Nanoscale characterization of unintentional doping of atomically thin layered semiconductors by scanning nonlinear dielectric microscopy

We use scanning nonlinear dielectric microscopy (SNDM) to visualize unintentional carrier doping of few-layer Nb-doped MoS$2$ mechanically exfoliated on SiO$2$⁠. SNDM enables imaging of the majority carrier distribution in as-exfoliated samples at the nanoscale. We show that, unlike thick MoS$2$ layers, atomically thin layers exhibit a $p$- to $n$-type transition as the thickness decreases. The level of the observed unintentional $n$-doping is estimated to be $1×1013$ $cm−2$⁠, in agreement with the results of previous independent studies. In addition, the influence of ultraviolet–ozone treatment on the majority carrier distribution is also investigated. The $n$-type doping is observed to progress with increasing processing time. SNDM can be readily applied to atomically thin layered semiconductors and will advance understanding of and the ability to predict device characteristics even at an early stage of the fabrication process.