Quasi-particle energy calculations were performed in order to study the chargetransition

levels of the oxygen vacancy (VO) in HfO2. The negative-U property of

VO could explain flat band voltage shifts and a threshold voltage (Vth) instability in

hafnium-based devices. In a p+ Si gate electrode, the Fermi level pinning varied by

up to 0.55eV, in good agreement with the measured values. Depending upon gate bias, the VO traps electrons or holes from the Si channel; causing the Vth instability.

It was suggested that short-term charge trapping/detrapping was due to metastable

VO

- centers, whereas stable VO

2- centers dominated long-term instability.

Charge-Transition Levels of Oxygen Vacancy as the Origin of Device Instability in

HfO2 Gate Stacks Through Quasi-Particle Energy Calculations. E.A.Choi,

K.J.Chang: Applied Physics Letters, 2009, 94[12], 122901