Inhomogeneous work-function hysteresis in chemical vapor deposition-grown graphene field-effect devices

The work function of graphene is known to be tuned by gate voltage on SiO₂ substrates. For a graphene field-effect transistor (FET), chemical species such as oxygen and water molecules induce electrostatic interactions between the graphene and SiO₂ substrate, and consequently, the conductance of graphene can be shifted with respect to the applied field effect. Furthermore, these shifts cause an inevitable hysteresis in the I–V characteristics of the graphene device, which degrades its performance. Herein, we study the work-function of graphene devices on SiO₂ substrates with chemical vapor deposition-grown graphene using Kelvin probe force microscopy and report the gate voltage-dependent work-function hysteresis, which is analogous to the hysteresis in the electronic transport of graphene FETs. The degree of work-function hysteresis is inhomogeneous depending on the positions on the graphene, and it originates from the inhomogeneous distribution of the chemical species such as H₂O and O₂ molecules at the interface of a graphene/SiO₂ substrate. This inhomogeneity of chemical species seems to be moderated by the gate voltage sweeping during the gate-voltage-dependent work-function measurements. The proposed study provides an advanced understanding of hysteresis phenomena in graphene devices and the guidance for developing controlled graphene devices with minimal influence from ambient conditions.