 23℃ (296.15K) and measures the peak position values ​​corresponding to the characteristic peaks G and 2D of the Raman spectrum of the graphene suspended region at this temperature. S22 raised the ambient temperature to 300K, 350K, 400K, 425K, 450K, 475K and 500K respectively, and measured the peak position values ​​of the G peak and 2D peak of the graphene suspended region at each temperature; S23 calculates the shift of the characteristic peak positions of graphene Raman spectra as a function of ambient temperature, and performs linear fitting on the relationship between peak position shift and ambient temperature change to obtain the ambient temperature coefficients of graphene corresponding to the G peak and 2D peak, respectively. and The unit is ; S24 measured the Raman spectral peak positions of the graphene suspended region when no voltage was applied at a set ambient temperature of 23℃. S25 applies a set voltage within the range of 0V to 15V to the suspended graphene microheater, with a step size of 0.5V, and measures the Raman spectral peak positions of the suspended graphene region at each voltage. S26 Calculates the shift of the characteristic peak positions of graphene Raman spectra at various set voltages compared to when no voltage is applied. According to the formula The lattice temperature of the graphene suspended region was calculated under each set voltage, where, It is the shift in the peak position of the characteristic peak in the graphene Raman spectrum under a set voltage compared to when no voltage is applied. It is the ambient temperature coefficient of graphene. This is the ambient temperature of graphene when no electricity is applied. This establishes a quantitative correlation between the applied voltage and the lattice temperature of the suspended graphene.

[0060] Example 2 The only difference between Example 2 and Example 1 is that in S1 of Example 2, the large-area chemical vapor deposition-grown monolayer graphene is transferred to the polished sample surface using a dry transfer method. That is, S1 of Example 2 prepares a monolayer suspended graphene microheater array.

[0061] Performance testing: Figure 2 The graph shows the modulation speed test results of the suspended graphene heater in Example 1. This test was conducted during the electric drive modulation process in S4 of Example 1, at which point the phase change material layer in S3 had been integrated with the suspended graphene to form a complete modulator structure.

[0062] Figure 2 The study shows that the fabricated suspended graphene heater, under conditions of 3V voltage and 90MHz frequency, has a modulation pulse width of only a few nanoseconds, exhibiting extremely fast response speed. For common microheaters, including metal microheaters and silicon-doped heaters, the modulation pulse width is typically in the tens to hundreds of microseconds. The nanosecond-level response time of the suspended graphene microheater not only means that the modulator can have a faster modulation speed but also that the modulator may have lower power consumption.

[0063] Figure 3 The graph shows the lattice temperature test results of the monolayer suspended graphene under different applied voltages in Example 2. Figure 3 The G and 2D peaks are the Raman G peak and 2D peak, respectively. This data was obtained through Raman spectroscopy calibration of S2 in Example 2. In the figure, the horizontal axis represents the electric field (V / μm), and the vertical axis represents the lattice temperature (K) of the suspended graphene.

[0064] Figure 3 The figure shows that the lattice temperature of monolayer suspended graphene exceeds 1200K under an applied electric field of 0.536V / μm. Conventional substrate-supported graphene microheaters, due to significant heat loss through the substrate in the out-of-plane direction, typically experience a temperature rise of no more than 500K under the same electric field strength. The temperature difference directly reflects the effect of the suspended structure on cutting off the heat dissipation channel of the substrate; Joule heat is confined within the graphene plane and cannot be conducted to the substrate, resulting in a much greater temperature increase with voltage compared to substrate-supported cases. This wide temperature range exceeding 1200K means that the suspended graphene microheater of this invention can not only drive phase change materials with low crystallization temperatures but also cover phase change material systems with high crystallization temperatures, exhibiting broad material compatibility. The figure also shows the temperature data obtained by fitting the Raman G peak and 2D peak, respectively; the results are consistent, verifying the reliability of the Raman calibration method.

[0065] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0066] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the appended claims.

[0067] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A modulator for controlling the optical properties of phase change materials based on suspended graphene, characterized in that, include: A substrate, wherein the upper surface of the substrate is provided with a channel array; A suspended graphene microheater array includes graphene functional strips spanning each channel in the channel array. The middle part of the graphene functional strip is suspended above the channel array to form a suspended region, and the two ends of the graphene functional strip are respectively disposed on the upper surface of the substrate on both sides of the channel. The source / drain electrode array includes multiple sets of source / drain electrodes, each set of source / drain electrodes being in contact with both ends of the corresponding graphene functional strip; A phase change material layer is disposed on the upper surface of the suspended region of the graphene functional strip. The phase change material layer undergoes a crystal phase transformation under thermal drive to change its optical properties.

2. The modulator based on suspended graphene to regulate the optical properties of phase change materials according to claim 1, characterized in that, The modulator based on suspended graphene to regulate the optical properties of phase change materials is prepared through the following steps: S1 etches a channel array on the upper surface of the substrate, fills the channel array with a sacrificial layer, and performs planarization on the substrate surface so that the sacrificial layer is flush with the surface of the substrate to form a continuous plane. Graphene is transferred onto the continuous plane and etched to form graphene functional strips spanning the channel array. The two ends of the graphene functional strips overlap the substrate surfaces on both sides of the channel array, and the middle region is attached to the surface of the sacrificial layer. Source and drain electrodes are fabricated at both ends of the graphene functional strips. The sacrificial layer in the channel array is removed by reacting it with high-temperature gas, so that the part of the graphene functional strip located above the channel array is suspended, thus obtaining a suspended graphene micro heater array. S2 performs Raman spectroscopy tests on the suspended region of the prepared suspended graphene microheater array. By measuring the peak position shift of the graphene Raman characteristic peaks under different ambient temperatures and different applied voltages, the heating capacity of the suspended graphene microheater array is calibrated, and a quantitative correspondence between the applied voltage and the temperature of the suspended graphene lattice is established. S3 is the deposition of a phase change material layer on the upper surface of the graphene corresponding to the suspended region of the suspended graphene microheater array. Based on the quantitative correspondence established in S2, S4 applies a set voltage to the suspended graphene microheater array that matches the crystallization temperature of the phase change material layer, and uses graphene Joule heating to drive the phase change material layer to undergo a crystal phase transition, thereby changing its optical properties and achieving light modulation.

3. A modulator based on suspended graphene to regulate the optical properties of phase change materials according to claim 2, characterized in that, The high-temperature gas mentioned in S1 is hydrogen chloride gas, and the sacrificial layer is a vanadium dioxide layer.

4. A modulator based on suspended graphene to regulate the optical properties of phase change materials according to claim 2, characterized in that, The etching depth of the channel array in S1 is 0.1μm to 1μm, and the width of the channel in the channel array is 1μm to 10μm.

5. A modulator based on suspended graphene to regulate the optical properties of phase change materials according to claim 2, characterized in that, The length of the graphene functional strip in S1 is greater than the width of the channel in the channel array, the two ends of the graphene functional strip are located on both sides of the same channel, and the width of the graphene functional strip does not exceed the channel length.

6. A modulator based on suspended graphene to regulate the optical properties of phase change materials according to claim 2, characterized in that, The process for calibrating the heating capacity of the suspended graphene microheater array described in S2 includes: S21 sets the initial ambient temperature and measures the Raman spectral peak position of the graphene suspended region at the current ambient temperature; S22 Updates the current ambient temperature to a value greater than the initial ambient temperature, and measures the Raman spectral peak positions of the graphene suspended region under different ambient temperatures; S23 calculates the shift of the characteristic peak position of graphene Raman spectrum as a function of ambient temperature, and performs a linear fit on the relationship between the shift and ambient temperature change to obtain the ambient temperature coefficient of graphene. , The unit is ; S24 measures the Raman spectral peak positions of the graphene suspended region under a set ambient temperature without applied voltage; S25 applies a set voltage to the suspended graphene microheater and measures the Raman spectral peak position of the suspended graphene region under the set voltage. S26 Calculates the shift of the characteristic peak position of graphene Raman spectrum under a set voltage compared to when no voltage is applied. According to the formula The lattice temperature of the graphene suspended region under a set voltage was calculated, where Δω is the shift of the characteristic peak position of the graphene Raman spectrum under the set voltage compared to when no voltage is applied. It is the ambient temperature coefficient of graphene. This refers to the ambient temperature of graphene when no electricity is applied. and All figures are in Kelvin.

7. A modulator based on suspended graphene to regulate the optical properties of phase change materials according to claim 2, characterized in that, The phase change material layer mentioned in S3 is an antimony selenide layer, and the thickness of the phase change material layer is 15nm~30nm.

8. A modulator based on suspended graphene to regulate the optical properties of phase change materials according to claim 2, characterized in that, The set voltage mentioned in S4 is the calibration voltage of the suspended graphene microheater array corresponding to the crystallization temperature of the phase change material layer.

9. A modulator based on suspended graphene to regulate the optical properties of phase change materials according to claim 2, characterized in that, The set voltage applied to the suspended graphene microheater array in S4 is a pulse voltage.

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