Exploring mechanism of defect engineering and metal-insulator transition in MoS₂ field effect transistors

Hao Wei Tu^1*, Jian-Jhong Lai¹, Ji-Chang Luo¹, Chin-Lung Lin¹, Geng-Li Lin¹, Wen-Bin Jian¹

¹Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan

* Presenter:Hao Wei Tu, email:tuhw0.ep05g@g2.nctu.edu.tw

The graphene devices¹ of fabrication technology is widely employed in emergent two-dimensional (2D) semiconducting materials to make field effect transistors (FETs). According to neoteric reports, graphene is not suitable for making FET devices cause it doesn’t possess any band gaps. In comparison, the single-layer MoS₂ exhibits a direct band gap of ~1.9 eV and an on/off ratio of up to 10⁶ in recent reports, indicating a potential candidate for applications on optical or logical electronic devices in the future².
In our research, we employ mechanical exfoliation to transfer few-layer thick MoS₂ flakes on SiO₂/Si substrates. The Au electrode is followed by a standard electron-beam patterning and thermal evaporation on MoS₂. The as-made MoS₂ FETs show an on/off ratio in the range between 10⁶ and 10⁸ and the highest current density of ~10 μA/μm at room temperature of the electrical characteristic. The mobility shows the range from 20 to 60 cm²V^-1s^-1 at temperatures ranging from 80 to 300 K. It is interesting that the MoS₂ FET devices can be categorized into two distinct groups. One group of MoS₂ FET devices exhibit a high mobility at room temperature, accompanied with an increased mobility with decreased temperature and the other group of MoS₂ FET devices exhibit a low mobility at room temperature accompanying with a decreasing mobility with a decrease of temperature.
The two different temperature behaviors of mobility indicate different disorder characteristic in low- and high-mobility MoS₂ FET devices³. The difference between this two group shows the metal-insulator transition in low disorder group. In the low disorder MoS₂ FET devices, the phonon scattering dominates while, in the high disorder devices, the impurity scattering dominates in electron transport and the defects of devices which can identify by HRTEM. On the other hand, The Mott’s 2D variable range hopping transport can well-described temperature behaviors of conductance⁴. We can successes estimate the disorder parameter T₀ of all devices and find that the room-temperature mobility of MoS₂ FET devices decrease with increasing T₀ parameter by a fitting curve. It indicates a strong dependency between the mobility and the disorder effect in MoS₂.

Reference:

1. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A., Electric field effect in atomically thin carbon films. science 2004, 306 (5696), 666-669.
2. Desai, S. B.; Madhvapathy, S. R.; Sachid, A. B.; Llinas, J. P.; Wang, Q.; Ahn, G. H.; Pitner, G.; Kim, M. J.; Bokor, J.; Hu, C., MoS2 transistors with 1-nanometer gate lengths. Science 2016, 354 (6308), 99-102.
3. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, i. V.; Kis, A., Single-layer MoS 2 transistors. Nature Nanotechnology 2011, 6 (3), 147.
4. Mott, N., Metal-insulator transition. Reviews of Modern Physics 1968, 40 (4), 677.

Keywords: MoS2, 2D material, MIT, high mobility, low disorder