Stable Ferroelectric Perovskite Structure with Giant Axial Ratio and Polarization in Epitaxial BiFe 0.6 Ga 0.4 O 3 Thin Films Zhen Fan, †,‡ Juanxiu Xiao, # Huajun Liu, † Ping Yang, § Qingqing Ke, † Wei Ji, ‡ Kui Yao,* ,‡ Khuong P. Ong, ⊥ Kaiyang Zeng, # and John Wang* ,† † Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore ‡ Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, Singapore # Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore § Singapore Synchrotron Light Source, National University of Singapore, 5 Research Link, Singapore 117603, Singapore ⊥ Materials Science & Engineering Department, Institute of High Performance Computing, 1 Fusionopolis Way, Singapore 138632, Singapore * S Supporting Information ABSTRACT: Ferroelectric perovskites with strongly elon- gated unit cells (c/a > 1.2) are of particular interest for realizing giant polarization induced by significant ionic off- center displacements. Here we show that epitaxial Bi- Fe 0.6 Ga 0.4 O 3 (BFGO) thin films exhibit a stable super- tetragonal-like structure with twinning domains regardless of film thickness and substrate induced strain, evidenced with high resolution X-ray diffractometry (HR-XRD), transmission electron microscopy (TEM) and piezoresponse force micros- copy (PFM). The origin of the structural stability of BFGO is investigated by the first-principles calculation. The ferroelectric properties of BFGO are studied by PFM, first-principles calculation and macroscopic polarization-electric field (P-E) hysteresis measurement. A giant ferroelectric polarization of ∼150 μC/cm 2 is revealed by the first-principles calculations and confirmed by experiments. Our studies provide an alternative pathway of employing Ga-substitution other than the extensively studied strain engineering to stabilize the supertetragonal structure in BiFeO 3 -based epitaxial thin films. KEYWORDS: BiFeO 3 , supertetragonal, giant polarization, epitaxial thin film, ferroelectric 1. INTRODUCTION Ferroelectric thin films with large polarization are of great interest, not only for studying the physical mechanisms underlying the polarization, 1 but also for realizing high performance devices, such as actuators and sensors, 2 non- volatile memories, 3 and devices based on tunneling junctions 4 and bulk photovoltaic effect. 5 To achieve large polarization, perovskite-type oxides having super-tetragonal(-like) unit cells with giant c/a ratios (>1.2) are the ideal objects, due to the significant ionic displacements. One of the extensively studied routes to develop the supertetragonal structures is through epitaxial strain induced by the substrate. 6 For example, BiFeO 3 (BFO), which shows rhombohedral symmetry in its poly- crystalline bulk state, can be tailored to a supertetragonal structure with c/a ∼ 1.24 and a giant polarization of ∼150 μC/ cm 2 under large compressive strains. 7-12 However, the strain- induced structure is often metastable, and it will relax toward its bulk when the film thickness exceeds certain value. Furthermore, when a bottom electrode has to be inserted between the substrate and the film for application, the strain effect is weakened and strain relaxation aggravates seriously. A recently demonstrated route is to stabilize supertetragonal BFO structures by the application of a β-Bi 2 O 3 (β-BO) layer which facilitates the growth of supertetragonal BFO. 13,14 However, because β-BO is highly conductive and is likely to crystallize within the BFO layer, the leakage issue arises. 14 It is well-known that formation of a solid solution can drastically tune the crystal structure and the physical properties, in comparison to the parent compositions, where Pb(Zr x Ti 1-x )- O 3 (PZT) is a classic example. In this letter, we demonstrate the stabilization of the desired super-tetragonal(-like) structure in BFO-based epitaxial thin films, by forming a Ga-substituted BFO solid solution, in contrast to the well-studied substrate strain approach. By using a combination of high resolution X- ray diffractometry (HR-XRD), transmission electron micros- copy (TEM), piezoresponse force microscopy (PFM), first- Received: November 4, 2014 Accepted: January 8, 2015 Published: January 8, 2015 Research Article www.acsami.org © 2015 American Chemical Society 2648 DOI: 10.1021/am509016w ACS Appl. Mater. Interfaces 2015, 7, 2648-2653