Graphene nanomechanical sensing of trap-assisted charge dynamics in leaky silicon oxide

Abstract

Nanomechanical resonators can detect electronic degrees of freedom through their sensitivity to small electrical forces. Here, we use few-layer graphene membranes as nanomechanical probes to investigate the slow dynamics of injected charge carriers in a trap-rich, leaky dielectric substrate. The dielectric consists of thermally grown SiO2 containing charge trapping centers introduced by the milling of cavities with a Ga+ focused ion beam. When suspended over these cavities, the resonators exhibit anomalous electromechanical behavior: their vibrational resonant frequencies are hysteretic when sweeping a dc gate voltage, and voltage steps result in an initial sharp increase in both frequency and static displacement, followed by a gradual relaxation toward steady values. We attribute this behavior to trap-assisted transport, in which injected charges undergo trapping and detrapping and gradually leak through the oxide, leading to a time-dependent evolution of the effective electrostatic force. Finally, we show that long-lived trapped charges give rise to a built-in potential that enables detection of the driven mechanical response even in the absence of an applied gate voltage, indicating opportunities for nanomechanical sensing of environmental charge dynamics.