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 (V-O) in HfO2. The negative-U property of V-O can explain flat band voltage shifts and threshold voltage (V-th) 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, V-O traps electrons or holes from the Si channel, causing the V-th instability. It is suggested that short time-scale charge trapping/detrapping is due to metastable V-O(-1) centers, whereas stable V-O(-2) centers dominate long time-scale instability.