Tailoring of Ferroelectric Coercive Field and Polarization With Ferroelectric and Antiferroelectric Hf_xZr_1–xO₂ Bilayer Structure

In this letter, the advantages of an antiferroelectric (AFE) and ferroelectric (FE) bilayer stack made of Hf<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub>Zr<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{1}-{\text {x}}}$ </tex-math></inline-formula>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>(HZO) with different compositions were reported. Compared to the monolayer ferroelectric control sample, Mo/FE/Mo, the Mo/FE/AFE/Mo, and Mo/AFE/FE/Mo stacks exhibited a significant decrease in the coercive field (E<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text {c}}\text {)}$ </tex-math></inline-formula>. A higher dielectric constant of AFE increased the voltage distribution across the FE layer in the bilayer HZO structure, leading to a decrease in E<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub>. Furthermore, the capacitor with Mo/AFE/FE/Mo exhibited 28% higher polarization, P<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> (2P<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\text {r}} \,\, = \,\, 45.9~\mu $ </tex-math></inline-formula>C/cm<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{{2}}\text {)}$ </tex-math></inline-formula>, than the control sample, while this significant increase of P<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> was not observed in the capacitor with Mo/FE/AFE/Mo. Electrical measurements of the capacitors with FE and AFE having various thicknesses showed that the dielectric constants and phase composition depend on the deposition sequence. A higher orthogonal phase ratio was achieved in the Mo/AFE/FE/Mo stack compared to the Mo/FE/AFE/Mo stack, resulting in higher polarization. Furthermore, bilayer capacitors with Mo/AFE/FE/Mo showed more robust long time reliability, such as endurance and retention.