The effects of grain size on electrical properties and domain structure of BiFeO3 thin films by sol-gel method

BiFeO3 (short for BFO) thin films with different grain sizes were fabricated via sol-gel spin-coating method. The effects of grain size on leakage behavior, dielectric, ferroelectric, piezoelectric properties and domain structure of BFO thin films have been investigated systematically. The X-ray diffraction results show that BFO thin films are rhombohedral distortion perovskite structure. Compared with the films annealed at 550 A degrees C, the grain size of BFO thin films annealed at 600 A degrees C is larger and the roughness is less, and the crystallinity and purity are higher. The leakage current density of BFO thin films with larger grain size is much lower than that of the films with smaller grain size. It is found that the conduction behavior of BFO thin films with smaller grain size transforms from Ohmic to space-charge-limited current and Fowler-Nordheim tunneling conduction as electric field increases. But there is the only transition from Ohmic conduction to space-charge-limited conduction for the thin films with larger grain size as electric field increase. The room temperature dielectric constant and remnant polarization of BFO thin films with larger grain size are higher than that of the films with smaller grain size. The ferroelectric domain size increases with the increase of grain size so that the ferroelectric polarization in BFO thin films with larger grain size enhances. Moreover, it is found that there are negatively charged "tail to tail" domain wall in BFO thin films with smaller grain size and positively charged "head to head" domain wall in the sample with larger grain size. The majority carriers including positively charged hole or oxygen vacancy assemble on negatively charged "tail to tail" domain wall in p-type BFO thin films with smaller grain size and result in relative higher leakage current. The piezoelectric coefficient of the films with larger grain size is much higher than that of the sample with smaller grain size.