FIELD OF THE INVENTION

The present disclosure relates to a chemical solution deposition system and method for practical device applications.

BACKGROUND OF THE INVENTION

Photovoltaic (PV) technology is regarded as the most promising renewable energy technology for the coming generations. While silicon-based solar cells have been the most common type of solar cell, other PV materials and mechanisms are being explored in the quest of cost reduction or higher efficiency. Oxide materials, which are inexpensive, abundant, and stable, and their tunable physical properties through chemical replacements make them excellent candidates for thin-film photovoltaic s. The extraordinary benefits of ferroelectric photovoltaic effect (FEPV) over traditional p-n junction-based PV systems, such as high output voltage, switchable photo-current, lower fabrication costs, and polarization control PV response, make these materials promising for the realization of the nextgeneration photovoltaic technology.

To date, much research has been conducted on the photovoltaic functionality of ferroelectric materials (such as BaTiCh, Pb (Zr, Ti)C>3, and Bi^isOn). However, the bandgap of such materials is often high (> 3 eV), resulting in poor absorption of visible light. Consequently, the power conversion efficiencies of ferroelectric photovoltaic systems remain too low for practical use. Most notable among the accessible ferroelectric materials, bismuth ferrite (BFO) stands alone owing to its exceptional features such as relatively lower optical bandgap (2.1-2.7 eV), more significant remnant polarization (100 pC/cm2) and multiferroic nature (i.e. simultaneous existence of ferroelectric and anti-ferromagnetic nature). These intriguing characteristics have fueled the interest of BFO towards PV applications among the global scientific community.

The fabrication of a pure phase BFO device is a difficult task since it is frequently associated with secondary phases (such as Bi2Fe40g, Bi2sFe04o) that are not always desirable. However, physical vapor deposition (PVD) techniques [such as pulsed laser deposition (PLD), radio-frequency (RF) sputtering, and molecular beam epitaxy (MBE)] have dominated the scene when it comes to demonstrating high-quality BFO thin films for device applications. The chemical solution deposition (CSD) technology, on the other hand, is of great industrial relevance because of its cheap cost, precise control of the precursor composition, and simplicity of processing for large-area wafers. However, achieving equivalent device performance and phase-pure BFO films using the CSD technique still remains challenging. In response to these problems, we began optimizing the fabrication process of monolayer BFO thin-films to achieve an effective device performance.

Several research groups suggest using an excess of Bi (5 - 10 mol%) to get rid of the secondary phases mostly associated with the BFO films. Our group previously reported the deposition of phase pure BFO thin films using a solution deposition approach followed by spin coating processing without using the excess amount of Bi. The deposition produced phase-pure BFO, but the amplitude of the diffraction peaks was substantially lower than that of films prepared using high-end techniques such as PED, RF sputtering, and MBE.

In the view of the forgoing discussion, it is clearly portrayed that there is a need to have a chemical solution deposition system and method for practical device applications.

SUMMARY OF THE INVENTION

The present disclosure seeks to provide a chemical solution preparation system and method for depositing exceptionally crystalline BiFcOs thin films.

In an embodiment, a chemical solution deposition system for practical device applications is disclosed. The system includes a mixing chamber equipped with a magnetic stirrer to dissolve and mix 3-6 grams of bismuth nitrate pentahydrate and 3-6 grams of iron nitrate nonahydrate in a mixed solvent 15-25mL of ethanolamine and 2-methoxyethnol uniformly, wherein 8-12mL of nitric acid is added to the solution while vigorously stirring it at room temperature for an hour. The system further includes a plate to receive precursor solution and heated along with aggressive stirring for 40 minutes at 120°C until it became dark red, wherein the resulting solution is aged for about 12 hours thereby tuning a set of deposition parameters to deposit a compact film, wherein the prepared films are pre-fired for 15 minutes at 300°C. The system further includes a tube furnace to anneal the produced films in an air and oxygen atmosphere for 40 minutes at 600°C.

In one embodiment, the set of deposition parameters are selected from a group of spinning rate, spinning duration, acceleration time, and quantity of coatings.

In one embodiment, the 3-6 grams of bismuth nitrate pentahydrate preferably 4.8507 grams, having a molecular weight of 485.07 grams/mol and 3-6 grams of iron nitrate nonahydrate preferably of 4.04 grams, having a molecular weight of 404.00 grams/mol are dissolved in the mixed solvent of 20 mL ethanolamine and 2-methoxyethnol.

In one embodiment, preferably 1.2216 grams of ethanolamine with a molecular weight of 61.8 grams/mol is mixed with 2-methoxyethnol to prepare 20 mL of mixed solvent.

In one embodiment, the pre-firing for 15 minutes is divided at three distinct phases of 5 minutes each.

In another embodiment, a method for fabricating thin-film device is disclosed. The method includes dissolving stoichiometric ratios of 3-6 grams of bismuth nitrate pentahydrate and 3-6 grams of iron nitrate nonahydrate in a mixed solvent of 15-25mL of ethanolamine and 2-methoxyethnol using a magnetic stirrer. The method further includes adding 8-12mL of Nitric acid to the solution while vigorously stirring it at room temperature for an hour. The method further includes transferring the precursor solution to a heated plate and aggressively stirring for 40 minutes at 120°C until it became dark red. The method further includes aging the resulting solution for about 12 hours before the actual deposition process. The method further includes tunning a set of coating parameters in order to deposit a pure, uniform, crack- free, and compact film. The method further includes pre-firing the prepared films for 15 minutes at 300°C. The method further includes annealing the produced films in an air and oxygen atmosphere for 40 minutes in a tube furnace at 600°C for fabricating the thin-film device.

In one embodiment, the set of coating parameters are selected from spinning rate, spinning duration, acceleration time, and quantity of coatings. In one embodiment, the thin film device comprises: an ITO coated glass substrate; a bismuth ferrite (BFO) layer; and a top electrode.

In one embodiment, the virtual valence fluctuation between Fe ²⁺ and Fe ³⁺ , via oxygen vacancies, plays a crucial role in describing the current conduction in the films, wherein the co-existence of Fe ²⁺ and Fe ³⁺ in the prepared films are in the ratio of 1:1.3.

An object of the present disclosure is to deposit exceptionally crystalline BiFcOs thin films.

Another object of the present disclosure is to endure a transition from Ohmic conduction to trap-filled space charge limited conduction.

Another object of the present disclosure is to trap-limited SCLC mechanism with increasing electric field.

Yet another object of the present invention is to deliver an expeditious and cost- effective system for depositing exceptionally crystalline BiFcOa thin films.

To further clarify advantages and features of the present disclosure, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail with the accompanying drawings.

BRIEF DESCRIPTION OF FIGURES

These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: Figure 1 illustrates a block diagram of a chemical solution deposition system for practical device applications in accordance with an embodiment of the present disclosure;

Figure 2 illustrates a schematic architecture of fabricated thin-film devices, and actual picture of the prepared device in accordance with an embodiment of the present disclosure;

Figure 3 illustrates XRD pattern of BFO film, (b) Rietveld refined XRD data, (c) SEM image of old BFO film, (d) SEM image of BFO film under investigation, (c) cross- sectional SEM image of BFO film, (c) bandgap determination of BFO film in accordance with an embodiment of the present disclosure;

Figure 4 illustrates measured current density bias voltage of -1.5 V to +1.5 V of (a) old BFO film, (b) BFO film under investigationin accordance with an embodiment of the present disclosure;

Figure 5 illustrates Fe-2p XPS spectra of (a) old BFO film, (b) BFO film under investigation in accordance with an embodiment of the present disclosure;

Figure 6 illustrates (a) Fog J vs log E plots of BFO film, (b) Langmuir-Child fitting of the J-E curve at lower field (polynomial fitting of order 2), (c) Langmuir-Child fitting of the J-E curve at higher field (polynomial fitting of order 2), (d) polynomial fitting of order 4 in accordance with an embodiment of the present disclosure;

Figure 7 illustrates Table 1 depicts the structural properties obtained from Rietveld refined analysis; and

Figure 8 illustrates a flow chart of a method for fabricating thin-film device in accordance with an embodiment of the present disclosure.

Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have necessarily been drawn to scale. For example, the flow charts illustrate the method in terms of the most prominent steps involved to help to improve understanding of aspects of the present disclosure. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

DETAILED DESCRIPTION: For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.

Reference throughout this specification to “an aspect”, “another aspect” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrase “in an embodiment”, “in another embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures or components proceeded by "comprises...a" does not, without more constraints, preclude the existence of other devices or other sub-systems or other elements or other structures or other components or additional devices or additional sub-systems or additional elements or additional structures or additional components.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting. Embodiments of the present disclosure will be described below in detail with reference to the accompanying drawings.

Referring to Figure 1, a block diagram of a chemical solution deposition system for practical device applications is illustrated in accordance with an embodiment of the present disclosure. The system 100 includes a mixing chamber 102 equipped with a magnetic stirrer 104 to dissolve and mix 3-6 grams of bismuth nitrate pentahydrate and 3-6 grams of iron nitrate nonahydrate in a mixed solvent 15-25mL of ethanolamine and 2-methoxyethnol uniformly, wherein 8-12mL of nitric acid is added to the solution while vigorously stirring it at room temperature for an hour.

In an embodiment, a plate 106 is employed to receive precursor solution and heated along with aggressive stirring for 40 minutes at 120°C until it became dark red, wherein the resulting solution is aged for about 12 hours thereby tuning a set of deposition parameters to deposit a compact film, wherein the prepared films are pre-fired for 15 minutes at 300°C.

In an embodiment, a tube furnace 108 to anneal the produced films in an air and oxygen atmosphere for 40 minutes at 600°C.

In another embodiment, the set of deposition parameters are selected from a group of spinning rate, spinning duration, acceleration time, and quantity of coatings.

In another embodiment, the 3-6 grams of bismuth nitrate pentahydrate preferably 4.8507 grams, having a molecular weight of 485.07 grams/mol and 3-6 grams of iron nitrate nonahydrate preferably of 4.04 grams, having a molecular weight of 404.00 grams/mol are dissolved in the mixed solvent of 20 mL ethanolamine and 2-methoxyethnol.

In another embodiment, preferably 1.2216 grams of ethanolamine with a molecular weight of 61.8 grams/mol is mixed with 2-methoxyethnol to prepare 20 mL of mixed solvent.

In another embodiment, the pre-firing for 15 minutes is divided at three distinct phases of 5 minutes each.

Figure 2 illustrates a schematic architecture of fabricated thin-film devices, and actual picture of the prepared device in accordance with an embodiment of the present disclosure. Bismuth nitrate pentahydrate and iron nitrate nonahydrate are used as starting materials.

Ethanolamine and 2-methoxy ethanol are used as stabilizers and solvents, respectively.

To begin with, stoichiometric ratios of bismuth nitrate pentahydrate (4.8507 grams, having a molecular weight of 485.07 grams/mol) and iron nitrate nonahydrate (4.04 grams, having a molecular weight of 404.00 grams/mol) are dissolved in a mixed solvent of ethanolamine and 2-methoxyethnol (20 mL) using a magnetic stirrer 104. (i.e., 1.2216 grams of ethanolamine is used with a molecular weight of 61.8 grams/mol). Nitric acid (10 mL) is also added to the solution while vigorously stirring it at room temperature for an hour. The precursor solution is then transferred to a heated plate and aggressively stirred for 40 minutes at 120°C until it became dark red. The resulting solution is aged for about 12 hours before the actual deposition process. The spinning rate, spinning duration, acceleration time, and quantity of coatings are all tuned in order to deposit a pure, uniform, crack-free, and compact film. Before the actual annealing procedure, the prepared films are pre-fired for 15 minutes (5 minutes each at three distinct phases) at 300°C. Finally, the produced films are annealed in an air and oxygen atmosphere for 40 minutes in a tube furnace 108 at 600°C.

The e-beam deposition technique is employed in conjunction with circular shadow masks to deposit the top Al-electrodes of the area (7.07 mm ) for JV characteristic measurements. An X-ray diffractometer (Rigaku Ultima-IV, Cu-k radiation source, = 1.5406A) is used to determine the crystalline structure. A profilometer is used to estimate the film thickness and electrode diameter. The film compactness and surface morphology are determined by using a field emission scanning electron microscope. X-ray photoelectron spectroscopy is used to assess the valence states of the components (Thermo Scientific K- Alpha). A UV-visible spectrophotometer is used to assess properties (Jasco, UV-670, FP- 6300). Finally, the current conduction is measured using a Keysight B2912A source meter.

Figure 3 illustrates XRD pattern of BFO film, (b) Rietveld refined XRD data, (c) SEM image of old BFO film, (d) SEM image of BFO film under investigation, (c) cross- sectional SEM image of BFO film, (c) bandgap determination of BFO film in accordance with an embodiment of the present disclosure. The high-quality polycrystalline nature of the BFO films is confirmed by x-ray diffraction patterns as given in Figure 3 (a). Rietveld refinement of the prepared film revealed that these films exhibit rhombohedrally distorted perovskite phase and are free of impurity phases, as shown in Figure 3(b). The extracted lattice parameters indicated are given in Table 1. Moreover, it appeared from these patterns that all the diffraction peaks have higher intensity in comparison to the reported literature. An increment of 16-fold in the intensity value of (110) peak is observed by adopting the current deposition technique more than the previously reported one. Additionally, the FESEM pictures (taken at different locations) demonstrated a compact and smooth surface with a uniform spread of the granular shapes and sizes, indicating that the films are well grown on ITO/glass substrates.

Figure 4 illustrates measured current density bias voltage of -1.5 V to +1.5 V of (a) old BFO film, (b) BFO film under investigationin accordance with an embodiment of the present disclosure. Figure 4 (a-c) depicts the surface morphology and cross-sectional images of the prepared films with a thickness of about 370 nm. Moreover, the UV-vis-NIR absorbance spectrum revealed a direct optical bandgap of 2.61 eV, as shown in Figure 4 (d), which agrees with the reported values.

The higher leakage current in the BFO thin-films dramatically diminishes their practical applications. As a result, several efforts have been undertaken to lower the leakage current of BFO thin films. Among them, cation substitutions at the Fe site of the BFO are effective in lowering leakage current to a considerable degree. However, enhancing the film quality is another important aspect in controlling leakage current density. For example, a two- order-of-magnitude drop is found in leakage current density following oxygenation in our previous work.

Figure 5 illustrates Fe’ ^2p XPS spectra of (a) old BFO film, (b) BFO film under investigation in accordance with an embodiment of the present disclosure. High-end device fabrication techniques (such as PLD, MBE, and RF- sputtering) result in very efficient device performance. However, employing the CSD approach, a comparable device performance of these films is obtained in the system. Figure 5 (a-b) shows the leakage current density curves of the current study and our previously reported work. The leakage current density of the BFO films under investigation is one order of magnitude lower than that stated in the literature. The current density of the BFO new and BFO old measured at room temperature at a voltage of 1.5 V is 2.2 x 10’ ⁵ A/cm ² and 1.7 x 10’ ⁶ A/cm ² , respectively. The reduced leakage current in these devices can be visualized by considering the current conduction and the role of defective states such as Fe ²⁺ ions and oxygen vacancies. Figure 6 illustrates (a) Log J vs log E plots of BFO film, (b) Langmuir-Child fitting of the J-E curve at lower field (polynomial fitting of order 2), (c) Langmuir-Child fitting of the J-E curve at higher field (polynomial fitting of order 2), (d) polynomial fitting of order 4 in accordance with an embodiment of the present disclosure. Current conduction in the BFO films is believed to be mostly driven by lattice defects, particularly the presence of oxygen vacancies, which are inevitable due to the annealing process in air. However, the existence of lower-charged impurities of Fe ²⁺ states is unavoidable due to their natural stability. More fascinatingly, the generation of these defects can be viewed as a mutual causality, in which one causes the other and vice versa, resulting in the existence of both Fe ²⁺ and oxygen vacancies to guarantee charge neutrality. The co-existence of both kinds of defects is essential to understand these devices’ current conduction mechanisms. The electron hopping conduction mechanism has been well understood in ferroic materials, such as FC3O4, where Fe ²⁺ occupies an octahedral site, and Fe ³⁺ occupies both octahedral and tetrahedral sites. Since octahedrons and tetrahedrons share faces, electrons may easily move from Fe ²⁺ to Fe ³⁺ . However, in the case of BFO, both Fe ²⁺ and Fe ³⁺ occupy the octahedral site, and these octahedrons are linked by vertices, such that there is one 02 in between Fe ²⁺ and Fe ³⁺ , thereby preventing direct electron jumping from Fe ²⁺ to Fe ³⁺ . However, electron hopping becomes conceivable in these materials if there exist oxygen vacancies. The oxygen vacancies bridge the electron transfer between Fe ²⁺ and Fe ³⁺ ions as these behave as ineffective electron trap centres. The oxygen vacancies act as positively charged centres and attract electrons from Fe ²⁺ , and quickly transfer them to Fe ³⁺ ion due to their (oxygen vacancy) ineffective electron holding capability. Moreover, these oxygen vacancies form trap levels in the forbidden band of BFO, which is located 0.6 eV below the conduction band edge, i.e., electrons from the oxygen vacancies can be emitted much more easily into the conduction band than those from the valance band since BFO has a large band gap value, which is found to be 2.61 eV. Based on this, it is plausible to presume that electron hopping (although virtually) from Fe ²⁺ to Fe ³⁺ plays an essential role in defining the current conduction of BFO-based films, and the coexistence of Fe ²⁺ ions and oxygen vacancies is an important aspect in realizing current conduction in such devices. Therefore, x-ray photoelectron spectroscopy is used to detect the valence states of all the constituents of prepared thin films. For comparative analysis, regarding the concentration of Fe ²⁺ states, the Fe 2p spectra of samples are plotted under investigation and the previously reported work. These spectra are presented in Figure 6. The positions of Fe 2p3/2 are anticipated to be at 710.7 eV for Fe ³⁺ and 709.4 eV for Fe ²⁺ states. The Gaussian fitting of the Fe 2p spectra confirmed the co-existence of Fe ²⁺ and Fe ³⁺ states in the deposited films. The concentration of the Fe ²⁺ states, on the other hand, is found to be significantly lower than in the previous study. The ratio of Fe ²⁺ : Fe ²⁺ states in this study and the previous one is estimated to be 1:1.3 and 1:1.1, respectively. Therefore, the reduction in Fe ²⁺ amount (thus oxygen vacancies) could be the possible reason for the observed lower leakage current density.

To further investigate the nature of current conduction in these films, several models are taken into consideration. However, it has been noticed that the results nicely matched the space-charge limited current conduction (SCLC) model. According to this model, the current density behaviour may be subdivided into several zones, and the slope of each zone (log J versus log E) specifies the conduction mechanism. Therefore, the logarithmic behaviour of J is plotted with the applied electric field (E) and observed that the overall curve could be analyzed with three conduction behaviours in different applied field regions, as shown in Figure 6(a). However, the SCLC effect dominates only when the injected carrier concentration (ni) exceeds (no) equilibrium carrier concentration (no). Therefore, at lower fields, since no > ni, the curve exhibited almost linear behaviour up to E = 0.95 x 106 V/m, with the slope value of 1.12. Since the slope value is close to unity, these results suggest that Ohmic conduction (J a m, m > 1) dominates at lower fields. However, at higher fields (from 0.96 x 106 V/m to 4.4 x 106 V/m), the graphs exhibited a considerable divergence from linear behaviour (J a m, m >1), and the obtained value of m is found to be 1.97, indicating that the SCLC mechanism is dominant in this region. Actually, the curve in this region follows the modified Langmuir-Child law (J = aE + bE2) as given in Figure 6(b), thereby confirming the SCLC mechanism is predominating. In this region, the concentration of injected electrons has substantially surpassed the equilibrium concentration (ni > no) in the thin film, contributing to the rise in leakage current. However, at further higher applied fields (beyond E = 4.4 x 106 V/m), an abrupt rise in current occurs. The slope values reach 3.7, which is much higher than to satisfy the Langmuir-Child law, indicating the possibility of trap-filled limited conduction mechanism in these samples. The slope of the curve in this region is found to be 3.7, as indicated in Figure 6, and such behaviour can be explained by Lampert’s triangle, (i) At lower fields, the carrier concentration of thermally generated charge carriers is greater than that of injected charge carriers, resulting in Ohmic conduction, (ii) The injected carrier concentration surpasses the thermally stimulated carriers at higher fields. When the number of injected carriers exceeds the number of background carriers (thermally generated carriers), the injected carriers spread and form a space-charge field. This field controls the currents, known as space charge limited currents SCLCs and obeys the square rule. However, If the sample contains traps, some of the injected carriers will be trapped while others will stay free. As the applied voltage grows, so does the number of injected charge carriers, and more and more traps are filled with carriers. The applied voltage at which all the traps are completely filled is called as Trap filled limit voltage (TFLV), and above this voltage, the current rises abruptly. Furthermore, we also attempted polynomial fitting of order two in the entire applied field region, as shown in Figure 6(c), but the fitted curve showed much variation. However, when we fitted the data using the polynomial fit of order 5, the curve fitted nicely, as depicted in Figure 6(d). These results indicate that the standard SCLC mechanism can not fully explain the conduction behaviour at higher fields; therefore, further investigations are required to understand the conduction behaviour at higher fields.

Figure 7 illustrates Table 1 depicts the structural properties obtained from Rietveld refined analysis. In conclusion, highly crystalline BFO films are deposited using low-cost and straightforward CSD techniques. The current conduction reveals a transition from Ohmic conduction to trap-limited SCLC mechanism with increasing electric field. The XPS findings imply the co-existence of Fe ²⁺ and Fe ³⁺ , indicating the virtual hopping conduction in these devices. These undoped films' relatively lower leakage current density makes them attractive for practical device applications.

Figure 8 illustrates a flow chart of a method for fabricating thin-film device in accordance with an embodiment of the present disclosure. At step 202, the method includes 200 dissolving stoichiometric ratios of 3-6 grams of bismuth nitrate pentahydrate and 3-6 grams of iron nitrate nonahydrate in a mixed solvent of 15-25mL of ethanolamine and 2- methoxyethnol using a magnetic stirrer.

At step 204, the method includes 200 adding 8-12mL of Nitric acid to the solution while vigorously stirring it at room temperature for an hour.

At step 206, the method includes 200 transferring the precursor solution to a heated plate and aggressively stirring for 40 minutes at 120°C until it became dark red. At step 208, the method includes 200 aging the resulting solution for about 12 hours before the actual deposition process.

At step 210, the method includes 200 tunning a set of coating parameters in order to deposit a pure, uniform, crack-free, and compact film.

At step 212, the method includes 200 pre-firing the prepared films for 15 minutes at 300°C.

At step 214, the method includes 200 annealing the produced films in an air and oxygen atmosphere for 40 minutes in a tube furnace at 600°C for fabricating the thin-film device.

In one embodiment, the set of coating parameters are selected from spinning rate, spinning duration, acceleration time, and quantity of coatings.

In one embodiment, the thin film device comprises: an ITO coated glass substrate; a bismuth ferrite (BFO) layer; and a top electrode.

In one embodiment, the virtual valence fluctuation between Fe ²⁺ and Fe ³⁺ , via oxygen vacancies, plays a crucial role in describing the current conduction in the films, wherein the co-existence of Fe ²⁺ and Fe ³⁺ in the prepared films are in the ratio of 1:1.3.

The drawings and the forgoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. For example, orders of processes described herein may be changed and are not limited to the manner described herein. Moreover, the actions of any flow diagram need not be implemented in the order shown; nor do all of the acts necessarily need to be performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. The scope of embodiments is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of embodiments is at least as broad as given by the following claims.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any component(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or component of any or all the claims.