Lead–zirconate–titanate-based metal/ferroelectric/insulator/semiconductor structure for nonvolatile memories

We have investigated the structural and electrical properties of the metal/ferroelectric/insulator/semiconductor (MFIS) structure that incorporates a ${\mathrm{MgO/SiO}}_2$ insulating buffer between a ferroelectric layer and Si substrate. Highly oriented lead–zirconate–titanate $[{\mathrm{Pb(Zr,Ti)O}}_3,$ or PZT] films were grown on the MgO-buffered oxidized silicon substrates with a rf magnetron sputtering technique. The x-ray diffraction and energy-dispersive x-ray spectroscopy analysis results show that a MgO buffer serves well not only as a template layer for growing oriented PZT films on an amorphous surface but also as a diffusion barrier between PZT and Si substrates. The memory window of the MFIS structure was characterized with a capacitance-versus-voltage method. Numerical analyses were also carried out to simulate the MFIS capacitor characteristics. In this simulation, the PZT films were assumed to have a two-layer structure in which the dielectric and ferroelectric properties of an initial layer are significantly weaker than those of the main layer part. By comparing the measurement data with the simulation result, we have extracted the parameters of this two-layer model (dielectric constant and the polarization-versus-electric-field characteristics) of the PZT films in the MFIS structure. The scalability of the memory window of the MFIS structure was investigated by varying the ferroelectric (PZT) layer thickness. Both the experimental and simulation results show that the PZT-based MFIS structure is suitable for nonvolatile memory field-effect transistors with low-voltage requirement (3 V or less) and large memory window (1–2 V).