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Register a new userHarnessing ferroelectric domains for negative capacitance
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The pressing quest for overcoming Boltzmann tyranny in low-power nanoscale electronics revived the thoughts of engineers of early 1930-s on the possibility of negative circuit constants. The concept of the ferroelectric-based negative capacitance (NC) devices triggered explosive activity in the field. However, most of the research addressed transient NC, leaving the basic question of the existence of the steady-state NC unresolved. Here we demonstrate that the ferroelectric nanodot capacitor hosts a stable two-domain state realizing the static reversible NC device thus opening routes for the extensive use of the NC in domain wall-based nanoelectronics.
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Integrating negative capacitance (NC) into the field-effect transistors promises to break fundamental limits of power dissipation known as Boltzmann tyranny. However, realization of the stable static negative capacitance in the non-transient regime without hysteresis remains a daunting task. Here we show that the failure to implement the NC stems from the lack of understanding that its origin is fundamentally related with the inevitable emergence of the domain state. We put forth an ingenious design for the ferroelectric domain-based field-effect transistor with the stable reversible static negative capacitance. Using dielectric coating of the ferroelectric capacitor enables the tunability of the negative capacitance improving tremendously the performance of the field-effect transistors.
It is well known that one needs an external source of energy to provide voltage amplification. Because of this, conventional circuit elements such as resistors, inductors or capacitors cannot provide amplification all by themselves. Here, we demonstrate that a ferroelectric can cause a differential amplification without needing such an external energy source. As the ferroelectric switches from one polarization state to the other, a transfer of energy takes place from the ferroelectric to the dielectric, determined by the ratio of their capacitances, which, in turn, leads to the differential amplification. {This amplification is very different in nature from conventional inductor-capacitor based circuits where an oscillatory amplification can be observed. The demonstration of differential voltage amplification from completely passive capacitor elements only, has fundamental ramifications for next generation electronics.
The so-called Boltzmann Tyranny defines the fundamental thermionic limit of the subthreshold slope (SS) of a metal-oxide-semiconductor field-effect transistor (MOSFET) at 60 mV/dec at room temperature and, therefore, precludes the lowering of the supply voltage and the overall power consumption. Adding a ferroelectric negative capacitor to the gate stack of a MOSFET may offer a promising solution to bypassing this fundamental barrier. Meanwhile, two-dimensional (2D) semiconductors, such as atomically thin transition metal dichalcogenides (TMDs) due to their low dielectric constant, and ease of integration in a junctionless transistor topology, offer enhanced electrostatic control of the channel. Here, we combine these two advantages and demonstrate for the first time a molybdenum disulfide (MoS2) 2D steep slope transistor with a ferroelectric hafnium zirconium oxide layer (HZO) in the gate dielectric stack. This device exhibits excellent performance in both on- and off-states, with maximum drain current of 510 {mu}A/{mu}m, sub-thermionic subthreshold slope and is essentially hysteresis-free. Negative differential resistance (NDR) was observed at room temperature in the MoS2 negative capacitance field-effect-transistors (NC-FETs) as the result of negative capacitance due to the negative drain-induced-barrier-lowering (DIBL). High on-current induced self-heating effect was also observed and studied.
Negative capacitance (NC) in ferroelectrics, which stems from the imperfect screening of polarization, is considered a viable approach to lower voltage operation in the field-effect transistors (FETs) used in logic switches. 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.
The electrostatics arising in ferroelectric/dielectric two-dimensional heterostructures and superlatitices is revisited here within a simplest Kittel model, in order to define a clear paradigmatic reference for domain formation. The screening of the depolarizing field in isolated ferroelectric or polar thin films via the formation of 180$^{circ}$ domains is well understood, whereby the width of the domains $w$ grows as the square-root of the film thickness $d$, following Kittels law, for thick enough films ($wll d$). This behavior is qualitatively unaltered when the film is deposited on a dielectric substrate, sandwiched between dielectrics, and even in a superlattice setting, with just a suitable renormalisation of Kittels length. As $d$ decreases, $w(d)$ deviates from Kittels law, reaching a minimum and then diverging onto the mono-domain limit for thin enough films, always assuming a given spontaneous polarization $P$ of the ferrolectric, only modified by linear response to the depolarizing field. In most cases of experimental relevance $P$ would vanish before reaching that thin-film regime. This is not the case for superlattices. Unlike single films, for which the increase of the dielectric constant of the surrounding medium pushes the deviation from the Kittels regime to lower values of $d$, there is a critical value of the relative thickness of ferroelectric/dielectric films in superlattices beyond which that behavior is reversed, and which defines the separation between strong and weak ferroelectric coupling in superlattices.
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