On the Contact Optimization of ALD-Based MoS FETs: Correlation of Processing Conditions and Interface Chemistry with Device Electrical Performance.

Despite the extensive ongoing research on MoS field effect transistors (FETs), the key role of device processing conditions in the chemistry involved at the metal-to-MoS interface and their influence on the electrical performance are often overlooked. In addition, the majority of reports on MoS contacts are based on exfoliated MoS, whereas synthetic films are even more susceptible to the changes made in device processing conditions. In this paper, working FETs with atomic layer deposition (ALD)-based MoS films and Ti/Au contacts are demonstrated, using current-voltage (-) characterization. In pursuit of optimizing the contacts, high-vacuum thermal annealing as well as O/Ar plasma cleaning treatments are introduced, and their influence on the electrical performance is studied. The electrical findings are linked to the interface chemistry through X-ray photoelectron spectroscopy (XPS) and scanning transmission electron microscopy (STEM) analyses. XPS evaluation reveals that the concentration of organic residues on the MoS surface, as a result of resist usage during the device processing, is significant. Removal of these contaminations with O/Ar plasma changes the MoS chemical state and enhances the MoS electrical properties. Based on the STEM analysis, the observed progress in the device electrical characteristics could also be associated with the formation of a continuous TiS layer at the Ti-to-MoS interface. Scaling down the Ti interlayer thickness and replacing it with Cr is found to be beneficial as well, leading to further device performance advancements. Our findings are of value for attaining optimal contacts to synthetic MoS films.