Optical synapse device based on optical phase change material and adjusting method, optical neural network

Abstract

The application discloses an optical synapse device based on an optical phase change material, a regulation method and an optical neural network, and belongs to the technical field of optical neural networks.The optical synapse device comprises a substrate, a waveguide, a phase change material layer, a heating layer, a covering layer and an electrode acting on the heating layer, which are sequentially stacked on the substrate, and the electrode is used for applying an electric signal to the heating layer to control the temperature of the heating layer; when light is transmitted in the waveguide, the phase change material realizes phase state modulation under the temperature control of the heating layer to control the output of the light; characterized in that the phase change material of the phase change material layer is nitrogen-doped Ge2Sb2Te5 phase change material, and the structural general formula is N x (Ge2Sb2Te5) 1‑x It is found through experiments that, compared with the traditional Ge2Sb2Te5 phase change material without nitrogen doping, the extinction coefficient of the Ge2Sb2Te5 phase change material after nitrogen doping is reduced.

Description

Technical Field

[0001] This invention belongs to the field of optical neural network technology, and more specifically, relates to an optical synaptic device and its adjustment method based on optical phase change materials, and an optical neural network. Background Technology

[0002] Neuromorphic computing systems, with their highly parallel and in-memory computing-integrated data processing capabilities that mimic the human brain, excel in various intelligent information processing environments and are currently a research hotspot in the field of high-performance computing. Among them, optical neuromorphic computing systems, with their advantages of high speed, parallelism, low crosstalk, low power consumption, and high interconnect bandwidth, as well as the rapid development of integrated optoelectronics in recent years, have become a new breakthrough in the field of high-performance intelligent computing.

[0003] Currently, traditional photonic chips primarily use silicon-based materials as substrates to construct optical components. This process is compatible with CMOS technology, facilitating device integration. Silicon-based integrated devices mainly achieve active modulation by altering the properties of silicon materials through thermo-optical effects or carrier dispersion effects. However, the response time of thermo-optical effects is relatively slow, typically on the order of milliseconds; while carrier dispersion effects have a fast response time, their refractive index modulation range is limited, usually on the order of 10⁻³, resulting in poor phase modulation. In recent years, chalcogenide phase change materials have gradually attracted attention in the field of integrated photonics, yielding some results. However, the currently used phase change material is mainly Ge₂Sb₂Te₅ (GST). Although it exhibits excellent optical performance, its high optical absorption and extinction coefficient in the communication band limit its further large-scale application. Developing an optical phase change material with a low extinction coefficient based on existing phase change material systems remains a challenge in the current technological field. Summary of the Invention

[0004] In view of the above-mentioned defects or improvement needs of the prior art, the present invention provides an optical synaptic device and modulation method based on optical phase change material, and an optical neural network, the purpose of which is to reduce the extinction coefficient of phase change material and further improve the modulation performance of optical synaptic device.

[0005] To achieve the above objectives, according to one aspect of the present invention, an optical synaptic device based on an optical phase change material is provided, comprising a substrate, a waveguide, a phase change material layer, a heating layer, a capping layer, and an electrode acting on the heating layer, wherein the electrode is used to apply an electrical signal to the heating layer to control the temperature of the heating layer; when light propagates in the waveguide, the phase change material achieves phase modulation under the temperature control of the heating layer to control the light output; the phase change material of the phase change material layer is a nitrogen-doped Ge2Sb2Te5 phase change material with the general structural formula N... x (Ge2Sb2Te5) 1-x .

[0006] In one embodiment, the value of x is in the range 0 < x ≤ 0.1.

[0007] In one embodiment, x ≥ 0.06.

[0008] In one embodiment, x = 0.08.

[0009] In one embodiment, the waveguide has a width of 450nm to 550nm and a thickness of 200nm to 240nm, and the phase change material layer has a thickness of 20nm to 40nm and a length along the waveguide direction of 2nm to 5µm.

[0010] In one embodiment, the phase change material layer is rectangular, and the photosynaptic device based on the optical phase change material performs unidirectional modulation.

[0011] In one embodiment, the phase change material layer is a plurality of discretely distributed strips, and the photosynaptic device based on the optical phase change material performs bidirectional modulation.

[0012] In one embodiment, the heating layer is a transparent conductive oxide, the electrode material is a multilayer electrode formed by stacking two of the following metals: gold, titanium, platinum, and chromium, and the covering layer is silicon dioxide.

[0013] According to another aspect of the present invention, a method for adjusting the extinction coefficient of an optical synaptic device based on an optical phase change material is provided. The optical synaptic device based on the optical phase change material is the aforementioned optical synaptic device based on an optical phase change material. The method for adjusting the extinction coefficient includes: adjusting the extinction coefficient of the phase change material by controlling the content of nitrogen.

[0014] According to another aspect of the present invention, an optical neural network is provided, comprising optical neurons and optical synaptic devices, wherein the optical synaptic devices are the aforementioned optical synaptic devices based on optical phase change materials.

[0015] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0016] This invention involves nitrogen doping of the traditional phase change material Ge2Sb2Te5. Experiments revealed that, compared to the undoped Ge2Sb2Te5, the extinction coefficient of the nitrogen-doped Ge2Sb2Te5 phase change material is reduced. This reduced extinction coefficient improves the on / off ratio of the photosynaptic device and allows for the acquisition of more state modulations. More state modulations result in greater programmability of the synaptic weights, leading to stronger tunability of the optical neural network utilizing this photosynapse.

[0017] Furthermore, the experiment also found that when the nitrogen doping content exceeds a certain value, the change in its extinction coefficient is not obvious. However, excessive nitrogen content will cause changes in the phase transition properties of Ge2Sb2Te5 phase transition material, resulting in reduced device performance or even failure. Therefore, limiting x ≤ 0.1 can avoid the problem of excessive nitrogen content.

[0018] Furthermore, the experiment also found that when the nitrogen doping content is too low, its effect on the extinction coefficient is not significant. Therefore, limiting x to ≥ 0.06 can achieve a relatively significant improvement in the extinction coefficient. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of an embodiment of an optical synaptic device;

[0020] Figure 2 This is a schematic diagram of bidirectional modulation according to one embodiment;

[0021] Figure 3 This is a comparison chart of the extinction coefficients of optical phase change materials in one embodiment;

[0022] Figure 4 This is an SEM image of an embodiment of an optical synaptic device;

[0023] Figure 5 A device switching performance diagram of an embodiment of an optical synaptic device;

[0024] Figure 6 A diagram illustrating the synaptic programmability of an embodiment of an optical synaptic device;

[0025] Figure 7 This is a comparison chart showing the effect of different nitrogen contents on the extinction coefficient in one embodiment. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0027] like Figure 1The diagram shows a schematic of an optical synaptic device in one embodiment. It includes a substrate, a waveguide, a phase change material layer, a heating layer, a capping layer, and electrodes acting on the heating layer, all sequentially stacked on the substrate. The electrodes apply electrical pulse signals to the heating layer to regulate its temperature. The heating layer is in contact with the phase change material layer, and the heating layer controls the temperature of the phase change material layer through thermal conduction, thereby changing the phase state of the phase change material. When light propagates in the waveguide, the light output power can be controlled by adjusting the phase state of the phase change material.

[0028] For example, initially, the phase change material is amorphous, and the phase change material layer is in a fully open state, resulting in the highest light output power. As the temperature rises, the phase change material gradually changes from amorphous to crystalline, and the phase change material layer is in a fully closed state, resulting in the lowest light output power. Continued heating and rapid cooling cause the phase change material to gradually change back from crystalline to amorphous. Simultaneously, as... Figure 2 As shown, during the transition from amorphous to crystalline state and vice versa, multiple intermediate states exist, each corresponding to different light output powers. These different light output powers enable different state modulations of the photosynapse. The more state modulations a photosynapse can achieve, the higher its programmability and flexibility. It's understandable that training a neural network essentially determines the weights of structures like synapses, and the weights of photosynapses correspond to their light output powers; that is, the weights of photosynapses correspond to their state modulations. The more modulated states a photosynapse can have, the more adjustable weights can be selected, resulting in stronger programmability and greater flexibility.

[0029] In this invention, the phase change material of the phase change material layer is a nitrogen-doped Ge2Sb2Te5 phase change material with the general structural formula N. x (Ge2Sb2Te5) 1-x The research team discovered in experiments that by doping nitrogen into the traditional Ge2Sb2Te5 phase change material, the extinction performance of photosynapses can be improved, the extinction coefficient can be reduced, and thus the optimal optical quality factor can be obtained. Adjusting the nitrogen content results in different degrees of reduction in the extinction coefficient; the appropriate nitrogen content can be selected based on the overall device design and performance requirements. A lower extinction coefficient increases the on / off ratio of the photosynaptic device, increases the number of modulotable states, and enhances programmability. Traditional techniques typically increase the on / off ratio by simply adjusting the refractive index of the device. This invention, however, increases the on / off ratio by reducing the extinction coefficient, specifically through nitrogen doping of the Ge2Sb2Te5 phase change material. The method is simple, easy to operate, and yields good results.

[0030] like Figure 3The figure shows a comparison of the extinction coefficients of optical phase change materials in one embodiment, where GST is an abbreviation for Ge2Sb2Te5. As can be seen from the figure, N doping reduces the extinction coefficient in both crystalline and amorphous states. A lower extinction coefficient increases the on / off ratio of the device, allowing it to have more modulated states.

[0031] Figure 4 This is an SEM image of a photosynaptic device in one embodiment. It can be seen that the device fabrication process is excellent and meets the expected structural design.

[0032] Figure 5 This is a device switching performance diagram of an embodiment of an optical synaptic device. The device exhibits very good reversible switching with an on / off ratio greater than 20dB, which is higher than the performance of similar devices reported in the prior art.

[0033] Figure 6 This is a synaptic performance diagram of an optical synaptic device in one embodiment. By applying an electrical pulse excitation to the device, 37 states can be modulated, which is higher than the maximum value reported previously (32 states).

[0034] In one embodiment, x is limited to ≤ 0.1, meaning that the nitrogen content should not be too high. If it exceeds 0.1, on the one hand, as the nitrogen content increases, the improvement in its extinction performance becomes less significant. On the other hand, since nitrogen is only a dopant element, if the content is too high, it will also cause the original phase change characteristics of the Ge2Sb2Te5 phase change material to change or even lose its phase change characteristics.

[0035] In one embodiment, x is limited to ≥ 0.06, meaning that the nitrogen content should not be too low. If the content is below 0.06, nitrogen doping has little effect on the extinction coefficient. Experiments show that when x ≥ 0.06, the extinction coefficient is significantly improved.

[0036] In one specific embodiment, the Ge2Sb2Te5 phase change material with a doping content of x = 0.08 exhibits a better extinction coefficient.

[0037] In one embodiment, the substrate can be an SOI substrate. Specifically, the SOI substrate structure, from top to bottom, consists of a 220nm top silicon layer, a 3µm silicon dioxide embedded layer, and a 700µm silicon substrate. The heating layer is a transparent conductive oxide with an ITO (9:1) thin film composition. Metal electrodes are deposited on both sides of the heating layer, and the electrode material is a multilayer electrode formed by stacking two of the following metals: gold, titanium, platinum, and chromium. The cladding layer is SiO2 with a film thickness of 50-100nm.

[0038] The waveguide has a width of 450nm to 550nm and a thickness of 200nm to 240nm.

[0039] In one embodiment, device performance is optimized by controlling the thickness, length, and shape of the optical phase change material above the waveguide, including:

[0040] When the control phase change material is rectangular in shape, with a film thickness of 20-40 nm and a length of 2-5 μm along the waveguide direction, the device can exhibit a high on / off ratio and a large number of modulation states, but it can only be unidirectionally modulated.

[0041] When the phase change material is controlled to have a discrete strip distribution, a film thickness of 20-40 nm, and a total length of 2-5 μm along the waveguide direction, more refined state modulation can be obtained, and the device state can be modulated bidirectionally.

[0042] Accordingly, the present invention also relates to a method for adjusting the extinction coefficient of a photosynaptic device, wherein the photosynaptic device is the aforementioned photosynaptic device, and the method for adjusting the extinction coefficient includes: adjusting the extinction coefficient of the phase change material by controlling the nitrogen content. Generally speaking, the higher the nitrogen content, the smaller the extinction coefficient; however, considering the overall performance of the device, the nitrogen content should not be too high to avoid affecting the original performance of the phase change material.

[0043] Accordingly, the present invention also relates to an optical neural network, comprising optical neurons and optical synaptic devices, wherein the optical synaptic device is the aforementioned optical synaptic device. The optical neuron device is a volatile threshold switching device, and the optical synaptic device is a non-volatile and state-tunable switching device; both are core unit devices for constructing neuromorphic computing chips. The aforementioned optical synaptic device, by improving the extinction coefficient and increasing the switching ratio, acquires more state modulations, making the training of the optical neural network more flexible.

[0044] The following is a description using specific embodiments.

[0045] Comparative Example

[0046] 1) Substrate cleaning: The SOI substrate was ultrasonically cleaned for 15 minutes each with deionized water, ethanol and acetone, and then dried with nitrogen gas.

[0047] 2) Fabrication of optical waveguide and grating: The grating and optical waveguide were fabricated on the above substrate using electron beam lithography and ICP etching. The waveguide width was 500nm and the height was 220nm. After the process, excess photoresist was removed by electron beam photoresist remover.

[0048] 3) Preparation of phase change material: A pattern of phase change material is etched onto the waveguide using ultraviolet lithography, and then N phase change material is deposited using magnetron sputtering. x (Ge2Sb2Te5) 1-x x = 0, thickness is 20-40nm, length along waveguide direction is 2-5um, and after completion, excess photoresist is washed away with acetone.

[0049] 4) Preparation of heating layer: The heating layer pattern is etched on the phase change material using ultraviolet lithography, and then an ITO thin film is deposited by magnetron sputtering. After completion, excess photoresist is washed away with acetone.

[0050] 5) Fabrication of metal electrodes: The electrode pattern is etched on both sides of the heating layer using ultraviolet lithography, and then a 100nm Ti / Pt thin film is deposited as the electrode using magnetron sputtering.

[0051] 6) Preparation of the capping layer: The pattern of the capping layer is etched on the heating layer using ultraviolet lithography, and then a 50-100nm SiO2 capping layer is deposited by magnetron sputtering or low-temperature PECVD.

[0052] Example 1

[0053] The preparation method and steps are the same as in Example 1, except that the content of N in the phase change material is controlled to be x = 0.06.

[0054] Example 2

[0055] The preparation method and steps are the same as in Example 1, except that the content of N in the phase change material is controlled to be x = 0.08.

[0056] Example 3

[0057] The preparation method and steps are the same as in Example 1, except that the content of N in the phase change material is controlled to be x = 0.1.

[0058] like Figure 7 The figure shows the effect of different nitrogen contents on the extinction coefficient in the comparative examples and embodiments above. As can be seen from the figure, compared with the undoped unit, doping with N element can reduce the extinction coefficient, and different doping contents have different degrees of influence on the extinction coefficient.

[0059] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements 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 communication band optical synapse device based on optical phase change materials, comprising a substrate, a waveguide, a phase change material layer, a heating layer, a capping layer, and an electrode acting on the heating layer, wherein the electrode is used to apply an electrical signal to the heating layer to control the temperature of the heating layer; when light propagates in the waveguide, the phase change material layer achieves phase modulation under the temperature control of the heating layer to control the light output; the phase change material of the phase change material layer is a nitrogen-doped Ge2Sb2Te5 phase change material with the general structural formula N... x (Ge2Sb2Te5) 1-x The value of x is in the range of 0.06≤x≤0.

1. In the reversible transition process between crystalline and amorphous states of Ge2Sb2Te5 phase change material, there are multiple intermediate states, corresponding to different light output powers, which realize the modulation of different states of photosynapses. Nitrogen doping reduces the extinction coefficient of Ge2Sb2Te5 phase change material, increases the on / off ratio of photosynaptic devices, increases the number of modulated states of the device, increases the number of adjustable weights, and enhances the programmability.

2. The optical synaptic device for communication bands based on optical phase change materials as described in claim 1, characterized in that, x=0.08。 3. The optical synaptic device for communication bands based on optical phase change materials as described in claim 1, characterized in that, The waveguide has a width of 450nm~550nm and a thickness of 200nm~240nm, and the phase change material layer has a thickness of 20nm~40nm and a length of 2~5μm along the waveguide direction.

4. The optical synaptic device for communication bands based on optical phase change materials as described in claim 1, characterized in that, The phase change material layer is rectangular, and the photosynaptic device based on the optical phase change material performs unidirectional modulation.

5. The optical synaptic device for communication bands based on optical phase change materials as described in claim 1, characterized in that, The phase change material layer consists of multiple discretely distributed strips, and the photosynaptic device based on the optical phase change material performs bidirectional modulation.

6. The optical synaptic device for communication bands based on optical phase change materials as described in claim 1, characterized in that, The heating layer is a transparent conductive oxide, the electrode material is a multilayer electrode formed by stacking two of the following metals: gold, titanium, platinum, and chromium, and the covering layer is silicon dioxide.

7. An optical neural network, comprising optical neurons and optical synaptic devices, characterized in that, The photosynaptic device is the communication band photosynaptic device based on optical phase change material as described in any one of claims 1 to 6.

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