Switching On/Off Negative Capacitance in Ultrathin Ferroelectric/Dielectric Capacitors

Ferroelectric (FE) and dielectric (DE) insulator bilayer stacks provide a promising gate for low-power microelectronic devices. To fully realize the FE polarization switching, the DE layer must be ultrathin in the FE/DE bilayer stack. Motivated by this, this work presents the first successful fabrication and characterization of Fe/FeO/Al₂O₃/Fe FE/DE bilayer capacitors using in vacuo atomic layer deposition (ALD) with a total FE/DE stack thickness <3-4 nm. A key tuning parameter in generating the FE/DE bilayer capacitors is the thickness of an Al wetting layer between the bottom Fe electrode and the ALD-Al₂O₃ DE layer. At a large thickness in exceeding 1.0 nm, high-quality conventional DE capacitors of 2.2 nm thick ALD-Al₂O₃ were obtained with dielectric constant (ε_r) ∼8.0 that is close to ε_r ∼ 9.2 for the Al₂O₃ bulk single crystal with an effective oxide thickness of 1.0 nm. By reducing the Al wetting layer thickness to below 1.0 nm, a thin ferroelectric FeO interfacial layer of a thickness of 1-2 nm forms, enabling the achievement of a FeO/Al₂O₃ FE/DE bilayer capacitor with static negative capacitance. Since all ferroelectric materials are piezoelectric, we show that a dynamic switching on/off of the negative capacitance can be achieved under the application of an external force on the ultrathin FE/DE capacitors through manipulation of the electric dipoles. This result not only provides a viable approach for generating ultrathin FE/DE bilayer capacitors but also offers a promising solution to low-power consumption microelectronics.