Domain Wall Effects on the Photon-induced Ultrafast Carrier Dynamics in Ferroelectric and Paraelectric BaTiO3

Ferroelectric materials are promising candidates in solar-energy conversion applications and the development of next-generation photoelectronic devices. The internal depolarization fields and associated domain walls are believed to significantly affect macroscopic photoelectrical properties of ferroelectric materials. However, their roles during the ultrafast relaxation of photon-generated carriers at its intrinsic excitation temporal scale are not yet fully understood. Using femtosecond time-resolved optical reflectivity measurements, we found that the carrier lifetime is ~200 picoseconds shorter in ferroelectric phase BaTiO3 thin-film than in the paraelectric phase. This difference cannot be fully explained by the commonly used trap-assisted and second-order recombination models. We propose a theoretical model to incorporate drifting of photoelectrons due to the depolarization field and recombination processes within the domain wall region. Our model provides excellent numeric fitting to the ultrafast optical reflectivity measurements across various pump fluences and specimen base temperatures. The method presented in this study can be generalized to the carrier relaxation dynamics of other ferroelectric materials, to provide better understanding on the role of domain walls on non-equilibrium relaxation dynamics of carriers. Additionally, the picosecond evolution of domain wall charges revealed by our model suggests the potential for developing GHz-to-THz tunable-gated photodevices with ultrafast optical control of ferroelectric BaTiO3.