  We present a theoretical description of the influence of misfit strain on
mobile defects dynamics in thin strained ferroelectric films. Self-consistent
solutions obtained by coupling the Poissons equation for electric potential
with continuity equations for mobile donor and electron concentrations and
time-dependent Landau-Ginzburg-Devonshire equations reveal that the Vegard
mechanism (chemical pressure) leads to the redistribution of both charged and
electro-neutral defects in order to decrease the effective stress in the film.
Internal electric fields, both built-in and depolarization ones, lead to a
strong accumulation of screening space charges (charged defects and electrons)
near the film interfaces. Importantly, the corresponding screening length is
governed by the misfit strain and Vegard coefficient. Mobile defects dynamics,
kinetics of polarization and electric current reversal are defined by the
complex interplay between the donor, electron and phonon relaxation times,
misfit strain, finite size effect and Vegard stresses.
