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Negative Capacitance for Ultra-low Power Computing

  • Author(s): Khan, Asif Islam
  • Advisor(s): Salahuddin, Sayeef
  • et al.

Owing to the fundamental physics of the Boltzmann distribution, the ever-increasing power dissipation in nanoscale transistors threatens an end to the almost-four-decade-old cadence of continued performance improvement in complementary metal-oxide-semiconductor (CMOS) technology. It is now agreed that the introduction of new physics into the operation of field-effect transistors--in other words, ``reinventing the transistor''-- is required to avert such a bottleneck. In this dissertation, we present the experimental demonstration of a novel physical phenomenon, called the negative capacitance effect in ferroelectric oxides, which could dramatically reduce power dissipation in nanoscale transistors. It was theoretically proposed in 2008 that by introducing a ferroelectric negative capacitance material into the gate oxide of a metal-oxide-semiconductor field-effect transistor (MOSFET), the subthreshold slope could be reduced below the fundamental Boltzmann limit of 60 mV/dec, which, in turn, could arbitrarily lower the power supply voltage and the power dissipation. The research presented in this dissertation establishes the theoretical concept of ferroelectric negative capacitance as an experimentally verified fact. \

The main results presented in this dissertation are threefold. To start, we present the first direct measurement of negative capacitance in isolated, single crystalline, epitaxially grown thin film capacitors of ferroelectric Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$. By constructing a simple resistor-ferroelectric capacitor series circuit, we show that, during ferroelectric switching, the ferroelectric voltage decreases, while the stored charge in it increases, which directly shows a negative slope in the charge-voltage characteristics of a ferroelectric capacitor. Such a situation is completely opposite to what would be observed in a regular resistor-positive capacitor series circuit. This measurement could serve as a canonical test for negative capacitance in any novel material system. Secondly, in epitaxially grown ferroelectric Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$-dielectric SrTiO$_3$ heterostructure capacitors, we show that negative capacitance effect from the ferroelectric Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ layer could result in an enhancement of the capacitance of bilayer heterostructure over that of the constituent dielectric SrTiO$_3$ layer. This observation apparently violates the fundamental law of circuit theory which states that the equivalent capacitance of two capacitors connected in series is smaller than that of each of the constituent capacitors. Finally, we present a design framework for negative capacitance field-effect-transistors and project performance for such devices.

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