Temperature-modulated crystallographic orientation and electrical properties of BiFeO <sub>3</sub> thick films sputtered on LaNiO <sub>3</sub>/Pt/Ti/SiO <sub>2</sub>/Si for piezo-MEMS applications
Hongyu Luo, Miaomiao Niu, Hanfei Zhu, Li Li, Hongbo Cheng, Chao Liu, Jianting Li, Yuyao Zhao, Chenxi Zhang, Xiao‐Jie Cao, Isaku Kanno, Qingguo Chi, Jun Ouyang
Abstract
In this work, thick BiFeO<sub>3</sub> films (~1 μm) were prepared on LaNiO<sub>3</sub>-buffered (111)Pt/Ti/SiO<sub>2</sub>/(100)Si substrates via radio-frequency magnetron sputtering without post-growth annealing. Effects of the substrate temperature on the film’s crystalline quality, defect chemistry, as well as the associated electrical properties were investigated. In contrast to the poorly crystallized BiFeO<sub>3</sub><sub> </sub>film deposited at 300 °C and the randomly-oriented & (111)-textured films deposited at 500 °C & 650 °C, respectively, a (001)-preferred orientation was achieved in the BiFeO<sub>3</sub><sub> </sub>film deposited at 350 °C. Such a film not only showed a dense, fine-grained morphology, but also displayed enhanced electrical properties due to the (001) texture and an improved defect chemistry. These properties include a reduced leakage current (<em>J</em> ~ 2.4´10<sup>−</sup><sup>5</sup> A/cm<sup>2 </sup>@ 200 kV/cm), a small dielectric constant (<em>ε</em><sub>r</sub> ~ 243-217) with a low loss (tan<em>d</em> ≤ 0.086) measured from 100 Hz to 1 MHz, and an nearly-intrinsic remnant polarization <em>P</em><sub>r</sub> ~ 60 μC/cm<sup>2</sup>. A detailed TEM analysis confirmed the <em>R</em>3<em>c </em>symmetry of the BFO films and hence ensured a good stability of their electrical properties. Particularly, single-beam cantilevers fabricated from the BiFeO<sub>3</sub>/LaNiO<sub>3</sub>/Pt/Ti/SiO<sub>2</sub>/Si heterostructures showed an excellent electromechanical performance, including a large transverse piezoelectric coefficient<em> e</em><sub>31,f</sub><em> </em>~ ‒2.8 C/m<sup>2</sup>, a high figure of merit parameter ~4.0 GPa, and a large signal-to-noise ratio ~1.5 C/m<sup>2</sup>. An in-depth analysis revealed the intrinsic nature of the <em>e</em><sub>31,f</sub><em> </em>piezoelectric coefficient, which is well fitted along a straight line of (<em>e</em><sub>31,f</sub>)<sub>ratio </sub>= (<em>P</em><sub>r</sub>×<em>e</em><sub>r</sub>)<sub>ratio</sub> with the representative results in the literature. These high-quality lead-free piezoelectric films processed with a reduced thermal budget can open up many possibilities for the integration of piezoelectricity into Si-based micro-electromechanical systems (MEMS).