Effects of Doping, Strain and Size on the Electrical Properties of MoS2 Nanoribbons

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as isolated monolayers or few-layers of MoS₂ and WSe₂, have recently gained intensive interest for their potential in future electronics [1]. The bulk of TMDs usually have a layer structure which is similar to graphene. Because the layers are bound together by weak van der Waals forces, an isolated monolayer of TMDs can be easily obtained by cleaving technique. Unlike graphene, these 2D materials have a significant band gap and exhibit attractive semiconductor properties. Therefore, the 2D TMDs have been widely considered for the ultimately thin channel materials in future Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) to suppress short channel effects. Especially, TMD monolayers usually have a direct band gap, very attractive for the mixed electronic and photonic application in future logic and memory devices [2].

During the integration of these 2D materials into electronic devices, e.g., as the channel materials in MOSFETs, they will be defined by lithography into nanoribbons, doped into n- or p-type, and stressed by the strain induced from the metal contacts and dielectric interface. In this work, we have studied the effects of these factors on the band structure and electrical properties of MoS₂monolayers by using a numerical simulation method based on density functional theory. We found that the doping, strain and size could induce significant variation in the electrical properties of these 2D materials. These will be very challenging issues for the 2D materials.

In this work, we first studied the size effect on the properties of MoS₂ monolayer nanoribbons. We found that the band gap of MoS₂ nanoribbons changed from 1.8 eV of an infinite MoS₂ sheet to 1.2 eV of a 10×10 (in molecule) MoS₂ nanoribbons or 0.5 eV of a 5×10 MoS₂ nanoribbons. We also found that the doping level and dopant position have a significant effect on the electrical properties of MoS₂ nanoribbons. The doping level and dopant position can induce large variation (30%) in sheet resistance. This variation in electrical properties may post a serious challenge on the application in logic and memory devices. In addition, we found that the strain will also significantly affect the electrical and carrier transport properties of MoS₂ monolayer. Under different strain, the MoS₂ will change from direct band-gap to indirect band-gap semiconductor. As the strain further increases to certain value, the MoS₂will become metallic.

In summary, this work demonstrated that the size, doping and strain have a significant effect on the 2D materials and will induce large variation in electrical properties. Therefore, the future integration of such 2D materials for logic or memory circuit application, the size, doping and strain must be well engineered and controlled in order to minimize the device variation.

References:

1. B. Radisavljevic et al., Nat. Nano. 6, 147 (2011)

2. Z. Yin et al., ACS Nano. 6, 74 (2012)