Ferroelectric BiFeO3 Thin Films Research Articles

Non-contact intelligent sensor for recognizing transparent and naked-eye indistinguishable materials based on ferroelectric BiFeO3 thin films

At present, the research on ferroelectric photovoltaic materials mainly focuses on photoelectric detection. In the context of the rapid development of the Internet of Things (IoT), it is particularly important to use smaller thin-film devices as sensors. In this work, an indium tin oxide/bismuth ferrite (BFO)/lanthanum nickelate device has been fabricated on an F-doped tin oxide glass substrate using the sol–gel method. The sensor can continuously output photoelectric signals with little environmental impact. Compared to other types of sensors, this photoelectric sensor has an ultra-low response time of 1.25 ms and ultra-high sensitivity. Furthermore, a material recognition system based on a BFO sensor is developed. It can effectively identify eight kinds of materials that are difficult for human eyes to distinguish. This provides new ideas and methods for developing the IoT in material identification.

Improved polarization retention in epitaxial BiFeO3 thin films induced by strain relaxation

This study investigated the impact of lattice mismatch strain (LMS) on the polarization retention of epitaxial ferroelectric BiFeO3 (BFO) thin films. The application of varying degrees of LMS on the BFO films was achieved by tuning the film thickness, which was verified through X-ray diffraction and reciprocal space mapping. As the strain was gradually eliminated, the film demonstrated a more saturated hysteresis loop with a reduced negative coercive electric field. Additionally, a significant increase in polarization retention was observed with increasing film thickness, strongly indicating the crucial role of LMS in driving the formation of preferred polarization, which is intimately connected with the built-in field induced by oxygen vacancies-relevant space charge alignment. Moreover, the anisotropic LMS state of the BFO films was utilized to horizontally reverse local polarization in a sub-micrometer scale, revealing that strain accelerates the back-switching of polarization to eliminate the artificially created ferroelastic domain walls. To counteract the strain effect, charge injection of electrons was proposed to enhance the retention of switched polarization by fully suppressing the built-in field and pinning the artificially created domain walls. The findings provide valuable insights into the impact of strain on polarization retention for the development of ferroelectric memories.

Mechanically Driven Reversible Polarization Switching in Imprinted BiFeO3 Thin Films

AbstractMechanically driven polarization switching via scanning probe microscopy provides a valuable voltage‐free strategy for designing ferroelectric nanodomain structures. However, it is still challenging to realize reversible polarization switching with mechanical forces. Here, the mechanically driven reversible polarization switching observed in imprinted ferroelectric BiFeO3 thin films is reported, i.e., up‐to‐down switching by a sharp scanning tip and down‐to‐up switching by a blunt tip. Free energy calculations, phase‐field simulations, and piezoresponse force microscopy reveal that reversible mechanical switching arises from the interplay among the flexoelectric effect, the piezoelectric effect, and the internal upward built‐in field in BiFeO3 films. This study gains a deeper insight into the mechanism and control of mechanically driven polarization switching, and provides guidance for exploring potential ferroelectric‐based electro‐mechanical microelectronics.

Size-dependent polarization retention in ferroelectric BiFeO3 domain wall memories

Reliable polarization retention in a single crystal-like ferroelectric thin film is crucial for the effective implementation of ferroelectric domain wall (DW) memories. This ensures that the saved information can be non-destructively read out by identifying the resistive state of the memory with a voltage signal below the coercive voltage (Vc) of the ferroelectric film. However, stabilizing a small volume of switched polarization for ultra-high-density information storage applications is still challenging due to its back-switching behavior at zero electric field. In this study, we investigated the polarization retention of ferroelectric BiFeO3 (BFO) thin film devices with size-variable Pt electrodes. With the scaling down of the device size, the switched polarizations became more susceptible to depolarization field and ferroelastic stress arising at the artificially created domain boundary. We found that BFO devices featuring a switched polarization area larger than 7 × 106 nm2 demonstrated both long-term polarization retention and a current rectification ratio of 4.5:1 induced by DW manipulation. As the device size decreases, the retention time of the switched polarization reduces gradually, which results in the disappearance of the device current due to the DW being eliminated by the back-switching of the polarization. A plot was created to illustrate the relationship between the current retention time and the switched polarization area on the surface of the BFO thin films. Furthermore, the unsustainable polarization retention facilitates a higher current rectification ratio of 48:1, which is suitable for non-destructive information readout with a voltage pulse larger than Vc in high-density ferroelectric DW memories.

Introduction of charged domain walls into BiFeO3 thin films using a pit-patterned SrTiO3 (001) substrate

This work demonstrated the artificial introduction of charged domain walls (CDWs) into a ferroelectric BiFeO3 (BFO) thin film by domain structure control using a pit-patterned SrTiO3 (STO) (001) surface. The pattern consisted of 1 × 1 μm square holes with sloped sides, fabricated on the STO (001) surface by electron beam lithography and Ar+ ion etching. Scanning electron and atomic force microscopy analyses demonstrated that the pit slopes had angles of 6.1°–7.6°, which were sufficient to limit the in-plane growth direction of the BFO at step edges on the STO surface, and thus control the domain structure. Lateral and vertical piezoresponse scanning force and transmission electron microscopy confirmed the artificial introduction of CDWs in the pit and showed that the sign of the CDWs could be reversed via ferroelectric polarization switching. This domain control technique based on a pit pattern provides a simple approach to the integration of ferroelectric DWs into functional devices.

Nanoscale excitonic photovoltaic mechanism in ferroelectric BiFeO3 thin films

We report an electrode-free photovoltaic experiment in epitaxial BiFeO3 thin films where the picosecond optical absorption arising from carrier dynamics and piezoelectric lattice distortion due to the photovoltaic field are correlated at nanoscale. The data strongly suggest that the photovoltaic effect in phase-pure BiFeO3 originates from diffusion of charge-neutral excitons and their subsequent dissociation localized at sample interfaces. This is in stark contrast to the belief that carrier separation is uniform within the sample due to the lack of center of symmetry in BiFeO3. This finding is important for formulating strategies in designing practical photovoltaic ferroelectric devices.

(Invited) Local Probing of Thermally Induced Phenomena in Inorganic/Biological Materials Based on Nonlinear Cantilever Dynamics

Atomic force microscopy (AFM) has been widely used to observe and manipulate physical phenomena at the nanoscale. In AFM, the cantilever is the lens for observing physical phenomena. If we understand how this lens behaves on the material surface, we can understand what kinds of physical phenomena are there and how they interact with the cantilever. Here, I will summarize our recent efforts on the probing of thermally induced phenomena in inorganic/biological samples at the nanoscale based on the nonlinear cantilever dynamics. I will present electrically driven thermal expansion in inorganic materials such as ferroelectric BiFeO3 thin films. It was found that electrically induced nonlinear responses are associated with Joule heating effects. Further, I will also show photo-activated thermal expansion in biological materials such as melanoma and Arabidopsis cells. In general, linear responses of the photo-activated thermal expansion have been widely used for observing chemical and/or thermal information of the organic and biological materials. However, it was found that nonlinear responses can be also useful for identifying local features of the biological materials. These results present that the nonlinear cantilever dynamics in AFM could provide new pathways for exploring fundamental physical properties.

Stripe domains in epitaxial BiFeO3 thin films on (100) SrTiO3 substrates

Highly crystallized ferroelectric BiFeO3 thin films were deposited on various miscut (100) SrTiO3 substrates. The domain structure of the films varies with substrate miscut angles and atomic-layer termination at the surface. The BiFeO3 thin films grown on 4° miscut substrates exhibit 71° periodic stripe domains consisting of two downward polarization variants. In contrast, four downward variants coexist in the films deposited on 0.2° miscut substrates, regardless of atomic-layer termination at the surface of the substrate. The introduction of an additional SrTiO3 repair layer on the same substrate results in a step-flow growth mode of the film. It is believed that the improved mobility of the BiFeO3 atomic species during the film growth leads to the appearance of preferred ferroelectric variants at the step edges. These preferred variants eventually form stripe domains. Our results reveal that both the miscut angle and the step-flow growth promote the formation of the two variant striped domains in BiFeO3 films deposited on decorated isotropic SrTiO3 substrates.

Photovoltaic and photo-capacitance effects in ferroelectric BiFeO3 thin film

A polycrystalline BiFeO3 film on Pt/Ti/SiO2/Si was fabricated using the spin coating technique. The film shows diode-like characteristics with and without poling measured under dark conditions. However, it exhibits a switchable photovoltaic effect with light illumination under poled conditions. The measured photovoltaic effect revealed an open circuit voltage of ∼0.47 V and a short circuit current of 3.82 μA/cm2 under the illumination of 165 mW/cm2 irradiance. The studies clarified the dominant role of the depolarization field rather than the interface in the photovoltaic characteristics of the BiFeO3 film. Significantly, the photo-capacitance effect was demonstrated with a substantial enhancement in capacitance (∼45%) in Au/BiFeO3/Pt geometry, which could open up a new window for BiFeO3 applications.

Chemical pressure effect in Sm and La substituted ferroelectric BiFeO3 thin films: Insights from infrared spectroscopy

We investigate the effects of Sm and La substitution in ferroelectric BiFeO3 thin films on the lattice dynamics by infrared reflection measurements at room temperature. The frequencies of the infrared-active phonon modes are studied as a function of Sm and La content in Bi1–x(Sm,La)xFeO3 composition spread films in the range from x = 0 up to x = 0.25, grown on SrTiO3 substrates by pulsed laser deposition. Substitution of the Bi3+ ions with small Sm3+ ions leads to the appearance of a new phase above x ≈ 0.09 coexisting with the ferroelectric BiFeO3 phase up to x ≈ 0.19. In contrast, for the substitution of Bi3+ ions with La3+ ions of similar size a continuous transition from the original BiFeO3 phase to a new phase takes place. In both cases, we assign the new phase to the paraelectric, orthorhombic phase. These findings are discussed in terms of the morphotropic phase boundary in Sm-doped BiFeO3 around x = 0.14 with a phase coexistence, which was suggested as the origin for enhanced piezoelectric properties.

Synthesis of BiFeO3 thin films on single-terminated Nb : SrTiO3 (111) substrates by intermittent microwave assisted hydrothermal method

We report on a simple and fast procedure to create arrays of atomically flat terraces on single crystal SrTiO3 (111) substrates and the deposition of ferroelectric BiFeO3 thin films on such single-terminated surfaces. A microwave-assisted hydrothermal method in deionized water and ammonia solution selectively removes either (SrO3)4− or Ti4+ layers to ensure the same chemical termination on all terraces. Measured step heights of 0.225 nm (d111) and uniform contrast in the phase image of the terraces confirm the single termination in pure and Nb doped SrTiO3 single crystal substrates. Multiferroic BiFeO3 thin films were then deposited by the same microwave assisted hydrothermal process on Nb : SrTiO3 (111) substrates. Bi(NO3)3 and Fe(NO3)3 along with KOH served as the precursors solution. Ferroelectric behavior of the BiFeO3 films on Nb : SrTiO3 (100) substrates was verified by piezoresponse force microscopy.

Ferroelectric and flexible barrier resistive switching of epitaxial BiFeO3 films studied by temperature-dependent current and capacitance spectroscopy

The resistive switching of epitaxial ferroelectric BiFeO3 thin films was investigated by the combined current and capacitance spectroscopy, in addition with the piezoresponse force microscopy and conducting atomic force microscopy. A strong correlation between the resistance states and the capacitance states was found in the temperature range from 77 to 300 K, revealing the role of interface traps. It is demonstrated that the trap-assisted current transport contributes to the large ON current of the ferroelectric and flexible barrier resistive switching. The combination of ferroelectric polarization reversal, electromigration of oxygen vacancies, and interface traps leads to the nonvolatile memory performance with the ON/OFF ratio of 102 for >105 s and the endurance of over 8000 cycles at room temperature.

Giant optical enhancement of strain gradient in ferroelectric BiFeO3 thin films and its physical origin.

Through mapping of the spatiotemporal strain profile in ferroelectric BiFeO3 epitaxial thin films, we report an optically initiated dynamic enhancement of the strain gradient of 105–106 m−1 that lasts up to a few ns depending on the film thickness. Correlating with transient optical absorption measurements, the enhancement of the strain gradient is attributed to a piezoelectric effect driven by a transient screening field mediated by excitons. These findings not only demonstrate a new possible way of controlling the flexoelectric effect, but also reveal the important role of exciton dynamics in photostriction and photovoltaic effects in ferroelectrics.

Heavy Mn-doping effect on spontaneous polarization in ferroelectric BiFeO3 thin films

Mn doping effect on the crystal structure and spontaneous polarization (Ps) in ferroelectric BiFeO3 (BFO) thin films are experimentally and theoretically investigated. Polarization hysteresis measurements for BiFe1−xMnxO3 (BFMO) films grown on (100) SrTiO3 substrates with SrRuO3 electrodes indicate that remanent polarization (Pr) remains unchanged in the range of 0 ≤ x ≤ 0.1. As a result of the density-functional theory (DFT) calculations for the rhombohedral, tetragonal, and monoclinic phases of BFO and BFMO (x = 0.5), we find that the monoclinic phase with compressive strain from the substrates yields the most reasonable Ps. The DFT calculations also indicate that heavy doping of up to 50% Mn does not lead to a considerable deterioration in Ps, which is consistent with the experiments. Our investigations demonstrate that the ferroelectric polarization in the BFMO system can be retained for a wide range of Mn content either in bulk or thin-film form.

Energy landscape scheme for an intuitive understanding of complex domain dynamics in ferroelectric thin films

Fundamental understanding of domain dynamics in ferroic materials has been a longstanding issue because of its relevance to many systems and to the design of nanoscale domain-wall devices. Despite many theoretical and experimental studies, a full understanding of domain dynamics still remains incomplete, partly due to complex interactions between domain-walls and disorder. We report domain-shape-preserving deterministic domain-wall motion, which directly confirms microscopic return point memory, by observing domain-wall breathing motion in ferroelectric BiFeO3 thin film using stroboscopic piezoresponse force microscopy. Spatial energy landscape that provides new insights into domain dynamics is also mapped based on the breathing motion of domain walls. The evolution of complex domain structure can be understood by the process of occupying the lowest available energy states of polarization in the energy landscape which is determined by defect-induced internal fields. Our result highlights a pathway for the novel design of ferroelectric domain-wall devices through the engineering of energy landscape using defect-induced internal fields such as flexoelectric fields.

Crystallinity Improvement of Ferroelectric BiFeO3 Thin Film by Oxygen Radical Treatment

BiFeO3 (BFO) thin film has been developed for realizing high-performance ferroelectric memory and/or sensors.[1-3] Furthermore, BFO film has received attention for realizing high open-circuit voltage solar cells.[4] Unlike other conventional ferroelectric materials such as PZT, BFO film does not contain Pb, which is a distinctive advantage to use BFO film. BFO thin films were mainly formed by the sol-gel method, pulsed laser deposition and sputtering deposition. After the BFO thin film deposition, there are oxygen vacancy defects in the BFO thin film. On the other hand, we developed high-performance ferroelectric Sr2(Ta,Nb)2O7 (STN) thin films obtained using the combination of magnetron sputtering deposition of STN film and subsequent oxygen radical treatment using microwave-excited high-density Kr/O2 plasma, where the oxygen radical treatment could dramatically reduce oxygen vacancy defects and improve crystallinity.[5]In this study, we applied this method to the BFO film formation, and discuss the effect of oxygen radical treatment on the BFO film property. BFO thin films (200 nm) were deposited on Pt(111)/Ta/SiO2/Si substrate at room temperature using the magnetron sputtering. BiFeO3 ceramic target was used. Before the sputtering deposition of BFO films, Ta (20 nm) and Pt (100 nm) were deposited on Si substrate with thermally grown SiO2 film (500 nm). In the BFO film depositions, a working pressure was 4 Pa (Ar), and plasma was excited by applying a 13.56-MHz rf power to target with a power density of 1.0 W/cm2. After the BFO thin film deposition, the radical oxidation treatment was carried out using the 2.45-GHz microwave-exited high-density plasma at 400 °C for 10 min at a working pressure of 133 Pa. An applied microwave power density was 1.0 W/cm2. After the oxygen radical treatment, BFO thin film was annealed at 600 °C for 10 min in the oxygen ambient. The crystalline structures of deposited films were analyzed by X-ray diffraction (XRD). Fig. 1 shows XRD patterns of BFO films with and without the oxygen radical treatment. The peaks of BFO (110) and (200) were clearly observed in the case with the oxygen radical treatment, while those peaks did not appear in the case without oxygen radical treatment. The results indicated that the perovskite phase could be formed in the case with oxygen radical treatment, and also, oxygen vacancy defects in the as-deposited film were oxidized by a large amount of oxygen radicals in the Kr/O2 plasma, which promoted crystallization of the BFO film. Further reduction of crystallization temperature can be expected by optimizing the condition of oxygen radical treatment. We consider that oxygen radical treatment is effective to develop ferroelectric memory and/or sensors. This work was supported by JSPS KAKENHI Grant Number 25630117. Reference [1] J. M. Park, S. Nakashima, M. Sohgawa, T. Kanashima, M. Okuyama: Jpn. J. Appl. Phys. 51(2012) 09MD05 [2] T. Kawae, H. Tsuda, H. Naganuma, S. Yamada, M. Kumeda, S. Okamura, A. Morimoto: Jpn. J. Appl. Phys. 47(2008) 7586 [3] J. H. Kim, H. Funakubo, Y. Sugiyama, H Ishiwara: Jpn. J. Appl. Phys. 48(2009) 09KB02 [4] S. Y. Yang, J. Seidel, S. J. Byrnes, P. Shafer, C.-H. Yang, M. D. Rossell, P. Yu, Y.-H. Chu, J. F. Scott, J.W. Ager, III, L. W. Martin, R. Ramesh: Nature Nanotechnology 5(2010) 143 [5] I. Takahashi, H. Sakurai, A. Yamada, K. Funaiwa, K. Hirai, S. Urabe, T. Goto, M. Hirayama, A. Teramoto, S. Sugawa, T. Ohmi: Jpn. J. Appl. Phys. 42 (2003) 2050 Figure 1

Crystallinity Improvement of Ferroelectric BiFeO3 Thin Film by Oxygen Radical Treatment

Oxygen radical treatment was applied to sputter-deposited BiFeO3 (BFO) thin film which is expected to be used for ferroelectric memories, sensors and solar cells. The oxygen radicals were produced by a microwave excited high-density Kr/O2 plasma. It was found from X-ray diffraction (XRD) that the crystallization temperature of the BFO film could be reduced to 350 °C by applying the oxygen radical treatment before the annealing. The (110) peak area which shows the crystallinity of BFO was by 4 times than that of the BFO film annealed at 500 °C without the oxygen radical treatment. From results of XRD and X-ray photoelectron spectroscopy (XPS), the stoichiometry was dramatically improved and this suggests that the oxygen radical treatment effectively creates the nucleation cites of BiFeO3 and reduces oxygen vacancy defects.

Ferroelectric polarization relaxation in Au/Cu2O/ZnO/BiFeO3/Pt heterostructure

The stability of polarization in ferroelectric BiFeO3 thin film stacked with a p-n junction of Cu2O/ZnO was studied in the Au/Cu2O/ZnO/BiFeO3/Pt heterostructure. It was observed that the downward ferroelectric polarization of BiFeO3 gradually relaxes once the external electric field is removed, which is driven by the depolarization effect induced by the reduction of compensating charges due to the charge redistribution within Cu2O/ZnO. This work contributes to an improved understanding on the polarization behavior in multilayer thin film structures comprising ferroelectrics and p-n junctions for guiding relevant device design and performance analysis.

Large resistive switching and switchable photovoltaic response in ferroelectric doped BiFeO3-based thin films by chemical solution deposition

The ferroelectric doped BiFeO3thin films exhibit large resistive switching (with ON/OFF ratios ∼104) and stably switchable photovoltaic response with good retention properties, providing multiple selections for non-destructive ferroelectric memory diveces.

Flexoelectric Control of Defect Formation in Ferroelectric Epitaxial Thin Films

Flexoelectric control of defect formation and associated electronic function is demonstrated in ferroelectric BiFeO3 thin films. An intriguing, so far never demonstrated, effect of internal electric field (Eint ) on defect formation is explored by a means of flexoelectricity. Our study provides novel insight into defect engineering, as well as allows a pathway to design defect configuration and associated electronic function.