Changing the Surface States of Few-layered Polycrystalline MoS₂ Thin Film
Yu-Kai Lin1*, Yang-Fang Chen1, Kuei-Hsien Chen2, Li-Chyong Chen3
1Department of Physics, National Taiwan University, Taipei, Taiwan
2Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
3Center of Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
* presenting author:Lin Yu-Kai,
Since the discovery of graphene, two-dimensional layered materials, especially the transition metal dichalcogenide (TMD), have drawn lots of attention for the study of new sciences and technological applications. Unlike the semi-metal graphene, molybdenum disulfide (MoS₂) is one of the TMD family that has a semiconductor behavior with a band gap between 1~2 eV depending on its thickness. Single-layer MoS₂ exhibits a high on/off current ratio exceeding 108 and a room-temperature carrier mobility of around 102 cm²/V-s which is comparable to silicon thin film. Although MoS₂ has shown its fascinating intrinsic properties for electronics, the interface between the contact metal and itself may severely limit the device performance. However, the surface/interface properties of MoS₂ are still vague so far. S. McDonnell et al. addressed that the intrinsic defects in MoS2 dominate the metal/MoS₂ interface resistance and provide a small Schottky barrier independent of metal contact work function. C. Gong et al. proposed that the partial Fermi level pinning occurred at the metal/MoS₂ interface due to the metal work function modification and the production of gap states. In our study, we reported for the first time the changes in surface states in few-layered polycrystalline MoS₂ thin film with various thicknesses grown on the SiO₂/Si substrate. Based on the XPS analyses, the Mo-3d and S-2p characteristic peaks all shift toward low binding energy as thickness increases, but the carbon, silicon (substrate), and oxygen (substrate) do not show such tendency. This phenomenon could be attributed to the inhomogeneous surface band bending resulting from the step-edge effects on MoS₂ surface as seen in the AFM images. We believe our finding could help clarify the surface/interface properties of MoS₂ and facilitate the development of MoS₂-based electronics.

Keywords: MoS2, surface properties, XPS