Achieving Robust Ferroelectric Properties of Hf0.6Zr0.4O2 Thin Films through the Pseudofield-Inducing Effect of Al0.85Sc0.15N Bottom Electrode

Although ferroelectric (FE) Zr-doped HfO2 (HZO) is a promising material for ferroelectric random-access memory (FeRAM), achieving high remnant polarization (Pr) with a low coercive field (EC) at film thicknesses ≲10 nm remains challenging. This work demonstrates a multifunctional bottom electrode (BE) strategy by inserting a pristine, down-poled ferroelectric Al1-xScxN (ASN) layer into the BE, thereby inducing high mechanical stress and an internal field during HZO crystallization. The pristine down-polarization of ASN increased the c-axis preference of the overlying HZO film, as indicated by the contrast in the electrical characteristics and surface potential responses depending on the ASN poling direction. The ASN-containing BE imposes additional in-plane tensile stress on HZO due to ASN's low thermal expansion coefficient, facilitating orthorhombic phase formation. Consequently, 9 nm-thick HZO on the ASN-containing BE exhibits a large 2Pr of 81 μC/cm2 by conventional polarization-electric field (P-E) measurement and 68 μC/cm2 by positive-up-negative-down (PUND) compared with that of the TiN BE (43 μC/cm2, and 41 μC/cm2) with a pristine negative-bias imprint. These effects enable a high 2Pr/EC ratio without a significant increase in EC after wake-up, resulting in faster switching characteristics. Furthermore, it also enables the 5 nm-thick HZO film to overcome the pristine pinched hysteresis associated with tetragonal-phase stabilization, thereby achieving robust ferroelectric performance while suppressing wake-up behavior.