Two-Dimensional Negative Capacitance-FETs with Ferroelectric HfZrO2

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2020

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Abstract

For decades, digital logic devices have been made from silicon-based metal-oxide- semiconductor field-effect transistors (MOSFETs). The development of MOSFETs has followed Moore’s law, doubling the number of transistors on an integrated circuit area every two years. Recently, progress has strayed from this path due to difficulties of overcoming the physical scaling limits and power dissipation issues. Two main concepts have been proposed to continue the scaling of transistor technology: (1) the exploration of new channel materials beyond silicon to continue miniaturization, and (2) the reduction of power consumption with a new device mechanism to overcome the thermionic switching limit in MOSFETs.

Two-dimensional (2D) semiconductors, such as molybdenum disulfide (MoS2), are

promising candidates for enabling aggressive miniaturization of field-effect transistors (FETs) because of their atomically-thin body thickness and facile integration into a junctionless transistor topology that offers enhanced electrostatic control of the channel, making them ideal candidates to enable aggressive miniaturization of FETs. Meanwhile, integrating a ferroelectric (FE) layer into the gate stack of a FET produces amplified internal voltage through the negative capacitance (NC) effect, enabling the resultant NC- FETs to operate with reduced supply voltage VDD by overcoming the 60 mV/dec thermal limit in switching behavior. Negative capacitance field-effect transistors with 2D semiconducting channels (2D NC-FETs) have become increasingly attractive due to their potential to yield sub-thermal switching behavior in a physically scalable device.

Combining these two advantages (2D channels with NC-FET switching) for steep- slope 2D NC-FETs has recently become more feasible using hafnium zirconium oxide layer (HZO) as the ferroelectric layer in the gate stack. 2D NC-FETs with HZO exhibit unique behavior that has been shown to improve both on- and off-states; however, the underlying operating mechanism in these devices, including the role of capacitance matching and the scaling of the channel length, is not well characterized or fully understood. The main objective of this dissertation work was to study and elucidate the factors driving the unique operation of 2D NC-FETs with HZO ferroelectric layers. The work begins by examining the FE behavior of HZO with various capping layers, thicknesses, and annealing temperatures. Then, bottom-gated 2D NC-FETs without a metallic interfacial layer were examined to investigate the effects of dielectric material composition, as well as ferroelectric and dielectric oxide thicknesses, on the operation and performance of the devices. Ultimately, 2D NC-FETs that achieve remarkable and robust short-channel behavior are demonstrated and analyzed, providing evidence that the NC effect enhances gate control and is beneficial to channel length scaling.

The results of this work contribute to the field in three major areas: (1) discovery of elemental metal capping layers for yielding ferroelectricity in HZO, which enables the integration of HZO into various device structures; (2) identification of the impact that FE layers and oxide thickness have on 2D NC-FETs from experimental evidence, yielding deeper understanding and support for previous simulation results in the field; and (3) demonstration of subthreshold switching improvement at short channel lengths in 2D NC-FETs along with an exploration of the mechanism that explains this unconventional behavior of scaling and the preferential performance of the 2D NC-FET.

In summary, this work involves investigating the performance and operation of 2D NC-FETs with HZO ferroelectrics in various gate stack configurations and at different dimensions. Future plans include exploring the unique benefits of 2D materials for NC- FETs, especially the impact on performance from different 2D materials with distinct quantum capacitances, as well as exploring the impact of the thickness of a 2D channel material. To further understand and improve the performance, a top-gated device structure that allows isolation of the contacts from the gate (i.e., no overlapping fields) should be studied. Overall, this work, in combination with the completed works discussed herein, provides an analysis of the operation of 2D NC-FETs based on HZO gate stacks and scaling.

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Lin, Yuh-Chen (2020). Two-Dimensional Negative Capacitance-FETs with Ferroelectric HfZrO2. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/22141.

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