Ultrathin transition metal dichalcogenides (TMDs) have recently been under intense research towards their electronic device applications. Some of TMDs such as MoS2 and WSe2 have band-gaps and retain semiconductor properties even if they are atomically thin. The application of TMDs to field effect transistors (FETs) need understanding how carrier type and carrier concentration are controlled with electric field effects in a nanoscale. Therefore, a tool for the nano-scale investigation of the electric field effects are important for boosting the studies in this field. Scanning nonlinear dielectric microscopy (SNDM)  is a scanning probe microscopy method, which can determine dominant carrier types and give information on carrier concentration distribution in high resolution. In this method, the electronic properties are reflected in a dC/dV image through the measurement of depletion layer capacitance locally modulated under the tip with ac voltage. In fact, it has recently been demonstrated that SNDM is applicable to a few-layer semiconductor . In addition, SNDM is possible to use for the investigation of electric field effects. In this presentation, we discuss the electric field effects in few-layer WSe2 on SiO2 substrates based on the SNDM imaging. It has been reported that WSe2 FET can show ambipolar behaviour .
Our sample was prepared by mechanically exfoliating WSe2 on a thermally oxidized Si substrate. The electric field effects were investigated by applying dc-bias voltages to the substrate relative to the tip. For no dc electric field, our few-layer WSe2 sample including a single layer area showed very weak dC/dV contrast, which implies that the sample can be seen an almost intrinsic semiconductor. On the other hand, we basically obtained n-(p-)type contrast for positive(negative) dc-bias. The observed polarities were consistent with those expected from the electric field effects, while spatial inhomogeneity was also seen. Charge injection from the tip may also be a possible cause of the observed carrier polarities. However, we did not consider charge injection was dominant in our case, which was suggested by simultaneous electrostatic force microscopy imaging. In addition, we found slow transient variation of carrier concentration after switching the polarity of dc-bias. This probably came from the interface charge states. These results demonstrate that SNDM is useful for the investigation of electric field effects on ultrathin semiconductors.
This work was partly supported by a Grant-in-Aid for Scientific Research (Nos. 15K04673, 16H02330) from the Japan Society for the Promotion of Science and the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University.
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