Control of polarization reversal temperature behavior by surface screening in thin ferroelectric films

Ferroelectric surfaces and interfaces are unique physical objects for fundamental studies of various screening mechanisms of spontaneous polarization by free carriers and possible ion exchange between the polar surface and ambient media. The theory of the polarization charge compensation at ferroelectric surface by ambient screening charges requires a detailed comparison of different screening models. In the article, we study the free energy of a thin ferroelectric film covered by a screening charge layer of different nature and calculate hysteresis loops of polarization and screening charge in the system at different temperatures. The dependence of the screening charge density on electric potential was considered for three basic models, namely for the linear Bardeen-type surface states (BS) and nonlinear the Fermi-Dirac (FD) density of states describing two-dimensional electron gas at the film surface, and the strongly nonlinear electrochemical Stephenson-Highland (SH) model describing the surface charge density of absorbed ions. Among considered surface screening models, the most various behavior of polarization and screening charge hysteresis loops is inherent to SH model. Obtained results give new insight to the understanding of bound charge compensation at ferroelectric surfaces at different temperatures, as well as they open the way to control the polarization reversal in thin ferroelectric films by appropriate choice of the surface charge nature and screening mechanism.