CN116249440A - Threshold value conversion device based on two-dimensional ion conductor and preparation method - Google Patents

Abstract

The invention belongs to the technical field of resistive random access memory devices, and particularly relates to a threshold value conversion device based on a two-dimensional ion conductor and a preparation method thereof. The device sequentially comprises a substrate, an oxide layer, a bottom electrode, a two-dimensional ion conductor functional layer, few-layer graphene and a top electrode from top to bottom; wherein the oxide layer is SiO ₂ The bottom electrode is a Cr electrode and a Cu electrode, the two-dimensional ion conductor functional layer is copper-based phosphosulfate, and the top electrode is a Cr electrode and an Au electrode. According to the invention, a copper electrode and copper-based phosphorus sulfate are introduced to form a van der Waals heterojunction with graphene; the depletion and accumulation of copper ions in the copper-based phosphorus sulfate are controlled by applying voltage, so that stable threshold resistance change behavior is realized. The introduction of the copper electrode increases the concentration gradient of copper ions, so that the functional layer can recover from a low-resistance state to a high-resistance state through copper ion diffusion when the voltage is reduced. The threshold value transition device has the characteristics of good stability, fatigue resistance, higher switching ratio, ultra-small subthreshold swing and the like.

Description

Threshold value conversion device based on two-dimensional ion conductor and preparation method

Technical Field

The invention belongs to the technical field of resistive random access memory devices, and particularly relates to a threshold value conversion device and a preparation method thereof.

Background

The rise of artificial intelligence and the Internet of things brings urgent needs for large-capacity data storage, and along with the development of storage technology, the resistive random access memory has the advantages of high reading and writing speed, simple device structure, high integration density and the like, and shows strong competitiveness in a new generation of storage devices. Resistive random access memories generally employ a cross-bar structure to increase their storage density, which is easy to manufacture, but also causes resistance crosstalk problems due to easy introduction of path currents, thereby affecting the accuracy of information reading and increasing the power consumption of the device. To solve the crosstalk problem, a "1S1R" structure is proposed, that is, a selector is connected in series with a resistive switching device, and the selector is used to avoid the path current. The commonly used selector is a resistance threshold switching device, i.e. enters a low resistance state at a certain voltage and returns to a high resistance state when the voltage decreases. A variety of materials are currently used as threshold transition devices, such as simple metal oxides, chalcogenides, and perovskite oxides. The resistance change mechanism of the devices mainly depends on a conductive filament structure formed by ion migration in a material, an electron conductive channel is constructed through the conductive filament, and then the resistance change conversion is realized along with the formation and annihilation of the conductive filament. But the formation of conductive filaments is generally random, thereby reducing the operational stability of the device. Meanwhile, the devices have the defects of high threshold voltage, poor durability, high processing temperature and the like, and the application of the devices is limited to a certain extent. Small struct.2021,2,2000109; nat. Electron.2018,1,274; phys.Lett.B 2020,34,2050115].

In recent years, the rise of two-dimensional materials has provided more options for constructing new threshold transition devices. Wherein, the metal phosphorus sulfur compound material, such as copper indium phosphorus sulfur, is a novel two-dimensional lamellar material. Because the metal cations in the crystal lattice have instability, the metal cations are easy to displace under the action of an electric field, so that ferroelectricity or antiferroelectricity is caused, and even ion conductivity can be caused under the action of a larger electric field. This ion mobility makes the metal phosphorus sulfur compound promising for application in resistive memory devices. Previous research results also show that the migration of copper ions in materials such as copper indium phosphorus sulfur can be controlled by the stimulation of an external field, and a resistor device with rectification characteristics is realized. However, the migration of copper ions is too sensitive to the electric field condition and is easily influenced by the electric field scanning speed, the electric field direction, the application history and the like, so that the stable resistance state control is still difficult to realize [ 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, it is possible to apply it to a resistive random access memory, and efforts are also required in the structural design of the device.

In order to solve the problems, the invention provides a preparation method of a novel resistive switching device based on a two-dimensional ion conductor. The method utilizes a metal copper electrode, a few-layer two-dimensional copper-based phosphorus sulfate and graphene to form a heterojunction, wherein copper and the few-layer graphene are respectively used as a bottom electrode and a top electrode. The copper electrode of the device is grounded, a forward scanning voltage is applied to the top electrode, and the initial state of the device is a high-resistance state because the intrinsic conductivity of the few-layer copper-based phosphosulfate material is low; when the applied voltage is larger than the threshold voltage, copper ions in the two-dimensional copper-based phosphorus sulfate are excited and drift motion occurs under the action of an electric field, and when the copper ions in the functional layer material are exhausted, the microstructure of the copper-based phosphorus sulfate is converted, and the microstructure is converted from an insulating state to a conductive state, so that the device is converted from a high-resistance state threshold value to a low-resistance state. When the applied voltage is reduced to a certain value, the drift motion of copper ions is weakened, meanwhile, the diffusion motion of copper ions from the interface to the inside is dominant because the concentration of copper ions near the interface of the copper electrode is far higher than that inside the material, and the material is converted from a low-resistance state to a high-resistance state because the copper ions are injected into the functional material. In this process, the introduction of the copper electrode increases the difference in concentration of copper ions between the interface and the interior, thereby enhancing the diffusion motion. Devices prepared by this method exhibit excellent threshold transition characteristics, including stable threshold transition and recovery processes, small operating voltages, high on-off ratios, and good fatigue resistance characteristics.

Disclosure of Invention

The invention aims to provide a threshold value transition device based on a two-dimensional ion conductor and a preparation method thereof, wherein the threshold value transition device is excellent in threshold value transition performance.

According to the threshold value conversion device based on the two-dimensional ion conductor, the copper electrode, the two-dimensional ion conductor copper-based phosphorus sulfate and the few-layer graphene are introduced to form the van der Waals heterojunction, and the depletion and injection of copper ions in the functional layer are realized by utilizing the drifting and diffusion movement of copper ions under the external bias, so that the threshold value conversion characteristics between high and low resistance states of the master device are realized. The threshold transition behavior realized based on ion migration has good stability, fatigue resistance and ultra-small subthreshold swing.

The invention provides a threshold value conversion device based on a two-dimensional ion conductor, which sequentially comprises the following structures from bottom to top: the device comprises a substrate, an oxide layer, a bottom electrode, a two-dimensional ion conductor functional layer, a few-layer graphene and a top electrode; wherein:

the substrate is a heavily doped p-type Si substrate, and the thickness of the substrate can be 0.5 millimeter;

the oxide layer is SiO ₂ The thickness is 285+/-15 nanometers;

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

the two-dimensional ion conductor functional layer is copper-based phosphosulfate with the thickness of 10-15 nanometers;

the thickness of the few-layer graphene is 8-15 nanometers;

the top electrode is a Cr electrode and an Au electrode, the thickness of Cr is 5-10 nanometers, and the thickness of Au is 50-60 nanometers.

The invention provides a preparation method of a threshold value conversion device based on a two-dimensional ion conductor, which comprises the following specific steps:

(1) Preparation of oxide layer

Preparing oxide layer silicon dioxide with the thickness of 285+/-15 nanometers on a heavily doped Si substrate by a thermal oxidation method;

(2) Cleaning of a substrate

Sequentially placing the substrate containing the oxide layer in acetone, isopropanol and deionized water for ultrasonic cleaning for 14-20 minutes, and placing the substrate in an ultraviolet ozone cleaning machine for cleaning for 100-130 seconds after cleaning is finished;

(3) Preparation of bottom electrode

Adopting spin coating and electron beam exposure, and preparing Cr/Cu electrode on the surface of the oxide layer by combining thermal evaporation and stripping processes;

(4) Preparation and transfer of two-dimensional ion conductor copper-based phosphorus sulfate

Cleavage of copper-based phosphorus sulfate crystals to a thin layer by adopting a mechanical stripping method, and transfer of even few-layer copper-based phosphorus sulfate to a bottom electrode by a wet transfer technology;

(5) Preparation and transfer of few-layer graphene

Transferring the few-layer graphene onto the two-dimensional ion conductor functional layer by a mechanical stripping and dry transfer technology;

(6) Preparation of top electrode

Preparing Cr/Au electrodes on the surface of the oxide layer by adopting spin coating and electron beam exposure and combining thermal evaporation and stripping processes to form a threshold conversion device based on ionic conductors in a sandwich structure;

(7) Post-processing of devices

And (3) placing the prepared device in a rapid annealing furnace in a nitrogen atmosphere for annealing for 1.5-2.5 hours, so that the direct van der Waals contact of the material is more sufficient to improve the performance.

The threshold transition device of the invention has the following mechanism:

the copper electrode is grounded, forward voltage scanning is applied to the top electrode, when the voltage is larger than a threshold value, cations in the ionic conductor migrate to the copper electrode, so that structural phase change occurs in the ionic conductor, the ionic conductor is caused to be converted into a conductive state from an insulating state, and the device is converted into a low-resistance state from a high-resistance state, and current limiting is required to be set to protect the device from being damaged. When the voltage is reduced to a certain value, the concentration of cations at the interface of the bottom electrode is higher, and the small voltage is insufficient to maintain ion drift movement, so that the cations are reversely diffused and injected into the ion conductor to restore the insulating state, and the device returns to the high-resistance state from the low-resistance state.

The invention has the advantages that:

based on the two-dimensional ionic conductor copper-based phosphosulfate, a Van der Waals heterojunction is formed by introducing a copper electrode, the copper-based phosphosulfate and graphene, and a threshold conversion device with a new mechanism is realized by controlling accumulation and exhaustion of copper ions through voltage, so that device instability caused by random formation of conductive filaments in a traditional device is avoided. Meanwhile, the concentration gradient of copper ions is increased by introducing the copper electrode, so that the copper ions are diffused under a small voltage, the transition from a low-resistance state to a high-resistance state is realized, and the cycling stability of the device is improved. In addition, the device has the characteristics of good fatigue resistance, low operating voltage, ultra-small subthreshold swing and the like.

Drawings

Fig. 1 is a schematic diagram of a threshold transition device structure and operating state based on a two-dimensional ionic conductor.

Fig. 2 is a graph of current-voltage characteristics of the device at a forward voltage. The arrow direction in the figure shows the hysteresis direction.

Fig. 3 is a schematic diagram of the device at operating voltage. Wherein (a) is the process of switching the high resistance state of the device to the low resistance state, and (b) is the process of switching the low resistance state of the device to the high resistance state.

Reference numerals in the drawings: 1 is a substrate, 2 is an oxide layer, 3 is a bottom electrode, 4 is a two-dimensional ion conductor, 5 is a few-layer graphene, and 6 is a top electrode.

Detailed Description

The invention will be further described with reference to examples and figures.

According to the novel threshold value conversion device based on the two-dimensional ionic conductor, which is developed by the invention, a Van der Waals heterojunction is formed by introducing a copper electrode and copper indium phosphorus sulfur of the ionic conductor, and a sandwich structure is formed by stacking few layers of graphene. The device is stably switched between a high-resistance state and a low-resistance state by utilizing migration of copper ions in copper indium phosphorus sulfur under the action of an electric field and diffusion caused by too high copper ion concentration at one side of a copper electrode, so that the threshold value transition device has excellent performances such as good stability, fatigue resistance, large switching ratio and the like.

The live broadcast specifically comprises the following steps:

1. substrate selection

Heavily doped p-type silicon with a thickness of 0.5 mm is selected as the substrate.

2. Preparation of oxide layer

Oxidizing an oxide layer SiO with a thickness of 285+/-15 nanometers on the surface of the silicon substrate by a thermal oxidation method ₂ The other surface of the Si substrate is polished.

3. Cleaning of substrate and oxide layer

And sequentially placing the substrate containing the oxide layer in acetone, isopropanol and deionized water for ultrasonic cleaning for 15 minutes, and placing the substrate in an ultraviolet ozone cleaning machine for cleaning for 120 seconds after cleaning.

4. Preparation of bottom electrode

The substrate (1X 1 cm) containing the protective layer was placed on a spin coater and attached with a pump, a small amount of polymethyl methacrylate (PMMA) solution was pipetted with a rubber head dropper and dropped onto the surface of the oxide layer, and after 6 seconds with 500 revolutions per minute, 30 seconds with 4000 revolutions per minute, and then baked for 3 minutes at 180 ℃. The substrate and oxide layer after spin coating were placed in an electron beam exposure system to expose the desired electrode pattern with a 300 picoampere electron beam. After the exposure was completed, the resultant was developed in a developer for 6 seconds. The developed substrate and oxide layer were placed in a thermal evaporation system to evaporate 5 nm of chromium and 40 nm of copper on the substrate at an evaporation rate of 1 angstrom per second. After evaporation was completed, peeling was performed with acetone.

5. Preparation and transfer of two-dimensional ionic conductors

The bulk copper indium phosphide sulfide was dissociated into thin layers with a blue tape, transferred to another clean substrate with the aid of Polydimethylsiloxane (PDMS), and found to be the desired uniform thin layer of copper indium phosphide sulfide (copper indium phosphide sulfide with a thickness of 10-20 nm appears deep blue or deep purple under a microscope). The target sample is fished by a polyvinyl alcohol film (PVA) and then transferred to a copper electrode at fixed points (the substrate is required to be heated to 70 ℃ in the transfer process). And (3) after the transfer is finished, soaking the PVA in deionized water for 8 hours to completely dissolve the PVA, cleaning the PVA with acetone, and drying the PVA by a nitrogen gun.

6. Preparation and transfer of few-layer graphene

The flake graphite was cleaved to a thin layer with a blue tape and transferred to PDMS, and a uniform few-layer graphene was found on PDMS with a microscope (graphene with a thickness of 10-20 nm appeared dark purple under the microscope). And then transferred to copper indium phosphorus sulfur at fixed points.

7. Preparation of top electrode

The process is the same as step 5, and finally a thermal evaporation system is used for evaporating 5 nm of chromium and 60 nm of gold.

The copper electrode is grounded, forward voltage for back scanning is applied to the top electrode, the electrical characteristic diagram is shown in the attached drawing 2 of the specification, it can be seen that when the voltage is greater than a certain threshold value, the device is changed from a high-resistance state to a low-resistance state, the current is increased by about 4 orders of magnitude (the platform is used for protecting the device from damage due to current limiting), when the voltage is reduced to a certain value, the device is changed from the low-resistance state to the high-resistance state, and the stability is good after a plurality of cycles.

The working principle of the threshold value transition device is illustrated in fig. 3 of the specification, when the voltage is greater than the threshold value, cations in the ionic conductor migrate to the copper electrode, so that structural phase transition occurs inside the ionic conductor and the ionic conductor is changed from an insulating state to a conductive state, and the device is changed from a high-resistance state to a low-resistance state (as shown in fig. a). When the voltage is reduced to a certain value, the voltage is too small to maintain ion migration due to the higher concentration of cations on one side, so that the cations will back diffuse to return the ion conductor to the insulating state, and the device will return to the high-resistance state from the low-resistance state (as shown in figure b).

Claims (2)

1. The threshold value conversion device based on the two-dimensional ion conductor is characterized in that the structure sequentially comprises a substrate, an oxide layer, a bottom electrode, a two-dimensional ion conductor functional layer, few-layer graphene and a top electrode from bottom to top; wherein:

the substrate is a heavily doped p-type Si substrate;

the oxide layer is SiO ₂ The thickness is 285+/-15 nanometers;

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

the two-dimensional ion conductor functional layer is copper-based phosphosulfate with the thickness of 10-15 nanometers;

the thickness of the few-layer graphene is 8-15 nanometers;

the top electrode is a Cr electrode and an Au electrode, the thickness of Cr is 5-10 nanometers, and the thickness of Au is 50-60 nanometers.

2. The method for manufacturing a threshold value transition device according to claim 1, comprising the specific steps of:

(1) Preparation of oxide layer

Preparing oxide layer silicon dioxide with the thickness of 285+/-15 nanometers on a heavily doped Si substrate by a thermal oxidation method;

(2) Cleaning of a substrate

Sequentially placing the substrate containing the oxide layer in acetone, isopropanol and deionized water for ultrasonic cleaning for 14-20 minutes, and placing the substrate in an ultraviolet ozone cleaning machine for cleaning for 100-130 seconds after cleaning is finished;

(3) Preparation of bottom electrode

Adopting spin coating and electron beam exposure, and preparing Cr/Cu electrode on the surface of the oxide layer by combining thermal evaporation and stripping processes;

(4) Preparation and transfer of two-dimensional ion conductor copper-based phosphorus sulfate

Cleavage of copper-based phosphorus sulfate crystals to a thin layer by adopting a mechanical stripping method, and transfer of even few-layer copper-based phosphorus sulfate to a bottom electrode by a wet transfer technology;

(5) Preparation and transfer of few-layer graphene

Transferring the few-layer graphene onto the two-dimensional ion conductor functional layer by a mechanical stripping and dry transfer technology;

(6) Preparation of top electrode

Preparing Cr/Au electrodes on the surface of the oxide layer by adopting spin coating and electron beam exposure and combining thermal evaporation and stripping processes to form a threshold conversion device based on ionic conductors in a sandwich structure;

(7) Post-processing of devices

And (5) placing the prepared device in a rapid annealing furnace in a nitrogen atmosphere for annealing for 1.5-2.5 hours.

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