Impact of Metal Contacts on the Performance of Multilayer HfS Field-Effect Transistors.

HfS is one of the emerging transition metal dichalcogenides and is very promising for low-power nanoelectronics and high-sensitivity optoelectronic device applications. We studied the band structures of 1T-HfS with different thicknesses by first principles simulation, and the impact of different metal contacts to the HfS device performance has been experimentally studied. Back-gate and top-gate HfS field-effect transistors (FETs) were fabricated, and better electrical characteristics have been achieved with the FETs with the Ti/Au contact as compared with the Pt-contacted FETs. Thin layers of Pt and Ti/Au films were deposited on HfS flakes to investigate the metal/HfS interface by using scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. A smoother Ti/Au film was formed on HfS, resulting in higher carrier injection and transport efficiency. The phonon behavior being dominated by the interface chemical bonding at the Ti/Au contact region has been confirmed with the more sensitive A phonon mode from the bilayer HfS.