Preparation and characterization of Ba₂TiSi₂O₈ ferroelectric films produced by sol–gel method

Abstract

Fresnoite (Ba₂TiSi₂O₈, BTS) thin films were grown on polished Si(100) substrates by sol–gel method. The films were characterized using Fourier transform infrared spectroscopy (FTIR), Raman scattering spectroscopy, X-ray diffraction (XRD) and atom force microscopy (AFM). The results reveal that the crystallinity of fresnoite thin films increases and their structures become more compact as post-annealing temperature increases. Combined with XRD data, the strong FTIR peaks and Raman bands assigned to Ti–O and Si–O vibration indicate the formation of fresnoite phase in the films at a temperature of 750 °C. Besides, the AFM observation showed the films have a smooth surface, fine grains and dense structure.

Introduction

Fresnoite, Ba₂TiSi₂O₈ (BTS), is a pyroelectric mineral belonging to the space group P4bm-C_4V² and having lattice constants a=b=0.852 nm and c=0.521 nm, and a tetragonality c/a of 0.61 [1], [2]. The positive and negative charges in the BST lattice are asymmetrically distributed, providing a spontaneous dipole of 1.14×10⁻²⁶ C m and good piezoelectric properties. Researchers studied the synthesis and properties of fresnoite, such as its optical second harmonic generation (SHG) [3], [4], UV luminescence with long-decay time [5], and, recently, the fresnoite ceramic powders showed ferroelectric hysteresis [6]. Particularly, Yamauchi et al. [7] reported that fresnoite thin films with c-axis orientation possess zero temperature coefficient of delay (TCD) and excellent surface acoustic wave (SAW) properties. Many efforts had been made to prepare fresnoite thin film. Yamauchi et al. [8] and Kawa [9] studied the deposition of fresnoite thin films from a ceramic target. Li et al. [10] produced fresnoite thin film with orientation by rf sputtering. They deposited fresnoite thin-film with c-axis orientation on Si (100) and Si (111) substrate by elevating the substrate temperature to 845 °C, and found the remarkable anisotropy in grain growth of fresnoite thin films. Ding et al. [11] crystallized Sr₂TiSi₂O₈ and fresnoite thin-film from a glass bulk by the use of ultrasonic surface treatment, and observed strong optical SHG from the STS thin films. However, sol–gel-derived fresnoite thin film has not yet been reported because of the difficulties in obtaining even, completely crystallized films.

In this paper, we fabricated fresnoite film by a sol–gel process with starting materials of Ba(CH₃COO)₂, Si(OC₂H₅)₄ and Ti(OC₃H₇)4. The effect of heat-treating temperature on the crystallinity of the fresnoite films was investigated. The fresnoite thin films were characterized by FT-IR, Raman scattering, XRD and AFM. It was found that crystalline fresnoite film was obtained at 750 °C and its crystallization was enhanced with increasing heat-treating temperature.