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

Abstract

We use scanning nonlinear dielectric microscopy (SNDM) to visualize unintentional carrier doping of few-layer Nb-doped MoS2 mechanically exfoliated on SiO2. SNDM enables imaging of the majority carrier distribution in as-exfoliated samples at the nanoscale. We show that, unlike thick MoS2 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.

Boxcar Averaging Scanning Nonlinear Dielectric Microscopy

“…Furthermore, we address a recent application of the proposed method to the imaging of the dominant carrier concentration distribution in atomically thin van der Waals semiconductors. As reported in greater detail in our recent paper [ 14 ], we were able to observe an anomalous doping effect on few-layer MoS 2 by utilizing the proposed method. Because of the similar imaging mechanism, the idea presented here can also be applied to the optimization of S/N ratios in other scanning near-field microwave microscopy such as scanning microwave impedance microscopy (SMIM) [ 30 ] combined with peak-force tapping AFM [ 25 ].…”

“…PFT-SNDM was able to visualize the distributions of dominant carrier concentration on atomically thin MoS 2 including single-layer structures. In addition to the capability of avoiding damaging the tip and the sample, the imaging stability of the SNDM channel is improved by suppressing the probability of charge injection from the tip to the sample, which can accidentally cause abrupt changes to the signal intensity [ 14 ]. Another benefit is that PFT-AFM had much better reproducibility in the measurement of topographic height differences in different stacking layers, which helps the identification of the layer number.…”

“…As already shown by the authors, SNDM is useful for the investigation of the anomalies in the doping levels on atomically thin van der Waals semiconductors [ 13 , 14 ]. Here, we introduce the application of the proposed BA-PFT-SNDM to the measurement of atomically thin natural and Nb-doped MoS 2 .…”

“…The detailed analysis and discussion on the measurement results are shown in greater detail in Ref. [ 14 ]. It is noted that PFT-AFM combined with SNDM has been known for its capability of quantitative nanomechanical mapping such as adhesion, modulus, and dissipation as well as topographic imaging [ 25 ].…”

“…SNDM was originally devised for imaging electric anisotropy of dielectrics such as ferroelectric domains [ 4 ] and has the potential to become a key technology for ferroelectric probe data storage enabling Tbit/inch 2 recording density [ 5 , 6 ]. The scope of applications has also extended to the nanoscale evaluation of semiconductor materials and devices, including dopant profiling in miniaturized transistors [ 7 , 8 ], imaging the stored charges in flash memories [ 9 ], carrier distribution imaging on SiC power transistors [ 10 ], amorphous and monocrystalline Si solar cells [ 11 , 12 ], and atomically-thin layered semiconductors [ 13 , 14 ]. SNDM and its potentiometric extension can show true atomic resolution in surface dipole imaging on a Si (111)-(7 × 7) surface [ 15 , 16 ] and single-layer graphene on SiC [ 17 ].…”