Critical thickness for ferroelectricity in perovskite ultrathin films

Abstract

The integration of ferroelectric oxide films into microelectronic devices^1,2, combined with the size reduction constraints imposed by the semiconductor industry, have revived interest in the old question concerning the possible existence of a critical thickness for ferroelectricity. Current experimental techniques have allowed the detection of ferroelectricity in perovskite films down to a thickness of 40 Å (ten unit cells), ref. 3. Recent atomistic simulations^4,5 have confirmed the possibility of retaining the ferroelectric ground state at ultralow thicknesses, and suggest the absence of a critical size. Here we report first-principles calculations on a realistic ferroelectric–electrode interface. We show that, contrary to current thought, BaTiO₃ thin films between two metallic SrRuO₃ electrodes in short circuit lose their ferroelectric properties below a critical thickness of about six unit cells (∼24 Å). A depolarizing electrostatic field, caused by dipoles at the ferroelectric–metal interfaces, is the reason for the disappearance of the ferroelectric instability. Our results suggest the existence of a lower limit for the thickness of useful ferroelectric layers in electronic devices.