Studies of crystal structure on mixed ferroelectric thin film using X-ray micro diffraction

Abstract

Ferroelectric materials with remnant polarization are widely used in transistors, sensors, actuators, and memory devices. The structural analysis in the single crystalline ferroelectric materials is well known through various measurements such as XRD, TEM, PFM, etc. But the structural analysis in the ferroelectric materials with mixed phase is still difficult because of the lack of decomposing various structures and observation of a slight change caused by an external factor such as temperature and external electric field. This thesis will report to the analysis crystal structure of organic and inorganic ferroelectric materials with mixed phases depending on the applied electric field and temperature. First, the mixed-phase ferroelectric material by strain relaxed thin films can create high piezoelectric properties in epitaxial BiFeO3 (BFO) thin films. A thin film grown above a critical thickness is not structurally stable but exists as a mixed phase of meta-stable phases. The meta-stable phase, such as the tilted monoclinic or S-polymorph phase between the rhombohedral and tetragonal, plays a vital role in the high piezoelectric response. However, the relation between meta-stable phase and piezoelectric response in mixed-phase was not understood because of the lack of a probe technique to observe the structural and electromechanical response. We will show that time-resolved X-ray diffraction can observe the piezo-response at each crystal structure and phase transition. Second, Quinuclidinium perrhenate (HQReO4), organic ferroelectric materials with mixed phases, has attracted flexible devices with a low coercive field. HQReO4 exhibits ferroelectricity when crystallized in only the intermediate temperature phase (ITP). However, this material has a Non-ferroelectric crystal structure as room temperature phase (RTP) and high-temperature phase (HTP). In this report, to stabilize the ferroelectric phase (ITP) in the temperature range, we use the highly oriented thin films of HQReO4 were fabricated via drop-casting methods using the solution process. We can observe the crystal structure and phase transition using reciprocal spacing mapping (RSM) using XRD.