Improvement of Memory Retention in Metal–Ferroelectric–Insulator–Semiconductor (MFIS) Structures

Abstract

A simple model has been derived to investigate the retention characteristics of an MFIS structure by considering the effects of currents through the insulator and ferroelectric layers. Simulated curves for the hold time dependence of the capacitance can reproduce experimental curves. Band diagram simulations for the MFIS structure have indicated that the retention characteristics can be severely degraded if the currents through the insulator and ferroelectric layers exceed certain values. A slow absorption current in the ferroelectric layer is also considered to degrade the retention. It is concluded from comparison of the experimental data with the simulation that the most serious origin of the retention characteristics can be attributed to the Schottky current through the metal–ferroelectric junction. In order to decrease the charge injected from the top metal electrode into the ferroelectric layer, the physical model for the MFIS structure has been modified and extended to an MIFIS structure. The calculation indicates that insertion of an ultrathin insulator film between the top metal electrode and the ferroelectric layer can be very effective in providing a longer retention time than that of the original MFIS structure. In order to increase the initially stored charge in the MFIS structure and reduce the depolarization field, substitution of a high-k film for the insulator layer in the MFIS structure has been investigated. The calculation indicates that a high-k insulator layer with ε _i =30 is expected to extend the MFIS retention time substantially. The effects of O₂ annealing on SBT thin films have been studied experimentally. The O₂ annealing improved the polarization retention characteristics of Pt/SBT/Pt capacitors, and decreased the current density of the Schottky contribution by an increase of the barrier height in the Pt/SBT/Pt capacitor. Consequently, the O₂ annealing is effective in improving the capacitance retention characteristics of MFIS structures. Moreover, an MFIS structure using an SBT film treated by RTA shows a very long memory retention time of 10⁵ s, which can be extrapolated to 1 year. In order to clarify the reason why O₂ annealing suppresses the current, UV-PYS analysis has been carried out on SBT films before and after O₂ annealing. We obtained the result that the barrier height for holes at the metal–ferroelectric junction is enhanced by the O₂ annealing and the Schottky current is suppressed.