Electrical Properties of Ozone-Treated MoOx/MoS2 Transistors
Pang-Chia Chang1*, Jian-Jhong Lai1, Li-Tsung Liu1, Po-Sheng Wang1, Bing-Shiuan Shie1, Wen-Bin Jian1, Yen-Fu Lin2, Kazuhito Tsukagoshi3
1Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan
2Department of Physics, National Chung-Hsing University, Taichung 40227, Taiwan
3WPI Center for Materials Nanoarchitechtonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
* Presenter:Pang-Chia Chang, email:iawia810130@yahoo.com.tw
   Molybdenum disulfide (MoS2) is representative of the family of layered transition metal dichalcogenides (TMDCs). By using mechanical exfoliation, atomically thin MoS2 films can be obtained and transferred to a SiO2/Si substrate. After electron-beam patterning and thermal evaporation, metal electrodes are deposited on MoS2 films so as to make MoS2 field-effect transistors (FETs). The devices demonstrate good ohmic contact, n-type conducting transfer characteristics, and high on/off current ratios of up to 108. Here, ozone exposure at temperatures above 150oC for several hours is treated on the MoS2 FET devices. Like that reported in the literatures,[1, 2] the oxidation results in etching of MoS2 and conversion from MoS2 to MoOx with x ≤ 3. It is reported that MoOx exhibits high work function potential that can be used as an efficient hole injection in organic devices.[3, 4]
   Scanning tunneling microscope equipped with tunneling spectroscopy is used to inspect the oxidized MoS2 surface. The topography images show either single- or double-layer-deep triangular pits while the tunneling spectra presents zero band gap in contrast to a clean band gap of MoS2 surface before oxidation. Additionally, the oxidation ratio of MoOx can be controlled by the time period and the substrate temperature during the ozone processing thus resulting in the variation of work function of metal electrodes on MoOx (Au/MoOx). After oxidation, MoOx/MoS2 FETs changes from its natively n-type to either ambipolar or p-type manners. The MoOx/MoS2 devices show a high on/off current ratio of up to 105. We examined the contact issue and found that the ambipolar MoOx/MoS2 FETs usually exhibit back-to-back Schottky contact features with an effective Schottky barrier height of ~0.3 eV. Both the carrier concentration and the electric field in the channel can be used to modulate the effective Schottky barrier. On the other hand, electron transport mechanism of MoOx/MoS2 transistors is examined. The carriers in both n- and p-channel show two-dimensional Mott’s variable range hopping transport. In this report, we present a simple ozone treatment to make complement FET devices on MoS2 films.
[1] M. Yamamoto, T. L. Einstein, M. S. Fuhrer, W. G. Cullen, Anisotropic Etching of Atomically Thin MoS2, J. Phys. Chem. C 117 (2013) 25643-25649.
[2] S. KC, R. C. Longo, R. M. Wallace, K. Cho, Surface Oxidation Energetics and Kinetics on MoS2 Monolayer, J. Appl. Phys. 117 (2015) 135301.
[3] S. Chuang, C. Battaglia, A. Azcatl, S. McDonnell, J. S. Kang, X. Yin, M. Tosun, R. Kapadia, H. Fang, R. M. Wallace, A. Javey, MoS2 P-type Transistors and Diodes Enabled by High Work Function MoOx Contacts, Nano Lett. 14 (2014) 1337-1342.
[4] M. T. Greiner, L. Chai, M. G. Helander, W. M. Tang, Z. H. Lu, Metal/Metal-Oxide Interfaces: How Metal Contacts Affect the Work Function and Band Structure of MoO3, Adv. Funct. Mater. 23 (2013) 215-226.

Keywords: Molybdenum disulfide, Ozone treatment, Field-effect transistor, Transport mechanism