| Work function is one of the most crucial factors in designing the stack structure of the metal oxide semiconductor field transistors(MOSFETs). In practical applications of MOSFETs, work function of n-type and p-type devices are desired to align with the bottom conduction band and top valence band o f substrate silicon respectively. But for the MOSFETs at present stage, gate stack is still not appropriate for applications due to the difficulty of band alignment and work function modulation. Moreover, many studies have reported that two-dimensional material MoS2 has great potential in thin film transistors(TFTs) owning to its excellent performance in combating short channel effect and gate leakage current. However, there are still many problems which need to be further explored for its practical applications. In this paper, we applied first-principle calculations to investigate the work function of the Pt/HfO2 stack and MoS2 channel. O n one hand, we explored the effect of a non- metallic heat treatment ambient gas element(N) on the effective work function modulation of the Pt/HfO2 interface. O n the other hand, the band structure and the work function of MoS2 channel depending on different thickness(atom layers) and interlayer distance was studied, and the contact type and barrier height when MoS2 interfacing with high-k material HfO2 are probed into. Our results are listed as follows:(1) We investigated the impact of gas ambient element N on the Pt/HfO 2 interface effective work function(Φeff) as well as work function shift(ΔΦ) using density functional theory calculations. First, we found that doped ambient gas element N tends to be favored at the interface and near the dielectric side, and the system stability increases with increasing dopant concentration. Second, work function shift of Pt/HfO2 stack is strongly correlated to N-doped position and concentration. The modulation of Φeff and ΔΦ is most effective when the first atom layer in HfO2 side doped, and the work function shift increases(more p-type) with an increase of doped concentration. Third, the N-induced change of work function originates from interface charge redistribution, resulting in additional interface dipole moment(Δμ). Our work shows a scheme to modulate the interface structure by changing the annealing gas N composition so as to adjust the interface effective work function.(2) We constructed the MoS2 structures with different thickness(layers) and interlayer distance, and the concerned parameters based on these structures were optimized step by step. We found that the band structure and vacuum work function of MoS2 depend strongly on the thickness of MoS2 atom- layers. Both the band structure and vacuum work function vary from a gradual process of quantitative change to qualitative change when the thickness of MoS2 increases from monolayer to bulk, i.e., the characterization of band structure of MoS2 changes gradually from the monolayer to the bulk, and the vacuum work function decreases from the monolayer level to the bulk level(specifically, from 5.86 e V to 5.68eV). Furthermore, we calculated the effective work function of MoS2/HfO2 interface, and come to the conclusion that the contact type of MoS2/HfO2 interface is Schottky barrier and its corresponding barrier height is about 1.57 eV by charge distribution analysis, which could be helpful to anti-leakage current performance for transistors. |
PDF Full Text Request