CN115504684B - Bismuth ferrite film doped with lanthanide metal atoms at A site and preparation method thereof - Google Patents
Bismuth ferrite film doped with lanthanide metal atoms at A site and preparation method thereof Download PDFInfo
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Classifications
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- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
- C03C17/00—Surface treatment of glass, not in the form of fibres or filaments, by coating
- C03C17/34—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions
- C03C17/3411—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions with at least two coatings of inorganic materials
- C03C17/3417—Surface treatment of glass, not in the form of fibres or filaments, by coating with at least two coatings having different compositions with at least two coatings of inorganic materials all coatings being oxide coatings
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L28/00—Passive two-terminal components without a potential-jump or surface barrier for integrated circuits; Details thereof; Multistep manufacturing processes therefor
- H01L28/40—Capacitors
- H01L28/55—Capacitors with a dielectric comprising a perovskite structure material
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
- C03C2218/00—Methods for coating glass
- C03C2218/10—Deposition methods
- C03C2218/11—Deposition methods from solutions or suspensions
- C03C2218/113—Deposition methods from solutions or suspensions by sol-gel processes
-
- C—CHEMISTRY; METALLURGY
- C03—GLASS; MINERAL OR SLAG WOOL
- C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
- C03C2218/00—Methods for coating glass
- C03C2218/10—Deposition methods
- C03C2218/11—Deposition methods from solutions or suspensions
- C03C2218/116—Deposition methods from solutions or suspensions by spin-coating, centrifugation
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- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Power Engineering (AREA)
- Materials Engineering (AREA)
- Life Sciences & Earth Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Chemical & Material Sciences (AREA)
- Geochemistry & Mineralogy (AREA)
- Organic Chemistry (AREA)
- Computer Hardware Design (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Inorganic Compounds Of Heavy Metals (AREA)
Abstract
The invention provides an A-site doped lanthanide metal atom bismuth ferrite film and a preparation method thereof, wherein the method comprises the following steps: s1, providing a substrate; s2, providing a precursor sol, wherein the precursor sol contains Bi ions, fe ions and lanthanide metal ions; and S3, spin-coating the precursor sol on the surface of the substrate, and performing heat treatment to obtain the A-site doped lanthanide metal atom bismuth ferrite film. According to the preparation method provided by the embodiment of the invention, the lanthanide metal element such as La is doped into the BFO film by a sol-gel technology, so that the bismuth ferrite film with no impurity phase and excellent physical properties is successfully prepared, the polarization value of the prepared doped BFO film is greatly improved, and the residual polarization value can reach 140.2 mu C/cm 2 Leakage current is suppressed to a large extent, three orders of magnitude lower than pure BFO.
Description
Technical Field
The invention relates to the technical field of ferroelectric materials, in particular to a bismuth ferrite film doped with lanthanide metal atoms at A site and a preparation method thereof.
Background
Bismuth ferrite (BiFeO) 3 BFO) is very large, along the polarity<111>The direction is 100 muC/cm 2 . This is the largest switchable polarization in all perovskite ferroelectrics, approximately twice as large as the most widely used material in ferroelectric memories, lead zirconate titanate (PZT).
BFO is currently the only room temperature single phase multiferroic material with both G-antiferromagnetic and ferroelectric properties. Thus, extensive research has been devoted to various forms of BiFeO over the last decade or more 3 Base materials including ceramic blocks, films and nanostructures. BiFeO 3 The films exhibit multifunctional structures and many interesting properties, in particular strong ferroelectricity, inherent magneto-electric coupling and emerging photovoltaic effects. The excellent physical properties make it hopeful to be widely applied toFerroelectric random access memory, surface acoustic wave device, pyroelectric detector, phase shifter, etc.
However, while the evolution of BFO presents very superior performance, there are important obstacles in BFO applications such as i) BFO possessing higher conductivity relative to PTZ and therefore large dielectric losses; ii) has a high fatigue tendency, severe film aging, and iii) has a high leakage current density. Leakage current suppresses the switching effect of ferroelectric polarization, resulting in a small tunneling resistance, which is detrimental to device performance.
Therefore, a method for effectively improving the leakage current of the BFO film is needed.
Disclosure of Invention
In view of the above, the present inventors have found through intensive studies that the loss of Bi atoms during heating and the change in valence state of Fe atoms result in a large leakage current. And a great number of repeated researches show that the bismuth ferrite film with high quality can be obtained by effectively reducing Bi loss and inhibiting chemical valence fluctuation of Fe atoms in a doping mode, and the invention is completed on the basis.
The invention provides a simple A-site doped lanthanide metal atom bismuth ferrite film (hereinafter sometimes referred to as BLFO film) capable of effectively improving leakage current of a BFO film and a preparation method thereof.
In order to solve the technical problems, the invention adopts the following technical scheme:
according to an embodiment of the first aspect of the invention, the preparation method of the bismuth ferrite film doped with lanthanide metal atoms at the A site comprises the following steps:
s1, providing a substrate;
s2, providing a precursor sol, wherein the precursor sol contains Bi ions, fe ions and lanthanide metal ions;
and S3, spin-coating the precursor sol on the surface of the substrate, and performing heat treatment to obtain the A-site doped lanthanide metal atom bismuth ferrite film.
That is, according to the preparation method of the present invention, the lanthanide metal atom is introduced into the a site of BFO by the sol-gel method, and according to the method, bi and the lanthanide metal atom undergo a gelation reaction together with Fe atoms from the precursor sol, so that the purity of the formed a-site doped lanthanide metal atom bismuth ferrite film can be improved without introducing the second phase impurity.
Further, the step S1 includes:
and cleaning and heat-treating the substrate to remove impurities on the surface of the substrate.
Further, the step S2 includes:
s21, providing a soluble Bi ion compound, a soluble Fe ion compound and a soluble lanthanide metal ion compound;
s22, fully dissolving the soluble Bi ion compound, the soluble Fe ion compound and the soluble lanthanide metal ion compound in a solvent to obtain a solution;
s23, adding a chelating agent, a dispersing agent and a stabilizing agent into the solution, and aging at room temperature to obtain the precursor sol.
Further, the soluble Bi ion compound is Bi (NO 3 ) 3 ·5H 2 O, the soluble Fe ion compound is Fe (NO) 3 ) 3 ·9H 2 O, the soluble lanthanide metal ion compound is La (NO) 3 ) 3 ·5H 2 O。
Further, the solvent is ethylene glycol methyl ether, acetic acid, or a mixture thereof, and Bi (NO 3 ) 3 ·5H 2 The concentration of O is 0.2-0.4mol/L.
Further, the Bi (NO 3 ) 3 ·5H 2 O：La(NO 3 ) 3 ·5H 2 O：Fe(NO 3 ) 3 ·9H 2 The molar ratio of O is 1.04-x:x:1, wherein x is in the range of 0.01-0.1.
Still further, the chelating agent is citric acid or a hydrate thereof, and the atomic ratio of the chelating agent to the sum of metal cations is (0.8-2): 1; the dispersing agent is glycol, and the volume ratio of the dispersing agent to the solvent is 1: (16-32); the stabilizer is ethanolamine, and the volume ratio of the stabilizer to the dispersing agent is 1:1.
Further, the step S3 includes:
s31, spin-coating the precursor sol on the surface of the substrate to form a wet film on the surface of the substrate;
s32, drying the substrate with the wet film, and annealing at 500-600 ℃ for 5-15min to form a dry film;
s33, repeating the above cycle 10-20 times by taking the steps S31 and S32 as one cycle, so as to form a film with a preset thickness on the surface of the substrate;
and S34, heating the substrate with the film formed with the preset thickness at 500-600 ℃ for 20min-1h to enable the film to be densified, and obtaining the bismuth ferrite film doped with lanthanide metal atoms at the A site.
Still further, the step S31 includes:
setting the substrate on an objective table of a spin coater and vacuumizing to fix the substrate;
spreading the precursor sol over the substrate;
spin coating the substrate at 500-800rpm for 5-10s, so that the precursor sol infiltrates the substrate;
then the rotating speed is increased to 1000-2000rpm, spin coating is continued for 5-10s, and redundant sol on the surface of the substrate is removed;
and then the rotating speed is further increased to 3500-4500rpm, spin coating is continued for 10-20s, and excessive sol is further removed, so that the wet film is formed on the surface of the substrate.
According to the bismuth ferrite film of the embodiment of the second aspect of the invention, the A site of bismuth ferrite is doped with lanthanide metal atoms, and the atomic ratio of the lanthanide metal atoms to the Bi atoms is (0.01-0.1): 1, the leakage current J value of the bismuth ferrite film doped with lanthanide series metal atoms at the A site is 4.8x10 -4 A/cm 2 The following is given.
The technical scheme of the invention has at least one of the following beneficial effects:
according to the preparation method of the invention, by introducing lanthanide metal atoms into the A site, oxygen vacancies are reduced,limiting leakage current and effectively inhibiting Fe 3+ To Fe 2+ The ferroelectric property of the prepared bismuth ferrite film is greatly improved, and the dielectric property is also improved.
Drawings
FIG. 1 is a graph of P-E hysteresis loops of BLFO films prepared at different precursor solution concentrations;
FIG. 2 is a graph of P-E hysteresis loops of BLFO films prepared with different amounts of chelating agent;
FIG. 3 is a graph of P-E hysteresis loops of BLFO films with different numbers of coating layers;
FIG. 4 is a graph of P-E hysteresis loops of the resulting BLFO film prepared at various annealing temperatures;
FIG. 5 is XRD patterns of the films obtained in examples 1 to 5 and comparative example;
FIG. 6 is a Raman spectrum of the films obtained in examples 1-5 and comparative examples;
FIG. 7 is XPS spectra of films obtained in examples 1-5 and comparative examples;
FIG. 8 is a graph showing leakage current density curves of films obtained in examples 1-5 and comparative examples;
FIG. 9 is a graph showing the room temperature P-E hysteresis curves of the films obtained in examples 1-5 and comparative examples;
FIG. 10 is a graph showing the frequency-dielectric properties of the films obtained in examples 1-5 and comparative examples;
FIG. 11 is an absorption spectrum of the films obtained in examples 1 to 5 and comparative example.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. It will be apparent that the described embodiments are some, but not all, embodiments of the invention. All other embodiments, which are obtained by a person skilled in the art based on the described embodiments of the invention, fall within the scope of protection of the invention.
Unless defined otherwise, technical or scientific terms used herein should be given the ordinary meaning as understood by one of ordinary skill in the art to which this invention belongs. The terms "first," "second," and the like, as used herein, do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Likewise, the terms "a" or "an" and the like do not denote a limitation of quantity, but rather denote the presence of at least one. The terms "connected" or "connected," and the like, are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "upper", "lower", "left", "right", etc. are used merely to indicate a relative positional relationship, which changes accordingly when the absolute position of the object to be described changes.
In the following, first, the preparation method according to the embodiment of the present invention is explained.
The preparation method comprises the following steps:
s1, providing a substrate.
As the substrate, there is no particular limitation. For example, from the standpoint of manufacturing a ferroelectric random access memory or the like, the thin film may be formed directly on the FTO substrate.
Furthermore, in order to avoid introducing impurities from the substrate, in some embodiments of the invention, the step S1 comprises:
and cleaning and heat-treating the substrate to remove impurities on the surface of the substrate.
Specifically, taking the substrate as FTO conductive glass as an example, for example, the substrate can be sequentially subjected to ultrasonic cleaning of acetone, alcohol and water to remove organic matters on the surface, and then the FTO layer is cleaned by using a special conductive glass cleaning agent.
Thereafter, the clean substrate was placed in a constant temperature muffle furnace at 500-600 ℃ and heat treated for 3min, cooled to room temperature in air.
Thereby, impurities on the surface of the substrate can be thoroughly removed. Further, by subjecting the substrate to heat treatment at a temperature at which gelation is subsequent, it can be ensured that impurities are not introduced into the thin film from the substrate in the subsequent heat treatment.
S2, providing a precursor sol, wherein the precursor sol contains Bi ions, fe ions and lanthanide metal ions.
That is, in order to form a thin film on a substrate, a precursor sol containing Bi ions and Fe ions, and lanthanide metal ions is prepared.
In some embodiments of the present application, the step S2 includes:
s21, providing a soluble Bi ion compound, a soluble Fe ion compound and a soluble lanthanide metal ion compound;
s22, fully dissolving the soluble Bi ion compound, the soluble Fe ion compound and the soluble lanthanide metal ion compound in a solvent to obtain a solution;
s23, adding a chelating agent, a dispersing agent and a stabilizing agent into the solution, and aging at room temperature to obtain the precursor sol.
Further, the soluble Bi ion compound may be Bi (NO) in view of solubility, stability, etc 3 ) 3 ·5H 2 O, the soluble Fe ion compound may be, for example, fe (NO) 3 ) 3 ·9H 2 O, the soluble lanthanide metal ion compound may be La (NO) 3 ) 3 ·5H 2 O。
As the soluble lanthanide metal ion compound, a compound other than a lanthanide metal compound may be used.
The solvent may be, for example, ethylene glycol methyl ether, acetic acid, or a mixture thereof. As an example, ethylene glycol methyl ether, for example, may be used: acetic acid is mixed solvent obtained by 3:1 mixing. The mixed solvent has better stability and better solubility of each precursor compound.
In addition, in terms of the concentration of the precursor solution, for example, bi (NO 3 ) 3 ·5H 2 The concentration of O is 0.2-0.4mol/L. The concentration of the solution has great influence on the gel time, the concentration is too low, the gelation time is prolonged, the viscosity of the sol is low, and the gel and the lining are mutually connectedPoor bottom bonding degree; and if the concentration is too high, the thickness of the single layer of the BFO film is too high, crystal grains grow abnormally in the annealing crystallization process, the surface is cracked, and the film performance is poor.
In addition, bi (NO 3 ) 3 ·5H 2 O：La(NO 3 ) 3 ·5H 2 O：Fe(NO 3 ) 3 ·9H 2 The molar ratio of O is 1.04-x:x:1, wherein x is in the range of 0.01-0.1. That is, the lanthanum element is selected to be doped with Bi at the A site, the doping amount is 0.01-0.1, and the Bi element is slightly higher than the chemical equivalent. This is to consider that a slightly higher than stoichiometric amount of Bi element is used, which is to consider that Bi has some volatilization during the heat treatment, and 4% excess bismuth is added in order to effectively avoid the generation of bismuth vacancies.
In addition, in order to maintain the stability of the sol and to controllably form bismuth ferrite at the rate of the subsequent heat treatment process, a chelating agent, a dispersing agent and a stabilizing agent may be added to the solution, and the precursor sol may be aged at room temperature.
Among them, as the chelating agent, for example, citric acid or a hydrate thereof is used, and the atomic ratio of the chelating agent to the total metal cations (i.e., the total of three metal cations) is (0.8-2): 1.
In addition, in order to avoid particle agglomeration during gelation, a dispersing agent, for example, ethylene glycol, is added to the sol, wherein the volume ratio of the dispersing agent to the solvent is 1: (16-32).
In addition, as the stabilizer, for example, ethanolamine may be used, and the volume ratio of the stabilizer to the dispersant is 1:1.
As an example, raw material Bi (NO 3 ) 3 ·5H2O、La(NO 3 ) 3 ·5H 2 O and Fe (NO) 3 ) 3 ·9H 2 O is as follows 1.04-x: x:1 in a mixed solvent of ethylene glycol methyl ether and acetic acid (solvent ratio is 3:1). The raw materials were sufficiently dissolved by a magnetic stirrer at room temperature. Stirring for 1-1.5 hr, adding chelating agent (citric acid monohydrate: C) 6 H 8 O 7 ·H 2 O) continuing stirring for 0.5h, adding dispersant glycol (C) 2 H 6 O 2 ) Stirring for 15min, dispersing, adding stabilizer ethanolamine (C) 2 H 7 NO), enhancing solution stability and viscosity. Finally, regulating the solution to a certain concentration, and aging for 24-48 hours at room temperature to obtain the sol.
And S3, spin-coating the precursor sol on the surface of the substrate, and performing heat treatment to obtain the A-site doped lanthanide metal atom bismuth ferrite film.
That is, after the precursor sol is prepared, it is spin-coated on the surface of the substrate and heated to gel, thereby forming a bismuth ferrite film doped with lanthanide metal atoms at the a-site on the substrate.
Among them, since the thickness of the thin film formed by one spin coating is limited, in order to form a bismuth ferrite thin film of a certain thickness in consideration of practicality, in some embodiments of the present invention, multiple spin coating is employed.
In addition, in order to improve the compactness, the film can be gelled by heat treatment after spin coating, and then spin coating, gelation, spin coating, gelation and the like are continued, so that the bismuth ferrite film with a certain thickness of A-site doped lanthanide metal atoms is finally formed.
Specifically, in some embodiments of the present invention, the step S3 includes:
s31, spin-coating the precursor sol on the surface of the substrate to form a wet film on the surface of the substrate;
s32, drying the substrate with the wet film, and annealing at 500-600 ℃ for 5-15min to form a dry film;
s33, repeating the above cycle 10-20 times by taking the steps S31 and S32 as one cycle, so as to form a film with a preset thickness on the surface of the substrate;
and S34, heating the substrate with the film formed with the preset thickness at 500-600 ℃ for 20min-1h to enable the film to be densified, and obtaining the bismuth ferrite film doped with lanthanide metal atoms at the A site.
In addition, in spin coating of each layer, in order to ensure that the sol has sufficient wettability with the previous layer or substrate and remove the superfluous sol on the surface, the step S31 preferably includes:
setting the substrate on an objective table of a spin coater and vacuumizing to fix the substrate;
spreading the precursor sol over the substrate;
spin coating the substrate at 500-800rpm for 5-10s, so that the precursor sol infiltrates the substrate;
then the rotating speed is increased to 1000-2000rpm, spin coating is continued for 5-10s, and redundant sol on the surface of the substrate is removed;
and then the rotating speed is further increased to 3500-4500rpm, spin coating is continued for 10-20s, and excessive sol is further removed, so that the wet film is formed on the surface of the substrate.
That is, in spin coating, the sol is first made to fully infiltrate the substrate at a low speed; then, spin coating at medium speed to remove a part of redundant sol; finally, spin coating is continued at 3500-4500rpm for 10-20s, and excess sol is further removed, so that a wet film formed by sol is obtained.
After the wet film is obtained, in step S32, the substrate on which the wet film is formed is first dried at a low temperature, for example, 60 to 85 ℃ for 5 to 15 minutes to remove a part of moisture and an organic substance, for example, a solvent, in order to avoid cracking or the like caused by rapid drying. After the preliminary drying, heating to 500-600 ℃ and annealing for 5-15min to form a dry film. Thereafter, it was naturally cooled to room temperature. The organic matters, nitrate ions and water remaining in the thin film are further removed by the high-temperature heat treatment of step S32, and the thin film is allowed to complete preliminary crystallization.
In addition, after repeated spin coating and gelation for a plurality of times, the substrate on which the film with a preset thickness is formed is heated at 500-600 ℃ for 20min-1h so as to densify the film, and the bismuth ferrite film with the A-site doped lanthanide metal atoms is obtained.
The method of the present invention and the BLFO film produced by the method are described in further detail below with reference to specific test examples, and comparative examples.
Test example:
1) Substrate processing
Selecting fluorine doped FTO/glass substrate with the size of 10 multiplied by 10mm and the sheet resistance of 8 ohm/cm 2 The transmittance is more than 83%, and the haze is 5%.
The substrate is cleaned and heat treated prior to spin coating. The substrate is sequentially subjected to ultrasonic cleaning of acetone, alcohol and water to remove organic matters on the surface, and then the FTO layer is cleaned by using a special conductive glass cleaning agent.
After cleaning, the clean substrate is subjected to a heat treatment. And (3) placing the substrate into a constant temperature muffle furnace at 550 ℃, performing heat treatment for 3-5 min, and cooling to room temperature in air.
2) Preparation of precursor liquid
Raw material Bi (NO) 3 ) 3 ·5H 2 O、La(NO 3 ) 3 ·5H 2 O and Fe (NO) 3 ) 3 ·9H 2 O is as follows 1.04-x: x:1 in a mixed solvent of 16ml of ethylene glycol methyl ether and acetic acid, and the solvent ratio is 3:1 (namely, the mixed solvent obtained by mixing 12ml of ethylene glycol methyl ether and 4ml of acetic acid).
The raw materials were sufficiently dissolved by a magnetic stirrer at room temperature.
Stirring for 1-1.5 hr, adding chelating agent (citric acid monohydrate: C) 6 H 8 O 7 ·H 2 And O), stirring for 0.5 h.
Thereafter, a dispersant of ethylene glycol (C 2 H 6 O 2 ) 0.5ml, stirring for 15min, dispersing, adding stabilizer ethanolamine (C) 2 H 7 NO) 0.5ml, and stirring is continued until uniform.
Finally, aging is carried out at room temperature for 48 hours. The solution is further complexed, uniform and stable.
3) Preparation of films
First, the cleaned FTO/glass is placed in a stage of a spin coater and evacuated to hold the substrate.
Thereafter, a sol is spread on the substrate, and the sol spreads over the substrate due to surface tension.
Thereafter, spin-coating at a low speed of 700rpm/min for 7s so that the sol completely wets the substrate; then spin-coating for 8s at a medium speed of 1500rpm/min, and removing a part of redundant sol; finally, spin-coating at 4000rpm/min for 15s at high speed, further removing excess sol to obtain a wet film.
Then, the substrate on which the wet film was formed was placed in a vacuum drying oven at 85 ℃ and dried for 10 minutes to remove organic matters and moisture in a part of the film. Then, further heat treatment at 550 ℃ is performed, annealing is performed for 10min, a dry film is obtained, and cooling is performed in air to room temperature. The high-temperature heat treatment can further remove organic matters, nitrate ions and water remaining in the film, and cause the film to complete preliminary crystallization.
In order to obtain a film with a certain thickness, a layer-by-layer spin coating and annealing method is adopted, and the steps are repeated for 10-20 times to obtain the film with a certain thickness.
And finally, placing the substrate subjected to the multilayer spin coating into a constant temperature muffle furnace at 550 ℃ for heat treatment for 30min, and improving the density of the film sample to obtain the A-site La-doped bismuth ferrite film (BLFO).
4) Preparation of top Pt electrode
In order to study the ferroelectric properties of the resulting BLFO film, it is necessary to study the ferroelectric properties by voltage drop between the top and bottom electrodes under the application of a voltage across the BLFO film.
Wherein the substrate FTO may serve as the bottom electrode. For this purpose, a sputtering apparatus was chosen to produce round Pt electrodes on the surface of the BLFO film. The radius of the electrode was 0.25cm. And (3) carrying out heat treatment on the sputtered BLFO film at 300 ℃ for 25-30 min.
In order to discuss the effect of various process conditions on film properties, the following study was conducted.
Influence of the precursor concentration on the ferroelectric properties of the thin film
To investigate the effect of different concentrations on the ferroelectric properties of thin films, in 2) above, the raw material Bi (NO 3 ) 3 ·5H 2 O and Fe (NO) 3 ) 3 ·9H 2 The concentration of O is Bi (NO 3 ) 3 ·5H 2 O calculation (wherein, bi (NO 3 ) 3 ·5H 2 O：Fe(NO 3 ) 3 ·9H 2 The molar ratio of O is 1.04: 1) Precursor solutions with different concentrations of 0.2,0.25 and 0.3mol/L are prepared, and BFO films are prepared and formed by utilizing the precursor solutions with different concentrations under the condition of the same other conditions.
The P-E hysteresis loop diagrams of BFO films prepared at different precursor solution concentrations are shown in fig. 1. As shown in fig. 1, the solution concentration greatly affects the BFO film P-E hysteresis loop. Wherein, the BFO film with 0.25M has the maximum value of the residual polarization intensity in the P-E hysteresis loop and the highest rectangle degree, which shows that the solution concentration and viscosity with 0.25M are the best. In contrast, the smaller 0.20M BFO film breaks down at low electric fields, probably because of the too low concentration, reduced viscosity, resulting in reduced bonding to the substrate; whereas the larger 0.30M BFO film surface was cracked.
(II) Effect of chelator usage on ferroelectric Properties of BFO thin films
Citric acid is added into the precursor solution as a complexing agent, and bismuth ferrite complex is generated through complexation reaction.
To investigate the effect of citric acid content on the ferroelectric properties of BFO films, in 2) above, BLFO films were prepared with atomic ratios of citric acid amount to metal cation sum of 0.85,1,1.15 and 3.
Figure 2 shows the P-E hysteresis loops of BLFO films prepared with different amounts of chelating agent. The experimental result of fig. 2 shows that the residual polarization value of the BFO film increases and then decreases with increasing citric acid content. When the atomic ratio is less than or equal to 1, the breakdown electric field of the BFO film is lower, the rectangle degree of the hysteresis loop is lower, and the hysteresis loop is approximately round, which is probably due to the fact that when the content of citric acid is lower, the raw material complexation reaction is lower, and the precursor stability is poorer, because the breakdown resistance of the film is lower. When the atomic ratio is increased to 1.15, the ferroelectric property of the BFO film is improved, and the rectangle degree of the hysteresis loop is high. When the atomic ratio, i.e., the citric acid content, is further increased, the breakdown voltage is not reduced, but an excessive amount of citric acid is added to the precursor solution to form a precipitate.
(III) influence of different spin-coating numbers on ferroelectric properties of BFO film
The number of spin-on layers determines the ferroelectric phase content of the BLFO film and has a large impact on the performance of the film material.
To explore the optimal number of spin-on layers, 10,12,14,16 layers of BLFO film were made under otherwise identical conditions. Figure 3 shows the P-E hysteresis loops of BLFO films of different layers.
The results of fig. 3 show that the ferroelectric properties of the spin-coated 14-layer BFO film are best. The reasons are as follows: too thin a film results in insufficiently dense films; while too thick results in a gradual accumulation of organics and a decrease in the quality of the sample.
(IV) influence of different annealing temperatures on ferroelectric properties of BFO film
The annealing temperature has a critical effect on the crystallization quality of the BLFO film, and too low an annealing temperature can result in insufficient crystallization of the spin-on film, and too high an annealing temperature can easily cause Bi to volatilize and introduce other impurity phases.
To investigate the effect of different annealing temperatures on the ferroelectric properties of BFO films, BFO films were prepared using annealing at 500, 525, 550, 575, and 600 ℃, respectively.
As can be seen from fig. 4, the thin film annealed at 550 ℃ has the best ferroelectric properties, while the sample breakdown voltage becomes lower as the temperature continues to rise to 600 ℃. This is probably due to the fact that the annealing temperature is too high, bi volatilizes seriously, a large number of bismuth vacancies and oxygen vacancies exist in the spin-coating film, so that the defects in the spin-coating film are too many, the inversion of ferroelectric domains is hindered, and the annealing temperature is too low, so that the crystallization quality is not high.
Based on the test results, the BFO film prepared under the conditions that the annealing temperature is 550 ℃, the coating layer number is 14, the chelating agent dosage ratio is 1.15, and the precursor solution concentration is 0.25mol/L has the best performance. It should be noted that since La is only introduced in a trace amount, the above test results should have the same or similar effects in the preparation of BLFO films.
Examples 1 to 5 and comparative examples
Based on the above test results, the present invention investigated the effect of different doping amounts on various properties in determining that the annealing temperature was 550 ℃, the number of coating layers was 14, the chelating agent dosage ratio was 1.15, and the precursor solution concentration was 0.25 mol/L.
That is, raw material Bi (NO 3 ) 3 ·5H 2 O、La(NO 3 ) 3 ·5H 2 O and Fe (NO) 3 ) 3 ·9H 2 O is mixed according to the mol ratio of 1.04-x: x:1 in a solvent, and preparing comparative examples and examples.
The values of x for each example and comparative example are shown in table 1 below.
TABLE 1 La (NO) used in each of examples and comparative examples 3 ) 3 Amount of 5H2O
| Comparative example | Example 1 | Example 2 | Example 3 | Example 4 | Example 5 | |
| Value of x | 0 | 0.01 | 0.02 | 0.03 | 0.04 | 0.05 |
(A) XRD pattern
Figure 5 shows the XRD patterns of the films obtained for each example and comparative example.
As can be seen from fig. 5 (a), the films all crystallized well, and no second phase impurity was generated other than FTO. Meanwhile, the main peak is matched with a standard card (NO: 72-2112), which shows that the prepared film has a distorted rhombic perovskite structure of R-3m space group. XRD results indicate that La doping does not alter the structure of BFO. In the course of the diffraction peak,is the strongest bimodal of (3).
Further, fig. 5 (b) shows a partial enlarged view of 31 to 33 °. It can be seen that as the La3+ content increases, the corresponding (110) peak gradually moves to a large angle, and finally, the peak moves to a large angleThe peaks are combined into one peak. This is probably due to the distortion of the crystal structure caused by La doping, indicating La 3+ Ions have successfully replaced part of the Bi ions. Furthermore, with La 3+ The content of the silicon dioxide is increased,the peak becomes sharper.
Table 2 shows lattice parameters and average grain sizes of the respective examples and comparative examples.
TABLE 2 lattice parameters and average grain sizes of the films obtained in examples and comparative examples
As shown in table 2, the lattice constant of the BLFO film was changed (error 0.001). As can be seen from Table 2, the lattice constant of the BLFO film is slightly lower than that of the BFO film (i.e., comparative example), due to La 3+ Is caused by the doping effect of (a). Due to La 3+ Radius of less than Bi 3+ />Therefore La 3+ Substitution at the Bi site will result in a lattice reduction of BLFO relative to BFO. That is, la 3+ The addition of ions results in a reduction of the average grain size. La (La) 3+ The introduction of ions suppresses the formation of crystals to some extent, which may be the cause of the decrease in average grain size. And with La 3+ The increase in ion concentration and the decrease in grain size are due to the suppression of oxygen vacancies.
In addition, the microstructure was also studied. For pure BFO, SEM micrograph (not shown) shows a number of pores and grain size ratio containing La 3+ The film is large and the density is low. And with La 3+ The surface becomes smoother, denser and the voids are reduced. Further, the thicknesses of the films of comparative example and example 2 were 1.014 and 0.875nm, respectively, as shown by SEM micrograph, which suggests that La doping would reduce the film thickness.
(B) Raman spectrum
Analysis of samples by Raman Scattering to further investigate La 3+ The effect of ion doping on the crystal structure, the result of raman scattering can also be complementary to XRD.
FIG. 6 is 50 to 800cm -1 Is a raman spectrum of (c). Based on group theory predictions, the original BFO has 13 active modes. The spectra obtained were in accordance with the Lorentzian model and, as expected, nine vibration modes were observed. The A mode is due to Bi-O bond at 400cm -1 Vibration in the following mode. The dominant E-mode transition can be seen here. The incorporation of ions has a significant effect on the vibration intensity and position of Fe-O and Bi-O. On the one hand, compared to the pure BFO sample (comparative example), la was doped 3+ A of ion sample 1 -1 and A 1 The peak intensity of-2 was changed. At the same time, A 1 The relative intensity of the-1 peak is significantly reduced, A 1 -2 peak moves to low angle andgradually forms a combined peak with the A1-1 peak, which shows that La is doped at Bi site 3+ The ions distort the Bi-O-Bi bond angle and squeeze the Bi-O length.
In addition, since the ionic radii of La and Bi are not matched, the lengths of La-O and Bi-O tend to vary, and the electronegativity of La-O and Bi-O are the same, resulting in peak shifts. On the other hand, with La 3+ The peak widths of the E-7, 8 and 9 vibrational modes increase gradually with increasing ion content, and the band shifts to higher wavenumbers. This phenomenon suggests that La has been successfully incorporated into BFO.
The above results indicate that La 3+ Successfully replace Bi 3+ Bi-O and Fe-O bonds vary due to changes in bond angles and bond lengths, consistent with XRD results.
(C) XPS (XPS Spectrum) map
XPS was used to evaluate elemental information in all samples. XPS fit plots of Fe 2p, bi 4f, la 3d and O1s orbitals for BLFO films are shown in FIGS. 7 (a) - (d). As shown in fig. 7 (a), the two prominent peaks of 158.06eV and 163.03eV, respectively, can be attributed to Bi 4f based on spin orbit interactions 7/2 And Bi 4f 5/2 . The presence of Bi-O bonds in the films reflects Bi in these samples 3+ Indicating the oxidation state of Bi 3+ The ion content is sufficient. The XPS peak signal of La 3d in the prepared BLFO sample is shown in FIG. 7 (b), indicating that La has been effectively incorporated into the LBFO film.
Under normal conditions, the valence state fluctuation of Fe has a direct influence on the oxygen vacancy concentration, which is an important factor in controlling the leakage current density. The Fe ion has two valence states, fe 2+ And Fe (Fe) 3+ Binding energies were 709.8 and 711.2eV, respectively. Fe (Fe) 2+ The presence of ions in the film is determined by Fe 3+ The valence state of the ions is converted to exist. Thus, the oxygen vacancies and valence states of the different extracts are changed:
fe in BLFO film 3+ /Fe 2+ Peak area ratios of 0.52, 0.55, 0.66, 0.62, 0.56 and 0.55, respectively, indicate lessLa doping in an amount effective to prevent Fe 2+ Is formed by the steps of (a).
Oxygen vacanciesIs affected by the absence of Bi element and the valence state of Fe ions.
XPS fit of O1s for BLFO film is shown in FIG. 7 (d). The asymmetric peak of the O1s spectrum is divided into lattice oxygen (O 2- )、And adsorbing oxygen. The most pronounced signal is O of 529eV 2- A signal. At 530.2eV, O (surface adsorbed oxygen) was found to be chemically bonded to Fe or Bi atoms. Furthermore, the peak at 531.4eV corresponds to +.>The experimental result shows that the method has the advantages of high yield,the ratios were 0.14, 0.13, 0.10, 0.12, 0.13 and 0.14, respectively. The concentration of oxygen vacancies of BLFO-2 is minimized due to the presence of the defect complex, preventing Fe 3+ To Fe 2+ Is transformed by the above method.
Experimental results show that La 3+ Successful BFO incorporation occupies Bi atoms and is achieved by reductionConcentration to inhibit Fe 2+ Is formed by the steps of (a). The removal of oxygen vacancies can greatly reduce leakage current and provide excellent ferroelectric properties in BFO films.
(D) Leakage current
Leakage current density (J) is found by mapping the insulating properties of the sample to strongly interact with polarization switching and rapidly destroy the electrically polarized sample.
As shown in fig. 8 (a), all J values of the BLFO film increase with increasing electric field (E). Meanwhile, for a given electric field, J was lower for all La-doped samples than for pure BFO films (comparative), and as x increased from 0 to 0.05, J decreased first and then increased.
The BLFO samples have J values of 9.6X10 for x from 0 to 0.05, respectively -3 、4.8×10 -4 、1.1×10 -6 、2.9×10 -6 、1.5×10 -5 And 6.5X10 -5 kV/cm。
Of all samples, example 2 (i.e., x=0.02) showed the lowest leakage current density, showing a significant improvement of three orders of magnitude lower than the comparative sample. Therefore, la addition improves the ferroelectric properties of BLFO samples, inhibits Bi 3+ Loss of (2) and Fe 3+ And Fe (Fe) 2+ Electron hopping between ions limitsIs formed by the steps of (a).
Fig. 8 (b) shows Log J-Log E curves to further investigate the leakage transport mechanism of BLFO samples. According to the formula J-Eα, the logarithm of the leakage current and the logarithm of the applied electric field are basically in a linear relation. The slope value may be used to determine the electrical leakage conduction mechanism in the film. The alpha values for ohmic conduction and Space Charge Limited Current (SCLC) conduction are 1 and 2, respectively. The slope of the curve of the BLFO film is shown in fig. 8 (b). The conduction of a pure phase BFO film at low electric fields is the SCLC conduction mechanism (α=2.62). However, at high electric fields, the α value drops to 1.94. The BLFO film exhibits ohmic conduction at low electric fields.
On the other hand, the conduction mechanism changes with an increase in the doping amount. The example 1 film exhibited an SCLC conduction mechanism at low electric fields, while the example 2 and example 3 films exhibited a hybrid ohm-SCLC conduction mechanism. The conduction mechanism of the example 4 film is SCLC at low electric field, but the conduction mechanism of the example 5 film is ohmic. As the electric field strength increases, more electrons are injected into the thin films and defects in these films, such as vacancies and interstitial ions, trap free carriers and create a localized electric field opposite the external electric field. When the injected carrier concentration exceeds the thermal equilibrium carrier concentration, the conduction mode is converted to SCLC.
(E) Room temperature P-E hysteresis loop
By using at room temperatureThe ferroelectric properties of the BLFO film were studied in the P-E loop at various bias voltages, and the results of comparative examples and examples 1 to 5 are shown in fig. 9 (a) - (f), respectively. All samples were loaded with the maximum voltage they can withstand to obtain a saturated P-E curve. The comparative thin film P-E loop (i.e., graph (a)) was unsaturated, indicating high leakage current and poor ferroelectric properties. Residual polarization of the film of comparative example (2P r) 168.76 μC/cm 2 Coercive field (2E c ) 734.4kV/cm. With increasing La doping amount, P of BLFO film r The rectangular shape of the ring increases sharply, as does the rectangular shape of the ring.
Table 2 shows the 2P of BLFO of each example and BFO of comparative example r And 2E c Is a test value of (a).
TABLE 2 ferroelectric parameters of films of examples and comparative examples
It can be seen from the data in table 2 that doping in sufficient amounts can increase the polarizability and ferroelectricity. 2P of example 2 r The highest value (280.4. Mu.C/cm) 2 ) And 2P r The value decreases with further increase in the La doping amount. This finding can be explained by the three factors listed below. (i) XRD data showed that La doping resulted in deformation of the perovskite structure. This structural deformation contributes to an increase in P s And P r Is a value of (2); (ii) La doping improves the crystalline quality of the film and reduces the grain size. The larger the number of grain boundaries, the smaller the grain size, the larger the oxygen mobility is limited, and as the number of grain boundaries increases, the leakage current density decreases. When x exceeds 0.02, grains are gradually formed, the blocking effect of grain boundaries on leakage current is reduced, J is increased, and ferroelectric performance is reduced; (iii) According to XPS data, la doping effectively inhibits Fe 2+ Ions and oxygen vacancies, resulting in a decrease in J. Therefore, ion doping can reduce grain boundary resistance, fe 2+ Concentration and oxygen vacancy concentration, thereby reducing leakage current and increasing polarization value.
(F) Dielectric Properties
The dielectric versus frequency relationship of the film is shown in fig. 10. Since more polarization switching is possible, the dielectric constant (ε r ) With the largest start value. Epsilon as the frequency increases r Sharply decreases and stabilizes around 1000 kHz.
Epsilon for samples at 200kHz r Increasing with increasing La concentration; epsilon r The maximum value of (a) appears in example 2 (i.e., x=0.02); however, when the doping amount exceeds 0.02 ε r Descending.
Epsilon for pure BFO-0 film (i.e., comparative example) at 1kHz r 65.46, and epsilon for the films of examples 1, 2, 3, 4 and 5 r 106.72, 161.77, 126.51, 118.30 and 112.09, respectively.
Increasing epsilon r One approach to reducing the number of defects, such as oxygen defects. As shown in fig. 10 (b), the dielectric loss (tan δ) of each sample was measured at room temperature to decrease with increasing frequency, and then increased with the frequency reaching 100 kHz. tan delta is divided into three parts: polarization loss, conduction loss, and resonance loss. At low frequencies leakage current losses are considerable, but at high frequencies polarization relaxation dominates. The data indicate that BLFO-2 has the lowest tan delta, probably due to leakage current andthe concentration was the lowest. As the La concentration increased from 0 to 0.05, tan delta was 0.276, 0.253, 0.091, 0.140, 0.166, and 0.193 at 1kHz, respectively. When La substitution is carried out on Bi sites, the film sample shows good dielectric characteristics and has high epsilon r And a relatively low tan delta.
(G) Ultraviolet absorption
The absorbance spectrum of the BLFO sample was measured in the 200 to 800nm region as shown in fig. 11 (a). As shown, the blue-green light is absorbed by the BLFO film with absorption edges spanning 300 to 560nm, with a peak at 480nm.
The absorption range of the BLFO film is enlarged, the absorption coefficient is improved along with the doping of La, the absorption of visible light is improved, and the possible application range of the material is widened.
The Tauc formula is used to calculate the bandgap (E g )：
(αhν) 2 ＝A(hν-E g )，
Wherein A represents a material correlation constant, h represents a Planck constant, v represents an optical frequency, and α is an absorption coefficient.
In addition, the optical band gap E of each sample g As shown in fig. 11 (b). The graph is shown on the y-axis (αhν) 2 H v is shown on the x-axis, where E g Calculated from the intersection of the linear component of the curve with the x-axis. E of pure BFO film g The value was 2.52.8eV. E when the La doping amount increases from 0 to 0.05 g The values varied slightly, 2.37, 2.24, 2.21, 2.26, 2.29 and 2.34eV respectively. La doping causes rearrangement of molecular orbitals and changes in energy band structure, leading to direct E g Is a variation of (c). Therefore, la doping can change the optical bandgap of BFO to some extent, which means that BLFO films have a broad application potential in photovoltaic systems.
As can be seen from the experimental results, the method of the invention can effectively reduce oxygen vacancies, limit leakage current and effectively inhibit Fe by doping La at A site by sol-gel method 3+ To Fe 2+ The ferroelectric property of the prepared bismuth ferrite film is greatly improved, and the dielectric property is also improved.
While the foregoing is directed to the preferred embodiments of the present invention, it will be appreciated by those skilled in the art that various modifications and adaptations can be made without departing from the principles of the present invention, and such modifications and adaptations are intended to be comprehended within the scope of the present invention.
Claims (7)
1. A preparation method of a bismuth ferrite film doped with lanthanide metal atoms at A site is characterized by comprising the following steps:
s1, providing a substrate;
s2, providing a precursor sol, wherein the precursor sol contains Bi ions, fe ions and lanthanide metal ions, and the step S2 comprises the following steps:
s21, providing a soluble Bi ionA daughter compound, a soluble Fe ion compound, and a soluble lanthanide metal ion compound, wherein the soluble Bi ion compound is Bi (NO 3 ) 3 ×5H 2 O, the soluble Fe ion compound is Fe (NO) 3 ) 3 ×9H 2 O, the soluble lanthanide metal ion compound is La (NO) 3 ) 3 ×5H 2 O；
S22, dissolving the soluble Bi ion compound, the soluble Fe ion compound and the soluble lanthanide metal ion compound in a solvent to obtain a solution, wherein the Bi (NO 3 ) 3 ×5H 2 O：La(NO 3 ) 3 ×5H 2 O：Fe(NO 3 ) 3 ×9H 2 The molar ratio of O is 1.04-x, x is 1, wherein x is in the range of 0.01-0.03;
s23, adding a chelating agent, a dispersing agent and a stabilizing agent into the solution, and aging at room temperature to obtain the precursor sol, wherein the chelating agent is citric acid or a hydrate thereof, and the atomic ratio of the chelating agent to the total metal cations is (1-2): 1;
and S3, spin-coating the precursor sol on the surface of the substrate, and performing heat treatment to obtain the A-site doped lanthanide metal atom bismuth ferrite film.
2. The method according to claim 1, wherein the step S1 comprises:
and cleaning and heat-treating the substrate to remove impurities on the surface of the substrate.
3. The method according to claim 1, wherein the solvent is ethylene glycol methyl ether, acetic acid, or a mixture thereof, and Bi (NO 3 ) 3 ×5H 2 The concentration of O is 0.2-0.4mol/L.
4. The method of claim 1, wherein the dispersant is ethylene glycol and the volume ratio of dispersant to solvent is 1: (16-32); the stabilizer is ethanolamine, and the volume ratio of the stabilizer to the dispersing agent is 1:1.
5. The method according to claim 1, wherein the step S3 comprises:
s31, spin-coating the precursor sol on the surface of the substrate to form a wet film on the surface of the substrate;
s32, drying the substrate with the wet film, and annealing at 500-600 ℃ for 5-15min to form a dry film;
s33, repeating the above cycle 10-20 times by taking the steps S31 and S32 as one cycle, so as to form a film with a preset thickness on the surface of the substrate;
and S34, heating the substrate with the film formed with the preset thickness at 500-600 ℃ for 20min-1h to enable the film to be densified, and obtaining the bismuth ferrite film doped with lanthanide metal atoms at the A site.
6. The method according to claim 5, wherein the step S31 includes:
setting the substrate on an objective table of a spin coater and vacuumizing to fix the substrate;
spreading the precursor sol over the substrate;
spin coating the substrate at 500-800rpm for 5-10s, so that the precursor sol infiltrates the substrate;
then the rotating speed is increased to 1000-2000rpm, spin coating is continued for 5-10s, and redundant sol on the surface of the substrate is removed;
and then the rotating speed is further increased to 3500-4500rpm, spin coating is continued for 10-20s, and excessive sol is further removed, so that the wet film is formed on the surface of the substrate.
7. A bismuth ferrite film characterized in that the bismuth ferrite film is prepared by the method for preparing the A-site doped lanthanide metal atom according to any one of claims 1 to 6, wherein the A-site doped lanthanide metal atom of the bismuth ferrite has leakage currentJThe value was 4.8X10 -4 A/cm 2 The following is given.
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| CN101587936A (en) * | 2009-06-10 | 2009-11-25 | 中国科学院宁波材料技术与工程研究所 | Resistive random access memory based on bismuth iron thin film system and manufacturing method thereof |
| CN102531405A (en) * | 2011-12-19 | 2012-07-04 | 陕西科技大学 | Preparation method of Sm-doped BiFeO3 ferroelectric film on surface of fluorine-doped tin oxide (FTO)/glass substrate |
| CN103693694A (en) * | 2013-12-20 | 2014-04-02 | 陕西科技大学 | Bi(1-x)DyxFeO3 low leakage current film and preparation method thereof |
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| CN101587936A (en) * | 2009-06-10 | 2009-11-25 | 中国科学院宁波材料技术与工程研究所 | Resistive random access memory based on bismuth iron thin film system and manufacturing method thereof |
| CN102531405A (en) * | 2011-12-19 | 2012-07-04 | 陕西科技大学 | Preparation method of Sm-doped BiFeO3 ferroelectric film on surface of fluorine-doped tin oxide (FTO)/glass substrate |
| CN103693694A (en) * | 2013-12-20 | 2014-04-02 | 陕西科技大学 | Bi(1-x)DyxFeO3 low leakage current film and preparation method thereof |
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