Enhanced conductivity at orthorhombic–rhombohedral phase boundaries in BiFeO3 thin films

Abstract

Enhanced properties in modern functional materials can often be found at structural transition regions, such as morphotropic phase boundaries (MPB), owing to the coexistence of multiple phases with nearly equivalent energies. Strain-engineered MPBs have emerged in epitaxially grown BiFeO 3 (BFO) thin films by precisely tailoring a compressive misfit strain, leading to numerous intriguing phenomena, such as a massive piezoelectric response, magnetoelectric coupling, interfacial magnetism and electronic conduction. Recently, an orthorhombic-rhombohedral (O-R) phase boundary has also been found in tensile-strained BFO. In this study, we characterise the crystal structure and electronic properties of the two competing O and R phases using X-ray diffraction, scanning probe microscope and scanning transmission electron microscopy (STEM). We observe the temperature evolution of R and O domains and find that the domain boundaries are highly conductive. Temperature-dependent measurements reveal that the conductivity is thermally activated for R-O boundaries. STEM observations point to structurally wide boundaries, significantly wider than in other systems. Therefore, we reveal a strong correlation between the highly conductive domain boundaries and structural material properties. These findings provide a pathway to use phase boundaries in this system for novel nanoelectronic applications. INTRODUCTIONMultiferroic BiFeO 3 (BFO) has attracted great interest as a promising lead-free functional material owing to its key properties, including electromechanical and magnetoelectric coupling, domain wall conductance, bias-induced semiconductor-insulator transitions and photovoltaic effects. 1-7 Bismuth ferrite is a room temperature multiferroic perovskite exhibiting antiferromagnetism that is coupled with ferroelectric order. 8 Below the Curie temperature, bulk BFO is considered to have a rhombohedral R3c space group structure that allows antiphase octahedral tilting and ionic displacements from the center of symmetry along the [111] pseudocubic direction. 9,10 The recent discovery of a strain-induced morphotropic phase boundary in BFO films grown on (001) LaAlO 3 has attracted further interest with the compressive strain imposed by an underlying substrate stabilizing a tetragonally distorted phase. 11-17 Mixed-phase regions exhibit the coexistence of both R-and T-like phases with the ability to selectively switch between the two phases by an external electric field and force. [18][19][20][21][22] Interestingly, the strong correlation between T-R interfaces and their enhanced properties, such as the finding of a large electric-field-induced strain 23 and a giant piezoelectric d 33 coefficient, led to extensive studies of bismuth ferrite films under compressive strain by both experimental and theoretical approaches. Moreover, a correlation between the elasticity at the T-R interfaces and electronic conduction was also reported. 24 Triggered by

The Effect of Bi2O3 and Fe2O3 Impurity Phases in BiFeO3 Perovskite Materials on Some Electrical Properties in the Low-Frequency Field

“…From Figure 8 , it can also be observed that E A,cond increases from 0.18 eV to 0.25 eV for sample S1, from 0.21 eV to 0.29 eV for sample S2 and from 0.23 eV to 0.32 eV (or sample S3, being in accordance with the values obtained by other authors [ 28 , 29 ], for similar samples. Therefore, the obtained results show that the thermal activation energy of the static conductivity of samples S2 (with Bi 2 O 3 impurities) and S3 (with Fe 2 O 3 impurities) is higher than that corresponding to the pure sample S1 (BFO), being in correlation with the σ DC values obtained for these samples (see Figure 4 ).…”

Electromechanical-mnemonic effects in BiFeO3 for electric field history-dependent crystallographic phase patterning

“…Varying the grid density across areas might provide a path to tailor magnetic and optical properties of larger regions of strained BFO. Moreover, the induced RT phase boundaries yield control over local charge transport [27]. Pre-poling also affects ferroelectric properties such as loop opening and coercive voltages even after multiple switching cycles and allows therefore to modify electromechanical behavior.…”

“…For example, the strong electromechanical coupling of mixed phase BFO [22,24] can enable lead-free micro-sensors and -actuators for biological applications, while its giant elastic tunability can be exploited in frequency-agile electroacoustic devices [2,21,25]. Enhanced conductivity at domain walls and phase boundaries [1,7,8,20,26,27] are envisioned to play a role in future nanoelectronic systems such as domain wall based memristors. Electrochromism and optical absorption both depend on crystallographic phases, yielding novel opportunities in photonics and acoustooptic 3 applications [3].…”

Structural and photoelectric properties of tensile strainedBiFeO3

“…Experimentally, a number of structural phases of BFO under tensile strain have been reported. These include monoclinic M B [19,20], orthorhombic [21,22], mixed orthorhombicrhombohedral [23,24] and even tetragonal [25]. Considered together, these do not provide a coherent picture.…”