Self-limited grain growth, dielectric, leakage and ferroelectric properties of nanocrystalline BiFeO3 thin films by chemical solution deposition
Introduction
Multiferroic materials, which exhibit the coexistence of at least two ferroic properties (ferroelectric, ferromagnetic or ferroelastic), have been studied extensively due to both their rich physics and their wide potential applications [1], [2]. Due to its high ferroelectric Curie temperature (1100 K), as well as its high magnetic Neél temperature (640 K), BiFeO3 (BFO) has attracted tremendous attention as a very important multiferroic single-phase material [3], [4], [5]. Although intensive works have been carried out on BFO ceramics since 1960, it is difficult to prepare pure, dense and highly resistive ceramics due to the narrow growth window, which results in low polarization and magnetization at room temperature [6], [7].
In 2003, Wang et al. [1] prepared BFO thin films with high remanent polarization and strong ferromagnetism at room temperature, and this has revived the investigation of BFO. Up to now, various methods have been used to prepare BFO thin films and to optimize their properties [8], [9], [10], [11], [12], [13], [14], [15], [16]. Among these methods, chemical solution deposition (CSD) is widely used due to its low cost, precise control of the stoichiometry, ability to achieve atomic-scale mixing and easy processing of large-area thin films [17], [18].
Although there have been many recent investigations of BFO thin films grown by CSD, the processing parameters are not quite the same, which results in obvious differences in microstructures as well as physical properties [13], [14], [19], [20]. It is suggested that the distinct differences are inherited from different microstructures, grain size/crystallite size, etc., which are needed to establish the relationships between processing, microstructure and properties. In general, the CSD-derived BFO thin films are nanocrystalline [15], [16] and it is found that the grain growth of nanostructured material is different from that of conventional materials [21], [22], [23]. Although several grain-growth modes have been proposed to elucidate the grain-growth mechanisms in nanocrystalline thin films, few reports have focused on the grain-growth mechanism of BFO materials, which is essential for optimizing the BFO thin films.
In this paper, grain growth of CSD-derived BFO thin films has been investigated by isothermal annealing. The BFO thin films show self-limited grain growth that can be well described by the relaxation model. The dielectric, leakage and ferroelectric properties have been investigated systematically. The results will elucidate the differences in the microstructures as well as the properties for the CSD-derived BFO thin films, and will provide an instructive route to optimize such BFO-based thin films.
Section snippets
Experimental
BFO thin films were prepared by the CSD method as in our previous reports [15], [16]. A 3 mol.% excess amount of bismuth nitrate pentahydrate was used to compensate the bismuth loss during processing. The final concentration of the solution was adjusted to 0.3 M. Thin films were deposited on Pt/Ti/SiO2/Si (1 0 0) substrates by spin coating. The BFO thin films were annealed at different temperatures (475, 500, 525 and 550 °C) with different dwell times (5, 10, 20, 40, 60, 120, 240, 480 and 720 min) in
Grain-growth kinetics
The XRD results show that all derived thin films are randomly oriented without parasitic phases. The average crystallite size was calculated through the three intense (0 0 1), (1 1 0) and (−1 1 0) peaks using the Scherrer equation, and all peaks were fitted by a Lorentz function. A silicon standard was used to account for the instrumental broadening. The Scherrer equation is only valid if there is no strain broadening, i.e. no local disorder associated with the microstrain. The microstrain
Summary and conclusions
Polycrystalline BiFeO3 thin films are prepared on Pt/Ti/SiO2/Si substrates by chemical solution deposition. It is observed that the derived BiFeO3 thin films are nanocrystalline. The growth mechanism has been investigated by isothermal annealing. The crystallite size grows rapidly while the microstrain decreases quickly within the first hour, and then tends slowly to saturated values with further annealing dwell time, which suggest self-limited grain growth in nanocrystalline BiFeO3 thin films.
Acknowledgements
This work was supported by the Director’s Fund under Contract No. Y14N381323 of Hefei Institutes of Physical Science, Chinese Academy of Sciences and National Science Foundation of China under Contract Nos. 11204316, 50802096. We would like to thank Dr Tania M. Silver at University of Wollongong for English improvement and Professor Li Chen at Institute of Solid State Physics, Chinese Academy of Sciences for helpful discussion about XRD results.
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