Static Negative Capacitance Response in Ferroelectric/Paraelectric Superlattices

Abstract

Recently Zubko et al.(1) reported a negative capacitance effect in PbTiO3/SrTiO3 superlattices. The negative capacitance of ferroelectrics is proposed as a solution to overcome the “Boltzmann tyranny” that holds back the process of miniaturization. Nowadays, the transistors are becoming too small, which increased the energy dissipation hence the self-heating of the electronic chips. The phenomenon of negative capacitance is gaining a great interest since it has been proposed as a way to reduce the power consumption of field-effect transistors(2), by replacing the conventional SiO2 gate insulator by an insulator that provides a negative capacitance effect.Actually, in ferroelectrics, “the negative capacitance state” takes place at the frontier between the two stable polarization states, during the transient processes of switching, and therefore is intrinsically thermodynamically unstableHence, negative capacitance cannot be evidenced for single layer ferroelectrics but should be stabilized in a series capacitance system. The ferroelectric capacitance can be stabilized in the negative regime, when a paraelectric capacitor and a ferroelectric capacitor are connected in series. Hence, the paraelectric/ferroelectric superlattices provide a model system to investigate the negative capacitance effect.The negative capacitance was shown to be strongly related to the existence of a 180° stripe domain structure. In fact, the depolarization field inside such structure is very strong and induces 180° stripe nanodomains formation in order to screen this field and to maintain the ferroelectric state. This nanodomains, in particularly, the domain wall motion greatly influences the dielectric properties of the heterostructure.Here we report an experimental study on a series of Pb(Zr0.2Ti0.8)O3 /SrTiO3 superlattices in which we evidenced 180° stripe nanodomains. The investigation of such system is motivated by the fact that leakage currents are reduced in PZT compared to the ferroelectric PbTiO3, investigated by Zubko et al.,and also by the possibility to vary the misfit strain related to PbTiO3 /SrTiO3 superlattices . For this study the thickness of the ferroelectric layers was fixed to 4 unit cells and those of the paraelectric layer was varied from 3 to 10 unit cells. The domain’s electrostatic effect in the superlattice is confirmed by a noticeable reduction of the Curie temperature. We will present an in-depth analysis of the dielectric response of these superlattices by impedance spectroscopy. The temperature dependent dielectric measurements will be presented and will emphasize a negative capacitance effect in these superlattices. References - Zubko et al., Nature, 2016, 534, 524–528.- S. Salahuddin and S. Datta, Nano Lett.8, 405 (2008)