Reducing non-linear effects in Kelvin Probe Force Microscopy of back-gated 2D semiconductors

TL;DR

This paper demonstrates that using a thin hBN dielectric in Kelvin probe force microscopy allows for accurate measurement of Fermi level and bandgap in 2D semiconductors by reducing nonlinear effects caused by probe voltage doping.

Contribution

It introduces a method to minimize nonlinear effects in KPFM measurements of 2D semiconductors using thin hBN dielectric, improving measurement accuracy.

Findings

KPFM signals align with theoretical expectations when using thin hBN dielectric.

Experimental results match literature values of WSe2 bandgaps.

Method enhances KPFM utility in 2D device analysis.

Abstract

In 2D field effect transistors the gate electrostatically dopes the 2D semiconductor (2DSC) channel, tuning the Fermi level. In principle, Kelvin probe force microscopy (KPFM) can detect the Fermi level, and its dependence on gate bias as well as position, potentially directly yielding band gaps, contact barriers, spatial nonuniformities, and sub-gap densities of states in such devices. However, KPFM relies on an oscillating probe voltage which itself electrostatically dopes the 2DSC, potentially creating a nonlinear response. Here, we show that when a suitably thin hBN back-gate dielectric is used, the KPFM signal agrees well with expectations, as explained by a quasistatic charge-balance model. Corresponding experimental results are consistent with the literature values of the bandgaps of monolayer and trilayer WSe2. With this approach, the widely available technique of KPFM should find improved utility and new uses in the study of 2D devices.