Relaxation of ferroelectric domains in epitaxial BiFeO3 thin films on vicinal SrTiO3 substrate

• We observed the relaxation of as-grown ferroelectric domain with time.
• The relaxation is related to the crystallographic direction of SrTiO3.
• We suggest that mobile space charges can promote the relaxation.

Epitaxial BiFeO3 (BFO) thin films with conducting SrRuO3 bottom layer were grown by using pulsed laser deposition on (001) SrTiO3 (STO) single crystal substrates with terrace widths of 150 and 250 nm. In this paper, we report instabilities of as-grown nanoscale ferroelectric domains observed by time evolution of piezoelectric force microscopy (PFM) data of BFO films stored at room-temperature. This ferroelectric polarization relaxation process shows time evolution of vertical responses during elapse of hundred hours after BFO film growth, where up-polarized domains appear from overall down-polarized domains formed by self-polarization effect. By comparing PFM data with X-ray diffractions of relaxed film, relaxed and strained structures of BFO films are associated to two orthogonal directions relative to the vicinal direction of STO. Topographic images show step-flow and step-bunching growths with respect to vicinal direction and the data suggest that the relaxation of ferroelectric polarization is related to this location and induced by the relaxed states along STO [010] step-flow direction. In addition, surface potential and displacement-electric field hysteresis measurements suggest that mobile space charges can promote this relaxation.