Graphene quantum dots as charge trap elements for nonvolatile flash memory

In this study, we investigated the charge storage properties of graphene quantum dots (GQDs) by embedding them between poly(methylmethacrylate) (PMMA) sheets as control and tunneling dielectric layers in a silicon metal-oxide-semiconductor-based nonvolatile memory device (Ag-Pd/PMMA/GQDs/PMMA/Si). The conduction mechanism through the gate dielectric stack embedded with GQDs could be explained by Fowler Nordheim (F-N) tunneling. The GQDs trapped electrons to yield a maximum memory window of ∼5.45 V (at ± 7 V) with a low leakage current density. A control experiment without GQDs obtained very low hysteresis (∼7 mV) even when the gate voltage was swept from +8.5 V to -8.5 V. Thus, the memory window obtained in the present study originated from the GQDs and not from the trapped states in the PMMA/Si interface. The maximum charge density was determined as 12.1 × 1011 cm−2, which is comparable to or even better than that found in conventional semiconductor devices. The endurance and charge retention characteristics were examined to assess the reliability of device operation when the devices were put under a constant stress of ±12 V. The flat-band voltage shifts were determined as 0.16 V and 0.23 V during the programming and erasing operations, respectively, and the charge loss was only ∼14.9% even after 105 s, thereby demonstrating the excellent charge confinement property of the GQDs.