| AVS 61st International Symposium & Exhibition | |
| Scanning Probe Microscopy Focus Topic | Thursday Sessions |
| Session SP+2D+AS+EM+MC+NS+SS-ThM |
| Session: | Probing Electronic and Transport Properties |
| Presenter: | Corentin Durand, Oak Ridge National Laboratory |
| Authors: | C.P. Durand, Oak Ridge National Laboratory J. Fowlkes, Oak Ridge National Laboratory S. Najmaei, Rice University J. Lou, Rice University A.P. Li, Oak Ridge National Laboratory |
| Correspondent: | Click to Email |
Transition metal dichalcogenides like molybdenum disulphide (MoS2) have attracted great interest as candidate to fill the need of 2 dimensional semiconductor materials. By controlling the thickness, the bandgap of MoS2 thin films can be tuned from 1.2 eV (bulk material, indirect bandgap) to 1.8 eV (monolayer film, direct bandgap). Recently, researchers succeeded in growing monolayered MoS2 by chemical vapor deposition (CVD) on silicon dioxide (SiO2) substrate, showing the possibility of low cost scalable device fabrication. However, the mobility reported on exfoliated MoS2 monolayers exceeds 200 cm2.V-1.s-1, whereas the measurements realized on CVD growth MoS2 monolayers reveal a mobility value that is usually 1-2 orders of magnitude lower. Here, we study the transport properties of CVD-grown monolayer on SiO2/Si substrate. We directly measure the resistivity and the mobility of the material with a field-effect transistor architecture by using a cryogenic four-probe scanning tunneling microscope (STM), the Si substrate being used as back-gate. In order to ensure reliable electrical contacts, we fabricate platinum pads (4x4 µm2) on individual MoS2 crystal domains by using an electron-beam induced deposition technique. The combination of the STM scanners and a scanning electron microscope (SEM) enables us to connect the STM tips on those pads and thereby establish the contacts on this material without any subsequent lithography process, avoiding contaminations introduced by other technological steps. An electron hopping process in localized charge trapping states appears to dominate the transport behavior. We performed temperature-dependent measurements in the range of 82 K to 315 K which demonstrate a variable range hopping (VRH) transport with a very low mobility. Furthermore, the effects of electronic irradiation are examined by exposing the film to electron beam in the SEM in an ultra-high vacuum environment. We found that the irradiation process affect the mobility and also the carrier concentration of the material, with conductance showing a peculiar time-dependent relaxation behavior. It is suggested that the presence of defects such as vacancies and antisites create charge trapping states, leading to the low mobility. This is consistent with recent density functional theory calculations where these defects are shown to create localized gap states that can act as scattering centers and thereby reduce the mobility.