Optimization of high-k composite dielectric materials of variable oxide thickness tunnel barrier for nonvolatile memory

Abstract

Downscaling the tunnel oxide thickness has become one of the innovative solutions to minimize the operational voltage with better the programming/erasing (P/E) operation time. However, the downscaling technique faces several challenges where the conventional SiO₂ tunnel layer has reached its limit. But a practical alternative has been introduced; Variable Oxide Thickness (VARIOT) technology in flash memory has been promising. VARIOT is one of tunnel barrier engineering technology for incorporating the high-k dielectric materials as a composite tunnel barrier. This paper presents the VARIOT concept to determine the optimum set of combination, the equivalent oxide thickness (EOT) and the low-k oxide thickness (T_ox) for alternate high-k materials. Fowler-Nordheim (F-N) tunneling coefficients are also extracted for various combinations of VARIOT, where in this work ZrO₂, HfO₂, Al₂O_3, La₂O_3, and Y₂O₃ are used. The VARIOT optimization is conducted using 3-Dimensional (3D) Silicon Nanowire Field-Effect-Transistor (SiNWFET) device structure and simulated in TCAD Simulation tools. From the simulation results, it has found out that the high-k materials of La₂O₃ asymmetric stack is the excellent dielectric material among four (4) other dielectric materials; ZrO₂, HfO₂, Al₂O₃ and Y₂O₃ for EOT=4nm and T_ox=1nm.