Charge Trapping at Deep States in Hf-Silicate Based High- κ Gate Dielectrics

We have observed charge trapping during constant voltage stress in Hf-based high- κ dielectrics at deep traps as well as at the shallow traps. Δ VFB and leakage current dependence on these deep traps further suggest that trapping at deep levels inhibits fast Δ VT recovery. The earlier findings where charge trapping seemed to be very transient due to the presence of a large number of shallow traps and trapped charge could be eliminated by applying a reverse direction electric field may no longer be valid. The experimentally observed trap energy levels from low-temperature measurements establish a relationship between the origin of the deep traps and their dependence on O vacancy formation in Hf-silicate-based films. Substrate hot electron injection gives rise to significant electron trapping and slow post-stress recovery under negative bias conditions, which confirms that O-vacancy-induced deep defects determine the transient behavior in Hf-silicate-based high- κ gate dielectrics. It is further shown that negative-U transition to deep defects is responsible for trap-assisted tunneling under substrate injection. A fraction of the injected electrons remains trapped at the deep defects and gives rise to significant Δ VFB. This has the potential to be the ultimate limiting factor for the long-term reliability of Hf-based high- κ gate dielectrics.