Radiation Effects in Graphene Field Effect Transistors (GFET) on Hexagonal Boron Nitride (hBN)
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Hexagonal Boron Nitride (hBN) has been proposed as a better substrate material in place of SiO2 for graphene electronic devices, especially Graphene Field Effect Transistors (GFETs), because of its lattice match to graphene and the absence of dangling chemical bonds on its planar surface. The performance of GFETs on hBN substrates is becoming increasingly well documented, but little is known about the tolerance of these devices to large doses of ionizing radiation, such as that encountered in many DoD satellite orbits. In this study, the current-voltage transconductance curves of top-gated GFET devices were measured before and after exposure to 1.7 Mrad (SiO2) total dose from 60Co gamma rays. Post-irradiation, the devices showed a distinct positive voltage shift in the charge neutrality point (a.k.a. Dirac point) of the transconductance curves averaging 1.3 volts. This shift suggests an increase in the effective hole doping of the graphene caused by a net trapped charge density of 3x1012 electrons cm-2 at one of the graphene interfaces, or possibly trapped electrons at the interface between the hBN layer and the underlying sapphire substrate. The voltage shift associated with this trapped charge was observed to build up for two days post-irradiation before saturating and remaining stable at room temperature. The post-irradiation magnitude of source-drain current showed a slight increase at the Dirac point, staying within 10% of the pre-irradiation levels. The carrier mobility extracted from the transconductance curves also showed a slight increase, remaining on average within 25% of pre-irradiation levels. Possible mechanisms for radiation-induced hole doping of the graphene are briefly considered, but additional experiments are required to distinguish between these possibilities. The observation of only modest changes in the transconductance curves following this relatively high total ionizing dose shows promise for the radiation resistance of these devices.