Density Functional Analysis of Threshold Voltage Control by Oxide Dipole Layers in Si- and MoS2-Based FETs

The control of the threshold voltage V_th of high-k/metal metal-oxide-semiconductor field-effect transistor (MOSFET) gate stacks for n-type or p-type polarities by oxide dipole layers is analyzed by density functional theory. It is found that oxides such as SrO, Y₂O₃, HfO₂, Nb₂O₅ or amorphous Al₂O₃ could shift V_th to give either n- or p-polarities due to the band alignments and charge neutrality levels of the intrinsic virtual gap states of the oxides. This use of oxide layers for V_th control can be extended from Si MOSFETs to those using 2D transition metal dichalcogenide (TMD) channels such as MoS₂. The implementation of the dipole layer approach could allow bipolar operation of TMD-based FETs. As the presence of sulfur vacancies in MoS₂ can give their MOSFETs an extrinsic n-type character, we find that this doping effect could be compensated by an Al₂O₃ layer that shifts E_F downward toward midgap.