Improvement of electrical properties of ferroelectric gate oxide structure by using Al2O3 thin films as buffer insulator

SrBi2Nb2O9 (SBN) and Al2O3 thin films were prepared by r.f.-sputtering as a ferroelectric material and as a buffer insulator for metal /ferroelectric/insulator/semiconductor (MFIS) structure, respectively. The electrical properties of those films on Si substrate were studied. Coercive field that decisively affects the memory window was greatly increased by inserting an Al2O3 insulator between SBN and Si, and thus the memory window also increased with the increasing electric field to the SBN. The Al2O3 intermediate layer between the perovskite SBN film and Si substrate prevent SBN from the serious inter-diffusion into Si substrate. Memory windows of MFIS structure were in the range of 0.7-3.4 V when the gate voltage varied from 3 to 9 V. Memory windows of MFIS structure were found to be dependent on the thickness and stoichiometry of the buffer layer. We obtained the maximum memory window in the MFIS structure with an optimized insulator thickness of 11.4 nm.