The development of smaller ferroelectric capacitors with additional capacity is at least partly held up by the presence of a ‘dead layer’ at the surface. This negative dead layer, however, can be reversed with certain material combinations, increasing – rather than reducing – the overall performance of a chip, according to a team lead by Nicola Spalding, at the University of California [Stengel et al., doi:10.1038/NMAT2429].
Depolarisation effects at the electrode-film interface, where a reduced capacitance is experienced at the electrode contact, leading to a ‘dead layer’, have created a barrier to the development of ultrathin ferroelectric capacitors. It is widely acknowledged that a depolarising field, caused by imperfect screening at the interface, suppresses ferroelectricity.
In developing its theory, the California team concentrated on ultrathin ferroelectric structures of PbTiO3 (PTO) or BaTiO3 (BTO) with metallic electrodes of SRO or Pt, covering a range of combinations and different behaviours. The interfacial dielectric response was found to be more complex than had previously been assumed, the microscopic effects at the contact point suggested a chemical environment dependence rather than simply the electronic screening properties. A theory of polarization was developed that took into account the interatomic force constants as well as the interface-specific electronic screening effects, to assess the overall capacitor performance.
A covalent bonding mechanism, based on the analysis, was created, that yielded a ferroelectric behaviour of the interface between AO-terminated films and simple metals, enhancing the capacitor’s performance. For example, for platinum metal contacts in combination with barium titanate capacitors, the overall capacitance is increased, not reduced.
Calculations showed that there exists a strong correlation between the overall ferroelectric response of a capacitor and the stiffness of the oxide bonds for the electrode. This suggests that, wherever the bonds are unstable, in the centro-symmetric reference structure, there will be an enhancement in the ferroelectric instability of the film. Thus, it can be seen that opportunities exist to engineer the electrical properties of thin film devices by taking advantage of the chemistry of metal oxide interfaces. It is anticipated that devices making use of the dead layers could lead to the production of significantly smaller memory chips with higher storage capacities.