Charge-transition levels of oxygen vacancy as the origin of device instability in HfO2 gate stacks through quasiparticle energy calculations

We perform quasiparticle energy calculations to study the charge-transition levels of oxygen vacancy (VO) in HfO2. The negative-U property of VO can explain flat band voltage shifts and threshold voltage (Vth) instability in hafnium based devices. In p+ Si gate electrode, the Fermi level pinning varies by up to 0.55 eV, in good agreement with the measured values. Depending on gate bias, VO traps electrons or holes from the Si channel, causing the Vth instability. It is suggested that short time-scale charge trapping/detrapping is due to metastable VO−1 centers, whereas stable VO−2 centers dominate long time-scale instability.