Photo-induced poly-domain to mono-domain switching in ultrathin PbTiO$_{3}$ films

Ferroelectric (FE) domain structures play a crucial role in polarization switching dynamics, ferroelectric random access memory (FRAM) data retention and polarization fatigue and it is therefore of practical importance to gain a more fundamental understanding of the origin of the transition between poly-domain and mono-domain states. Methods that promote the mono-domain state in the FE films are also of interest for model studies of ferroelectricity and its dimensional scaling. We investigate a novel poly-domain to mono-domain (P-M) switching mechanism in thin ferroelectric PbTiO$_{3}$ (PTO) films induced by ultraviolet (UV) light illumination. The switching occurs within minutes if a sample is irradiated with light of wavelength $<$ 390 nm which corresponds to a photon energy that is roughly equivalent to the band gap of PbTiO$_{3}$ (3.2 eV). Based on a developed electrostatic model, we show that the observed phenomenon is related to photo generation of free carriers at the PbTiO$_{3}$/SrTiO$_{3}$ interface which effectively screens the PTO polarization charge at the interface and therefore promotes the transition of the ferroelectric film from a poly-domain to a mono-domain state. The model successfully describes not only the observed illumination effects on PTO stripe domain patterns but also a longer-term poly-domain to mono-domain (P-M) transition that occurs without intentional illumination.