Negative capacitance in multidomain ferroelectric superlattices

Abstract

*These authors contributed equally to this work.The stability of the spontaneous electrical polarisation characteristic of ferroelectrics is fundamental to a multitude of their current applications, ranging from the simple electrical cigarette lighter to non-volatile random access memories 1 . Yet, the technological potential of these materials is far from being exhausted as research on nanoscale ferroelectrics reveals their properties to be profoundly different from those in bulk, giving rise to fascinating new phenomena with exciting prospects for future 2 devices 2-4 . As ferroelectrics become thinner, maintaining a stable polarisation becomes increasingly challenging. On the other hand, intentionally destabilising this polarisation can cause the effective electrical permittivity of a ferroelectric to become negative 5 , enabling it to behave as a negative capacitance when integrated in a heterostructure.Negative capacitance has been garnering increasing attention following the realisation that it could be exploited to overcome fundamental limitations on the power consumption of field effect transistors 6 . Experimentally, however, demonstrations of this phenomenon are still contentious 7 . The prevalent interpretations based on homogeneous polarisation models are difficult to reconcile with the expected strong tendency for domain formation 8,9 , while the effect of domains on negative capacitance has received surprisingly little attention 5,10-12 . Here we report the observation of negative capacitance in a model system of multidomain ferroelectric-dielectric superlattices across a wide range of temperatures, in both the ferroelectric and paraelectric phases. Using a phenomenological model we show that domain-wall motion not only gives rise to negative permittivity but can also enhance, rather than limit, its temperature range. Furthermore, our first-principles-based atomistic simulations provide detailed microscopic insight on the origin of this phenomenon, identifying the dominant contribution of near-interface layers and paving the way for its future exploitation.Negative capacitance (NC) has its origins in the imperfect screening of the spontaneous polarisation 5,10,13,14 . Imperfect screening is intrinsic to any semiconductor-ferroelectric or even metal-ferroelectric interfaces because of their finite effective screening lengths 15,16 .Alternatively, it can be engineered in a controlled manner by deliberately inserting a dielectric layer of relative permittivity ߳ ௗ between the ferroelectric and the electrodes as suggested by Salahuddin and Data 6 and shown in Fig. 1a. The physical separation of the 3 ferroelectric bound charge from the metallic screening charges creates a depolarizing field inside the ferroelectric, destabilizing the polarisation and lowering the ferroelectric transition temperature. The effect of the dielectric layer can be understood by considering the free energy of the bilayer capacitor with the usual assumption of a uniform polarisation ܲ (see Methods). Below the bulk transitio...

Polar nature of stress-induced twin walls in ferroelastic CaTiO3

“…29 It would be also noticed that the layered structure of non-polar domains and polar TWs could be a possible candidate of the negative capacitance material, which is very important for MOS-FET applications with a small driving voltage. 45 On this line of research, we are also going to investigate temperature dependences of the polar nature of TWs and their over-all and local dielectric responses.…”

Transient nature of negative capacitance in ferroelectric field-effect transistors

“…In this paper, we discuss the implications of the transient nature of negative capacitance for its practical application. It is suggested that the NC effect needs to be characterized at the proper time scale to identify the type of circuits where functional NC-FETs can be used effectively.Index Terms-Fe-FET, ferroelectric, MOSFET, NC-FET, negative capacitance, screening, steep subthreshold, transient.Recently, there has been much interest in the negative capacitance (NC) phenomenon in ferroelectrics, which could potentially reduce the energy consumption in field-effect transistors (FETs) due to the enhanced subthreshold slope [1][2][3]. While there is much debate on the physics, interpretation, and applications of this NC effect, there is a general consensus that the phenomenon is metastable in nature [4].…”

“…Recently, there has been much interest in the negative capacitance (NC) phenomenon in ferroelectrics, which could potentially reduce the energy consumption in field-effect transistors (FETs) due to the enhanced subthreshold slope [1][2][3]. While there is much debate on the physics, interpretation, and applications of this NC effect, there is a general consensus that the phenomenon is metastable in nature [4].…”

“…More importantly, application of NC-FETs needs to be examined in the context of real-circuit performance where the NC transient nature is taken into account. One of the attractive approaches for realization of the NC-FET devices could be employment of the multidomain ferroelectric superlattices [3] where polarization screening is inherently suppressed, potentially giving rise to a more steady negative capacitance effect. …”

Frontiers in strain-engineered multifunctional ferroic materials

“…A particularly emergent example is the use of the negative capacitance time domain regime of ferroelectric switching [130] and the potential to leverage nanodomain structures to produce negative capacitance over a large temperature range. [131] Such approaches will enable us to, at least temporarily, overcome the 60 mV/decade limit that is current in CMOS transistors.…”