Ferroelectric phase transitions in films with depletion charge

We consider ferroelectric phase transitions in both short-circuited and biased ferroelectric-semiconductor films with a space (depletion) charge which leads to some unusual behavior. It is shown that in the presence of the charge the polarization separates into "switchable" and "nonswitchable" parts. The electric field, appearing due to the space charge, does not wash out the phase transition, which remains second order but takes place at a reduced temperature. At the same time, it leads to a suppression of the ferroelectricity in a near-electrode layer. This conclusion is valid for materials with both second- and first-order phase transitions in pure bulk samples. Influence of the depletion charge on the coercive field reduces mainly to the lowering of the phase-transition temperature, and its effect is negligible. The electric field estimated in the literature and supposed to favor the switching actually corresponds to the nonswitchable part of the polarization and is obviously irrelevant to the problem The depletion charge can, however, facilitate an appearance of the domain structure which would be detrimental for device performance. We discuss some issues of conceptual character, which are generally known but were overlooked in previous works. The present results have general implications for small systems with depletion charge.