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Register a new userStudy of strain effect on in-plane polarization in epitaxial BiFeO3 thin films using planar electrodes
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Epitaxial strain plays an important role in determining physical properties of perovskite ferroelectric oxide thin films. However, it is very challenging to directly measure properties such as polarization in ultrathin strained films using traditional sandwich capacitor devices, because of high leakage current. We employed a planar electrode device with different crystallographical orientations between electrodes along different electric field orientation to directly measure the in-plane polarization-electric field (P-E) hysteresis loops in fully strained thin films. At high misfit strains such as -4.4%, the pure Tetrogonal-like phase is obtained and its polarization vector is constrained to lie in the (010) plane with a significantly large in-plane component, ~44 {mu}C/cm2. First-principle calculations are carried out in parallel, and provide a good agreement with the experimental results. Our results pave the way to design in-plane devices based on T-like BFO and the strategy proposed here can be expanded to study all other similar strained multiferroic ultrathin films.
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We report a study on the thermodynamic stability and structure analysis of the epitaxial BiFeO3 (BFO) thin films grown on YAlO3 (YAO) substrate. First we observe a phase transition of MC-MA-T occurs in thin sample (<60 nm) with an utter tetragonal-like phase (denoted as MII here) with a large c/a ratio (~1.23). Specifically, MII phase transition process refers to the structural evolution from a monoclinic MC structure at room temperature to a monoclinic MA at higher temperature (150oC) and eventually to a presence of nearly tetragonal structure above 275oC. This phase transition is further confirmed by the piezoforce microscopy measurement, which shows the rotation of polarization axis during the phase transition. A systematic study on structural evolution with thickness to elucidate the impact of strain state is performed. We note that the YAO substrate can serve as a felicitous base for growing T-like BFO because this phase stably exists in very thick film. Thick BFO films grown on YAO substrate exhibit a typical morphotropic-phase-boundary-like feature with coexisting multiple phases (MII, MI, and R) and a periodic stripe-like topography. A discrepancy of arrayed stripe morphology in different direction on YAO substrate due to the anisotropic strain suggests a possibility to tune the MPB-like region. Our study provides more insights to understand the strain mediated phase co-existence in multiferroic BFO system.
A morphotropic phase boundary driven by epitaxial strain has been observed in a lead-free multiferroic BiFeO3 thin films and the strain-driven phase transitions were widely reported to be iso-symmetric Cc-Cc ones by recent works. In this paper, we suggest that the tetragonal-like BiFeO3 phase identified in epitaxial films on (001) LaAlO3 single crystal substrates is monoclinic MC. This MC phase is different from MA type monoclinic phase reported in BiFeO3 films grown on low mismatch substrates, such as SrTiO3. This is confirmed not only by synchrotron x-ray studies but also by piezoresponse force microscopy measurements. The polarization vectors of the tetragonal-like phase lie in the (100) plane, not the (110) plane as previously reported. A phenomenological analysis was proposed to explain the formation of MC Phase. Such a low symmetry MC phase, with its linkage to MA phase and the multiphase coexistence open an avenue for large piezoelectric response in BiFeO3 films and shed light on a complete understanding towards possible polarization rotation paths and enhanced multiferroicity in BiFeO3 films mediated by epitaxial strain. This work may also aid the understanding of developing new lead-free strain-driven morphotropic phase boundary in other ferroic systems.
Crystal and domain structures of tensile-strained BiFeO3 films grown on orthorhombic (110)o PrScO3 substrates were investigated. All films possess a MB-type monoclinic structure with 109o stripe domains oriented along the [=i10]o direction. For films thicknesses less than ~40 nm, presence of well-ordered domains is proved by the detection of satellite peaks in synchrotron x-ray diffraction studies. For thicker films, For thicker films, only the Bragg reflections from tilted domains were detected. This is attributed to the broader domain size distribution in thicker films.Using planar electrodes,the in-plane polarization of the MB phase is determined to be 85 uC/cm2, which is much larger than that of compressive strained BiFeO3 films. Our results further reveal that the substrate monoclinic distortion plays a major role in determining the stripe domain formation of the rhombohedral ferroic epitaxial thin films, which sheds light to the understanding of elastic domain structure evolution in many other functional oxide thin films as well.
BiFeO3 thin films with various thicknesses were grown epitaxially on (001) LaSrAlO4 single crystal substrates using pulsed laser deposition. High resolution x-ray diffraction measurements revealed that a tetragonal-like phase with c-lattice constant ~4.65 {AA} is stabilized by a large misfit strain. Besides, a rhombohedral-like phase with c-lattice constant ~3.99 {AA} was also detected at film thickness of ~50 nm and above to relieve large misfit strains. In-plane piezoelectric force microscopy studies showed clear signals and self-assembled nanoscale stripe domain structure for the tetragonal-like regions. These findings suggest a complex picture of nanoscale domain patterns in BiFeO3 thin films subjected to large compressive strains.
BiFeO3 thin films have been deposited on Pt/sapphire and Pt/Ti/SiO2/Si substrates with pulsed laser deposition using the same growth conditions, respectively. Au was sputtered as the top electrode. The microscopic structure of the thin film varies by changing the underlying substrate. Thin films on Pt/sapphire are not resistively switchable due to the formation of Schottky contacts at both the top and the bottom interface. However, thin films on Pt/Ti/SiO2/Si exhibit an obvious resistive switching behavior under forward bias. The conduction mechanisms in BiFeO3 thin films on Pt/sapphire and Pt/Ti/SiO2/Si substrates are discussed to understand the different resistive switching behaviors.
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