Enhancement of ferroelectricity at metal-oxide interfaces

The development of ultrathin ferroelectric capacitors for use in memory applications has been hampered by depolarization effects arising from the electrode-film interfaces. These can be characterized in terms of a reduced interface capacitance, or equivalently an 'effective dead layer' in contact with the electrode. Here, by performing first-principles calculations on four capacitor structures based on BaTiO3 and PbTiO3, we determine the intrinsic interfacial effects responsible for destabilizing the ferroelectric state in ultrathin-film devices. Although it has been widely believed that these are governed by the electronic screening properties at the interface, we show that they also depend crucially on the local chemical environment through the force constants of the metal oxide bonds. In particular, in the case of interfaces formed between AO-terminated perovskites and simple metals, we demonstrate a novel mechanism of interfacial ferroelectricity that produces an overall enhancement of the ferroelectric instability of the film, rather than its suppression as is usually assumed. The resulting 'negative dead layer' suggests a route to thin-film ferroelectric devices that are free of deleterious size effects.