Giant Voltage Amplification Under Scrutiny

If a capacitor contains both ferroelectric and dielectric layers then, under certain conditions, the drop in the electrical potential across the dielectric can be higher than the external voltage applied to the capacitor. This is recently of practical interest in relation to field-effect-transistors (FET) for example. Such a “voltage amplification”, which can be seen as arising due to a “negative-capacitance effect” associated with the ferroelectric, has been mostly discussed in relation to multidomain ferroelectricity (A. M. Bratkovsky and A. P. Levanyuk, Appl. Phys. Lett. 89, 253108 (2006)). However, it can also be realized when the ferroelectric is in its paraelectric phase but such that it would be in the ferroelectric state had it been in a short-circuited capacitor without a dielectric and with ideal electrodes (A. Cano and D. Jimenez, Appl. Phys. Lett. 97, 133509 (2010)). In a recent paper by Graf et al. (Nature Materials 21, 1252 (2022), a “giant voltage amplification” with a factor ~12 is reported for a special case of an infinite PbTiO₃/SrTiO₃ multilayer in the paraelectric state. In this presentation, drawing from the results of our earlier work (A. P. Levanyuk and I. B. Misirlioglu, J. Appl. Phys. 110, 114109 (2011)), we report on our theoretical investigation so as to how this result transforms when applied to finite systems that are closer to those of interest for applications. Using the Landau-Ginzburg-Devonshire (LGD) theory we consider a uniaxial ferroelectric with different dielectric surroundings. We find that the voltage amplification depends strongly on the configuration of the system and the characteristics of the electrodes of the capacitor. In a special case of symmetrical configuration of the dielectric layers and ideal electrodes our results reproduce the giant voltage amplification reported by Graf et. al. In asymmetric configurations, however, we find that the voltage amplification decreases substantially.