Negative U traps in HfO2 gate dielectrics and frequency dependence of dynamic BTI in MOSFETs

We report for the first time the following new findings in charge trapping in HfO₂ gate dielectrics: 1) Using an ultra-fast electronic method to measure the MOSFET V_th, two different traps (fast and slow) are identified, each leading to charge trapping characteristics with different dependence on the frequency of dynamic stress; 2) First-principle calculation of oxygen vacancy related traps in HfO₂ reveals the negative U (-U) property of traps in the strongly ionized HfO₂ dielectric. Each trap can trap two electrons and lower the trap energy due to a large lattice relaxation; 3) A physic-based model for the frequency dependence of dynamic BTI is established. The BTI frequency dependence of the slow traps is explained by the -U property of these traps. Excellent agreement between the simulation and experimental data was achieved.