Band engineering of advanced materials for semiconductor devices

As the downscaling of metal-oxide-semiconductor field-effect transistors (MOSFETs) continues, the short-channel effects (SCEs) and contact resistance severely degrade device performance. It is crucial to understand the physics at various interfacial regions of MOSFETs to provide guidance for overcoming these limitations. In this thesis, the representative materials and their contacts employed in the development of MOSFETs are studied using density functional calculations, with an emphasis on understanding the electronic behaviours of metal-semiconductor junctions and heterojunction. The thesis studies the structural, electronic, and optical properties of nine polymorphs of SiO₂, employing both the traditional generalised gradient approximation (GGA) and the state-of-the-art screened exchange (sX) functional. Calculations using the sX functional accurately reproduce the experimental band gap values, whereas GGA is more effective in describing the optical properties. The advanced sX method and the more efficient GGA + U scheme are applied to several important oxides (ZnO, CdO, SrO, and MgO) to address the underestimated band gaps of oxides by the traditional GGA functional. The GGA + U scheme is further applied to calculate the Schottky barrier heights (SBHs) at various metal-oxide interfaces. The metal-induced gap states (MIGS) model is demonstrated to be a reliable simplified approach for predicting the pinning effect. A similar investigation involving high-κ CaF₂ is carried out, which confirms the accuracy of the sX method in characterising wide band gap materials. Moreover, the computed electronic properties of Si-CaF₂ and metal-CaF₂ interfaces obtained using the GGA + U scheme are consistent with the MIGS predictions. Studies are also conducted on p-type and ambipolar monolayer WSe₂ contacting with various metals. Through a rational design approach, weakly pinned, low-resistance metal-WSe₂ contacts are achieved, offering potential applications in 2D semiconductor devices.