Ferroelectric polarization retention with scaling of Hf_0.5Zr_0.5O₂on silicon

In this paper, we investigate the polarization retention of Hf0.5Zr0.5O2 (HZO)-based metal-ferroelectric-insulator-Si (MFIS) capacitors with scaling of the ferroelectric (FE) layer thickness from 5 nm to 20 nm. The capacitors have a constant interface layer capacitance of ∼24 μF/cm2, developed due to the integration of HZO on a degenerated Si as a bottom conducting electrode. It is observed that 20 nm HZO films show a small change (∼5%) in FE polarization (PFE) between short (10 μs) and long (6 s) retention time, while 5-nm-thick films exhibit a large difference (∼90%). The dependence of PFE retention loss on the FE thickness can be understood by the presence of a built-in electric field in the FE layer, generated due to charge continuity between the FE and the interface layers in the ground state without any external bias. A direct experimental observation also confirms that a residual voltage is developed at the node between the metal-ferroelectric-metal and metal-oxide-semiconductor capacitors connected in series, in the ground state with zero external bias. It is expected that a proper understanding of the built-in field developed in the FE layer in an MFIS stack is crucial for FE memory retention characteristics.