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

The control of the threshold voltage Vth 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 Vth 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 Vth 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 EF downward toward midgap.