Structural and ferroelectric properties of bismuth ferrite thin films deposited by direct current reactive magnetron sputtering
Introduction
BiFeO3 (BFO) thin films with perovskite structure have lots of attention during the last years. BFO is assigned to multiferroic material group. Multiferroics have combined two (or more) properties of ferroelectric and ferromagnetic, simultaneously [1]. Therefore, multiferroics have interesting correlation between the polarization and magnetization processes, as well as strain in the materials. Multiferroics are characterized as having magneto-electric effect that is to say electric field induces magnetization, and magnetic field induces electric polarization. One multiferroic is not necessarily equal to the other multiferroic, therefore, these materials have attracted interest in material science for potential applications. BFO is ferroelectric material below Curie temperature TC ~ 1100 K temperature [1], [2], it's Neel temperature TN ~ 370 °C [2], [3]. BFO films display a greater remnant polarization (Pr) and lower energy band gap than regular ferroelectric materials (PZT) [2]. Ferroelectric properties of BFO thin films depend on substrate materials, deposition method, and measurement frequencies. The high remnant polarization (Pr ~ 90 μC/cm2, 1 kHz) was observed in the BFO thin films, deposited on indium tin oxide coated glass as the substrate, using radio frequency magnetron sputtering [2]. Some investigations assert that the weak ferromagnetism in BFO nanostructured films was observed [3], [4]. A lot of different deposition techniques are used for BiFeO3 thin film formation: RF magnetron sputtering [2], [5], [6], [7], [8], [9], [10], pulsed DC magnetron sputtering [11], pulsed laser deposition [12], [13], [14], sol–gel technique [15], [16], [17], chemical solution deposition [18], [19] and low-energy cluster beam deposition [3]. Many formation processes till now have problems with film density, crystal structure, and stoichiometric and ferroelectric properties. The properties of BFO thin films can be affected greatly by various oxide seed layers (few nm of thickness) [20]. Reactive magnetron layer-by-layer deposition method can be one of the most perspective methods for the formation of ferroelectric thin films. This method allows to get large area thin films with the same thickness and stoichiometry. However, the information about the BFO thin film growing conditions and properties deposited by reactive direct current magnetron sputtering is lacking. To obtain optimal deposition conditions of this method will allow forming perovskite BiFeO3 films with desirable properties for mass production.
Therefore, the aim of this work was to synthesize BFO thin films in situ using layer-by-layer direct current reactive magnetron sputtering and to obtain the optimal parameters for the synthesis of the highest quality of morphology and ferroelectric properties of bismuth ferrite thin films. The dependence of structure and electrical properties on deposition temperature and composition of thin films were investigated.
Section snippets
Experiment
The bismuth ferrite thin films were deposited on platinized silicon substrate by reactive magnetron layer-by-layer deposition in O2 gas environment (p = 1.33 Pa) at various substrate temperatures. Multilayer system Pt/Ti/SiO2/Si was used as the substrate with the thickness of Pt, TiO2 and SiO2 layer of 200 nm, 50 nm, and 1 μm, respectively. The SiO2 thin film on Si (100) substrate was grown by thermal oxidation method. The Pt and TiO2 layers deposited by magnetron sputtering at room temperature were
Results and discussions
The crystalline nature of BiFeO3 thin films strongly depends on the substrate temperature when films are grown by sputtering. The samples were heated during this magnetron deposition at (400 to 600) °C. This temperature interval was chosen because it is optimal for perovskite phase formation. The perovskite phase at 400 °C and lower substrate temperature was not obtained via too low formation energy [21], [22]. The samples, deposited at higher temperatures than 550 °C, show no perovskite phase
Conclusions
Perovskite phase of BiFeO3 thin films (600 nm thickness) was prepared using in situ layer-by-layer DC reactive magnetron sputtering. The optimal parameters were found in order to achieve the highest structural quality of perovskite thin films without post-annealing. It was obtained that the 550 °C substrate temperature and Bi:Fe (50:50) composition lead to the formation of dense and uniform bismuth ferrate thin film with desirable ferroelectric properties. It was shown that microstructure
Acknowledgements
This research was funded by a grant (no. MIP-069/2013) from the Research Council of Lithuania.
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