Researchers developed a method for fabricating nanographene with a density estimated at more than a trillion (10^12) nanographene islands per square centimeter. Their strategy uses a technique called plasma etching to create large numbers of defects as well as extended defects along the edges of the main defects.
The large number of charge trapping sites provided by the defects enabled the researchers to fabricate a memory device with a very competitive memory performance. One measure of large capacitance is a large memory window, which indicates that a large number of charge carriers have been trapped. Tests here revealed that the new memory has the largest ever memory window (9 volts) reported to date for a graphene-based charge trapping memory. In addition, this large memory window was maintained even after 1,000 program/erase cycles.
Overall, the researchers hope that this high-density memory will provide a path toward shrinking flash memory to even smaller scales.
“Our future research plan in this area is to realize a footprint as small as the tip of an atomic force microscope,” Meng said.
Nanographene is a promising alternative to metal nanoparticles or semiconductor nanocrystals for charge trapping memory. In general, a high density of nanographene is required in order to achieve high charge trapping capacity. They demonstrate a strategy of fabrication for a high density of nanographene for charge trapping memory with a large memory window. The fabrication includes two steps:
(1) direct growth of continuous nanographene film; and
(2) isolation of the as-grown film into high-density nanographene by plasma etching. Compared with directly grown isolated nanographene islands, abundant defects and edges are formed in nanographene under argon or oxygen plasma etching, i.e. more isolated nanographene islands are obtained, which provides more charge trapping sites.
As-fabricated nanographene charge trapping memory shows outstanding memory properties with a memory window as wide as ~9 V at a relative low sweep voltage of ±8 V, program/erase speed of ~1 ms and robust endurance of over 1000 cycles. The high-density nanographene charge trapping memory provides an outstanding alternative for downscaling technology beyond the current flash memory.
Left) Atomic force microscope image of the nanographene film with a high density of nanographene islands, which provide more charge-trapping sites to increase store capacity. (Right) Structure of the nanographene-based charge trapping memory. Credit: Meng, et al. ©2015 IOP Publishing