Investigation of the Origin of $V_{T}/V_{\rm FB}$ Modulation by $\hbox {La}_{2}\hbox {O}_{3}$

The role of La₂O₃ capping in the VT/V_FB shift with various high- k and metal gate electrodes was systematically investigated. It was found that the La concentration at the high-k/SiO₂ interface is mainly responsible for the VT/V_FB modulation in NMOS devices, whereas the effect of the host high-k and gate electrodes on VT/V_FB is minimal. A 400-mV shift in VT from the control HfO₂ device with minimal degradation in mobility was obtained when a La₂O₃ layer was inserted between the high-k and SiO₂ layers. It was also found that the incorporation of La₂O₃ in the dielectric stack improves device reliability in terms of breakdown and positive-bias temperature instability characteristics. The main key for the V_FB shift is the ability of La diffusion through the host high-k material.