Density functional theory based study of biaxially strained ferroelectric materials and applications to negative capacitance FETs

Abstract

Over the past few decades, the numerous works have delved into the steep-slope device that can surmount the “Boltzmann tyranny,” and negative capacitance FETs (NCFETs) have recently drawn a lot of attention as a most probable steep-slope device. Of the most technologically important factor determining the performance of NCFETs is the functionality of the ferroelectric layer integrated into the gate stack. Among a few factors that can conspicuously change the functionality of ferroelectric materials, the epitaxial strain is one of the crucial issues because it can be imposed either naturally or artificially. In this thesis, we report device simulations conducted to study the performance of biaxially strained ferroelectric-based NCFETs. We adopted Pb($Zr_xTi_{1-x}$)$)$O_3$ (PZT) and $HfO_2$ as representative ferroelectric materials and applied biaxial strain using the first-principles method. We have found that mainly due to the simple polarization switching pathway of PZT, PZT-based NCFETs show monotonous variation to the biaxial strain. However, unlike the PZT, $HfO_2$ has multiple switching pathways and strain dependence of each pathway is noticeably different. We have found that $P4_2$/nmc pathway-based $HfO_2$ exhibits a large variation in negative capacitance with biaxial strain, whereas Pbcm pathway-based $HfO_2$ is insensitive to the biaxial strain. As a result, $HfO_2$-based NCFETs show large variations depending on the switching pathways and strain. Especially, we suggest that the performance difference between $P4_2$/nmc pathway-based NCFETs and Pbcm pathway-based NCFETs can be noticeably decreased by inducing the tensile strain along the x-direction. In this work, we have identified that switching pathway of $HfO_2$ is a critical factor when the strain engineering is conducted on $HfO_2$.