Graphene-quantum-dot nonvolatile charge-trap flash memories

Scientists have used graphene quantum dots instead of nanocrystals as the discrete charge trap material. Graphene quantum dots in particular are very new materials. As bits of graphene extracted from bulk carbon, graphene quantum dots can be engineered with specific electronic and optical properties for different purposes.

Here, the researchers prepared graphene quantum dots of three different sizes (6, 12, and 27 nm diameters) between silicon dioxide layers. The researchers found that the memory properties of the dots differ depending on their sizes. For instance, while the 12-nm dots exhibit the highest program speed, the 27-nm dots exhibit the highest erase speed, as well as the highest stability.

(a) Schematic diagram and (b) image of a graphene quantum dot flash memory. Charge storage in discrete charge traps, such as graphene quantum dots, offers the potential for high-density data storage. Credit: Joo, et al. ©2014 IOP Publishing

Nonvolatile flash-memory capacitors containing graphene quantum dots (GQDs) of 6, 12, and 27 nm average sizes (d) between SiO2 layers for use as charge traps have been prepared by sequential processes: ion-beam sputtering deposition (IBSD) of 10 nm SiO2 on a p-type wafer, spin-coating of GQDs on the SiO2 layer, and IBSD of 20 nm SiO2 on the GQD layer. The presence of almost a single array of GQDs at a distance of ~13 nm from the SiO2/Si wafer interface is confirmed by transmission electron microscopy and photoluminescence. The memory window estimated by capacitance–voltage curves is proportional to d for sweep voltages wider than ± 3 V, and for d = 27 nm the GQD memories show a maximum memory window of 8 V at a sweep voltage of ± 10 V. The program and erase speeds are largest at d = 12 and 27 nm, respectively, and the endurance and data-retention properties are the best at d = 27 nm. These memory behaviors can be attributed to combined effects of edge state and quantum confinement.

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