Tristate Memory Using Ferroelectric-Insulator-Semiconductor Heterojunctions for 50% Increased Data Storage

Abstract

Ferroelectric random-access memory (FeRAM) is considered to be one of the best candidates for universal memory. However, difficult scaling of the memory cell size has hindered the realization of high density FeRAM. Given that size scaling is inherently limited by the complicated crystal structure and processing of ferroelectric materials, the highly stable and step-wise three memory state of one cell can be another pathway to high-density FeRAM. A feasible structure and actual operation of a tristate memory function for high-density FeRAM is presented that uses stacked ferroelectric Pb(Zr,Ti)O3/insulating Al2O3/semiconducting ZnO layers with Pt top and bottom electrodes. The complicated electrical responses of the stacked structure to external stimuli are well understood based on the separated trapping of the compensating charges at the Pb(Zr,Ti)O3/Al2O3 and Al2O3/ZnO interfaces and the discrete dissipation of the trapped charges during polarization switching in one direction. This unique function of the structure induces three discrete charge states that can be used to increase the memory density by 50% compared to conventional FeRAM at a given cell size.