(Invited) Understanding the Switching Mechanism in RRAM Devices and the Dielectric Breakdown of Ultrathin High-k Gate Stacks from First Principles Calculations

Abstract

RRAM devices received increased interest lately as advanced non-volatile memory technologies in terms of low operating power, high density, better non-volatility, fast switching speed, and compatibility with conventional CMOS process. However, up to date the fundamental physical principles controlling the switching are not well understood. We have employed first-principles simulations based on density functional theory (DFT) to elucidate the effect of oxygen vacancy defects on the electronic structure of rutile TiO2 and NiO using the local density approximation with correction of on-site Coulomb interactions (LDA+U). The vacancy filament induces several defect states within the band gap, which can lead to the defect-assisted electron transport and account for on-state low resistance conduction in bulk rutile TiO2 and NiO. For CMOS devices on the other hand the reliability of the gate stack is becoming a significant challenge with the continuous scaling of transistors, due to the ultrathin oxides and defects in the gate stack. The degradation of the gate oxides has been observed under electrical stress, due to traps generated by defects, e.g. oxygen vacancies present in these materials. First principles methods based on density functional theory are used to determine the location of the defect states in the band gap when these defects are at the various interfaces of the gate stack and how they contribute to the oxide breakdown in ultrathin gate stacks.