Inhomogeneous hysteresis in local STM tunnel conductance with gate-voltage in single-layer MoS$_2$ on SiO$_2$

TL;DR

This study investigates the inhomogeneous hysteresis in local tunnel conductance of single-layer MoS$_2$ on SiO$_2$, revealing how trap distributions and sweep directions influence electronic properties at the nanoscale.

Contribution

It provides the first detailed STM/S analysis of local hysteresis and inhomogeneities in MoS$_2$/SiO$_2$, linking trap behavior to conductance thresholds and Fermi energy pinning.

Findings

Hysteresis depends on gate voltage sweep direction and rate.

Spatial variations indicate inhomogeneous trap densities and energy distributions.

Rare p-doping regions show unusual hysteresis behavior.

Abstract

Randomly distributed traps at the MoS$_2$/SiO$_2$ interface result in non-ideal transport behavior, including hysteresis in MoS$_2$/SiO$_2$ field effect transistors (FETs). Thus traps are mostly detrimental to the FET performance but they also offer some application potential. Our STM/S measurements on atomically resolved few-layer and single-layer MoS$_2$ on SiO$_2$ show n-doped behavior with the expected band gap close to 2.0 and 1.4 eV, respectively. The local tunnel conductance with gate-voltage $V_{\rm g}$ sweep exhibits a turn-on/off at a threshold $V_{\rm g}$ at which the tip's Fermi-energy nearly coincides with the local conduction band minimum. This threshold value is found to depend on $V_{\rm g}$ sweep direction amounting to local hysteresis. The hysteresis is, expectedly, found to depend on both the extent and rate of $V_{\rm g}$-sweep. Further, the spatial variation in the local $V_{\rm g}$ threshold and the details of tunnel conductance Vs $V_{\rm g}$ behavior indicate inhomogenieties in both the traps' density and their energy distribution. The latter even leads to the pinning of the local Fermi energy in some regions. Further, some rare locations exhibit a p-doping with both p and n-type $V_{\rm g}$-thresholds in local conductance and an unusual hysteresis.