Effect of annealing temperatures on the structure and leakage mechanisms of BiFeO3 thin films prepared by the sol–gel method

Abstract

In this paper, we report the fabrication of polycrystalline BiFeO3 (BFO) thin films on Pt(111)/Ti/SiO2/Si(100) substrates by the sol–gel method. The effect of annealing temperature (T a ) on the structural, optical, morphological, ferroelectric and leakage properties of the films were analyzed by X-ray diffraction (XRD), Raman spectroscopy, Atomic force microscope (AFM) and ferroelectric measurements. XRD patterns and micro-Raman spectra demonstrated that all films had a single perovskite-type rhombohedral structure. A distortion in crystal lattice constants and a contraction in unit cell volume were observed with the increase of annealing temperatures. AFM images showed that dense and uniform grains were obtained. This reveals that the film is well crystallized. The data of ferroelectric test indicated that the double remanent polarization (2P r ) value of the thin film at T a = 600 °C was 22 μC/cm2 and the leakage current density was 9.0 × 10−8 A/cm2 at an applied electric field of 1.0 × 105 V/cm. The leakage mechanisms of thin films have been studied in order to identify the cause of high leakage currents such as Ohmic conduction, the space-charge-limited current, the Schottky emission, the Fowler–Nordheim tunneling and the Poole–Frenkel emission. In low electric field region (<1.0 × 105 V/cm), the conduction behavior was found to be dominated by Ohmic conduction for the thin film annealed at 500 °C. The Ohmic conduction and space-charge-limited current dominated the leakage behavior for the thin films at T a = 550 and 600 °C. Fowler–Nordheim tunneling was responsible for the leakage behavior of the thin films at T a = 550, 600 °C in high electric field region (>1.0 × 105 V/cm). The results demonstrated that the microstructure, surface morphology and ferroelectric properties of BFO thin films have a strong dependence on annealing temperatures.