The mechanism of the flatband voltage shift by capping a thin layer of Me₂O₃ (Me=Gd, Y and Dy) on SiO₂ and HfO₂-based dielectrics
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Continuing to scale down the transistor size makes the introduction of high-k dielectric necessary. However, there are still a lot of problems with highk transistors such as worse reliability and Fermi-level pinning. In HfO₂, low crystallization temperature, fixed charge in the bulk and low quality of the Si/HfO₂ interface cause reliability problems. Fermi-level pinning results in high threshold voltage. For the first work in this dissertation, forming Hf1-xTaxO through doping HfO₂ with Ta is used to improve the crystallization temperature and electron mobility. Then, the fluorine passivation of high-k dielectrics is studied. With fluorine passivation, the electron mobility was improved in NMOSFETs with gate stacks of poly-Si/TaN/HfO₂/p-Si with thin TaN layers. Inserting a 1.5nm layer of HfSiON between TaN and HfO₂ completely blocked the fluorine atoms so that they could not reach the Si interface. Thus, no mobility was improved even with fluorine implantation. In order to decrease threshold voltage, we must study mechanisms of Fermi level pinning (FLP) in high-k gate stacks. We summarize three FLP mechanisms: (1) the dipole formation at the interface between metal gate and high-k dielectric due to hybridization; 2) the dipole formation through oxygen vacancy mechanism; (3) the dipole formation at the interface between high-k dielectric and interfacial SiO₂. The rest of dissertation focuses on the mechanism of Vfb shift by capping a thin layer of Me₂O₃ (Me=Gd, Y and Dy) on SiO₂ and HfO₂-based high-k dielectrics with TaN, W and Pt metal gate. It is proposed that the negative Vfb shift with TaN metal gate be due to the dipole formation at the interface between Me₂O₃ and the interfacial SiO₂. An XPS (X-ray photoelectron spectroscopy) study of Gd₂O₃ capping on SiO₂ indicates clear Si, O and Gd related bonding state change at the interface between Gd₂O₃ (or GdSiO) and the interfacial SiO₂. So the bonding state change is the root cause of the dipole formation. When there is an oxygen deficiency in Me₂O₃, another dipole formation through oxygen vacancy mechanism can also be observed. For a full understanding of the Vfb shift, all three FLP mechanisms must be considered.