Carrier transport properties of MoS2 field-effect transistors produced by multi-step chemical vapor deposition method

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 (T-PA). The films worked as ambipolar transistors below T-PA = 1000 degrees C. Meanwhile, the transistor operation transited from ambipolar to n-type transport at a T-PA of 1000 degrees C. X-ray photoelectron spectroscopy measurements revealed that the films annealed below 1000 degrees 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 degrees 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 degrees C had another distinct effect, i.e., it improved the crystallinity of the MoS2 films. The electron mobility consequently reached 0.20 +/- 0.12 cm(2)/Vs. Published by AIP Publishing.