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Extracted EWF of the devices used in this study for three different gate dielectrics: HfO2, HfSixOy, and SiO2. Four different capping layers of 1 nm have been used: AlN, Al2O3, Gd2O3, and La2O3. Uncapped devices, marked as No Cap, have been used as reference. For the capped devices, there is clear shift of the EWF as the % SiO2 in the gate dielectric increases. Al-based capping layers cause a positive shift on the EWF, while the rare earth-based capping layers shift in the opposite direction.
Magnitude of the shift on the EWF for the capped devices as a function of % SiO2 present in the gate dielectric. 0% SiO2 corresponds to devices built with HfO2 gate dielectric, while the 60% SiO2 corresponds to the HfSixOy dielectric. The magnitude of the shift in the Al-based capping layers is higher than the rare earth based ones.
Interfacial fixed charge density extracted from the plots of VFB versus EOT for all the devices in this study. The interaction between the capping layer and the underlying gate stack increases with % SiO2 in the gate dielectric, resulting in more charges at the interface. Fixed charge density is higher for Al-containing capping layers compared to the lanthanide capping layers. This may be due to the lower melting point and smaller size of any elemental Al, allowing more extensive diffusion and subsequent interfacial reactions.
O 2p PDOS for (a) 0%, (b) 50%, and (c) 100% La substitution at the HfO2/SiO2 interface. The valence band offset (VBO) is measured from the top ofvalence band of O 2p states in HfO2 with respect to the O 2p states in SiO2 (indicated by the vertical solid lines in the PDOS graphs). The VBO of pure HfO2/SiO2 interface, which indicates the magnitude of the dipole that is present in the uncapped stack, is 0.6 eV. 50% La doping shows a VBO of 0.5 eV, while 100% La doping shows VBO of 1.1 eV. The magnitude of the VBO increases with the concentration of dopants, matching experimental observation. The corresponding relaxed structure is shown on top of the PDOS graphs. Similar behavior was obtained using Al-based capping layers.
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