CN111547676B

CN111547676B - Preparation method of novel ferroelectric vortex nano island array

Info

Publication number: CN111547676B
Application number: CN202010280816.XA
Authority: CN
Inventors: 高兴森; 杨文达; 田国
Original assignee: South China Normal University
Current assignee: South China Normal University
Priority date: 2020-04-10
Filing date: 2020-04-10
Publication date: 2023-06-02
Anticipated expiration: 2040-04-10
Also published as: CN111547676A

Abstract

A preparation method of a novel ferroelectric vortex nano island array comprises the following steps: s1: depositing an SRO conductive layer on the STO single crystal substrate in the (001) direction by adopting a pulse laser deposition method to serve as a bottom electrode; s2: depositing a diamond-shaped square-phase BFO film on the SRO conductive layer by adopting a pulse laser deposition method; s3: spreading a single-layer PS (polystyrene) pellet on the surface of the BFO film in the step S2 as a mask plate, then carrying out oxygen plasma etching treatment, then placing the mask plate in an ion beam etching machine for etching, and finally removing the residual single-layer PS pellet mask plate to obtain a rhombic phase BFO nano island array; s4: and obtaining the ferroelectric vortex nano island array with the vortex topological domain structure through electric field regulation. The rhombic phase BFO nano island array prepared by the method realizes the transformation of a vortex topological domain structure and a common domain structure under the regulation and control of an external electric field, realizes the storage of data by utilizing the conductivity difference of the rhombic topological domain structure and the common domain structure, and has high stability and good application prospect.

Description

Preparation method of novel ferroelectric vortex nano island array

Technical Field

The invention relates to the technical field of ferroelectric materials, in particular to a preparation method of a novel ferroelectric vortex nano island array.

Background

In the context of the big data age, the size of the traditional semiconductor chip is approaching the quantum limit, and the traditional semiconductor chip is difficult to further develop, so that people are promoted to search for a memory device under a novel semiconductor electronic material system to meet the increasing memory requirement of human beings. The ferroelectric memory under the ferroelectric material system is a novel memory. In ferroelectric materials, ferroelectric domain walls have many novel physical properties as an ultrathin heterojunction, such as good conductivity, easy human erasure and generation, etc., so that the data storage can be greatly improved by domain wall control, response time is shortened, and system energy consumption is reduced.

In recent years, as the requirement of storage capacity of a storage device is continuously increased, it is very important to realize ordered high-density ferroelectric domain wall regulation under the nanoscale to increase the density of a storage device. Because of the characteristics of high stability, small size and the like, the topological domain structure can be prevented from being damaged by other external sites except the regulating field, and the size of the memory is reduced. At present, various methods for preparing ferroelectric topological domains are tried, such as obtaining a single vortex domain on a film or a block body through electric field induction, but the method has the defects of low density, limited structure, difficult integration and the like of the obtained ferroelectric topological domains; and for example, a vortex domain array is formed in a superlattice, but the method is difficult to regulate and control, has complex process and has undefined electrical property. Thus, achieving the preparation of high density, regulatable topological domain walls remains a great challenge.

Disclosure of Invention

Based on the above, the invention aims to provide a preparation method of a novel ferroelectric vortex nano island array, which is used for preparing a vortex type ferroelectric domain nano island array with high density and ordered arrangement. The nano array structure has obvious advantages on a storage device: has high density, high order, easy individual regulation and control, and the like, and is more similar to the requirement of integrated devices.

The technical scheme adopted by the invention is as follows: a preparation method of a novel ferroelectric vortex nano island array comprises the following steps:

s1: STO (SrTiO) in (001) direction by pulsed laser deposition ₃ ) Depositing a layer of SRO (SrRuO) on a monocrystalline substrate ₃ ) The conductive layer is used as a bottom electrode;

s2: depositing a diamond square phase BFO (BiFeO) layer on the SRO conductive layer by pulse laser deposition method ₃ ) A film;

s3: spreading a single-layer PS (polystyrene) pellet on the surface of the BFO film in the step S2 as a mask plate, then carrying out oxygen plasma etching treatment, then placing the mask plate in an ion beam etching machine for etching, and finally removing the residual single-layer PS pellet mask plate to obtain the ordered rhombic phase BFO nano island array ferroelectric material;

s4: electric field regulation: and inducing the ferroelectric material of the rhombic phase BFO nano island array by a piezoelectric response force microscope (PFM) to obtain the ferroelectric vortex nano island array with a vortex topological domain structure.

Compared with the prior art, the method prepares rhombohedral phase BFO (BiFeO) by a pulse laser deposition method ₃ ) The thin film is then used for preparing a rhombic phase BFO nano island array by a PS (polystyrene) pellet auxiliary ion etching method, and the single nano island is regulated and controlled by an external electric field to control the existence of vortex domains, so that a ferroelectric vortex topological domain structure with high density, order and mutual independence is obtained. And the difference of the conductivity is used as the basis for data storage and reading. In addition, the vortex domain can still stably maintain the vortex structure after four months under the condition of not performing external field regulation and control, and has good stability. It is hopeful to be applied to design a novel ferroelectric domain wall memory with high density, high stability, high response speed and low energy consumption.

Further, step S4 includes the steps of:

s41: applying negative bias to the ferroelectric material obtained in the step S3 by adopting a conductive probe of a piezoelectric response force microscope to represent the domain structure of the ferroelectric material;

s42: the conductive probe of the piezoelectric response force microscope is adopted to apply forward bias voltage to the ferroelectric material obtained in the step S3, and the domain structure is represented;

s43: the topological domains obtained in the steps S41 and S42 are respectively subjected to conductivity characterization by adopting a Conductive Atomic Force Microscope (CAFM).

Further, the thickness of the rhombohedral phase BFO film in the step S2 is 30nm. Under the thickness, the substrate stress of the BFO film is well released, the BFO film is in a uniform rhombic phase, and meanwhile, the surface is relatively flat, so that the subsequent preparation of the nano structure can be well carried out.

Further, the thickness of the SRO conductive layer in the step S1 is 20nm-40nm. The thickness of the SRO conductive layer is controlled in the range, so that the SRO conductive layer is easy to operate, and the conductive effect can be ensured.

Further, step S3 includes the steps of:

s31: dripping a mixed solution of PS pellets with the diameter of 500nm and ethanol into a culture dish filled with deionized water, and adding a dispersing agent to enable the PS pellets to be arranged in a single layer on the surface of the deionized water;

s32: treating the BFO film prepared in the step S2 with oxygen plasma for 3 minutes;

s33: placing the treated BFO film sample under a single-layer PS pellet by using tweezers, and then slightly and horizontally lifting out; after the water naturally evaporates, forming a layer of PS pellets with a single layer and close arrangement on the surface of the BFO film;

s34: placing the BFO film with the PS globule mask plate in an oxygen plasma etching machine for etching treatment for 25-35 minutes, thereby reducing the diameter of the PS globules and separating the PS globules which are closely arranged;

s35: placing the sample obtained in the step S34 in an ion beam etching machine for etching;

s36: and removing the residual single-layer PS globule mask plate to obtain the ordered rhombic phase BFO nano island array which is in a vortex topological domain structure.

The steps utilize Polystyrene (PS) pellets as an etching template, so that the preparation is convenient and time-saving; the method has the advantages that the ion etching technology is utilized to directly carry out etching, the arrangement pattern of the polystyrene beads is directly transferred to the substrate material, a sacrificial layer structure is not required to be introduced, the nano structure is prepared in one step, the operation is simple and convenient, and the working procedure is simple; meanwhile, the ion etching does not need to introduce chemical reaction gas, does not introduce new impurities to pollute the film microstructure, and has no toxicity and harm to the operating environment, low preparation cost and high safety of operators.

Further, in step S35, the vacuum degree is 8.0X10 ^-4 Pa, maintaining the cathode current of the ion beam etching system to be 15.7A, the anode voltage to be 50V, the screen electrode voltage to be 250V, the accelerating voltage to be 250V, the neutralizing current to be 13A and the bias current to be 1.2A at room temperature, and etching for 3-4 minutes.

Further, in step S36, the sample obtained in step S35 is respectively placed in chloroform, alcohol and deionized water, and subjected to ultrasonic treatment for 8-12 minutes, and then is dried by a nitrogen gun, and then the surface is cleaned by low-power oxygen plasma for 4-6 minutes, so that the ordered rhombic phase BFO nano island array which is in a vortex topological domain structure is obtained.

Further, in step S1, the preparation parameters of the pulsed laser deposition method are as follows: energy of 300mJ/cm ³ The pulse frequency was 8Hz, the temperature was 680℃and the oxygen pressure was 15Pa.

Further, in step S2, pulse laser deposition methodThe preparation parameters of (2) are as follows: energy of 300mJ/cm ³ The pulse frequency was 8Hz, the temperature was 680℃and the oxygen pressure was 15Pa.

For a better understanding and implementation, the present invention is described in detail below with reference to the drawings.

Drawings

FIG. 1 is a schematic diagram of a preparation process of a rhombic BFO nano island array prepared in example 1;

FIG. 2 is a schematic drawing of RSM and XRD of the rhombohedral phase BFO film prepared in example 1;

FIG. 3 is a schematic diagram showing SEM and AFM morphology characterization of the rhombohedral phase BFO nanodot array prepared in example 1;

fig. 4 is a schematic diagram of a hysteresis loop and a piezoelectric butterfly loop of a single nano island of rhombohedral phase BFO prepared in example 1;

FIG. 5 is an in-plane phase diagram and a corresponding conductivity signal diagram of the rhombic BFO nano island vortex array prepared in example 1 measured by a piezoelectric stress microscope and a conductive atomic force microscope when the rhombic BFO nano island vortex array is placed at 0 degrees and 90 degrees;

FIG. 6 is a schematic diagram of the current variation and I-V curve obtained by conducting atomic force microscopy test of the rhombic phase BFO single nano island prepared in example 1 under variable temperature conditions;

FIG. 7 is a diagram of the energy band structure of the diamond-phase BFO single nano-island vortex domain prepared in example 1;

FIG. 8 is a schematic diagram of the rhombohedral phase BFO single nano islands prepared in example 1 regulated by different externally applied electric fields;

fig. 9 is a graph of the controllability and stability analysis data of the single nano island vortex domain of rhombohedral phase BFO prepared in example 1.

Detailed Description

Examples

The preparation method of the novel ferroelectric vortex nano island array in the embodiment, referring to fig. 1, specifically includes the following steps:

s1: preparing a rhombic phase BFO film by adopting a pulse laser deposition method: STO (SrTiO) for selecting (001) direction ₃ ) Monocrystalline substrate, on which a layer of 20nm is deposited by laser pulse depositionThick SRO (SrRuO) ₃ ) The conductive layer is used as a bottom electrode, and the preparation parameters of the pulse laser deposition method are as follows:

energy (mJ/cm) ³ )	Pulse frequency (Hz)	Temperature (. Degree. C.)	Oxygen pressure (Pa)
				300	8	680	15

S2: depositing a 30nm thick rhombic phase BFO (BiFeO) on the SRO conductive layer by pulse laser deposition ₃ ) The preparation parameters of the film and the pulse laser deposition method are as follows:

The obtained film was confirmed to be rhombohedral phase BFO (as shown in fig. 2) by RSM and XRD characterization, and the lattice constant a=3.91 a and c=4.06A were calculated. The ferroelectric film sample has good ferroelectricity by testing the electric hysteresis loop and the piezoelectric butterfly loop (shown in fig. 4), and the coercive field is about 3.0V.

S3: the preparation of the vortex BFO nano island array specifically comprises the following steps:

s33: placing the BFO film sample treated in the step S32 below a single-layer PS pellet by using tweezers, and then slightly and horizontally lifting out; after the water naturally evaporates, forming a layer of PS pellets with a single layer and close arrangement on the surface of the BFO film;

s34: placing the BFO film with the PS pellet mask plate in an oxygen plasma etching machine for etching treatment for 30 minutes, thereby reducing the diameter of the PS pellets and separating the PS pellets which are closely arranged;

s35: at a vacuum degree of 8.0X10 ^-4 Under the condition of Pa and room temperature, keeping the cathode current of an ion beam etching system to be 15.7A, the anode voltage to be 50V, the screen electrode voltage to be 250V, the accelerating voltage to be 250V, the neutralizing current to be 13A and the bias current to be 1.2A, and placing the sample obtained in the step S34 into an ion beam etching machine to be etched for 200 seconds;

s36: removing the residual single-layer PS bead mask plate: and (3) respectively placing the sample obtained in the step (S35) in chloroform, alcohol and deionized water for ultrasonic treatment for 10 minutes, taking out, drying by a nitrogen gun, and cleaning the surface by using low-power oxygen plasma for 5 minutes to obtain the large-area ordered rhombic phase BFO nano island array (shown in figure 3).

S4: electric field regulation: inducing to obtain the ferroelectric vortex nano island array with the vortex topological domain structure, which comprises the following steps:

s41: applying negative bias of-3.5V to the obtained rhombic phase BFO nano island array ferroelectric material by adopting a conductive probe of a piezoelectric response force microscope, and representing the domain structure of the ferroelectric material;

s42: the positive bias voltage of +3.5V applied to the obtained rhombic phase BFO nano island array ferroelectric material by adopting a conductive probe of a piezoelectric response force microscope is used for representing the domain structure of the rhombic phase BFO nano island array ferroelectric material;

Regulation and characterization result analysis:

the piezoelectricity response force microscope is a microscope for detecting the electric deformation quantity of the sample surface at a microscopic scale. The mode is mainly used for representing the domain structure of ferroelectric materials, and the principle is to detect the mechanical deformation of a sample under an external electric field due to the inverse piezoelectric effect. By applying alternating voltage to the conductive needle point, the local tested sample under the needle point is periodically deformed, and the deformation causes the cantilever connected with the needle point to be distorted, so that the reflected optical signal is influenced, and the signal is collected by the photodiode and is subjected to subsequent analysis by the lock-in amplifier, thereby realizing detection. Alternatively, the tip may be biased with a DC voltage during contact scanning with the sample to reverse the polarization of the ferroelectric material. The domain structure of the rhombic BFO nano island array prepared in the example 1 is changed from an initial state to a vortex state by adopting a piezoelectric response force microscope through needle tip bias, and the domain structure of the rhombic BFO nano island array prepared in the example 1 (hereinafter referred to as an example 1 sample) is characterized.

The out-of-plane polarization inversion is induced by negative externally applied voltage slightly higher than the coercive field, and a vortex domain structure is spontaneously formed on a single nanometer island under the action of a larger unshielded depolarization field due to lower surface charge aggregation at the moment so as to reduce the system energy.

The vortex domain structure is induced by applying-3.5V DC bias to the sample of the embodiment 1 by using a piezoelectric microscope, and the domain structure is characterized. As shown in fig. 5, the induced upper vortex domain of the BFO nano-island is composed of four domains rotating clockwise by 90 degrees around the vortex center, and the four domains are divided by two mutually staggered 71-degree domain walls. Under a piezoelectricity response force microscope, a sample placed at 0 degree has an in-plane domain structure of an upper half and a lower half, and an in-plane domain shows a left half and a right half structure when placed at 90 degrees. The out-of-plane domains are single domains. The successful induction of vortex domains on the BFO nano-island array can be determined by the double-angle test of the in-plane domains and the out-of-plane domains.

The domain structure of the sample of the embodiment 1 can be reciprocally switched between a vortex state and an initial state by applying +/-3.5V bias to the sample of the embodiment 1 through a needle point, namely, the generation and disappearance of vortex domains can be realized through electric field regulation and control, and the regulation and control on ferroelectric materials are realized.

The conductive atomic force microscope is that a conductivity test function is added on a common contact atomic force microscope. During the scanning of the sample, the sample is biased while the needle tip is grounded, and the current flowing through the sample is read with a built-in source meter. The invention tests conductivity of vortex domain of sample of example 1 by conductive atomic force microscope, researches conductivity mechanism and realizes mechanism of data storage function.

According to the invention, by applying a 2V forward voltage to the sample of the embodiment 1, the current of about 3nA at the vortex center of the BFO nanometer island vortex domain is detected, the current of about several picoamps at the 71-degree common domain wall forming the vortex is detected, and the current of other domain areas is lower than 1pA. The sample of example 1 was repeatedly scanned for electrical conductivity at a temperature varying from 25 ℃ to 150 ℃ and found that the current at the vortex core showed an exponential decrease with increasing temperature, which was similar to the metal conductivity, confirming that the source of high conductivity was probably due to the formation of a one-dimensional-like electron gas channel at the vortex core.

Referring to FIG. 6, the samples of example 1 were subjected to surface conductivity testing and localized I-V curve testing at 25, 60, 90, 120, 150, respectively. The results show that the conductivity at the vortex core exhibits an exponential decrease with increasing temperature, similar to the metal conductivity and so on, further corroborating that the vortex core conductivity originates from the generation of a quasi-one-dimensional electron gas channel. Fig. 7 is a diagram of the energy band structure of a single nano-island vortex domain, showing that the conduction band at the vortex center of the single nano-island vortex domain falls below the fermi level, where electrons can be used as carriers to achieve a metalloid conduction characteristic.

In the reciprocating control of the sample of example 1, it was observed that the conductivity at the scroll center was stably controlled, and the conductivity did not decrease with the increase in the number of cycles over 50 cycles (as shown in fig. 9). While the initial BFO nano islands have extremely low conductivity, which is in clear contrast to the vortex core. The difference between the high and low resistance states corresponds to the "0" and "1" states in the data storage, so that the high-density and stable storage of the data under the nano-size is realized (as shown in fig. 8).

Compared with the prior art, the method prepares rhombohedral phase BFO (BiFeO) by a pulse laser deposition method ₃ ) The thin film is then used to prepare rhombic BFO nanometer island array via polystyrene ball auxiliary ion etching, and negative applied voltage slightly higher than coercive field is used to induce out-of-plane polarization turning, so that vortex topological domain structure is formed spontaneously on single nanometer island. The novel ferroelectric vortex nano island array structure prepared by the invention reaches the nano level, and is a high-density and ordered nano dot array structure. The ferroelectric vortex topological domain structures are mutually independent and can be regulated and controlled by an external electric field, the domain structure obtained by regulation and control is stable for a long time, and after more times of circulation regulation and control, the conductivity of the domain structure is not reduced along with the increase of the circulation times, so that the domain structure has better stability. The vortex domain has different conductivity through the regulation and control of an external electric field, the difference of conductivity is used as the basis for data storage and reading, the storage function is realized, and the method is expected to be applied to the preparation of a novel ferroelectric memory with high density, order, easy regulation and control, small volume and high stability.

The present invention is not limited to the above-described embodiments, but, if various modifications or variations of the present invention are not departing from the spirit and scope of the present invention, the present invention is intended to include such modifications and variations as fall within the scope of the claims and the equivalents thereof.

Claims

1. The preparation method of the novel ferroelectric vortex nano island array is characterized by comprising the following steps of:

s1: depositing an SRO conductive layer on the STO single crystal substrate in the [001] direction by adopting a pulse laser deposition method to serve as a bottom electrode;

s2: depositing a diamond-shaped square-phase BFO film on the SRO conductive layer by adopting a pulse laser deposition method;

s3: spreading a single-layer PS (polystyrene) pellet on the surface of the BFO film in the step S2 as a mask plate, then carrying out oxygen plasma etching treatment, then placing the mask plate in an ion beam etching machine for etching, and finally removing the residual single-layer PS pellet mask plate to obtain the rhombic BFO nano island array ferroelectric material; the method comprises the following steps:

s35: placing the sample obtained in the step S34 in an ion beam etching machine for etching: at a vacuum degree of 8.0X10 ^-4 Under the condition of Pa and room temperature, maintaining the cathode current of an ion beam etching system to be 15.7A, the anode voltage to be 50V, the screen electrode voltage to be 250V, the accelerating voltage to be 250V, the neutralizing current to be 13A and the bias current to be 1.2A, and etching3-4 minutes;

s36: removing the residual single-layer PS small sphere mask plate to obtain an ordered rhombic phase BFO nano island array which is in a vortex topological domain structure;

s4: electric field regulation: inducing the ferroelectric material of the rhombic phase BFO nano island array through a piezoelectric response force microscope to obtain a ferroelectric vortex nano island array with a vortex topological domain structure; the method comprises the following steps:

s43: the topological domains obtained in the steps S41 and S42 of the conductive atomic force microscope are used for conducting characterization.

2. The method for preparing the novel ferroelectric vortex nano island array according to claim 1, wherein the thickness of the SRO conductive layer in the step S1 is 20nm-40nm.

3. The method for preparing the novel ferroelectric vortex nano island array according to claim 1, wherein in step S36, the samples obtained in step S35 are respectively placed in chloroform, alcohol and deionized water for ultrasonic treatment for 8-12 minutes, blow-dried by a nitrogen gun, and then the surface is cleaned by low-power oxygen plasma for 4-6 minutes, so that the ordered rhombic phase BFO nano island array which is in a vortex topological domain structure is obtained.

4. The method for preparing the novel ferroelectric vortex nano island array according to claim 1, wherein in step S36, the samples obtained in step S35 are respectively placed in chloroform, alcohol and deionized water for ultrasonic treatment for 8-12 minutes, blow-dried by a nitrogen gun, and then the surface is cleaned by low-power oxygen plasma for 4-6 minutes, so that the ordered rhombic phase BFO nano island array which is in a vortex topological domain structure is obtained.

5. The method for preparing a novel ferroelectric vortex nano island array according to any one of claims 1 to 4, wherein in step S1, the preparation parameters of the pulsed laser deposition method are as follows: energy of 300mJ/cm ³ The pulse frequency was 8Hz, the temperature was 680℃and the oxygen pressure was 15Pa.

6. The method for preparing a novel ferroelectric vortex nano island array according to any one of claims 1 to 4, wherein in step S2, the preparation parameters of the pulsed laser deposition method are as follows: energy of 300mJ/cm ³ The pulse frequency was 8Hz, the temperature was 680℃and the oxygen pressure was 15Pa.