Patent Application
20240417273
This application claims priority to Korean Patent Application No. 10-2023-0076849 filed in the Korean Intellectual Property Office on Jun. 15, 2023, and all the benefits accruing therefrom under 35 U.S.C. § 119, the entire contents of which are incorporated herein by reference.
The present research has been conducted with support from the Samsung Research Funding & Incubation Center for Future Technology (Project Number: SRFC-MA2101-02).
The present research relates to a perovskite oxide thin film and its manufacturing method using selective A-site atomic gradients, and to a universal method that can generate large electric polarization and implement polar functionality in perovskite oxide ABO3.
Conventionally, the following methods are generally used to artificially break inversion symmetry or create polarization inside a material.
One example a method for controlling an oxygen octahedral rotation pattern. In this method, when a material with an oxygen octahedral rotation pattern (e.g., a−b+a−) without polar symmetry is grown in the form of a thin film on a substrate with a pattern with polar symmetry (e.g., a−a−a−), the material follows the same pattern as the substrate to achieve polar symmetry.
When a non-polar CaTiO3 material with an oxygen octahedral rotation pattern of a−b+a− in Glazer notation is grown on a LaAlO3 with the a−a−a− pattern, it shows polarity while having the same a−a−a− pattern as the substrate.
Another example is a Schottky junction method. The Schottky junction method refers to a case of joining a metal material and a semiconductor material. This method implements polar properties through a built-in-field induced by band-bending occurring in a process for adjusting different Fermi levels. For example, if the built-in-field is generated by joining a semiconductor of niobium (Nb)-doped SrTiO3 or TiO2 with gold, piezoelectricity and pyroelectricity may occur.
In addition, still another example may be a flexoelectric effect. This method utilizes a phenomenon that electrical polarization occurs in all materials due to a strain gradient, that is, the flexoelectricity. While the piezoelectricity limitedly exists only in 20 crystal point groups, the flexoelectricity may occur advantageously in all 32 point groups. In addition, because the strain gradient is inversely proportional to a length unit, the flexoelectricity that more significantly acts at nano units and the electrical polarization that occurs through this may be variously used at the nanoscales.
Furthermore, yet another example may be a conventional compositional gradient method, which is a method of inducing a compositional gradient in a thickness direction through compositional ratio changes of two different constituent atoms and artificially breaking the inversion symmetry. This method has been conventionally used in metal alloys, etc. but recently used in some oxides. This method has an advantage of artificially/universally breaking the inversion symmetry through various material combinations.
However, the Schottky junction method or the oxygen octahedral rotation pattern control method respectively has limitations of working at junctions of metals and semiconductors and using only a substrate with a polar structure.
In addition, currently-known methods of utilizing the flexoelectric effect caused by a strain gradient have limitations of inevitably accompanying many crystallographic point defects and working only when used with other devices such as a scanning probe microscope.
In addition, the previously studied compositional gradient methods, have not yet been experimentally verified with respect to strong polarity (electrical polarization) and polar functionality artificially generatable therethrough and have limitations in a size of the polarity (electrical polarization) and functionality that can be implemented.
Accordingly, it is necessary to verify a method of universally inducing the strong polarity (electrical polarization) without being limited by a substrate, physical properties, or equipment, etc. and its functionality.
One aspect of the present disclosure provides a perovskite oxide thin film that may universally generate inversion symmetry breaking inside a material without using a substrate having a special oxygen pattern or without help of other devices, may form polarization by flexoelectricity, and may achieve improved dielectric properties and pyroelectric effect.
A perovskite oxide thin film according to one aspect comprises a compound represented by Chemical Formula 1, and has a layered perovskite structure, wherein a ratio of A′ atoms to all A-sites in one A-site atomic layer is different from three or more other A-site atomic layers in the thickness direction.
A1-xA′xBO3 [Chemical Formula 1]
In Chemical Formula 1, A and A′ are different divalent or trivalent cations, B is a tetravalent or trivalent cation, and 0≤x≤1.
A and A′ may each independently include strontium (Sr), calcium (Ca), barium (Ba), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promerium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutepium (Lu), or a combination thereof.
B may include titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), or a combination thereof.
One A-site atomic layer may be an A-site atomic layer (first atomic layer) located inside in a thickness direction of the perovskite oxide thin film.
Three or more different A-site atomic layers may be, respectively, an A-site atomic layer (second atomic layer) that is located on one surface of the perovskite oxide thin film, an A-site atomic layer (third atomic layer) that is located on the other surface of the perovskite oxide thin film, and an A-site atomic layer (fourth atomic layer) that is located inside in the thickness direction of the perovskite oxide thin film and is different from the first atomic layer.
Ratios (R1) of A′ atoms to all A-sites in the first atomic layer and the fourth atomic layer may each independently be greater than 0 and less than about 1.
A ratio (R2) of A′ atoms to all A-sites in the second atomic layer may be greater than 0 and less than about 1.
A ratio (R3) of A′ atoms to all A-sites in the third atomic layer may be greater than about 0 and less than or equal to about 1.
The first atomic layer may be an A-site atomic layer of the compound represented by Chemical Formula 1.
The second atomic layer may be an A-site atomic layer of a compound represented by Chemical Formula 2.
ABO3 [Chemical Formula 2]
In Chemical Formula 2, A is a divalent or trivalent cation different from A′, and B is a tetravalent or trivalent cation.
The third atomic layer may be an A-site atomic layer of the compound represented by Chemical Formula 3.
A′BO3 [Chemical Formula 3]
In Chemical Formula 3, A′ is a divalent or trivalent cation different from A, and B is a tetravalent or trivalent cation.
The fourth atomic layer may be an A-site atomic layer of the compound represented by Chemical Formula 4.
A1-zA′zBO3 [Chemical Formula 4]
In Chemical Formula 4, A and A′ are different divalent or trivalent cations, B is a tetravalent or trivalent cation, 0<z<1, and z≠x.
The compound represented by Chemical Formula 1 may be Ba1-xCaxTiO3, the compound represented by Chemical Formula 2 may be BaTiO3, the compound represented by Chemical Formula 3 may be CaTiO3, and the compound represented by Chemical Formula 4 may be Ba1-zCazTiO3.
The perovskite oxide thin film may have a thickness direction gradient in which the ratio of A′ atoms to all A-sites in one A-site atomic layer changes in the thickness direction.
The perovskite oxide thin film may have a thickness gradient of 0.5% to 50%, calculated by Equation 1.
In Equation 1, ΔG is a gradient in the thickness direction, ALn is a ratio of A′ atoms to all A-sites in any one A-site atomic layer, and ALn+1 is a ratio of A′ atoms to all A-sites in other A-site atomic layers separated by less than or equal to about 0.4 nm.
A total thickness of the perovskite oxide thin film may be about 1.2 nm to about 100 nm.
An atomic ratio of A atoms to A′ atoms in the entire perovskite oxide thin film may be about 30:70 to about 70:30.
An average magnitude of polarization induced throughout the perovskite oxide thin film may be greater than or equal to about 20 uC/cm2.
A dielectric loss throughout the perovskite oxide thin film may be less than or equal to about 0.1 under measurement conditions of a frequency of 1 kHz to 2 MHz and an AC level of 10 mV to 200 mV.
A pyroelectric coefficient of the entire perovskite oxide thin film may be greater than or equal to about 106 uC/m2K under measurement conditions of a sinusoidal temperature change period of 20 seconds to 120 seconds and low frequency period (LFP) with a temperature change of 0.5 K to 8 K.
According to another aspect, a method of preparing a perovskite oxide thin film that comprises a compound represented by Chemical Formula 1 and has a layered perovskite structure, on a substrate wherein a ratio of A′ atoms to all A-sites in one A-site (A+A′) atomic layer is different from that of three or more other A-site atomic layers in a thickness direction.
A1-xA′xBO3 [Chemical Formula 1]
In Chemical Formula 1, A and A′ are different divalent or trivalent cations, B is a tetravalent or trivalent cation, and 0≤x≤1.
The perovskite oxide thin film may be formed by a physical vapor deposition (PVD) method, a chemical vapor deposition (CVD) method, a molecular beam epitaxy (MBE) method, an atomic layer deposition (ALD) method, a pulsed laser deposition (PLD) method, or a sputter method.
The perovskite oxide thin film may be formed using PLD (Pulsed laser deposition) equipment equipped with a high-pressure-high-speed reflection electron diffraction device (reflection high energy electron diffraction), under certain conditions of a rate of 200 pulses/unit cell to 2 pulses/unit cell.
According to embodiments, the perovskite oxide thin film can universally generate inversion symmetry breaking inside the material and form polarization by flexoelectricity without using a substrate with a special oxygen pattern or without help of other devices, has improved dielectric properties, and can generate a pyroelectric effect.
Hereinafter, with reference to the attached drawings, various embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. The invention may be implemented in many different forms and is not limited to the embodiments described herein. In order to clearly explain the present invention, parts that are not relevant to the description are omitted, and identical or similar components are assigned the same reference numerals throughout the specification.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms, including “at least one,” unless the content clearly indicates otherwise. Therefore, reference to “an” element in a claim followed by reference to “the” element is inclusive of one element as well as a plurality of the elements.
“At least one” is not to be construed as limiting “a” or “an.” “Or” means “and/or.” As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof.
It will be understood that, although the terms “first,” “second,” “third” etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, “a first element,” “component,” “region,” “layer” or “section” discussed below could be termed a second element, component, region, layer or section without departing from the teachings herein.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
A perovskite oxide thin film 100 according to an embodiment will be described with reference to
Referring to
A1-xA′xBO3 [Chemical Formula 1]
In Chemical Formula 1, A and A′ are different divalent or trivalent cations, B is a tetravalent or trivalent cation, and 0≤x≤1. At this time, B may be a cation different from A and A′.
For example, in Chemical Formula 1, A and A′ may each independently include an alkaline earth metal such as strontium (Sr), calcium (Ca), or barium (Ba); a rare earth metal such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promerium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutepium (Lu); or a combination thereof. Herein, A and A′ may be different from each other, and for example, A may be barium (Ba) and A′ may be calcium (Ca).
B may be a transition metal including titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), or a combination thereof.
x represents the atomic percent of A′ atoms and may be 0≤x≤1.
The perovskite oxide thin film 100 has a layered perovskite structure.
For example, in the layered perovskite structure, A atoms or A′ atoms are located between BO6 octahedrons that share each edge in three dimensions. For reference,
The layered perovskite structure includes a plurality of A-site atomic layers including A atoms, A′ atoms, or both between BO6 octahedrons in the thickness direction. Additionally, A-site atomic layers may include additional oxygen atoms because they share oxygen atoms with BO6 octahedrons.
Herein, the ratio of A′ atoms to all A-sites in one A-site atomic layer may be different from three or more other A-site atomic layers in the thickness direction. Herein, the thickness direction may be a direction perpendicular to the substrate (not shown) or a direction parallel to the growth direction of the perovskite oxide thin film 100. The perovskite oxide thin film may be a thin film grown in the 001 plane direction, a thin film grown in the 110 plane direction, or a thin film grown in the 111 plane direction. Hereinafter, the thin film grown in the 001 plane direction will be mainly described, but is not limited thereto.
As an example, one A-site atomic layer may be an A-site atomic layer (hereinafter referred to as “first atomic layer AL1”) located inside in the thickness direction of the perovskite oxide thin film 100. Three or more other A-site atomic layers may include, respectively, an A-site atomic layer (hereinafter referred to as “second atomic layer AL2”) located on one surface of the perovskite oxide thin film 100, an A-site atomic layer (hereinafter referred to as “third atomic layer AL3”) located on the other surface of the perovskite oxide thin film 100, and an A-site atomic layer (hereinafter referred to as “fourth atomic layer AL4”) located inside in the thickness direction of the perovskite oxide thin film 100 and being different from the first atomic layer.
That is, the second atomic layer AL2 and the third atomic layer AL3 may be located on one surface and the other surface in the thickness direction of the perovskite oxide thin film 100, respectively, and the first atomic layer AL1 and the fourth atomic layer AL4 may be located between the second atomic layer AL2 and the third atomic layer AL3.
For example, the first atomic layer AL1 may be an A-site atomic layer of the compound represented by Chemical Formula 1. Herein, the ratio (R1) of A′ atoms to the all A-sites in the first atomic layer AL1 may be greater than about 0 and less than about 1, for example greater than or equal to about 0.14 and less than about 1, greater than or equal to about 0.28 and less than about 1, greater than or equal to about 0.58 and less than about 1, greater than or equal to about 0.72 and less than about 1, or greater than or equal to about 0.86 and less than about 1, greater than about 0 and less than or equal to about 0.14, greater than about 0 and less than or equal to about 0.28, greater than about 0 and less than or equal to about 0.58, greater than about 0 and less than or equal to about 0.72, or greater than about 0 and less than or equal to about 0.86. As an example, the compound represented by Chemical Formula 1 included in the first atomic layer AL1 may be Ba1-xCaxTiO3.
In the second atomic layer AL2, a ratio (R2) of A′ atoms to all A-sites may be greater than or equal to about 0 and less than about 1, for example, greater than or equal to about 0.14 and less than about 1, greater than or equal to about 0.28 and less than about 1, or greater than or equal to about 0.58 and less than about 1, greater than or equal to about 0.72 and less than about 1, or greater than or equal to about 0.86 and less than about 1.
The second atomic layer AL2 may be an A-site atomic layer of a compound represented by Chemical Formula 2.
ABO3 [Chemical Formula 2]
In Chemical Formula 2, A is a divalent or trivalent cation different from A′, and B is a tetravalent or trivalent cation. For example, the compound represented by Chemical Formula 2 may be BaTiO3.
In the third atomic layer AL3, a ratio (R2) of A′ atoms to all A-sites may be greater than about 0 and less than or equal to about 1, for example, greater than about 0 and less than or equal to about 0.14, greater than about 0 and less than or equal to about 0.28, greater than about 0 and less than or equal to about 0.58, greater than about 0 and less than or equal to about 0.72, or greater than about 0 and less than or equal to about 0.86.
The third atomic layer AL3 may be an A-site atomic layer of a compound represented by Chemical Formula 3.
A′BO3 [Chemical Formula 3]
In Chemical Formula 3, A′ is a divalent or trivalent cation different from A, and B is a tetravalent or trivalent cation. For example, the compound represented by Chemical Formula 3 may be CaTiO3.
The fourth atomic layer AL4 may be an A-site atomic layer of the compound represented by Chemical Formula 4.
A1-zA′zBO3 [Chemical Formula 4]
In Chemical Formula 4, A and A′ are different divalent or trivalent cations, B is a tetravalent or trivalent cation, 0<z<1, and z≠x.
Herein, in the fourth atomic layer AL4, a ratio (R1) of A′ atoms to all A-sites may be greater than about 0 and less than about 1, for example greater than or equal to about 0.14 and less than about 1, greater than or equal to about 0.28 and less than about 1, greater than or equal to about 0.58 and less than about 1, greater than or equal to about 0.72 and less than about 1, or greater than or equal to about 0.86 and less than about 1, greater than about 0 and less than or equal to about 0.14, greater than about 0 and less than or equal to about 0.28, greater than about 0 and less than or equal to about 0.58, greater than about 0 and less than or equal to about 0.72, or greater than about 0 and less than or equal to about 0.86. As an example, the compound represented by Chemical Formula 4 included in the fourth atomic layer AL4 may be Ba1-zCazTiO3, where z is different from x in Chemical Formula 1.
In this way, the ratio of A′ atoms to all A-sites in the first atomic layer AL1 is different from the ratio of three or more other second atomic layer AL2, third atomic layer AL3, and fourth atomic layer AL4 in the thickness direction.
Additionally, the first atomic layer AL1 and the fourth atomic layer AL4 may be the closest A-site atomic layers adjacent to each other. That is, the perovskite oxide thin film 100 may have a different ratio of A′ atoms to all A-sites in units of A-site atomic layers.
As an example, the perovskite oxide thin film 100 may have a thickness direction gradient in which the ratio of A′ atoms to all A-sites in one A-site atomic layer changes in the thickness direction.
For example, the ratio of A′ atoms to all A-sites of the first atomic layer AL1 may stepwise or gradually decrease as it approaches the second atomic layer AL2, and the ratio of A′ atoms to all A-sites may increase stepwise or gradually as it approaches the third atomic layer AL3.
That is, the thickness direction gradient calculated by Equation 1 may be about 0.5% to about 50%, or about 2% to about 20%.
In Equation 1, ΔG is a gradient in the thickness direction, ALn is a ratio of A′ atoms to all A-sites in any one A-site atomic layer, and ALn+1 is a ratio of A′ atoms to all A-sites in other A-site atomic layers separated by less than or equal to about 0.4 nm. For example, ALn may be the ratio of A′ atoms to all A-sites in the first atomic layer AL1, and ALn+1 may be the ratio of A′ atoms to all A-sites in the fourth atomic layer AL4.
As an example,
For example, in the second atomic layer AL2, the shading of the A′ atoms is 0%, indicating that the atomic ratio (A:A′) of A atoms to A′ atoms is 1.00:0.00, in the third atomic layer AL3, the shading of the A′ atom is 100%, indicating that the atomic ratio (A:A′) of the A atoms and A′ atoms is 0.00:1.0, in the first atomic layer AL1, the shading of A′ atoms is 58%, indicating that the atomic ratio (A:A′) of A atoms to A′ atoms is 0.42:0.58, and in the fourth atomic layer AL4, the shading of A′ atoms is 42%, indicating that the atomic ratio of (A:A′) of A atoms to A′ atoms is 0.58:0.42.
In this way, the perovskite oxide thin film 100 selectively and precisely controls the ratio of A atoms and A′ atoms having different ionic radius sizes for each A-site atomic layer, resulting in a very large strain gradient in the nano-thickness unit.
Therefore, it is possible to universally break the inversion symmetry inside the material and form polarization by flexoelectricity without using a substrate with a special oxygen pattern or without the help of other devices. For example, when making an atomic gradient thin film with a thickness of about 3 nm using Ba2+ ions with an ionic radius of 0.161 nm and Ca2+ ions with an ionic radius of 0.131 nm at the A position in the titanate thin film of ATiO3, the lattice gradient is approximately 108/m, and considering that the general flexoelectric coefficient is 10−9 C/m to 10−8 C/m, a large polarization of about 10 uC/cm2 to about 100 uC/cm2 can be generated. This method is universally applicable regardless of the ion at the B position that determines the physical properties, and thus it can be used to combine various physical properties and polar structures.
As an example, a total thickness of the perovskite oxide thin film 100 may be about 1.2 nm to about 100 nm, for example, about 2.4 nm to about 20 nm, or about 4.3 nm to about 10 nm. That is, as the ratio of A′ atoms to all A-sites in the A-site atomic layer changes in units of A-site atomic layers, the perovskite oxide thin film 100 according to an embodiment can induce strong and stable electrical polarization even in ultra-thin films of about 4 nm.
The atomic ratio of A atoms to A′ atoms in the entire perovskite oxide thin film 100 may be about 30:70 to about 70:30, for example, about 40:60 to about 60:40, or about 50:50. For example, CaTiO3 is a non-polar material without polarization, and BaTiO3 has less than or equal to about 20 uC/cm2 at a thin film thickness. In the case of a (Ba0.5Ca0.5)TiO3 solid-solution thin film in which CaTiO3 and BaTiO3 are evenly mixed, with a Ba:Ca ratio of 50:50, there is no polarization. On the other hand, in the case of the perovskite oxide thin film 100 according to an embodiment, as the ratio of A′ atoms to all A-sites in the A-site atomic layer changes in units of A-site atomic layers, even when the atomic ratio of A atoms to A′ atoms in the entire perovskite oxide thin film 100 is 50:50, flexoelectricity can be exhibited.
Accordingly, an average magnitude of the polarization induced throughout the perovskite oxide thin film 100 may be greater than or equal to about 20 uC/cm2, for example, greater than or equal to about 30 uC/cm2, or greater than or equal to about 50 uC/cm2, and a maximum magnitude of polarization induced throughout the perovskite oxide thin film 100 may be greater than or equal to about 60 uC/cm2, greater than or equal to about 70 uC/cm2, or greater than or equal to about 100 uC/cm2.
Because polarization is formed during thin film synthesis at high temperature, as the perovskite oxide thin film 100 according to an embodiment is a thin film at high temperature as the ratio of A′ atoms to all A-sites in the A-site atomic layer changes in units of A-site atomic layers, charged defects (for example, oxygen vacancies) move toward the interface, leading to a very clean, defect-free state inside the thin film. In addition, because the perovskite oxide thin film 100 has a large polarization gradient, a bulk bound charge density (ρb=−∇·P) with a large negative value is induced, and therefore, electrons, which are major extrinsic carriers in oxides, are depleted inside the thin film. As a result, the perovskite oxide thin film 100 has improved dielectric properties even as a very thin and ultra-thin film.
Accordingly, the dielectric loss in the entire perovskite oxide thin film 100 is less than or equal to about 0.1, for example, less than or equal to about 0.01, or less than or equal to about 0.001 under measurement conditions of a frequency of 1 kHz to 2 MHz and an AC level of 10 mV to 200 mV. On the other hand, in thin films with a thickness of approximately 5 nm manufactured using conventional methods, leakage current is very large due to trap charges, making it impossible to measure dielectric and functionality-related results.
In addition, since the perovskite oxide thin film 100 according to an embodiment inevitably has a region inside the thin film where a paraelectric-ferroelectric phase transition occurs within the temperature range for measuring pyroelectricity, within the measurement temperature range, the dielectric constant, the flexoelectric coupling coefficient, and the flexoelectric coefficient proportional to the product of these two values may change rapidly, and the changes in flexoelectric coefficient in response to temperature changes may diverge. As a result, the perovskite oxide thin film 100 has ultra-large pyroelectric properties even as a very thin and ultra-thin film.
Accordingly, the pyroelectric coefficient of the entire perovskite oxide thin film 100 may be greater than or equal to about 105 uC/m2K, for example greater than or equal to about 106 uC/m2K, or greater than or equal to about 107 uC/m2K under measurement conditions of low frequency cycle (LFP) with a sinusoidal temperature change period of 20 seconds to 120 seconds and a temperature change of 0.5 K to 8 K. On the other hand, in thin films with a thickness of about 5 nm manufactured by conventional methods, trap charges generate currents that are out of phase with the pyroelectric current, making it impossible to measure pyroelectricity and functionality-related results.
The perovskite oxide thin film 100 according to an embodiment may be formed by a physical vapor deposition (PVD) method, a chemical vapor deposition (CVD) method, a molecular beam epitaxy (MBE) method, an atomic layer deposition (ALD) method, a pulsed laser deposition (PLD) method, or a sputter method.
As an example, the perovskite oxide thin film 100 may be formed by using a pulsed laser deposition (PLD) equipment equipped with a high pressure-high energy reflection electron diffraction device, under conditions of 200 pulses/unit cell to 2 pulses/unit cell, or under conditions of 50 pulses/unit cell or more.
When using PLD (Pulsed laser deposition) equipment equipped with high-pressure-high energy electron diffraction, by using only the first target including A atoms and the second target including A′ atoms, materials with dozens of different composition ratios of A atoms:A′ atoms can be controlled at the single atomic layer level. That is, there is no need to use N targets to manufacture N-layer thin films as in the prior art.
Hereinafter, specific examples of the invention are presented. However, the examples described below are only for illustrating or explaining the invention in detail, and should not limit the scope of the invention.
A (Ba,Ca)TiO3 epitaxy thin film with A-site atomic layer gradients were grown on a commercially available SrTiO3(001) monocrystalline substrate in a pulse laser deposition (PLD) method.
The SrTiO3 substrate was prepared by soaking in deionized water for 1 hour and etching with buffered hydrofluoric acid for 30 seconds.
Subsequently, the substrate was annealed in oxygen at 1000° C. for 6 hours to fabricate an atomically smooth single TiO2 surface with unit cell steps.
Then, each SrRuO3 layer with a thickness of 20 nm and 5 nm was deposited respectively as lower and upper electrodes.
Herein, the SrRuO3 thin film was deposited by using a KrF excimer laser (λ=248 nm) with energy density of 1.5 J/cm2 or less at a repetition rate of 3 Hz.
During the growth process, a temperature and an oxygen partial pressure were respectively maintained at 675° C. and 100 mTorr.
In the PLD system, a high-pressure reflection high-energy electron diffraction (RHEED) was mounted to continuously monitor a growth rate.
Based on RHEED vibration data of each BaTiO3 and CaTiO3 film growth on the 20 nm-thick SrRuO3 lower electrode, the (Ba,Ca)TiO3 thin film with A-site atomic layer gradients was controlled to have a deposition rate of 0.2 uc/pulse or less (
The (Ba,Ca)TiO3 thin film with A-site atomic layer gradients was grown at a temperature of 600° C. with laser energy density of 1.0 J/cm2 or less at an oxygen pressure of 10 mtorr at a repetition speed of 1 Hz.
BaTiO3, a solid-solution Ba0.5Ca0.5TiO3 thin film, and a superlattice BaTiO3/CaTiO3 alternately grown with a thickness of 1 uc were prepared under the same conditions as in the example.
After the growth process, the obtained film was slowly cooled to room temperature at 100 Torr of oxygen.
In order to measure dielectric properties according to frequency, a 60 nm-thick Pt layer was deposited on top of the SrRuO3 layer through electron beam deposition.
The upper Pt of the SrRuO3 electrode was patterned by electron beam lithography and ion-milling and had a dimension of a diameter of 15 μm and a thickness of 70 nm or less for the measurement.
In order to measure pyroelectric characteristics according to sinusoidal temperature changes, after depositing a 20 nm-thick SrRuO3 lower electrode, a (Ba,Ca)TiO3 thin film with A-site atomic layer gradients, a (Ca,Ba)TiO3 thin film with atomic layer gradients in an opposite direction thereto, and a solid-solution Ba0.5Ca0.5TiO3 thin film, a Pt electrode with a thickness of 60 nm and a diameter of 50 μm was formed in a lift-off method.
Referring to
Referring to
Referring to
In order to measure dielectric properties of the (Ba,Ca)TiO3 thin film with A-site atomic layer gradients, a metal-insulator-metal structure was prepared. This structure consisted of a 60 nm-thick Pt layer and a 5 nm-thick SrRuO3 layer as an upper electrode.
For comparison, BaTiO3 (BTO), solid-solution Ba0.5Ca0.5TiO3 (BCTO), and superlattice BaTiO3/CaTiO3 (BCTO) with the same thickness of 11 uc were prepared.
Referring to
In order to measure pyroelectric characteristics of the (Ba,Ca)TiO3 thin film with A-site atomic layer gradients, a metal-insulator-metal structure was prepared. As an upper electrode, Pt with a thickness of 60 nm and a diameter 50 μm was formed in a lift-off method right respectively on the (Ba,Ca)TiO3 thin film with A-site atomic layer gradients (atomic gradient BCTO), a (Ca,Ba)TiO3 thin film with A-site atomic layer gradients (atomic gradient CBTO), and a solid-solution Ba0.5Ca0.5TiO3 thin film.
Referring to
In conclusion, in the present example embodiment, the A-site atomic layer gradient thin film was successfully designed by controlling A site atoms in an ATiO3 perovskite structure.
The (Ba,Ca)TiO3 thin film with A-site atomic layer gradients exhibited that the atom gradients induced a strain gradient of 20×106/m in an 11 uc-thick film without structural defects.
As a result, a flexo electric field generate a large local electrical polarization of 60 μC cm−2 in the (Ba,Ca)TiO3 thin film with A-site atomic layer gradients.
The (Ba, Ca)TiO3 thin film with A-site atomic layer gradients had a dielectric constant of about 220 with a low loss (e.g., tan 6 of less than 0.1) within a frequency range of 1 kHz to 2 MHz, which was similar to a 2D nanosheet.
In addition, the pyroelectric coefficient of the (Ba, Ca)TiO3 thin film and the (Ca,Ba)TiO3 thin film having A-site atomic layer gradient was measured to be about 2.55×106 uC/m2K under the conditions of a temperature change period of 30 seconds and a temperature change of 7.5 K.
While this invention has been described in connection with what is presently considered to be practical example embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2023- 0076849 | Jun 2023 | KR | national |
