We report the transistor properties of MoS2 thin films formed with a multi-step chemical vapor deposition (CVD) method. The established multi-step CVD technique has four steps: MoO3 thermal evaporation, annealing for MoO3 crystallization, sulfurization, and post-annealing. We found that the MoS2 transistor properties were greatly affected by the post-annealing temperature (TPA). The films worked as ambipolar transistors below TPA = 1000 °C. Meanwhile, the transistor operation transited from ambipolar to n-type transport at a TPA of 1000 °C. X-ray photoelectron spectroscopy measurements revealed that the films annealed below 1000 °C had sulfur-rich compositions (S/Mo > 2). The excess S atoms were reduced by elevating the annealing temperature to produce an almost stoichiometric composition (S/Mo = 2) at 1000 °C. These results indicate that excess sulfurs are responsible for the ambipolar operation by acting as acceptors that generate holes. Moreover, the high-temperature annealing at 1000 °C had another distinct effect, i.e., it improved the crystallinity of the MoS2 films. The electron mobility consequently reached 0.20 ± 0 .12 cm2/V s.

No abstract

Transition metal dichalcogenide MoS2 is a two-dimensional material, attracting much attention for next-generation applications thanks to rich functionalities stemming from its crystal structure. Many experimental and theoretical works have focused on the spin-orbit interaction which couples the valley and spin degrees of freedom so that the spin-states can be electrically controllable. However, the spin-states of charge carriers and atomic vacancies in devices have not been yet elucidated directly from a microscopic viewpoint. Here, we report the spin-states in thin-film transistors using operando electron spin resonance spectroscopy. We have observed clearly different electron spin resonance signals of the conduction electrons and atomic vacancies, and distinguished the corresponding spin-states from the signals and theoretical calculations, evaluating the gate-voltage dependence and the spin-susceptibility and g-factor temperature dependence. This analysis gives deep insight into the MoS2 magnetism and clearly indicates different spin-scattering mechanisms compared to graphene, which will be useful for improvements of the device characteristics and new applications.