A threshold transition device based on two-dimensional ionic conductor and its preparation method

Abstract

The present invention belongs to the technical field of resistive memory devices, specifically a threshold shift device based on a two-dimensional ionic conductor and its preparation method. The device comprises, from top to bottom, a substrate, an oxide layer, a bottom electrode, a two-dimensional ionic conductor functional layer, few-layer graphene, and a top electrode. The oxide layer is SiO2, the bottom electrode is Cr and Cu electrodes, the two-dimensional ionic conductor functional layer is a copper-based phosphosulfate, and the top electrode is Cr and Au electrodes. The present invention introduces a copper electrode to form a van der Waals heterojunction with the copper-based phosphosulfate and graphene. The depletion and accumulation of copper ions in the copper-based phosphosulfate are controlled by applying a voltage, thereby achieving stable threshold resistive switching behavior. The introduction of the copper electrode increases the copper ion concentration gradient, allowing the functional layer to recover from a low-resistance state to a high-resistance state through copper ion diffusion when the voltage decreases. This threshold shift device exhibits excellent stability and fatigue resistance, a high on / off ratio, and an ultra-small subthreshold swing.

Description

Technical Field

[0001] The present invention belongs to the technical field of resistive memory devices, and in particular relates to a threshold transition device and a preparation method thereof. Background Art

[0002] The rise of artificial intelligence and the Internet of Things (IoT) has created an urgent need for large-capacity data storage. With the advancement of storage technology, resistive random access memory (RRAM) has become a highly competitive memory device due to its advantages, including fast read / write speeds, simple device structure, and high integration density. RRAM typically utilizes a crossbar structure to increase storage density. While this structure is easy to manufacture, it also easily introduces current paths, leading to crosstalk, which affects data readout accuracy and increases device power consumption. To address this crosstalk issue, the "1S1R" structure has been proposed. This structure connects a selector in series with the RRAM to prevent current paths. The commonly used selector is a resistive threshold-shift device (RTD), which enters a low-resistance state at a certain voltage and returns to a high-resistance state when the voltage decreases. Currently, a variety of materials are used as RRAMs, including simple metal oxides, chalcogenides, and perovskite oxides. The resistive switching mechanism of these devices relies on the formation of conductive filaments within the material, which form electronic conduction channels. The formation and annihilation of these filaments then result in a resistive switching transition. However, the formation of conductive filaments is often random, which reduces the device's operational stability. These devices also suffer from disadvantages such as high threshold voltage, poor durability, and high processing temperatures, which to some extent limit their application in devices. [Small Struct. 2021, 2, 2000109; Nat. Electron. 2018, 1, 274; Phys. Lett. B 2020, 34, 2050115].

[0003] In recent years, the rise of two-dimensional materials has provided more options for constructing novel threshold-shift devices. Metal phosphosulfide materials, such as copper indium phosphosulfide, are a new type of two-dimensional layered material. Due to the instability of the metal cations within their lattice, they are easily displaced under the action of an electric field, resulting in ferroelectricity or antiferroelectricity, and even ionic conductivity under large electric fields. This ion migration property makes metal phosphosulfide promising for use in resistive memory devices. Previous research results have also shown that the migration of copper ions in materials such as copper indium phosphosulfide can be controlled by external field stimulation, resulting in resistor devices with rectifying properties. However, because copper ion migration is extremely sensitive to electric field conditions and is easily affected by factors such as the electric field scanning speed, direction, and application history, achieving stable resistance state control remains difficult [ACS Nano 2022, 16, 15347; Nat. Commun. 2022, 13, 574; Nano Lett. 2021, 21, 995]. Therefore, in order to improve the stability of the device and make it possible to apply it to resistive random access memory, efforts need to be made in the device structure design.

[0004] In order to solve the above problems, the present invention proposes a method for preparing a novel resistive switching device based on a two-dimensional ionic conductor. The method uses a metal copper electrode, a few-layer two-dimensional copper-based phosphosulfate, and graphene to form a heterojunction, wherein copper and few-layer graphene serve as the bottom electrode and top electrode, respectively. The copper electrode of the device is grounded, and a positive scanning voltage is applied to the top electrode. Due to the low intrinsic conductivity of the few-layer copper-based phosphosulfate material, the initial state of the device is a high-resistance state; when the applied voltage is greater than the threshold voltage, the copper ions inside the two-dimensional copper-based phosphosulfate are excited and drift under the action of the electric field. When the copper ions inside the functional layer material are depleted, the microstructure of the copper-based phosphosulfate undergoes a transformation, and a transition occurs from an insulating state to a conductive state, so that the device transitions from the high-resistance threshold to a low-resistance state. When the applied voltage decreases to a certain value, the drift motion of the copper ions weakens. At the same time, because the copper ion concentration near the copper electrode interface is much higher than that inside the material, the diffusion motion of the copper ions from the interface to the interior becomes dominant. As the copper ions are injected into the functional material, the material undergoes a transition from a conductive state to an insulating state, that is, from a low-resistance state back to a high-resistance state. In this process, the introduction of the copper electrode increases the copper ion concentration difference between the interface and the interior, thereby enhancing the diffusion motion. The device prepared by this method exhibits excellent threshold transition characteristics, including stable threshold transition and recovery process, low operating voltage, high on-off ratio, and good fatigue resistance. Summary of the Invention

[0005] The purpose of the present invention is to provide a threshold transition device based on a two-dimensional ionic conductor with excellent threshold transition performance and a preparation method thereof.

[0006] The threshold transition device based on a two-dimensional ionic conductor, provided by the present invention, forms a van der Waals heterojunction by introducing a copper electrode, a two-dimensional ionic conductor copper-based phosphosulfate, and few-layer graphene. The device utilizes the drift and diffusion of copper ions under an applied bias to achieve depletion and injection of copper ions within the functional layer, thereby enabling the device to achieve threshold transition characteristics between high and low resistance states. This threshold transition behavior, achieved through ion migration, exhibits excellent stability, fatigue resistance, and ultra-small subthreshold swing.

[0007] The threshold transition device based on a two-dimensional ionic conductor provided by the present invention has a structure from bottom to top of: substrate, oxide layer, bottom electrode, two-dimensional ionic conductor functional layer, few-layer graphene, and top electrode; wherein:

[0008] The substrate is a heavily doped p-type Si substrate with a thickness of 0.5 mm;

[0009] The oxide layer is SiO2 with a thickness of 285±15 nanometers;

[0010] The bottom electrode is a Cr and Cu electrode, the thickness of Cr is 5-10 nanometers, and the thickness of Cu is 30-40 nanometers;

[0011] The two-dimensional ionic conductor functional layer is copper-based phosphosulfate with a thickness of 10-15 nanometers;

[0012] The thickness of the few-layer graphene is 8-15 nanometers;

[0013] The top electrode is a Cr and Au electrode, the thickness of Cr is 5-10 nanometers, and the thickness of Au is 50-60 nanometers.

[0014] The present invention provides a method for preparing a threshold transition device based on a two-dimensional ionic conductor, comprising the following specific steps:

[0015] (1) Preparation of oxide layer

[0016] An oxide layer of silicon dioxide with a thickness of 285 ± 15 nm was prepared on a heavily doped Si substrate by thermal oxidation;

[0017] (2) Cleaning of substrate

[0018] The substrate containing the oxide layer is ultrasonically cleaned in acetone, isopropyl alcohol, and deionized water for 14-20 minutes in sequence, and then cleaned in a UV-ozone cleaner for 100-130 seconds.

[0019] (3) Preparation of bottom electrode

[0020] Cr / Cu electrodes were prepared on the oxide layer surface by using a coating and electron beam exposure process combined with thermal evaporation and lift-off.

[0021] (4) Preparation and transfer of two-dimensional ionic conductor copper-based phosphosulfate

[0022] The copper-based phosphosulfate crystals were cleaved into thin layers by mechanical exfoliation, and a uniform few-layer copper-based phosphosulfate was transferred to the bottom electrode by wet transfer technology.

[0023] (5) Preparation and transfer of few-layer graphene

[0024] Transferring few-layer graphene onto a two-dimensional ionic conductor functional layer through mechanical exfoliation and dry transfer technology;

[0025] (6) Preparation of top electrode

[0026] Cr / Au electrodes were prepared on the oxide layer surface using a combination of coating and electron beam exposure, thermal evaporation, and lift-off processes to form a sandwich-structured threshold transition device based on ionic conductors.

[0027] (7) Post-processing of devices

[0028] The prepared device is placed in a rapid annealing furnace in a nitrogen atmosphere and annealed for 1.5-2.5 hours to make the direct van der Waals contact of the materials more sufficient to improve the performance.

[0029] The threshold transition device of the present invention has the following threshold transition mechanism:

[0030] The copper electrode is grounded, and a forward voltage sweep is applied to the top electrode. When the voltage exceeds the threshold, the cations in the ionic conductor migrate toward the copper electrode, causing a structural phase transition within the device. This transition causes the ionic conductor to transition from an insulating state to a conductive state, and the device to transition from a high-resistance state to a low-resistance state. At this point, a current limit is required to protect the device from damage. When the voltage decreases to a certain value, the high cation concentration at the bottom electrode interface and the low voltage are insufficient to maintain ion drift motion. Consequently, the cations diffuse in the opposite direction and are injected into the ionic conductor, restoring the insulating state and returning the device from a low-resistance state to a high-resistance state.

[0031] The advantages of the present invention are:

[0032] Based on a two-dimensional ionic conductor, copper-based phosphosulfate, a van der Waals heterojunction is formed with a copper electrode, copper-based phosphosulfate, and graphene. Voltage-controlled accumulation and depletion of copper ions enable a novel threshold transition device, avoiding the device instability caused by the random formation of conductive filaments in traditional devices. The introduction of the copper electrode also increases the concentration gradient of copper ions, allowing them to diffuse at low voltages, achieving a transition from a low-resistance state to a high-resistance state and improving device cycling stability. The device also exhibits excellent fatigue resistance, a low operating voltage, and an ultra-small subthreshold swing. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Schematic diagram of the structure and working state of the threshold transition device based on two-dimensional ionic conductor.

[0034] Figure 2 The current-voltage characteristic diagram of the device under forward voltage. The arrow in the figure shows the hysteresis direction.

[0035] Figure 3 Schematic diagram of the device under operating voltage. (a) shows the device transition from high-resistance to low-resistance state, and (b) shows the device transition from low-resistance to high-resistance state.

[0036] Numbers in the figure: 1 is the substrate, 2 is the oxide layer, 3 is the bottom electrode, 4 is the two-dimensional ion conductor, 5 is the few-layer graphene, and 6 is the top electrode. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the embodiments and drawings.

[0038] The novel threshold-shift device developed by this invention, based on a two-dimensional ionic conductor, forms a van der Waals heterojunction with a copper electrode and the ionic conductor copper indium phosphide (CIPS), which is then stacked with few-layer graphene to form a sandwich structure. The device stably switches between high-resistance and low-resistance states by utilizing the migration of copper ions within the CIPS under the influence of an electric field and the diffusion caused by excessive copper ion concentration on one side of the copper electrode. This results in excellent stability, fatigue resistance, and a high on / off ratio, among other superior properties.

[0039] The specific steps for live streaming are as follows:

[0040] 1. Substrate selection

[0041] A heavily doped p-type silicon substrate with a thickness of 0.5 mm was selected as the substrate.

[0042] 2. Preparation of Oxide Layer

[0043] An oxide layer SiO2 with a thickness of 285±15 nanometers is oxidized on the surface of the silicon substrate by a thermal oxidation method, and the other side of the Si substrate is polished.

[0044] 3. Cleaning of substrate and oxide layer

[0045] The substrate containing the oxide layer was ultrasonically cleaned in acetone, isopropyl alcohol, and deionized water for 15 minutes in sequence, and then cleaned in a UV-ozone cleaning machine for 120 seconds.

[0046] 4. Preparation of Bottom Electrode

[0047] A substrate (1×1 cm) containing a curing layer is placed on a coating machine and pumped to hold it in place. A small amount of polymethyl methacrylate (PMMA) solution is then dripped onto the oxide layer using a rubber-tipped dropper. The coating is then uniformly applied at 500 rpm for 6 seconds, then at 4000 rpm for 30 seconds. The coating is then baked at 180°C for 3 minutes. The coated substrate and oxide layer are then exposed to the desired electrode pattern using an electron beam at 300 picoamperes. After exposure, the coating is then placed in a developer for 6 seconds. The developed substrate and oxide layer are then placed in a thermal evaporation system, where 5 nanometers of chromium and 40 nanometers of copper are evaporated onto the substrate at a rate of 1 angstrom per second. After evaporation, the coating is stripped using acetone.

[0048] 5. Preparation and Transfer of Two-Dimensional Ionic Conductors

[0049] Use blue tape to dissociate the bulk copper indium phosphide into thin layers. Transfer them to another clean substrate with the aid of polydimethylsiloxane (PDMS). Using a microscope, locate the desired uniform thin layer of copper indium phosphide (10-20 nanometers thick, appearing dark blue or purple under a microscope). Use a polyvinyl alcohol (PVA) film to scoop out the target sample and transfer it to the copper electrode at a fixed point (heat the substrate to 70°C during the transfer process). After transfer, soak in deionized water for 8 hours to completely dissolve the PVA, then rinse with acetone and blow dry with a nitrogen gun.

[0050] 6. Preparation and transfer of few-layer graphene

[0051] Use blue tape to cleave the graphite flakes into thin layers and then transfer them to PDMS. Use a microscope to find uniform few-layer graphene on the PDMS (graphene with a thickness of 10-20 nanometers appears dark purple under the microscope). Then transfer them to copper indium phosphide in a targeted manner.

[0052] 7. Preparation of Top Electrode

[0053] The process is the same as step 5. Finally, 5 nanometers of chromium and 60 nanometers of gold are evaporated using a thermal evaporation system.

[0054] The copper electrode is grounded and a forward voltage is applied to the top electrode in a back-and-forth scanning manner. The electrical characteristic diagram is shown in the appendix of the manual. Figure 2 It can be seen that when the voltage is greater than a certain threshold, the device changes from a high-resistance state to a low-resistance state, and the current increases by nearly 4 orders of magnitude (the platform is due to current limiting to protect the device from damage). When the voltage decreases to a certain value, the device changes from a low-resistance state to a high-resistance state, and the stability is good after multiple cycles.

[0055] Instructions attached Figure 3The working principle of this threshold-shift device is explained. When the voltage exceeds the threshold, the cations in the ionic conductor migrate toward the copper electrode, causing a structural phase transition within the device, which switches the ionic conductor from an insulating state to a conductive state. The device then switches from a high-resistance state to a low-resistance state (see Figure a). When the voltage decreases to a certain value, the cation concentration on one side is too high and the voltage is too low to sustain ion migration. As a result, the cations diffuse in the opposite direction, returning the ionic conductor to an insulating state. The device then switches from a low-resistance state to a high-resistance state (see Figure b).

Claims

1. A threshold transition device based on a two-dimensional ionic conductor, characterized in that: The structure from bottom to top is substrate, oxide layer, bottom electrode, two-dimensional ion conductor functional layer, few-layer graphene, and top electrode; among them: The substrate is a heavily doped p-type Si substrate; The oxide layer is SiO2 with a thickness of 285±15 nanometers; The bottom electrode is a Cr and Cu electrode, the thickness of Cr is 5-10 nanometers, and the thickness of Cu is 30-40 nanometers; The two-dimensional ionic conductor functional layer is copper-based phosphosulfate with a thickness of 10-15 nanometers; The thickness of the few-layer graphene is 8-15 nanometers; The top electrode is a Cr and Au electrode, the thickness of Cr is 5-10 nanometers, and the thickness of Au is 50-60 nanometers.

2. The method for preparing a threshold transition device according to claim 1, wherein: The specific steps are: (1) Preparation of oxide layer The oxide layer of silicon dioxide was prepared by thermal oxidation on a heavily doped Si substrate with a thickness of 285 ± 15 nm; (2) Cleaning of substrate The substrate containing the oxide layer is ultrasonically cleaned in acetone, isopropyl alcohol, and deionized water for 14-20 minutes in sequence, and then cleaned in a UV-ozone cleaner for 100-130 seconds. (3) Preparation of bottom electrode Cr / Cu electrodes were prepared on the oxide layer surface by using a coating and electron beam exposure process combined with thermal evaporation and lift-off. (4) Preparation and transfer of two-dimensional ionic conductor copper-based phosphosulfate The copper-based phosphosulfate crystals were cleaved into thin layers by mechanical exfoliation, and a uniform few-layer copper-based phosphosulfate was transferred to the bottom electrode by wet transfer technology. (5) Preparation and transfer of few-layer graphene Transferring few-layer graphene onto a two-dimensional ionic conductor functional layer through mechanical exfoliation and dry transfer technology; (6) Preparation of top electrode Cr / Au electrodes were prepared on the surface of the oxide layer using a coating and electron beam exposure process combined with thermal evaporation and lift-off to form a sandwich-structured threshold transition device based on ionic conductors. (7) Post-processing of devices The prepared device was placed in a rapid annealing furnace in a nitrogen atmosphere and annealed for 1.5-2.5 hours.

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