Theoretical study of HfO₂ as a gate material for CMOS devices
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The continual downscaling of the thickness of the SiO₂ layer in the complementary metal oxide semiconductor (CMOS) transistors has been one of the main driving forces behind the growth of the semiconductor industry for past 20-30 years. The gate dielectric works as a capacitor and therefore the reduction in thickness results in increase of capacitance and the speed of the device. However, this process has reached the limit when the further reduction of the SiO₂ thickness will result in a leakage current above the acceptable limit, especially for mobile devices. This problem can be resolved by replacing SiO₂ with materials which have higher dielectric constants (high-k). The leading candidates to replace SiO₂ as a gate material are hafnium dioxide and hafnium silicate. However, several problems arise when using these materials in the device. One of them is to find p and n type gate metals to match with the valence and conduction band edges of silicon. This problem can be rooted in lack of our understanding of the band alignment and its controlling mechanisms between the materials in the gate stack. Theoretical simulations using density functional theory can be very useful to address such problems. In this dissertation present a theoretical study of the band alignment between HfO₂ and SiO₂ interface. We identify oxygen coordination as a governing factor for the band alignment. Next, we discuss effects of Al incorporation on the band alignment at the SiO₂/HfO₂ interface. We find that one can tune the band alignment by controlling the concentration of Al atoms in the stack. We also perform a theoretical study of HfO₂/Metal interface in case of Rh. We identify Rh as a good candidate for a p-type gate metal due to its large work-function and the low oxidation energy. Finally, we report a study of the stability of oxygen vacancies across the gate stack. We model a gate stack composed of n-Si/SiO₂/HO₂/Rh. We find that oxygen vacancies are easier to create in SiO₂ than in HfO₂. Also, vacancies in HfO₂ modify the band alignment, while in SiO₂ they have no effect.